MODIFICATION OF BIODEGRADABLE POLYMER FILMS · Modification of biodegradable polymer films iii were...

MODIFICATION OF BIODEGRADABLE

POLYMER FILMS

Camille Fromageot

Master in Materials Science

Submitted in fulfillment of the requirements for the degree

of Doctor of Philosophy

Faculty of Science and Engineering

Queensland University of Technology

2019

Modification of biodegradable polymer films i

Keywords

1,8-Diazabicyclo[5.4.0]undec-7-ene, 2-oxepane-1,5-dione, accelerated artificial

ageing, biodegradable polymers, blends, chain scission, copolymers, crosslinking,

degradation, L-lactide, organocatalysis, photodegradation, photoprodegradant,

photooxidation, polyesters, poly(L-lactide), reactive extrusion, ring-opening

polymerisation, thermo-oxidation, tin (II) octanoate, transesterification, Ultraviolet

irradiation.

ii Modification of biodegradable polymer films

Abstract

Poly(L-lactide) (PLLA), a biodegradable and compostable polyester, features versatile

properties that support its use as a sustainable alternative to common polyolefins.

However, to reduce environmental impact of dispersed polymer fragments, PLLA-

based polymers need to be developed that can degrade within tailored life-times and

without requiring additional waste treatment. Therefore, the global objective of this

PhD project was to accelerate the photodegradation of PLLA by adding ketone

moieties into the polymer matrix. A lactone-type molecule that featured a ketone

within its structure, 2-oxepane-1,5-dione (OPD), was selected to either be blended with

poly(L-lactide) or copolymerized with L-lactide to produce photodegradable PLLA-

based materials.

OPD was first used as an additive that was blended with a commercial grade PLLA.

Films were produced with an OPD content ranging from 0 to 10 wt%, and then

artificially aged under conditions mimicking natural outdoor exposure. A faster

embrittlement was observed for films containing 4 to 10 wt% OPD compared to neat

PLLA and films with 2 wt% OPD. Gel permeation chromatography, differential

scanning calorimetry as well as spectroscopic techniques enabled the photosensitizing

role of OPD, when used as an additive, to be assessed.

Following the successful use of OPD for increasing the photodegradation rate of

PLLA, its incorporation into the backbone of PLLA was then investigated. Both in-

melt modification and copolymerisation were employed. First, transesterification

reactions between the ester groups of PLLA and OPD were investigated via reactive

extrusion in an effort to prepare poly(L-lactide-co-OPD) copolymers. However,

spectroscopic techniques demonstrated the absence of incorporated OPD while

revealing evidence of thermo-oxidative degradation. In order to limit the extent of such

degradation, ring-opening polymerisation of L-lactide and OPD were carried out under

milder conditions. Two sets of conditions were investigated: in the bulk at 110 ºC with

tin (II) octanoate as catalyst; and, in solution at room temperature using 1,8-

diazabicyclo[5.4.0]undec-7-ene (DBU) and benzyl alcohol as catalyst and initiator,

respectively. Experiments carried out in the bulk resulted in limited incorporation of

OPD and for the DBU-benzyl alcohol system, although poly(L-lactide) homopolymers

Modification of biodegradable polymer films iii

were obtained within short times, no copolymer could be obtained. In both cases, the

ketone moiety hindered the polymerisation by competing with the ester functional

groups from both monomers for reaction with the catalysts.

In order to confirm the hindering effect of the ketone moiety of OPD during

copolymerisation, a modified OPD was employed where the ketone was protected

using ethylene ketal groups. Copolymerisation reactions of this protected OPD

monomer with L-lactide were performed in the bulk using tin (II) octanoate and benzyl

alcohol as catalyst and initiator, respectively. Copolymers with various compositions

were obtained, and subsequent deprotection steps afforded poly(L-lactide-co-OPD).

Accelerated ageing of copolymers revealed increased rates of photodegradation

compared to PLLA itself, with a mechanism based on crosslinking events rather than

chain scissions.

Overall, this PhD project showed that 2-oxepane-1,5-dione could increase the

photodegradation rate of PLLA when used as an additive or when incorporated into

the PLLA backbone, with each method of incorporation showing a different

mechanism of degradation. OPD, as an additive, efficiently accelerated the

photodegradation of poly(L-lactide) to the point of embrittlement via both cross-

linking and chain scission, while only crosslinking of the copolymer was observed

when OPD was incorporated into the PLLA backbone.

iv Modification of biodegradable polymer films

Table of Contents

Keywords .................................................................................................................................. i

Abstract .................................................................................................................................... ii

Table of Contents .................................................................................................................... iv

List of Figures ......................................................................................................................... vi

List of Schemes ...................................................................................................................... xii

List of Tables ......................................................................................................................... xiv

List of Abbreviations ............................................................................................................ xvii

Statement of Original Authorship .......................................................................................... xx

Acknowledgements ............................................................................................................... xxi

Chapter 1: Literature Review ............................................................................. 1

1.1 Introduction .................................................................................................................... 1

1.2 Polylactide ...................................................................................................................... 3 1.2.1 Evolution of the Polylactide Market .................................................................... 3 1.2.2 From Lactic Acid to Polylactide .......................................................................... 4 1.2.3 Thermal Properties and Crystallinity of Polylactide .......................................... 10 1.2.4 Mechanical Properties ........................................................................................ 13

1.3 Degradation of Polylactide ........................................................................................... 14 1.3.1 Biodegradation ................................................................................................... 15 1.3.2 Thermal Degradation ......................................................................................... 17 1.3.3 Photodegradation ............................................................................................... 20

1.4 Tailoring the Degradation of Poly(L-Lactide) .............................................................. 25 1.4.1 Accelerating the Biodegradation Rate ............................................................... 25 1.4.2 Improving the Thermal Resistance and Mechanical Properties ......................... 26 1.4.3 Accelerating the Photodegradation Rate ............................................................ 28

1.5 Tailoring Degradability ................................................................................................ 30

1.6 Project Proposal ........................................................................................................... 34

1.7 List of References ........................................................................................................ 37

Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(L-

lactide) 47

2.1 Background .................................................................................................................. 47

2.2 Results and Discussion ................................................................................................. 49 2.2.1 2-Oxepane-1,5-Dione as a Photosensitizer ........................................................ 49 2.2.2 Initial Characteristics of the Films of Poly(L-lactide) and 2-Oxepane-1,5-

Dione .................................................................................................................. 51 2.2.3 Photodegradation of PLLA - OPD Blends ......................................................... 61 2.2.4 Influence of Temperature on the Degradation Behaviour of the Blends ........... 75 2.2.5 Mechanism of Photodegradation ....................................................................... 80

2.3 Summary ...................................................................................................................... 84

2.4 Experimental ................................................................................................................ 84

Modification of biodegradable polymer films v

2.5 List of References .........................................................................................................87

Chapter 3: Reactive Extrusion of Poly(L-lactide) With 2-Oxepane-1,5-

Dione 93

3.1 Background ...................................................................................................................93

3.2 Results and Discussion .................................................................................................94 3.2.1 Melt-Modification of Poly(L-lactide) with 2-Oxepane-1,5-Dione .....................94 3.2.2 Thermal Stability of 2-Oxepane-1,5-Dione ......................................................122

3.3 Summary .....................................................................................................................124

3.4 Experimental ...............................................................................................................124

3.5 List of References .......................................................................................................130

Chapter 4: Functionalization of Poly(L-lactide) with 2-Oxepane-1,5-

Dione 133

4.1 Background .................................................................................................................133

4.2 Results and Discussion ...............................................................................................136 4.2.1. Transition Metal-Catalysed Copolymerisation of L-lactide and 2-

Oxepane-1,5-Dione in the Bulk ........................................................................136 4.2.2. Organocatalysed Copolymerisation of L-Lactide and OPD in Solution ...........153

4.3 Summary .....................................................................................................................163

4.4 Experimental ...............................................................................................................164


Chapter 5: Photodegradation of Functionalized Poly(L-Lactide) with 2-

Oxepane-1,5-Dione ................................................................................................. 175

5.1 Background .................................................................................................................175

5.2 Results and Discussion ...............................................................................................177 5.2.1 Synthesis of 1,4,8-Trioxaspiro[4.6]-9-Undecanone .........................................177 5.2.2 Synthesis of Poly(L-Lactide-co-TOSUO) ........................................................180 5.2.3 Synthesis of Poly(L-Lactide-co-2-Oxepane-1,5-Dione) ...................................190 5.2.4 Photodegradation of Poly(L-lactide-co-OPD) ..................................................196 5.2.5 Mechanism of Photodegradation ......................................................................210

5.3 Summary .....................................................................................................................212

5.4 Experimental ...............................................................................................................213 5.4.1 Material ............................................................................................................213 5.4.2 Methods ............................................................................................................213


Chapter 6: Conclusions and Future Research Directions ............................ 222

6.1 Conclusions ................................................................................................................222

6.2 Future Research Directions .........................................................................................224

Appendices .............................................................................................................. 225

vi Modification of biodegradable polymer films

List of Figures

Figure 1.1. Structures of selected biodegradable polyesters: poly(ɛ-

caprolactone) (a), poly(-hydroxybutyrate) (b) and polylactide (c). ............. 2

Figure 1.2. Stereoisomers of lactic acid and lactide.30 ................................................ 5

Figure 1.3. Structure of the different stereoisomers that can be obtained from

polymerisation of lactide: (a) poly(L-lactide) (PLLA), (b) poly(D-

lactide) (PDLA) and (c) poly(D,L-lactide) (PDLLA). .................................. 11

Figure 2.1. Structure of 2-oxepane-1,5-dione. .......................................................... 48

Figure 2.2. ATR-FTIR average spectrum of 2-oxepane-1,5-dione (average of 9

spectra after baseline correction). ................................................................ 50

Figure 2.3. UV-Visible spectrum of OPD showing an absorbance maximum at

273 nm (measured in methanol at 1 mmol·L-1; a baseline spectrum

was measured in methanol). ......................................................................... 51

Figure 2.4. Visual aspects of the films of PLLA - OPD blends before

accelerated ageing. a: 0 wt%; b: 2 wt%; c: 4 wt%; d: 6 wt%; e: 8 wt%;

f: 10 wt% OPD. ............................................................................................ 52

Figure 2.5. GPC traces of PLLA - OPD 0 - 10 wt% films before UV

degradation, measured in chloroform (the percentage values

correspond to the concentration of OPD in the blends). .............................. 54

Figure 2.6. DSC thermograms from the second heating cycle for PLLA - OPD

blend films before accelerated ageing (the percentage values

correspond to the concentration of OPD in the blends). .............................. 55

Figure 2.7. ATR-FTIR average spectra of PLLA - OPD 0-10 wt% films before

degradation (average of 9 spectra per film after baseline correction

and normalization with the -CH3 bending band at 1455 cm-1). ................... 58

Figure 2.8. Carbonyl band in the ATR-FTIR spectra of the PLLA - OPD (0 -

10 wt%) blend films before ageing revealing the shoulder from 1725

to 1690 cm-1 due to the OPD ketone moiety (average of 9 spectra per

film after baseline correction and normalization with the -CH3

bending band at 1455 cm-1). ......................................................................... 59

Figure 2.9. UV-Visible spectra of PLLA - OPD films before accelerated

ageing, showing an increase in absorbance in the range 250 - 300 nm

due to the n-π* transition of the ketone moiety of OPD. ............................. 60

Figure 2.10. Effect of UV exposure on the films of PLLA - OPD (0-10 wt%)

before (top) and after 14 days (bottom) of UV exposure using a QUV

device (UV-A 340 lamps, 50 °C) with whitening and embrittlement

observed. ...................................................................................................... 62

Figure 2.11. Evolution of the GPC distributions of each film before (plain line)

and after ten irradiation days (dashed line) in the QUV, revealing a

shift towards low molecular weight for films containing OPD. .................. 63

Modification of biodegradable polymer films vii

Figure 2.12. Decrease in the number average molecular weight of the aged

PLLA - OPD films (0 - 10 wt% OPD) versus irradiation days, as

measured by GPC in chloroform (data were obtained from three

different batches of films and averaged). ..................................................... 64

Figure 2.13. Evolution of the polydispersities of the PLLA - OPD blend films

during artificial ageing. ................................................................................ 65

Figure 2.14. 1/𝑀𝑛 vs irradiation days of the PLLA - OPD films (0 - 10 wt%). ...... 66

Figure 2.15. Evolution of the number of chain scissions of the aged films of

PLLA - OPD (0 - 10 wt%) in the QUV, calculated from the 𝑀𝑛

measured by GPC in chloroform. ................................................................ 68

Figure 2.16. Evolution of the melting temperature of the PLLA - OPD films as

a function of irradiation time in the QUV. ................................................... 69

Figure 2.17. Evolution of the glass transition of the PLLA - OPD films as a

function of irradiation time in the QUV. ..................................................... 70

Figure 2.18. ATR-FTIR average spectra of PLLA film before and after one

and ten irradiation days (average of 9 spectra after baseline correction

and normalization with the -CH3 bending band at 1452 cm-1). ................... 71

Figure 2.19. ATR-FTIR average spectra of PLLA - OPD 2 wt% film before

and after one and ten irradiation days (average of 9 spectra after

baseline correction and normalization with the -CH3 bending band at

1452 cm-1). ................................................................................................... 72

Figure 2.20. ATR-FTIR average spectra of PLLA - OPD 10 wt% film before

and after one and ten irradiation days (average of 9 spectra after


1452 cm-1). ................................................................................................... 73

Figure 2.21. UV-visible spectra from the PLLA only film as a function of

irradiation time in the QUV. ........................................................................ 74

Figure 2.22. UV-visible spectra of the PLLA - OPD 10 wt% film as a function

of irradiation time in the QUV. .................................................................... 75

Figure 2.23. Comparison of the GPC traces of neat PLLA when covered (C)

and uncovered (U) before and after ten days in the QUV. .......................... 76

Figure 2.24. Comparison of the GPC traces of PLLA - OPD 10 wt% when

covered (C) and uncovered (U) before and after ten days in the QUV. ...... 78

Figure 3.1. ATR-FTIR average spectrum of POPD revealing the ester and

ketone bands at 1723 and 1700 cm-1, respectively (nine spectra were

collected, baseline-corrected and averaged). ............................................... 95

Figure 3.2. Evolution of apparent viscosity during the extrusion of neat PLLA

at 190 °C for 10 minutes (six extrusions were performed and values of

apparent viscosity were averaged). .............................................................. 98

Figure 3.3. Evolution of apparent viscosity during the extrusion of pure PLLA,

PLLA with tin (II) octanoate and PLLA - OPD with tin (II) octanoate

formulations at 190 °C for 10 minutes (the percentages account for the

OPD initial feed; 0 wt% corresponds to the PLLA – tin (II) octanoate

formulation without OPD). .......................................................................... 99

viii Modification of biodegradable polymer films

Figure 3.4. ATR-FTIR spectra of extrudates of PLLA - OPD - tin (II)

octanoate at 190 °C for 10 minutes after one purification step (the

weight percentages represent the OPD initial feed; 9 spectra were

measured per film, baseline corrected and normalized to the -CH3

bending band at 1455 cm-1). ....................................................................... 101

Figure 3.5. Evolution of the ketone stretching shoulder at 1717 cm-1 in the

ATR-FTIR spectra of extrudates of PLLA - OPD 15 wt% - tin (II)

octanoate at 190 °C for 10 minutes with the number of purification

steps (average of 9 spectra per film after baseline correction and

normalization to the -CH3 bending band at 1454 cm-1). ............................ 102

Figure 3.6. Evolution of the 1H NMR spectra of the extrudates resulting from

the extrusion of PLLA with OPD 15 wt% at 190 ºC for 10 minutes

(top: crude extrudate; middle: extrudate after two purification steps;

bottom: extrudate after three purification steps), measured in CDCl3. ...... 103

Figure 3.7. GPC traces of purified extrudates of PLLA and OPD 0 - 15 wt% at

190 °C for 10 minutes measured in chloroform (the traces were

baseline-corrected and normalized). .......................................................... 104

Figure 3.8. DSC thermograms from the second heating cycle of purified

extrudates of PLLA - OPD (the percentages account for the OPD

initial feed; 0 wt% corresponds to the PLLA – tin (II) octanoate

formulation without OPD). ........................................................................ 107

Figure 3.9. Photographs of extrudates collected every 30 minutes of a reactive

extrusion experiment of PLLA - OPD (15.1 wt%) catalysed by tin (II)

octanoate, revealing the change of colour over time. ................................ 109

Figure 3.10. ATR-FTIR spectra of double-purified extrudates after various

residence times revealing the broadening of the carbonyl band (1820 -

1660 cm-1) and the appearance of a broad band between 3700 to 2700

cm-1 (average of 9 spectra per film after baseline correction and

normalization to the -CH3 bending band at 1454 cm-1). ............................ 110

Figure 3.11. 1H NMR spectra of double-purified extrudates of PLLA - OPD -

tin (II) octanoate after various residence times at 190 ºC, measured in

CDCl3. ........................................................................................................ 111

Figure 3.12. GPC traces of purified extrudates of PLLA - OPD 15 wt%,

collected every 20 minutes at 190 °C, measured in chloroform (the

traces were baseline-corrected and normalized). ....................................... 112

Figure 3.13. DSC thermograms from the second heating cycle of double-

purified extrudates after various residence times. ...................................... 113

Figure 3.14. Evolution of the apparent viscosity vs residence time for

extrudates of PLLA - OPD 10 wt% without and with titanium (IV)

tetrabutoxide as the transesterification catalyst. ........................................ 116

Figure 3.15. ATR-FTIR spectra of twice-purified extrudates without and with

titanium (IV) tetrabutoxide as transesterification catalyst (average of

nine spectra after baseline-correction and normalization with the –CH

stretching band at 1454 cm-1). .................................................................... 117

Modification of biodegradable polymer films ix

Figure 3.16. 1H NMR spectra of twice-purified extrudates: using titanium (IV)

tetrabutoxide (top) and without any transesterification catalyst

(bottom), measured in CDCl3. ................................................................... 118

Figure 3.17. GPC traces of PLLA - OPD 10 wt% with or without

transesterification catalyst, measured in chloroform (the traces were


Figure 3.18. DSC thermograms of purified extrudates of PLLA - OPD 10 wt%

with and without titanium (IV) tetrabutoxide as the transesterification

catalyst. ...................................................................................................... 121

Figure 3.19. TGA trace of 2-oxepane-1,5-dione measured from 0 to 1000 °C

under nitrogen showing a decomposition step around 160 ºC with an

inflection point at 184.1 °C. ....................................................................... 122

Figure 3.20. DSC thermogram of OPD on a nonisothermal run at a heating rate

of 10 ºC·min-1 under nitrogen showing both the melting (Tm) and

decomposition (Tdecomposition) phases. ......................................................... 123

Figure 4.1. 1H NMR spectra of various poly(L-lactide-co-OPD) with initial

OPD concentration of 5 (bottom); 10 (middle) and 20 mol% (top),

measured in CDCl3. ................................................................................... 138

Figure 4.2. (a) 1H NMR spectrum of poly(L-lactide-co-OPD); (b) PGSE NMR

spectra of poly(L-lactide-co-OPD) using 3 % magnetic field gradient

pulse; (c) PGSE NMR spectra of poly(L-lactide-co-OPD) using 95 %

magnetic field gradient pulse, measured in CDCl3. ................................... 140

Figure 4.3. ATR-FTIR spectra of the different poly(L-lactide-co-OPD)s

revealing the characteristic bands of poly(L-lactide) (the mol%

represents the concentration of incorporated OPD within the

copolymer). ................................................................................................ 141

Figure 4.4. Enlarged view of the carbonyl region of the ATR-FTIR spectra of

the different poly(L-lactide-co-OPD)s revealing the OPD shoulder at

1725 - 1700 cm-1 (the two maxima observed around 1750 cm-1 for 7

mol% OPD was due to noise resulting from the resolution (4 cm-1)

used to run the spectra and the normalization process to the band at

1453 cm-1 assigned to -CH3 bending). ....................................................... 142

Figure 4.5. GPC traces of synthesized poly(L-lactide) and poly(L-lactide-co-

OPD)s measured in chloroform (the traces were baseline-corrected

and normalized). ........................................................................................ 144

Figure 4.6. DSC thermograms of purified poly(L-lactide) 1 and poly(L-lactide-

co-OPD) 2, 3, 4 under nitrogen on a second heating run. .......................... 145

Figure 4.7. ATR-FTIR average spectrum of the product 5, resulting from the

ROP of L-lactide with an initial OPD feed of 50 mol% (average of 9

spectra per film after baseline correction and normalization with the -

CH3 bending band at 1456 cm-1). ............................................................... 147

Figure 4.8. ATR-FTIR average spectrum of the product 6, resulting from the

ROP of L-lactide with an initial OPD feed of 75 mol% (average of 9

spectra per film after baseline correction and normalization with the -

CH3 bending band at 1456 cm-1). ............................................................... 148

x Modification of biodegradable polymer films

Figure 4.9. GPC trace of products 5 and 6, resulting from the ROP of L-lactide

with an OPD initial feed of 50 and 75 mol%, respectively, measured

in chloroform (the traces were baseline-corrected and normalized). ......... 149

Figure 4.10. 1H NMR spectra of (a) purified compound 7; (b) purified

compound 8, measured in CDCl3. .............................................................. 151

Figure 4.11. Concentrations of analytes in red complexes measured by ICP-

OES, revealing tin and silicon as the main components. ........................... 152

Figure 4.12. Homopolymerisation of L-lactide in DCM at room temperature

(monomer conversion calculated from 1H NMR spectroscopy) under

the following conditions: [LLA]0 = 2.072 mol·L-1, LLA / BDU = 15,

LLA / benzyl alcohol = 87. ......................................................................... 155

Figure 4.13. Representative ATR-FTIR average spectrum of synthesized

poly(L-lactide) using DBU and benzyl alcohol as catalyst and initiator,

respectively. ............................................................................................... 156

Figure 4.14. ATR-FTIR averaged spectra of purified polymers with OPD

initial feed ranging from 0 to 77 mol% (average of 9 spectra after


1454 cm-1). ................................................................................................. 158

Figure 4.15. Enlarged view of the carbonyl region in the ATR-FTIR spectra of

purified polymers with an OPD initial feed of 0 to 77 mol%. ................... 159

Figure 4.16. GPC traces of copolymers of L-lactide and OPD with an initial

OPD concentration of 0 to 23 mol% measured in chloroform (the

traces were baseline-corrected and normalized). ....................................... 160

Figure 4.17. Structure of 1,4,8-trioxaspiro[4.6]-9-undecanone. ............................. 161

Figure 4.18. 1H NMR spectrum of the crude product of the ROP of L-lactide

and protected OPD revealing the conversion of L-lactide only,

measured in CDCl3. .................................................................................... 163

Figure 5.1. Structure of 1,4,8-trioxaspiro[4.6]-9-undecanone. ............................... 175

Figure 5.2. 1H NMR spectrum of TOSUO, measured in CDCl3. ............................ 179

Figure 5.3. 13C NMR spectrum of TOSUO, measured in CDCl3. ........................... 179

Figure 5.4. Representative 1H NMR spectrum of poly(L-lactide-co-TOSUO),

measured in CDCl3. .................................................................................... 182

Figure 5.5. ATR-FTIR spectra of different poly(L-lactide-co-TOSUO)s

(average of 9 spectra per film after baseline correction and

normalization with the -CH3 bending band at 1455 cm-1). ........................ 184

Figure 5.6. Enlarged view of the carbonyl region (1850 – 1650 cm-1) in the

ATR-FTIR spectra of the different poly(L-lactide-co-TOSUO)s (the

two maxima observed around 1755 cm-1 were due to noise resulting

from the resolution (4 cm-1) used to run the spectra and the

normalization process). .............................................................................. 185

Figure 5.7. GPC traces of purified poly(L-lactide-co-TOSUO) measured in

chloroform (the traces were baseline-corrected and normalized). ............. 186

Modification of biodegradable polymer films xi

Figure 5.8. DSC thermograms from the second heating cycle of purified

poly(L-lactide-co-TOSUO). ....................................................................... 189

Figure 5.9. Comparison of the 1H NMR spectra of copolymer before (referred

to as A) and after deprotection of the ketones in DCM at room

temperature (referred to as B). ................................................................... 192

Figure 5.10. ATR-FTIR spectra of poly(LLA-co-TOSUO) and poly(LLA-co-

OPD) with an enlarged view of the carbonyl region (1850 – 1650 cm-

1) revealing a shoulder at 1715 cm-1 corresponding to the C=O

stretching band of the ketone moity of OPD, a shoulder at 1775 cm-1

corresponding to the carbonyl stretching of a lactone-type product

resulting from transesterification during the deprotection step. ................ 193

Figure 5.11. Comparison of the GPC traces of the copolymer before and after

deprotection of the ketones, measured in chloroform (the traces were


Figure 5.12. Evolution of the GPC traces of poly(L-lactide-co-OPD) (5.2

mol% OPD segments) as a function of irradiation time, measured in

THF (the traces were baseline-corrected and normalized). ....................... 198

Figure 5.13. Evolution of the GPC traces of poly(L-lactide-co-OPD) (8 mol%

OPD segments) as a function of irradiation time, measured in THF

(the traces were baseline-corrected and normalized). ................................ 199

Figure 5.14. DSC thermograms from the second heating cycle of poly(L-

lactide-co-OPD) with 5.2 mol% OPD with increasing irradiation time

in the QUV. ................................................................................................ 202

Figure 5.15. Enlarged view of the glass transitions in the DSC thermograms

from the second heating cycle of poly(L-lactide-co-OPD) with 5.2

mol% OPD with increasing irradiation time in the QUV. ......................... 203

Figure 5.16. DSC thermograms from the second heating cycle of poly(L-

lactide-co-OPD) with 8 mol% OPD with irradiation days in the QUV. .... 204

Figure 5.17. ATR-FTIR average spectra of poly(L-lactide-co-OPD) (5.2 mol%

OPD) before and after ten days of UV irradiation (average of 9 spectra

after baseline correction and normalization to the -CH3 bending band

at 1454 cm-1). ............................................................................................. 207

Figure 5.18. Enlarged view of the carbonyl band in the ATR-FTIR spectra of

poly(L-lactide-co-OPD) (5.2 mol% OPD): anhydride region 1900 -

1810 cm-1 and ketone region 1740 - 1690 cm-1. ........................................ 208

Figure 5.19. ATR-FTIR average spectra of poly(L-lactide-co-OPD) (8 mol%

OPD) powder before and after irradiation (average of 9 spectra after


1454 cm-1). ................................................................................................. 209

Figure 5.20. Enlarged view of the carbonyl band in the ATR-FTIR spectra of

poly(L-lactide-co-OPD) (8 mol% OPD): anhydride region 1900 - 1810

cm-1 and ketone region 1740 - 1690 cm-1. ................................................. 210

xii Modification of biodegradable polymer films

List of Schemes

Scheme 1.1. Microbial fermentation of L-lactic acid.13 ............................................... 4

Scheme 1.2. Synthesis of lactide: polycondensation of lactic acid followed by

depolymerisation via backbiting.31 ................................................................ 6

Scheme 1.3. Anionic ring-opening polymerisation of lactide. .................................... 8

Scheme 1.4. Cationic ring-opening polymerisation of lactide.23 ................................. 9

Scheme 1.5. Ring-opening polymerisation of L-lactide following a

coordination-insertion mechanism.23 ........................................................... 10

Scheme 1.6. Hydrolysis of an ester linkage. .............................................................. 15

Scheme 1.7. (a) Intramolecular transesterification resulting in the formation of

lactide, oligomers, acetaldehyde, and carbon monoxide; (b)

intermolecular transesterification; (c) hydrolysis.62, 82 ................................. 19

Scheme 1.8. Norrish type II mechanism for the photodegradation of PLLA

under UV-C light. ........................................................................................ 21

Scheme 1.9. Proposed mechanism of racemization occurring both at the

hydroxyl chain end (left) and the carboxyl chain end (right) during the

photodegradation of PLLA exposed to UV-C light.99 ................................. 22

Scheme 1.10. Photodegradation mechanism of PLA based on hydrogen

abstraction from the carbon in the α-position to the carbonyl group

with formation of macroradicals leading to anhydride as a main

photodegradation product (X represents chromophoric defects). ................ 24

Scheme 1.11. Photodegradation of ethylene-carbon monoxide copolymers via

Norrish type I and II. .................................................................................... 32

Scheme 1.12. Synthesis of poly(2-oxepane-1,5-dione) via the ROP of TOSUO.

Conditions and reagents: a. Al(OiPr)3, toluene, 25 °C, H3O+; b.

(C6H5)3CBF4, dichloromethane, 25 °C, 1 hour. ........................................... 33

Scheme 1.13. Synthesis of poly(ɛ-caprolactone-co-2-oxepane-1,5-dione).

Conditions and reagents: a. tin (II) octanoate, toluene, 90 °C. .................... 33

Scheme 1.14. Ring-opening polymerisation of L-lactide and 2-oxepane-1,5-

dione to afford poly(L-lactide-co-2-oxepane-1,5-dione). Conditions

and reagents: a. tin (II) octanoate, 110-160 °C, in the bulk; b. DBU,

benzyl alcohol, DCM, room temperature. .................................................... 35

Scheme 1.15. Synthesis of poly(L-lactide-co-OPD) via two steps: a. Ring-

opening polymerisation of L-lactide and TOSUO to afford poly(L-

lactide-co-TOSUO). b. Deprotection of the acetal groups of poly(L-

lactide-co-TOSUO) to afford poly(L-lactide-co-OPD). ............................... 37

Scheme 2.1. Synthesis of 2-oxepane-1,5-dione. Conditions and reagents: 1,4-

cyclohexanedione, mCPBA, DCM, 40 °C, 4 h, 45 % yield. ........................ 49

Scheme 2.2. Norrish type I and II cleavages of ketones. ........................................... 81

Modification of biodegradable polymer films xiii

Scheme 2.3. Proposed photodegradation mechanism for PLLA - OPD blends

initiated by the Norrish type I cleavage of OPD, releasing radicals that

attack the PLLA backbone leading to hydrogen abstraction. The rest

of the mechanism is based on previous reports, leading to PLLA chain

scission and anhydride formation.13, 14 ......................................................... 84

Scheme 4.1. Conversion of tin (II) octanoate into tin (II) alkoxide via reaction

with alcohol or residual protic impurities. ................................................. 134

Scheme 4.2. Ring-opening polymerisation of lactide using DBU and an alcohol

as catalyst and initiator, respectively. ........................................................ 135

Scheme 4.3. Tin (II) octanoate-catalysed ROP of L-lactide and OPD at 110 °C

in the bulk to afford poly(L-lactide-co-OPD). ........................................... 137

Scheme 5.1. Two step synthesis of poly(L-lactide-co-OPD): Conditions and

reagents: a. tin (II) octanoate, in the bulk, 110 ºC; b.

triphenylcarbenium tetrafluoroborate (TPFB), DCM, room

temperature, 2 hours................................................................................... 177

Scheme 5.2. Baeyer-Villiger oxidation of 1,4-cyclohexane monoethylene

acetal by mCPBA to afford TOSUO and the mCPBA by-product, 3-

chlorobenzoic acid. .................................................................................... 178

Scheme 5.3. ROP of L-lactide and TOSUO in the bulk at 110 °C to afford

poly(L-lactide-co-TOSUO) using tin (II) octanoate and benzyl alcohol

as the catalyst and the initiator respectively. ............................................. 180

Scheme 5.4. Conversion of tin (II) octanoate into tin (II) alkoxide via reaction

with alcohol or residual protic impurities. ................................................. 187

Scheme 5.5. Chemical structure of triphenylcarbenium tetrafluoroborate

(TPFB). ...................................................................................................... 190

Scheme 5.6. Mechanism of the deprotection using TPFB involving a hydride

abstraction from the ethylene acetal that affords an oxonium ion that is

subsequently quenched by aqueous work-up.30, 31 ..................................... 191

Scheme 5.7. Deprotection of the ketone acetal groups of poly(L-lactide-co-

TOSUO) using TPFB in DCM at room temperature to afford poly(L-

lactide-co-OPD). Conditions and reagents: a. poly(L-lactide-co-OPD),

TPFB (1.5 equivalents of ethylene ketal groups), DCM, 2 hours, room

temperature, 80 - 85 %. .............................................................................. 191

Scheme 5.8. Norrish type I and II cleavages of the ketone of ring-opened OPD. ... 212

xiv Modification of biodegradable polymer films

List of Tables

Table 1.1. Comparison of the physical properties of lactic acid and lactide

enantiomers.26, 32, 33 ........................................................................................ 7

Table 1.2. Thermal properties of random stereoisomers of PLA (neither

molecular weight values nor accuracy of the thermal transitions values

were reported in the literature).39 ................................................................. 11

Table 1.3. Characteristics of the three crystal forms of PLLA. ................................. 13

Table 2.1. Average values of 𝑀𝑛 , 𝑀𝑤 and polydispersity of three batches of

PLLA -OPD blend films (OPD: 0 - 10 wt%) before accelerated

ageing, measured by GPC in chloroform. .................................................... 54

Table 2.2. Evolution of the glass transition and melting temperature of the

films with OPD content obtained by DSC before ageing (the

measurements were performed on three different batches of films and

the values were averaged). ........................................................................... 57

Table 2.3. Comparison of the thermal properties of the transparent and opaque

sections of the PLLA - OPD 10 wt% film. .................................................. 57

Table 2.4. ATR-FTIR band assignment of poly(L-lactide) based on reported

literature.13, 47 ................................................................................................ 59

Table 2.5. r2 values and rate constants k determined from the 𝑀𝑛 measured by

GPC for the six formulation films. ............................................................... 67

Table 2.6. Comparison of the 𝑀𝑛 and the polydispersity of the uncovered and

covered films of PLLA - OPD blends (0 - 10 wt%) as a function of

irradiation days. ............................................................................................ 78

Table 3.1. Formulations of the extrusions of PLLA and OPD 0 - 15 wt% using

tin (II) octanoate as the transesterification catalyst. ..................................... 96

Table 3.2. Molecular weight of three-times-purified extrudates of PLLA with

OPD (processed via reactive extrusion for 10 minutes at 190 °C,

measured by GPC in chloroform (three measurements were performed

and values were averaged; the percentages account for the OPD initial

feed; 0 wt% corresponds to the PLLA – tin (II) octanoate formulation

without OPD). ............................................................................................ 105

Table 3.3. Thermal properties of purified extrudates obtained by DSC on a

second heating cycle (three measurements were performed and values

were averaged). .......................................................................................... 108

Table 3.4. Molecular weight of purified extrudates of PLLA - OPD 15 wt% at

190 °C collected every 20 minutes, measured by GPC in chloroform

(three measurements were performed and values were averaged)............. 112

Table 3.5. Evolution of the thermal properties of purified extrudates collected

every 20 minutes, as measured by DSC on a second heating run (three

measurements were performed and values were averaged). ...................... 114

Modification of biodegradable polymer films xv

Table 3.6. Formulations of extrudates featuring titanium (IV) tetrabutoxide as

the transesterification catalyst. ................................................................... 115

Table 3.7. Molecular weight of twice-purified extrudates of PLLA - OPD 10

wt% for 10 minutes at 190 °C with or without titanium (IV)

tetrabutoxide measured by GPC in chloroform (three measurements

were performed and values were averaged). .............................................. 119

Table 3.8. Thermal properties of extrudates of PLLA - OPD 10 wt% with and

without titanium (IV) tetrabutoxide, as measured by DSC in the

second heating run (three measurements were performed and values

were averaged). .......................................................................................... 121

Table 3.9. Formulations of the different extrudates. ............................................... 125

Table 4.1. Conditions and results of the ring-opening polymerisations of L-

lactide and OPD 0 – 20 mol% at 110 ºC catalysed by tin (II) octanoate

in the bulk. ................................................................................................. 137

Table 4.2. Molecular weight and polydispersities of PLLA and poly(L-lactide-

co-OPD)s measured in chloroform (two measurements were

performed and values were averaged). ...................................................... 144

Table 4.3. Thermal properties of purified poly(L-lactide-co-OPD) measured by

DSC on a second heating cycle (two measurements were performed

and values were averaged). ........................................................................ 146

Table 4.4. Comparison of characteristic bands in the ATR-FTIR spectra of L-

lactide and poly(L-lactide) with their assignments based on reported

literature.34, 35 ............................................................................................. 148

Table 4.5. Evolution of the conversion and number average molecular weight

over time of the ROP of L-lactide using DBU and benzyl alcohol as

catalyst and initiator, respectively, DPn 87. ............................................... 154

Table 4.6. Conditions and results of the batch polymerisations of L-lactide and

OPD in solution at room temperature with DBU and benzyl alcohol as

catalyst and initiator, respectively.............................................................. 157

Table 4.7. Molecular weight and polydispersities of the purified products of

the batch polymerisations of L-lactide and OPD using DBU as the

catalyst. ...................................................................................................... 160

Table 4.8. Conditions of batch copolymerisations in DCM at room temperature

using DBU and benzyl alcohol as catalyst and initiator, respectively. ...... 169

Table 5.1. Copolymerisation of L-lactide and TOSUO in the bulk at 110 °C

catalysed by tin (II) octanoate. ................................................................... 183

Table 5.2. Comparison of the theoretical 𝑀𝑛 and the measured values for the

different poly(L-lactide-co-TOSUO) copolymers. ..................................... 186

Table 5.3. Thermal properties of the different poly(L-lactide-co-TOSUO)

(three measurements were performed and the values were averaged). ..... 189

Table 5.4. Molecular weights of copolymers before and after deprotection, as

measured by GPC in chloroform. .............................................................. 195

xvi Modification of biodegradable polymer films

Table 5.5. Thermal transitions of poly(L-lactide-co-OPD) after the

deprotection of the ketone acetal groups (three measurements were

performed and values were averaged). ...................................................... 195

Table 5.6. Evolution of the number and weight averaged molecular weights,

polydispersity and the chain scission of poly(L-lactide-co-OPD) as a

function of irradiation time in the QUV. .................................................... 200

Table 5.7. Thermal properties of aged poly(L-lactide-co-OPD) copolymers

determined by DSC for samples before irradiation and after UV

irradiation for 2-10 days (two measurements were performed and

values were averaged). ............................................................................... 205

Table 5.8. Conditions of the ROP of L-lactide and TOSUO in the bulk at 110

°C. .............................................................................................................. 215

Modification of biodegradable polymer films xvii

List of Abbreviations

𝑀𝑒 Entanglement molecular weight (g·mol-1)

𝑀𝑛 Number average molecular weight (g·mol-1)

𝑀𝑤 Weight average molecular weight (g·mol-1)

𝑇𝑔,∞ Glass transition of polylactide having an infinite molecular weight

𝜒𝑐 Crystallinity (%)

∆𝐻𝑐𝑐 Cold crystallization enthalpy (J·g-1)

∆𝐻𝑚 Melting enthalpy (J·g-1)

∆𝐻𝑚0 Melting enthalpy of 100 % crystalline PLA sample

Al(OiPr)3 Aluminium isopropoxide

AR Analytical reagent

BnOH Benzyl alcohol

br Broad

CaSO4 Calcium sulfate

CDCl3 Deuterated chloroform

CH2Cl2 Dichloromethane

Ð Dispersity

Da Dalton

DBU 1,8-Diazabicyclo[5.4.0]undec-7-ene

DCM Dichloromethane

DMF-DMA N,N-dimethylformamide dimethyl acetal

DPn Degree of polymerisation

DSC Differential scanning calorimetry

EVOH Poly(ethylene-co-vinylalcohol)

FDA Food and Drug Administration

FTIR Fourier transform infrared

GPC Gel permeation chromatography

GWP Global warming potential

HPLC High-performance liquid chromatography

ICP-OES Inductively coupled plasma optical emission spectroscopy

IR Infrared

xviii Modification of biodegradable polymer films

LCA Life cycle assessment

LDPE Low-density polyethylene

LLA L-lactic acid

m Multiplet (NMR)

mCPBA 3-Chloroperbenzoic acid

mol% Mol percentage (%)

NIR Near-infrared

NMR Nuclear magnetic resonance

OPD 2-oxepane-1,5-dione

PBAT Poly(butylene adipate-co-terephtalate)

PCL Poly(ε-caprolactone)

PDLA Poly(D-lactic acid)

PDLLA Poly(D,L-lactic acid)

PE Polyethylene

PEG Poly(ethylene glycol)

PGSE NMR Pulsed field gradient spin-echo nuclear magnetic resonance

PHB Poly(β-hydroxybutyrate)

PhD Doctor of Philosophy

PLLA Poly(L-lactic acid)

POPD Poly(2-oxepane-1,5-dione)

PTOSUO Poly(1,4,8-trioxaspiro[4.6]-9-undecanone)

PP Polypropylene

RH Relative humidity

RH Hydrodynamic radius

ROP Ring-opening polymerisation

RT Room temperature

s Chain scission

Sn(Oct)2 Tin (II) octanoate

st Strong

TBD 1,5,7-Triazabicyclo[4.4.0]dec-5-ene

Tcc Cold crystallization temperature (ºC)

Tg Glass transition temperature (ºC)

TGA Thermal gravimetric analysis

Modification of biodegradable polymer films xix

Ti(OBu)4 Titanium (IV) tetrabutoxide

TiO2 Titanium dioxide

Tm Melting temperature (ºC)

TMPD N,N,N’,N’-tetramethyl-1,4-phenylenediamine

TOSUO 1,4,8-trioxaspiro[4.6]-9-undecanone

TPFB Triphenylcarbenium tetrafluoroborate

UV Ultra-violet

w Weak

wt% Weight percentage (%)

ZnO Zinc oxide

𝑤 Weight fraction

xx Modification of biodegradable polymer films

Statement of Original Authorship

The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any other higher education institution. To the best

of my knowledge and belief, the thesis contains no material previously published or

written by another person except where due reference is made.

Signature:

Date: ________05/07/2019_________________

QUT Verified Signature

Modification of biodegradable polymer films xxi

Acknowledgements

This PhD project would not have been possible without the following persons.

My principal supervisor, Prof. Steven Bottle, for his guidance and support. I would

like to thank my associate supervisors as well, Dr Melissa Nikolic, Dr John Colwell

and Dr Christiane Lang. Thanks to the four of them for being available to discuss,

suggest ideas or help throughout my PhD project.

The Cooperative Research Centre for Polymers for the financial support, not only as a

scholarship but also for opportunities to travel and present my work during the Annual

Meetings, as well as attending several workshops to develop my communication skills.

Prof Graeme George, for giving me advice and sharing his sound knowledge and

experience during discussions.

The Cooperative Research Centre for Polymers staff in Brisbane, including Emilie

Gautier, Jorja Cork and Michael Murphy for their great support and friendship.

Other QUT academics in the chemistry discipline, Dr Llew Rintoul, and Dr Mark

Wellard for their help and discussions.

The technical staff, Dr Chris Carvahlo, Dr Lauren Butler, Mr Peter Hegarty and Ms

Leonora Newby for their availability and assistance.

My friends for their unconditional support, regardless of the distance.

My family, thank you for your presence throughout the distance and time, your

considerable support and understanding that helped me over this PhD journey.

Chapter 1: Literature Review 1

Chapter 1: Literature Review

1.1 INTRODUCTION

The world production of polymers has evolved from 230 million tons in 2005 to 335

million tons in 2016 and is expected to reach 400 million tons in 2020.1, 2 Their

durability, ease of fabrication and low cost justify the extensive use of polymers in a

broad range of applications such as packaging, textiles, medical items, and building

materials.2 However, their versatile properties have also contributed to one of the

world’s major issues: pollution.

Common polymers, such as polypropylene and polyethylene, can persist longer than

the time range required by the application fields. An appropriate waste treatment is

therefore required. To date, plastic waste is either thermally treated, recycled, or

decomposed in landfills.2 Cumulative waste of primary and recycled polymers reached

6,300 Mt between 1950 and 2015, with 60 % accumulating in landfills. If the

worldwide production and waste management follow the same trend, about 12,000 Mt

of plastic waste are estimated to accumulate in the environment by 2050.3 Plastic

debris are increasingly gathered in oceans, in concentrations reaching up to 1.0 - 2.5

kg·km-2 in the Pacific Ocean, in the west of the United States of America, or in the

Atlantic Ocean, between South America and Africa, and between Cuba and Europe.2,4

These plastics are typically not readily degraded, and come from sources such as

shopping bags, agricultural mulch films, or fishing nets and fishing lines. This leads

not only to drastic visual pollution in the environment but also to fatal consequences

for animals including turtles and whales.5

Different approaches are emerging to solve this issue. For instance, a landfill ban has

been decided by European countries, such as Denmark, Switzerland, and Germany.2

Plastic bags have also been subjected to taxation or banned, leading to a 90 % reduction

in use.6

Biodegradable plastics may provide a renewable alternative to petroleum-based

polymers and a solution to plastic pollution. These polymers emerged in 1926 with the

first report of bio-based polymers for poly(-hydroxybutyrate) (PHB) and were

developed further in the 1970s due to a petroleum crisis.7 In 2011, biodegradable

2 Chapter 1: Literature Review

plastics represented 30.1 Mt (8.2 %) of the plastics market.1 These polymers degrade

via depolymerisation of their chains through hydrolysis, and assimilation of oligomers

by microorganisms.

Polyesters have experienced a growing interest as biodegradable polymers due to the

degradability of the ester bond by hydrolysis. Several biodegradable polyesters have

been or are being commercially developed, including poly(ɛ-caprolactone) (PCL),

poly(-hydroxybutyrate) (PHB) and polylactide (PLA) (Figure 1.1).8

Figure 1.1. Structures of selected biodegradable polyesters: poly(ɛ-caprolactone) (a),

poly(-hydroxybutyrate) (b) and polylactide (c).

Polylactide is an aliphatic, thermoplastic polyester produced from lactic acid, typically

derived from renewable resources (fermented corn starch). The mechanical properties,

tensile strength and elastic modulus, are similar to polystyrene and poly(ethylene

terephthalate).9 The biodegradability and compostability of PLA strongly support its

use as a sustainable alternative to common polyolefins. Moreover, progress in

manufacturing of the monomer from fermentation has reduced the price of PLA to

$2.50 - 3.00 per kg compared to other biodegradable polyesters, such as poly(ɛ-

caprolactone) (> $9 per kg).10 The ease of fabrication and the versatile mechanical

properties have resulted in a wide range of applications, from textiles to medical

devices, or food packaging.11, 12

The environmental impact of polylactide has been quantified by life cycle assessment

(LCA) tools, from the raw material to end-of-life scenarios. Different global warming

potentials (GWP) have been assessed for polylactide, depending on the models used


in the LCA. While some studies showed little differences between PLA and their

comparable petroleum alternatives (3.0 - 3.1 CO2 eq. per kg for PLA compared to 3.5

for polystyrene), others revealed a GWP of 0.6 CO2 eq. per kg of polylactide compared

to 1.6 for polypropylene, or 2.3 for polystyrene.13 14 In terms of end-of-life scenarios,

polylactide (under dry packaging conditions) hardly degrades in landfill over a period

of hundred years, as revealed by extrapolation from collected data.15 Polylactide can

be degraded under composting conditions, at an industrial scale where conditions, such

as temperature, humidity, and microorganisms are controlled.14 However, to be

competitive to commodity polymers, PLA-based polymers need to be developed that

can degrade within tailored life-times and without requiring additional waste treatment

(e.g. in physical removal, landfill, mechanical recycling).

This literature review will focus on the different steps in the life cycle of polylactide,

except for the waste treatment phase. The production of polylactide will be reviewed

from the raw material to the polymerisation and the physico-chemical properties of the

polyester. Subsequently, three degradation pathways will be discussed, i.e. thermal,

biological and photolytic degradation, including studies that have been undertaken to

modify their rates of degradation.

1.2 POLYLACTIDE

1.2.1 Evolution of the Polylactide Market

Carothers first synthesized a low molecular weight poly(L-lactide) (PLLA) in 1932.7

Dupont (United States of America) further developed PLA manufacturing and

patented the production of a high molecular weight polyester in 1954.16 In 1966,

poly(L-lactide) was reported to have a non-toxic tissue response when implanted in

animals, resulting in developments of sutures and orthopedic applications in 1971.17

In the late 1980s, improvements in process technology for production of lactic acid

and reduction of costs broadened the application range to disposable packaging

applications, fibres for textile applications, and even more durable goods for the

automotive industry.18-22

Polylactide is now produced worldwide by different companies, such as Mitsui

Chemicals and Shimadzu Corporation in Japan, Purac Biochem in the Netherlands,

and Boeringer Ingelheim in Germany.23 NatureWorks (United States of America) is


the main manufacturer with 140,000 tons produced annually. They have launched

research and development programs to produce lactic acid via commercialization of a

new fermentation process, based on methane rather than agricultural feedstocks.24 The

development of the process, the reduction of costs, and the broadening of the

applications all suggest a growing market for PLA. Production is estimated to reach

12 million tonnes by 2020; three times more than in 2011.1

The variety of applications may be explained by the diversity of processing techniques.

The processing of PLA can be achieved by extrusion, injection moulding, blown film

extrusion, thermoforming, as well as film and sheet casting.25

1.2.2 From Lactic Acid to Polylactide

1.2.2.1. Lactic Acid and Lactide

Lactic acid (2-hydroxy propionic acid) is a hydroxycarboxylic acid with a stereo-

centre, leading to two optically active forms: the L- and D- enantiomers, respectively

(Figure 1.2). It is produced by either chemical synthesis or microbial fermentation.26

The former produces a racemic mixture of the two enantiomers while the latter results

in the natural and optically pure form of L-lactic acid.7 The stereochemical purity of

lactic acid is of primary importance as it impacts the stereochemistry of polylactide,

which influences the thermal and mechanical properties (refer to sections 1.2.3 and

1.2.4). Therefore, microbial fermentation is chosen as the main production process.27

Microbial fermentation starts with a carbon source (Scheme 1.1).26 Glucose, starch or

lignocellulose are the typical substrates from which lactic acid is derived.

Scheme 1.1. Microbial fermentation of L-lactic acid.13


However, the cost-efficiency of lactic acid production from starch and lignocellulose

has made them the principal carbon sources.28 Starch is obtained from corn, wheat or

potatoes. Hydrolysis of starch yields dextrose or maltose, which are subsequently

fermented by microorganisms to afford lactic acid.7 The microorganisms that produce

the lactic acid cannot hydrolyse the starch itself, making the enzymatic hydrolysis a

necessary step. Those microorganisms need to meet a few requirements such as a high

activity to reduce the fermentation time and a high conversion yield to reduce

feedstock costs. α-Amylase and glucoamylase are among the enzymes used for the

fermentation step and are commercially available.26

The choice of microorganism influences the stereochemical purity of the obtained

lactic acid. For instance, Lactobacilli (bacteria) and Rhizopus (fungus) produce the L-

lactic acid form in up to 99 % purity.27, 28 D-lactic acid can also be produced in 99.4 %

purity by microbial fermentation using special bacterial species, such as

Sporolactobacillus inulinus or Lactobacillus delbrueckii.26, 29

Polylactide polymerisation commences from a cyclic derivative of lactic acid, lactide

(3,6-dimethyl-1,4-dioxane-2,5-dione). The chirality of lactic acid leads to the

production of lactide in three stereoisomeric forms: L-lactide, D-lactide and a meso

form D,L-lactide (Figure 1.2).

Figure 1.2. Stereoisomers of lactic acid and lactide.30


Lactide is obtained by condensation of lactic acid under reduced pressure (70 - 250

mbar) and high temperature (190 °C) (Scheme 1.2). Oligomers of lactic acid (degree

of polymerisation (DPn) around 10) are obtained at first. Lactide is then afforded by

the depolymerisation of the prepolymer via a backbiting process, most commonly

catalysed by tin (II) octanoate.26, 31 However, the required catalyst and high

temperatures promote racemization.31 Purification is thus necessary not only to

separate lactide from the by-products (lactic acid, oligomers and water), but also

separate different isomeric forms of lactide. Distillation, solvent crystallization or melt

crystallization are used to isolate a stereochemically pure lactide.26 The stereochemical

purity not only impacts the thermal properties of the monomers, it will also influence

the purity of the resulting polylactide.

Scheme 1.2. Synthesis of lactide: polycondensation of lactic acid followed by

depolymerisation via backbiting.31

The stereochemistry of both lactic acid and lactide impacts their thermal properties

(Table 1.1). Meso-lactic acid features a melting point of 16.8 °C instead of 53 °C for

both pure L-lactic acid or D-lactic acid. L-Lactide and D-lactide have a melting point of


96 - 97 °C while the meso and racemic mixture have a melting point of 53 - 54 °C and

122 - 126 °C, respectively.

In terms of solubility, lactic acid is miscible with various solvents including

chloroform, ether, ethyl acetate, and hexanol, while lactide is soluble in benzene,

toluene, ethyl acetate, methanol, and acetone, among others.32

Table 1.1. Comparison of the physical properties of lactic acid and lactide

enantiomers.26, 32, 33

Lactic acid Lactide

Isomer L D L,D L D meso rac

Melting point

(°C)

53 53 16.8 95 - 98 95 - 98 53 - 54 122 - 126

Optical rotation

(°)

+ 2.5 - 2.5 - - 260 + 260 - -

1.2.2.2. Synthesis of Polylactide

Poly(lactic acid) is obtained by polycondensation of lactic acid while polylactide

results from the ring-opening polymerisation (ROP) of lactide.

1.2.2.2.1. Polycondensation of Lactic Acid

Polycondensation relies on the reaction of the hydroxyl and carboxylic acid groups of

lactic acid to afford poly(lactic acid) and water. The reaction is subjected to an

equilibrium; the water produced during the process needs to be removed to achieve a

better yield. The obtained molecular weight is inversely proportional to the conversion,

according to the Carothers equation:

𝐷𝑃𝑛 =

1

1 − 𝑝

With 𝐷𝑃𝑛 the average degree of polymerisation and p the conversion of monomer.34

Obtaining high molecular weight poly(lactic acid) is only achievable for high

conversions. The long reaction times and high temperatures favour side reactions such

as the formation of lactide by back-biting.35 As a result, only low molecular weight


poly(lactic acid) is formed by this synthetic method, limiting the number of

applications in which it can be used.

1.2.2.2.2. Ring-Opening Polymerisation of Lactide

The ring-opening polymerisation of lactide overcomes the issues identified above for

polycondensation. For instance, high average molecular weight polymers up to

100,000 Da can be obtained by ROP.7 Carothers first reported the ROP of lactide in

1932.36 Further improvement in the purification of lactide enabled high molecular

weight PLA to be obtained and thus the use of this method on a commercial scale.7

Three different mechanisms are generally accepted depending on the type of catalyst

used: anionic, cationic, and coordination-insertion mechanism.

The anionic mechanism is triggered by the attack of a nucleophilic initiator on the

carbon of the carbonyl group, resulting in cleavage of the C-O bond and an oxygen-

centred anion on the chain end. Repeated anionic ring-opening of the cyclic monomer

yields linear polylactide (Scheme 1.3). Nucleophilic initiators include organometallic

compounds such as alkyl magnesium bromide, alkyl lithium, metal amides, as well as

alkoxides.23, 37

Scheme 1.3. Anionic ring-opening polymerisation of lactide.


ROP via a cationic mechanism is initiated by the protonation of the carbonyl oxygen-

atom, resulting in an electrophilic activation of the O-CH bond. The propagation

occurs via cleavage of the alkyl-oxygen bond by the nucleophilic attack of another

lactide (Scheme 1.4). Typical electrophilic initiators are Brønsted or Lewis acids, such

as triethyloxonium tetrafluoroborate or trifluoroacetic acid.23, 37 Trifluoromethane

sulfonic acid and methyl trifluoromethane sulfonic acid have been reported as the only

electrophilic reagents capable of polymerising lactide.7

Scheme 1.4. Cationic ring-opening polymerisation of lactide.23

ROP following a coordination-insertion mechanism is the most common route,

yielding polylactides with controlled architectures and molecular weights. The

polymerisation is initiated through the temporary coordination of the exocyclic oxygen

of lactide with the metal atom of the catalyst. The increase in nucleophilicity of the

alkoxide part of the initiator and the electrophilicity of the carbonyl group of the

monomer results in the cleavage of the acyl-oxygen bond of the lactide. The resulting

lactide chain is then inserted into the metal-oxygen bond of the initiator. The

polymerisation proceeds by the opening of lactide monomers and their subsequent

insertion into the bond between the metal atom and the adjacent oxygen atom (Scheme

1.5).23, 38

Metal alkoxides or carboxylates are the most commonly used catalysts for ring-

opening polymerisation via this mechanism. Tin (II) octanoate is the catalyst most

widely used, including at an industrial scale by NatureWorks in their patented

process.31 The high catalytic activity, high solubility in lactide and approval by the

United States Food and Drug Administration (FDA) justify its wide use. 10 ppm of


residual tin (II) octanoate after bulk polymerisation of lactide is considered as non-

toxic.29

Scheme 1.5. Ring-opening polymerisation of L-lactide following a coordination-

insertion mechanism.23

1.2.3 Thermal Properties and Crystallinity of Polylactide

The thermal properties of polylactide are highly related to its stereochemistry.

Polymerisation of lactide yields stereoisomers with various percentages of the different

isomers, depending on its optical purity, the catalyst used and the type of


polymerisation mechanism. The common nomenclature of some stereoisomers is

illustrated in Figure 1.3.

Figure 1.3. Structure of the different stereoisomers that can be obtained from

polymerisation of lactide: (a) poly(L-lactide) (PLLA), (b) poly(D-lactide) (PDLA)

and (c) poly(D,L-lactide) (PDLLA).

Poly(L-lactide) and poly(D-lactide) present similar thermal properties with a glass

transition (Tg) in the range of 50 - 70 °C and a melting temperature (Tm) between 170

and 190 °C. However, random copolymers with higher stereocomplexity display

different thermal transitions depending on the composition. The reduction of L-units

results in lowering both the Tg and Tm (Table 1.2).39

Table 1.2. Thermal properties of random stereoisomers of PLA (neither molecular

weight values nor accuracy of the thermal transitions values were reported in the

literature).39

Copolymer ratio Glass transition (°C) Melting temperature (°C)

100 / 0 (L/D,L)-PLA 63 178

95 / 5 (L/D,L)-PLA 59 164

90 / 10 (L/D,L)-PLA 56 150

85 / 15 (L/D,L)-PLA 56 140


In addition to the tacticity, the glass transition is also sensitive to the molecular

weight.40 Fambri et al.41 reviewed the evolution of Tg and Tm as a function of number

average molecular weights ranging from 2 to 110 kDA for PLLA samples. As the

molecular weight increased, the values for the glass transitions evolved from about 25

to 60 °C, and for the melting temperatures from 130 to 180 °C. The differences were

attributed to limited chain mobility of the amorphous phase caused by the crystalline

regions (no correlation with an increase in crystallization was reported in the review).41

Polylactide is generally described as a semi-crystalline polymer. However, the

crystallinity is also influenced by the stereochemistry. Both PLLA and PDLA are

semi-crystalline polymers with a degree of crystallinity of about 37 %. The spherulite

radius is 100 - 1,000 µm for films obtained by solution-casting.42 PDLLA is

amorphous because of the irregularity of the structure.41 Such differences in

crystallinity between stereoisomers strongly affect the degradation kinetics, either the

oxidation or the hydrolysis rates. Indeed, both of which will preferentially occur in the

amorphous regions due to the limited permeation of oxygen or water diffusion,

respectively, in the crystalline regions (refer to section 1.3.1).

Three crystal forms (α, β and γ) were reported for PLLA depending on the

crystallization conditions (Table 1.3). The α form is the most common as it develops

from the melt or solution casting process, thus at industrial conditions.43, 44 The β and

γ forms require specific conditions to develop. The β form is produced during melt

solution or solution electrospinning whereas the γ form occurs through epitaxial

crystallization.45, 46

In addition to these three forms, a distorted crystal form was reported as α’.48 This

crystal form requires temperatures ranging from 90 to 120 °C to crystallize along with

the α form. Below 100 °C, only the α’ form crystallizes whereas from 100 to 120 °C,

both α and α’ forms crystallize and coexist.47, 49 At 150 °C, a solid-solid phase

transition α’-α occurs.49 The less ordered chain packing of the α’ crystal impacts the

mechanical properties of PLLA by decreasing the Young’s modulus and increasing

the elongation at break.50


Table 1.3. Characteristics of the three crystal forms of PLLA.

Crystal

form

Crystal system Chain

conformation

Cell parameters References

α Pseudo-

orthorombic

103 helical a = 1.07 nm

b = 0.645

nm

c = 2.78 nm

α = 90 °

β = 90 °

γ = 90 °

43, 47

β Orthorombic 31 helical a = 1.031

nm

b = 1.821

nm

c = 0.90 nm

α = 90 °

β = 90 °

γ = 90 °

46, 47

γ Orthorombic 31 helical a = 0.995

nm

b = 0.625

nm

c = 0.88 nm

α = 90 °

β = 90 °

γ = 90 °

45, 47

1.2.4 Mechanical Properties

Polylactide is characterized by mechanical properties that enable its use in applications

where conventional polymers are usually chosen, for example in packaging. The

modulus of elasticity ranges from 3 to 4 GPa, the tensile strength from 50 to 70 MPa

and the elongation at break from 2 to 5 %.51, 52 In comparison, polystyrene and

poly(ethylene terephthalate) feature elastic moduli of 3.2 and 2.8 - 4.1 GPa and

elongations at break ranging between 3 and 300 %, respectively (no accuracy on these

values was reported).53 The mechanical properties depend on a number of intrinsic

properties of polylactide, such as stereochemistry, crystallinity and molecular weight.

Concerning the effect of stereochemistry on mechanical properties, semi-crystalline

PLLA presents a tensile strength ranging from 50 to 70 MPa; whereas, amorphous

PDLLA is characterized by values ranging from 40 to 53 MPa.54 In terms of

crystallinity, an annealed PLLA (molecular weight 20,000 Da) with a degree of

crystallinity of 70 % featured a modulus of elasticity of 4,100 MPa and a tensile


strength of 47 MPa. A non-treated PLLA sample of similar molecular weight (23,000

Da) with a degree of crystallinity of 9 % presented a modulus of elasticity of 3,550

MPa and a tensile strength of 59 MPa (no accuracy on these values were reported).54

Regarding the influence of the molecular weight on the mechanical properties, a

general trend is observed to increase the values of the tensile properties (yield strength,

tensile strength, yield elongation, elongation at break and modulus of elasticity) with

increasing molecular weight. For instance, an increase from 50 to 100 kg·mol-1 for

PLLA doubled the values of tensile strength and modulus of elasticity. However, the

flexural strength was reported to reach a plateau for a molecular weight of 35,000

g·mol-1 for both amorphous PDLLA and PLLA, and 55,000 g·mol-1 for semi-

crystalline PLLA.54 Regarding very low molecular weights, the polymer loses its

strength and starts to embrittle when the molecular weight reaches a critical value, the

chain entanglement molecular weight (𝑀𝑒 ), with the polymer strength being inversely

proportional to the molecular weight. When the polymer strength reaches zero, the 𝑀𝑛

corresponds to 𝑀𝑒 .55 𝑀𝑒

was reported to be in the range of 8 to 10 kg·mol-1 for

polylactide.56 A PLLA with 98:2 L:D enantiomer content presented a value of 9 kg·mol-

1.57

A factor that was shown to have no influence on the mechanical properties was the

method used to produce polylactide. The mechanical properties of PLA samples of

similar molecular weight synthesised by the direct condensation polymerisation of

lactic acid and the ring-opening polymerisation of lactide did not differ.58

To summarize, semi-crystalline poly(L-lactide) features appropriate mechanical

properties for packaging applications compared to amorphous poly(D,L-lactide).

Molecular weight substantially impacts the mechanical properties. 𝑀𝑛 should exceed

the entanglement molecular weight (9 - 10 kg·mol-1) to produce films while lower

values result in the loss of polymer strength and embrittlement.

1.3 DEGRADATION OF POLYLACTIDE

Degradation is defined as “a deleterious change in the chemical structure, physical

properties or appearance of a polymer, which may result from chemical cleavage of

the macromolecules forming a polymeric item regardless of the mechanism of chain

cleavage” according to the Standards PD CEN/TR 155351:2006 and ASTM D883.59,60

Degradation involves different processes depending on the causal factors: thermal


degradation, mechanochemical degradation, photodegradation, catalytic degradation,

or biodegradation.61 Three main degradation pathways will be reviewed here:

biodegradation, thermal degradation, and photodegradation. Biodegradation, a biotic

process, results in the disappearance of the polymer by fragmentation and assimilation

by micro-organisms. Whereas thermal degradation and photodegradation are both

abiotic processes. Thermal degradation can already occur at early stages of the

polymer life cycle during processing, resulting in negative consequences, such as the

loss of molecular weight and mechanical properties. Photodegradation occurs when

the polymer is exposed to UV irradiation at any point during its lifecycle.62 While

thermal degradation during processing and manufacture should be minimized to

broaden the application range, both photodegradation and biodegradation may be used

advantageously to avoid pollution by polymer residues. These three degradation

pathways will be described and studies undertaken to monitor their rates will be

discussed.

1.3.1 Biodegradation

Biodegradation is divided into three steps: deterioration, fragmentation and

assimilation. These steps overlap to gradually decrease the molecular weight, resulting

in the loss of mechanical properties and embrittlement of the polyester.63 Deterioration

and fragmentation steps rely on hydrolysis of the ester linkage of polylactide (Scheme

1.6).

Scheme 1.6. Hydrolysis of an ester linkage.

The hydrolytic mechanism strongly depends on the sample thickness, with the critical

thickness Lcritical, where the mechanism changes from a bulk erosion to a surface

erosion mechanism for a thickness greater than this value. Bulk erosion occurs when

the rate of water diffusion through the polymer is higher than the rate of hydrolysis.

On the contrary, the rate of hydrolysis exceeds the rate of water diffusion in a surface

erosion mechanism resulting in a decrease in thickness.55 For polylactide, a sample

thickness lower than 0.5 - 2 mm leads to a bulk erosion mechanism, while thicknesses


greater than 7.4 cm result in a surface erosion mechanism. A core-accelerated erosion

mechanism proceeds for thicknesses between those values.64

Intrinsic properties of polylactide affect the hydrolysis rate, such as stereochemistry.

For instance, comparative studies of the hydrolytic degradation between amorphous

films of PLLA, PDLA and PDLLA of similar molecular weights in a phosphate-

buffered solution at 37 °C revealed a faster reduction in the molecular weight for

PDLLA than for PLLA and PDLA films. The faster hydrolysis rate was attributed to

the difference in tacticity with lower interaction between PDLLA chains than for

PLLA or PDLA.64 The presence of the D-enantiomer within a PLLA matrix also

impacted the hydrolysis rate by lowering the optical purity, thus modifying the chain

packing, and facilitating the water diffusion between the polymer chains.63 For

instance, an amorphous PLLA with low amounts of D-units (0.2 and 1.2 %) showed a

faster decrease in molecular weight compared to optically pure amorphous PLLA

when placed in a phosphate-buffered solution at 37 °C.65 The hydrolysis rate is also

impacted by the crystallinity. Pantani et al.66 investigated the rates of water diffusion

and the global biodegradation rate (in compost conditions) between amorphous and

crystalline PLLA samples (with 4 % of D-enantiomer). Hydrolysis of both samples led

to a decrease in Tg because of the plasticization effect of the water, diffusing and

enhancing chain mobility. Moreover, both samples embrittled, suggesting a loss in

molecular weight. The crystallinity did not affect the early stages of water diffusion,

but influenced the swelling of the samples after a few days and significantly decreased

the biodegradation rate. The majority of biodegradation studies of PLLA demonstrated

that the amorphous regions are predominantly hydrolysed, forming oligomers and

monomers.67

The hydrolysis of ester linkages induces a reduction in molecular weight with the

formation of oligomers, loss of mechanical properties, and subsequent fragmentation.

Once fragmented, PLA is assimilated by microorganisms through enzymatic chain

cleavage involving proteases (e.g. Amycolatopsis) or lipases (e.g. Bacillus siniithii).68

The microorganisms attack the chain ends of the polymers, the number of end groups

being inversely proportional to the molecular weight. This assimilation process leads

to the formation of biomass, carbon dioxide and water.62, 69 The stereochemistry of

PLA influences the assimilation process. Several studies used Proteinase K, an

enzyme isolated from a fungus, Engyodontium, which is stable over a pH range of 4 -


12. Proteinase K proved to efficiently cleave LL-, DL- and LD- bonds but not DD-

bonds.70 The DD- bonds could be assimilated by another thermophile called Strain 73,

from Geobacillus stearothermophilus.68, 71

Biodegradation not only depends on intrinsic properties of polylactide, it is

considerably impacted by external factors. pH dictates the mechanism type. For

instance, hydrolysis occurs via a surface erosion mechanism in alkaline media or via a

bulk erosion mechanism in acidic media.72, 73 For a hydroxyl-terminated PLLA,

cleavage preferentially occurred at the first ester bond in acidic media whereas the

second ester bond was cleaved in alkaline media.74 In terms of kinetics, the hydroxide

ions in alkaline media catalyse the cleavage of the ester linkages.75 Temperature also

impacts the rate of hydrolytic degradation. For temperatures between the glass

transition and melting temperature, water diffusion is facilitated by enhanced chain

mobility. Temperatures above the melting temperature accelerate the rate of

homogeneous hydrolysis by melting the crystalline regions.67 As a result, the influence

of external factors (temperature, pH, relative humidity) results in different

biodegradation rates depending on the environmental conditions. PLA degrades within

three to four weeks in compost where the conditions are controlled (average

temperature 60 ºC, average relative humidity 65 %, pH 7.5).76, 77 However, in landfill

conditions in Thailand, the fragmentation of PLA started only after six months.78 Its

biodegradation rate in soil, when parameters like temperature, humidity, pH and the

nature of the microorganisms are not controlled, is slower due to its Tg being around

60 °C, which is higher than the temperature under these conditions. An increase in the

environmental temperature leads to an increase in the hydrolysis rate, decreasing the

molecular weight of the polymer. This leads to a higher mobility of the polymer chains

and therefore leads to a higher rate of PLA biodegradation.79 Ho et al.76 investigated

the degradation of PLA films in two sites with different temperatures and relative

humidities. The molecular weight loss was faster in the place with the higher

temperature and humidity, suggesting the prodegradant effects of those two

parameters. However, the microbial types should significantly explain these results as

well (no precision of microbial types was given).

1.3.2 Thermal Degradation

Polylactide is subject to thermal degradation at different stages of its lifecycle.

Thermolysis occurs at high temperatures when only a limited amount of oxygen is


present, for example during processing, while thermo-oxidation takes place at longer

timescales, under oxygen, and at lower temperatures during storage, for example. This

review focusses on the degradation occurring at early stages of the lifecycle during

processing. As the melting temperature ranges from 170 to 180 ºC, manufacturers

recommend processing temperatures between 185 to 250 ºC.7

McNeill et al.80, 81 investigated the thermal degradation of PLA at temperatures

ranging from 230 to 440 °C. They reported the formation of oligomers as the main

products besides acetaldehyde, water, and carbon dioxide above 230 °C, or methyl

ketene above 320 °C. It was suggested that unzipping depolymerisation (a non-radical

backbiting ester interchange with the hydroxyl chain ends) occurred in the temperature

range of 230 - 440 °C. These transesterification reactions were further confirmed as

the dominant pathway for degradation at temperatures above 200 ºC (Scheme 1.7 (a)).

Other reactions were suggested, such as cis-elimination forming acrylic acid and

acyclic oligomers, radical reactions and depolymerisation catalysed by residual tin (II)

octanoate.82 Since the temperatures required for processing do not exceed 250 ºC, non-

radical transesterification reactions are considered to be the predominant reason for

the observed loss in molecular weight. This reduction is promoted by higher

temperatures (Scheme 1.7 (b)).83 The transesterification reactions were reported to be

catalysed by transition metals. Tin (II) octanoate, a catalyst for the ROP of PLA, also

catalyses PLA thermal degradation. An increase in tin (II) octanoate content was found

to lead to a decrease in the degradation temperature of PLLA, and the greater the

amount of tin (II) octanoate, the more selective the production of lactides.84 Other

transition metals, such as zinc, aluminium, iron, titanium and zirconium were found to

catalyse chain transfer, intra- and intermolecular transesterification, and

depolymerisation reactions at temperatures above 240 °C. Comparisons of their

efficiency as transesterification catalysts led to inconsistent conclusions in the

literature. A first order of efficiency was suggested: tin < zinc < aluminium < iron.85

In contrast, another order of efficiency was stated: tin > zinc > zirconium > titanium >

aluminium.86 This difference could be explained by the form in which these metals

were used or the range of their concentration. The latter order was based on the use of

the metals as alkoxides, organic acids and enolate salts while the first order stated their

use simply as metals, lacking a more precise indication of their form. Further studies

on the effects of processing parameters on the thermal degradation of PLLA at 210


and 240 ºC revealed the strong impact of residual moisture and residence time in the

extruder. The molar mass loss was higher for longer residence times and residual

moisture levels, which provoked chain scission by hydrolysis of the ester linkages

(Scheme 1.7 (c)).87

Scheme 1.7. (a) Intramolecular transesterification resulting in the formation of

lactide, oligomers, acetaldehyde, and carbon monoxide; (b) intermolecular

transesterification; (c) hydrolysis.62, 82


Thermal degradation of PLA follows first-order law kinetics with an apparent

activation energy ranging from 22 to 28.5 kcal·mol-1 in air.81, 88 However, recent

studies demonstrated better fitting second-order law kinetics with an apparent

activation energy around 25 kcal·mol-1.89

Racemization is a consequence of thermal degradation of PLLA. For instance, the

pyrolysis of PLA between 400 and 600 °C afforded racemization due to the production

of L-lactide.90 For lower temperatures (250 - 290 °C), PLLA samples sealed under

reduced pressure confirmed the depolymerisation process to afford L-lactide at first.

However, the increase in both temperature and degradation time resulted in the

formation of D- and meso-lactide (1 and 8 % after 5 hours to 10 and 33 % after 15

hours, respectively) and a decrease in L-lactide (from 91 % after 5 hours to 57 % after

15 hours).91

The loss in molecular weight during thermo-oxidation of PLLA at 70, 100, 130, and

150 ºC provoked a decrease in the glass transitions and the mechanical properties, such

as the strain at break. For instance, the strain at break was reduced from 20 % before

thermal ageing to about 0 % after 500 hours at 100 ºC, or 150 hours at 150 ºC,

respectively.92

To summarize, the temperature range required for PLA extrusion already induces

thermal degradation to a certain extent, where transesterification reactions are the main

pathway of the degradation. These reactions can be catalysed by transition metals such

as tin, aluminium or zinc.

1.3.3 Photodegradation

In some applications, polymers may be exposed to UV irradiation. While the product

should feature UV stability during its use, UV exposure could be used to accelerate

the degradation at the end of the useful life and result in a faster removal of the polymer

from the environment. Poly(L-lactide) features a carbonyl group C=O in its repeating

unit. This group absorbs UV irradiation at 280 nm via the n-π* transition with the

corresponding extinction coefficient ɛ at that wavelength of less than 100 L·mol-1·cm-

1.93 Although ɛ is low, inducing a relative stability towards UV, PLA does undergo

structural and mechanical changes upon UV irradiation. The UV spectrum is divided

into three domains: UV-C 100 - 280 nm totally absorbed by the Earth’s atmosphere;


UV-B 280 - 315 nm partially blocked by the ozone layer; UV-A 315 - 400 nm, which

is the largest portion of natural UV light, not blocked by the ozone layer.94 Two main

photooxidation mechanisms have been discussed in the literature for polylactide,

depending on the type of UV light used.

Ikada et al.95-97 investigated the photodegradation of PLLA using light sources in the

UV-C domain. They observed an increase in the absorbance bands in the IR spectra at

3290 cm-1 and 990 cm-1, which were assigned to carboxylic acid and C=C double

bonds, respectively. An increase in the number of chain scissions as a function of

irradiation time was also observed based on gel permeation chromatography (GPC)

analysis. Photocleavage via a Norrish type II mechanism was then suggested to occur

at the carbonyl group via UV absorption at the ester linkage (n-π* transition) (Scheme

1.8). A comparative study of solvent-casted films of amorphous and melt-crystallized

PLLA irradiated with UV-C ( < 300 nm, 255 W·m-2) revealed that the

photodegradation proceeded as a bulk process in both, the amorphous and the

crystalline regions.98

Scheme 1.8. Norrish type II mechanism for the photodegradation of PLLA under

UV-C light.

Racemization converted L-lactic acid units into D-units as revealed by the loss of

optical purity during UV-C (254 nm) irradiation of PLLA films.99 The authors

suggested a racemization occurring at both hydroxyl and carboxyl chain ends, with

one D-lactate unit being formed for every chain scission (Scheme 1.9).


Scheme 1.9. Proposed mechanism of racemization occurring both at the hydroxyl

chain end (left) and the carboxyl chain end (right) during the photodegradation of

PLLA exposed to UV-C light.99

Another photodegradation mechanism was proposed based on studies using UV-A

light, which is more relevant to natural outdoor conditions. Bocchini and coworkers

studied the photooxidation of PLA at wavelengths above 300 nm and followed the

structural changes using infrared spectroscopy.100 The appearance of a shoulder at

1845 cm-1 on the carbonyl band, assigned to anhydrides, was noticeable during ageing.

The photodegradation mechanism proposed included initiation by chromophoric

impurities in the polymer and hydrogen abstraction from the carbon in the α-position

to the carbonyl group on the polymer backbone as the main degradation pathway. The

formed macroradicals then reacted with oxygen to afford hydroperoxides. Subsequent

photolysis of those hydroperoxides was proposed to form anhydrides as the main

photodegradation product (Scheme 1.10). Gardette et al.101 confirmed this mechanism

and observed the occurrence of chain scission, resulting in a reduction in molecular

weight. However, the molecular weight decreased only slowly from 80,000 to about

70,000 g·mol-1 after 400 hours of UV irradiation (using a Sepap 12.24 unit, under


normal atmosphere at 60 °C), revealing a certain stability of polylactide towards UV

light.

External factors, such as temperature and relative humidity, have been reported to

enhance the photooxidation of PLA when exposed to UV light (315 nm) by increasing

the rate of hydrolysis.102 The combined increase in temperature and relative humidity

(RH) resulted in faster molecular weight loss compared to films stored in the dark.

When exposed to UV light, at 100 % RH and 60 °C, the molar mass loss was faster,

with values reaching 4 % of the initial value after fifteen weeks compared to 15 %

when stored in the dark.102 Reduction in molecular weight and loss of mechanical

properties (stiffness and strength) was confirmed by other researchers when PLA was

exposed to UV light in the range of 295 - 400 nm.103

Biodegradation, thermal and photo-degradation occur at different stages of the life

cycle of PLA via different mechanisms, but all result in lowering the molecular weight.

Consequently, the mechanical properties are altered and embrittlement starts. The rates

of the different types of degradation also differ. Thermal degradation during

processing results in a loss of mechanical or thermal properties. This process needs to

be minimized to obtain PLLA that meets commercial mechanical property

requirements for each intended application. Both photodegradation and biodegradation

are beneficial pathways in terms of degrading the polymer at the end of its life.

Although the biodegradability is also influenced by intrinsic properties of PLLA

(stereochemistry, crystallinity), external factors, including temperature, humidity, and

soil composition play a critical role in the rate of degradation. Regarding

photodegradation, the reported loss of molecular weight and occurrence of chain

scissions happen rather slowly. Similarly to biodegradation, both intrinsic properties

of PLLA and external conditions impact the degradation rate. While external factors

can hardly be controlled in outdoor applications, poly(L-lactide) can be modified to

accelerate the degradation rate. The following section of this review will focus on

studies aiming at either increasing or minimizing the effects of the three degradation

pathways by modifying the structure of the polyester.


Scheme 1.10. Photodegradation mechanism of PLA based on hydrogen abstraction

from the carbon in the α-position to the carbonyl group with formation of

macroradicals leading to anhydride as a main photodegradation product (X

represents chromophoric defects).


1.4 TAILORING THE DEGRADATION OF POLY(L-LACTIDE)

Poly(L-lactide) degrades via three main processes whose rates can be challenging to

predict. For instance, PLLA degrades within three or four weeks in compost but the

degradation time can increase to up to six months under landfill conditions (refer to

section 1.3.1). However, the degradation behaviour can be tailored either by

modifying the polymer backbone via copolymerisation or by blending with suitable

polymers or additives.

1.4.1 Accelerating the Biodegradation Rate

Copolymerisation or blending with poly(ɛ-caprolactone) was extensively investigated

to enhance the biodegradation of poly(L-lactide). L- and D,L-lactide were

copolymerized with ɛ-caprolactone (40 / 60; 20 / 80) and hydrolysed in phosphate-

buffered solution at pH 7.0 at 23 and 37 °C. Kinetic studies based on the loss in

molecular weight revealed higher rate constants for the copolymers than for neat PLLA

and PCL, respectively. Higher temperatures and a greater D-unit content significantly

increased the rate constant.104 When hydrolysed in a phosphate-buffered solution at

pH 7.4 and at 37 °C, films of PLLA / PCL blends (50 / 50 and 75 / 25) revealed the

highest rate constants for hydrolysis compared to neat PLLA, as well as faster weight

loss, molecular weight reduction and reduction in tensile strength. This enhanced

hydrolytic degradation was attributed to the chain-end carboxylic group of poly(ɛ-

caprolactone).105 Moreover, biodegradation in soil was impacted by poly(ɛ-

caprolactone) segments, which were preferentially attacked by selected enzymes at pH

values ranging between 7.3 and 6.8 (total carbon content 5.7 %, study conducted in

Japan for twenty months).106

The hydrolysis of polylactide was also accelerated using additives as nanofillers. For

instance, hydrolysis of a PLA melt-blended with 3 wt% of an organically modified or

a non-modified Montmorillonite (Cloisite®), respectively, proceeded faster in the bulk

than neat PLA in a phosphate-buffered solution for five months. Such acceleration was

attributed both to their hydrophilicity and their morphology facilitating or not the water

diffusion (clay platelets or intercalated).107 Compost studies of melt-blended PLA

nanocomposite with similar Montmorillonite (Cloisite and Nanofil, 5 wt%)

demonstrated visual signs of degradation (surface modification, whitening) after three

weeks. The number average molecular weight decreased by 55, 79, and 41 % for neat


PLA, PLA - Cloisite and PLA - Nanofil, respectively, after seventeen weeks. The

observed differences were suggested to arise from the heterogeneous dispersion of the

nanofillers in the PLLA matrix. The enhanced biodegradation in compost was

attributed to the presence of hydroxyl groups in the silicate layers.108 The dispersion

of the nanofillers influences the rate as the hydrolysis was reported to occur at the

interface between the PLA matrix and the nanofiller.109

1.4.2 Improving the Thermal Resistance and Mechanical Properties

As reviewed in section 1.3.2, thermal degradation proceeds through various reactions,

such as hydrolysis and back-biting transesterification. Moisture present during

processing results in random chain scission as a result of hydrolysis. The molecular

weight is reduced and, consequently, the mechanical properties of the processed

polymers are altered. NatureWorks recommends processing PLLA with a moisture

content less than 250 ppm to prevent viscosity degradation.24 Back-biting

transesterifications slowly reduce the molecular weight by depolymerisation and

random chain scission starting from the carboxyl and hydroxyl chain-end of

polylactide. End-group modification of polylactide has proved to successfully prevent

those reactions from occurring. For instance, acetylation of hydroxyl-end groups has

been performed on PLLA samples. Thermogravimetric analysis revealed that

acetylated PLLA presented a degradation profile starting at 360 °C instead of 260 °C

for neat PLLA. However, the acetylation of hydroxyl groups involved elimination of

residual traces of tin, which contributed even more to enhance the thermal stability.110

Modification of hydroxyl end-groups into cinnamate esters proved to be more efficient

than acetylation for improving the thermal stability of polylactide. Hydroxyl end-

groups of PLLA were reacted with cinnamoyl chloride to afford cinnamate esters as

end-groups. 10 % weight loss for the modified PLLA occurred at 320 °C instead of

240 °C for the unmodified samples. The modified PLLA still contained a quite high

amount of tin after treatment (600 ppm instead of 1000 ppm before treatment,

measured by Inductively Coupled Plasma Atomic Emission spectroscopy).111

On an industrial scale, additives are used to improve the thermal resistance of PLA

during processing. For instance, Dupont commercialized several additives (Biomax®)

which improve the thermal stability to 95 °C when mixed at 2 - 4 wt% concentration

range.112 Acrylic-based additives, commercialized by Arkema, enhanced the melt-


strength of PLA during processing by forming networks via entanglement of acrylic

and PLA chains, impeding chain scissions.112

Polylactide is characterized by high values for tensile strength and tensile modulus,

but it is a glassy and brittle polymer with low tensile toughness and elongation at break

(refer to section 1.2.4). Toughening polylactide can be achieved by plasticization. This

method improves the processability by increasing the flexibility and the elongation at

break of polylactide. The glass transition is lowered as a result of the blending of the

plasticizer with the polylactide.53

Lactide is an extensively used plasticizing agent for polylactide, because of their

similar chemical structures.53 Citrate esters have also proved to increase the elongation

at break (610 % with 30 wt% of triethyl citrate compared to 7 % for neat PLA) and

reduce the glass transition temperature.113, 114 However, small molecules, such as

lactide, tend to migrate within the polymer matrix. For instance, lactide was reported

to migrate to the surface, then out of the matrix, leading to stiffening over time.53 To

overcome this issue, oligomers of higher molecular weight have been investigated as

plasticizers for polylactide. Oligomers of lactic acid with carboxyl or hydroxyl end-

groups (𝑀𝑛 of 1,179 and 1,050 Da, respectively) were employed as plasticizers to

increase the ductility of polylactide. A single glass transition measured by Differential

Scanning Calorimetry (DSC) proved the miscibility between both types of oligomers

and polylactide. The elongation at break increased from 5 % for neat PLA to 430 %

and 480 %, respectively, when blended with 20 wt% of each type of oligomer.115

Other oligomers, such as poly(ethylene glycol) (PEG), poly(propylene glycol) or

poly(diethylene adipate) efficiently plasticize polylactide.53, 116 However,

concentrations exceeding 20 wt% are often required, leading to phase separation and

alteration of the mechanical properties. To overcome these limitations, acrylated-

poly(ethylene glycol) was successfully grafted onto the PLLA backbone using radical

initiators via reactive blending, to improve the miscibility and avoid phase

separation.117 Another study involved the maleation of PLLA via reactive extrusion

using radical initiator, followed by the esterification between the anhydride functions

with hydroxyl-terminated PEG to improve the compatibility of the PLLA / PEG

blends.118 Reactive blending of PLLA and poly(butylene adipate-co-terephtalate)


(PBAT) were successfully performed using transesterification reactions to synthesise

low amounts of PLLA - PBAT copolymer to increase interfacial adhesion between

PLLA and PBAT.119, 120

At an industrial scale, impact modifiers have been developed to improve the toughness

of polylactide. BlendexTM 338 additive (NatureWorks LLC), an acrylonitrile-

butadiene-styrene terpolymer, increased the notched Izod impact strength from 26.7

J·m-1 of notch to 518 J·m-1 of notch and the elongation at break from 10 % to 281 %

when blended at 20 % with polylactide.53 Dupont commercialized Biomax® Strong,

an ethylene-butyl acrylate copolymer, which aimed to improve the toughness without

altering the transparency of polylactide. The Spencer impact, which is a measure of

resistance to impact-puncture penetration in a film, increased from 1250 g·mm-1 to

3500 g·mm-1 for blends of poly(L-lactide) with 2 wt% of Biomax® Strong 100.53

1.4.3 Accelerating the Photodegradation Rate

A number of nanofillers have been used to reinforce the mechanical or thermal

properties of polylactide and thus to extend the application range. Their low cost and

ease of processability enable the production of composite films by melt-blending or

solvent-casting techniques. However, accelerated weathering studies of such films

revealed prodegradant effects of the nanofillers on the PLA matrix. For instance, melt-

blended films of PLLA with Montmorillonite, Sepiolite and fumed silica, 5 wt% of

fillers, were aged using a SEPAP ageing device (Atlas) under oxidative conditions (

> 300 nm, 60 °C, under air). Infrared (IR) spectroscopy of the irradiated samples

revealed the disappearance of the -CH2 stretching bands at 2922 and 2853 cm-1 of the

nanofillers without an induction period. Moreover, a linear increase in the absorbance

at 1845 cm-1, assigned to anhydride, was observed with irradiation time. The anhydride

formation was faster for the composites than for neat PLLA.100, 121 Therias and

coworkers investigated the photooxidation of extruded PLLA with a thermal stabilizer

(Ultranox 626A, 0.3 wt%) and ZnO nanofillers in a content range of 0 - 3 wt% using

a SEPAP ageing device (Atlas) ( > 300 nm, 60 °C, under air). The monitoring of the

films photodegradation by IR spectroscopy demonstrated similar results, with a faster

formation of anhydrides as a function of irradiation time for the composite films

compared to neat PLLA. Moreover, GPC measurements revealed a decrease in

molecular weight of about 20 % for neat PLLA, 30 % for PLLA 1 wt% with ZnO, and


60 % for PLLA with 3 wt% ZnO, after 300 hours in the Sepap, suggesting the

occurrence of chain scissions.122 In another study, PLLA was melt-blended with

halloysite nanotubes in the range of 0 - 12 wt% and the resulting films were aged using

the same device and conditions as above. Similar results could be observed, with an

increase in anhydride formation for the modified PLLA.123 Gardette et al.101 melt-

compounded PLLA with a different nanocomposite filler, CaSO4 (content range 0 - 40

wt%), obtained films by compression moulding, and subsequently aged using the

above-mentioned ageing device under the same conditions. The IR monitoring

revealed the formation of anhydrides and faster kinetics for the modified PLLA than

for the neat one. GPC measurements confirmed a drop in the molecular weight due to

random chain scissions. All studies featured similar outcomes, that is a faster

anhydride formation with increasing concentration of fillers, resulting in random chain

scissions throughout the ageing process. The presence of transition metals as

chromophoric impurities in the different fillers were suggested to accelerate the

photodegradation without any induction period, by catalysing the hydroperoxide

decomposition from which anhydrides were formed. The type of nanofillers, their

concentration and degree of dispersion influenced the extent of anhydride

formation.100, 101, 121-123

The same effect could be shown with TiO2 as the nanofiller. Solvent-cast films of

PLLA with 0.5, 1, 2, 5 and 10 wt% TiO2 were artificially aged under UV-A light (365

nm) and their weight loss was recorded over time. A linear relationship between weight

loss and irradiation days was demonstrated, as well as a faster degradation with

increased concentration of TiO2. TiO2 induced photodegradation by generating active

oxygen species that subsequently attacked the polymer chains and resulted in chain

scission.124 However, this photocatalytic effect could be inhibited by the ability of TiO2

to block UV light at the surface of the films.125 Another key factor was the surface area

of the nanofiller compared to the PLLA matrix, as observed in a comparative

photodegradation study ( = 254 nm) of solvent-casted films of PLLA with either TiO2

nanoparticles or nanowires. The larger surface area of TiO2 nanowires favoured

recombination of electron-hole pairs in the bulk and reduced the probability of the

generation of reactive oxygen species.126

Photosensitizers were investigated as prodegradants for PLLA as well, such as

N,N,N’,N’-tetramethyl-1,4-phenylenediamine (TMPD). Sakai and coworkers aged


solvent-casted films of PLLA - TMPD 0.4 wt% under UV-A light (356 nm) and

observed a decrease in molecular weight by GPC. The photoionization of TMPD

enabled the release of a free electron, leading to the formation of radicals and

subsequent chain scissions.127 Tsuji et al.128 compared the efficiency of TMPD on the

photodegradation of amorphous and crystallized PLLA films with an initial TMPD

concentration in the range of 0 - 1 wt%. GPC measurements of the irradiated films

(using UV-C) confirmed the reduction in molecular weight, with a larger decrease with

higher TMPD content. Both amorphous and crystallized PLLA films underwent chain

scissions, in contrast to hydrolytic degradation (refer to section 1.3.1).

1.5 TAILORING DEGRADABILITY

Bio- and photo-degradation are processes that can degrade polymer fragments during

and after the service life. Accelerating one of either biodegradation or

photodegradation will impact the overall degradation rate. Concerning biodegradation,

external conditions play a considerable role in the degradation of both neat and

modified poly(L-lactide). Thus, accelerating the biodegradation rate requires finding

the optimum external conditions to obtain the highest rate possible. However, the rate

of photodegradation could be substantially increased by adding prodegradants that

initiate the degradation and result in chain scissions. Therefore, the project introduced

in this thesis aims at accelerating the photodegradation rate of PLLA by modifying the

backbone via the addition of chromophores.

Similar strategies have been employed for polyolefins. Photodegradable polyolefins

are classified into two categories: photoinduced photodegradable and intrinsically

photodegradable. The first category includes polymers without chromophores on their

backbone, in contrast to the second category where polymers contain chromophores

within their structures.129

Photoinduced photodegradable polyolefins require prodegradants to catalyse their

degradation. Polypropylene (PP) or polyethylene (PE) are typical polyolefins lacking

chromophores on their backbones. Extensive research on understanding the effects of

antioxidants and light stabilizers on the formation of hydroperoxides led to the

development of the Scott-Gilead photo-biodegradation process. This process involved

formulations based on PP or PE containing a balance of anti- and pro-oxidants that

catalyse the fragmentation after a predictable induction time. Iron, manganese or


nickel are typical transition metals composing the pro-oxidant salts, while cobalt or

zinc are part of the UV light stabilizer complexes.130-132 UV irradiation of Plastor films,

a commercial PE with iron dithiocarbamate, resulted in the combined formation of

carboxylic acids as products of hydroperoxide decomposition and embrittlement of the

film due to reduction of molecular weight.133 Not only do the transition-metal salts

accelerate photodegradation, the level of oxidation catalysed by the transition metals

also dictates the hydrophilic nature of the film that enables the microorganisms to

interact with the polymer matrix.134-136 These modified polymers are referred to as

oxobiodegradable.

Intrinsically photodegradable polymers feature chromophores in their backbone that

can interact with light and do not require additional prodegradants to degrade. Ketones

are among the most efficient chromophores characterized by an accessible n-π*

transition, relevant for photochemical studies in outdoor conditions (UV-A). They

undergo chemical change via various reactions, such as -cleavage, known as Norrish

type I and intramolecular elimination, referred to as Norrish type II. The Norrish type

I reaction yields free radicals whereas type II comprises an intramolecular

rearrangement and results in a methyl ketone and a C=C double bond as the

fragmentation products.137 Guillet et al.138 investigated the photodegradation of

ethylene-carbon monoxide copolymers and reported a Norrish type II reaction

mechanism at room temperature and a Norrish type I mechanism at elevated

temperatures (Scheme 1.11). Dupont patented a polyketone-type polymer based on

ethylene with carbon monoxide in the concentration range of 0.5 - 1.6 wt%.129 Shell

commercialized aliphatic polyketones named Carilon, alternating copolymers of

ethylene and carbon monoxide featuring small amounts of incorporated polypropylene

units.129 Photodegradation studies undertaken in outdoor conditions (in Italy) revealed

a degradation mechanism based on Norrish type I, involving the formation of radicals

that subsequently reacted with oxygen to form hydroperoxides. Decomposition of

hydroperoxides resulted in chain scission, and embrittlement of the irradiated films.139,

140 Guillet and coworkers further investigated ketones as chromophores for

poly(ethylene terephthalate) (PET) by copolymerising it with in-chain and side-chain

ketone-containing molecules and irradiating the copolymers under UV-A light. A

faster decrease in molecular weight was observed for the copolymers featuring the

ketone in the backbone compared to the side-chain ketone copolyesters, demonstrating


the efficient prodegradant effect of the ketone on the photodegradation of the

polyester.141

Scheme 1.11. Photodegradation of ethylene-carbon monoxide copolymers via

Norrish type I and II.

Following the polyketone-type polymer strategy, Tian and coworkers investigated the

synthesis of novel aliphatic polyesters with photodegradable potential, specifically

poly(2-oxepane-1,5-dione) (POPD).142 POPD presents a similar structure to poly(-

caprolactone), but features a ketone to increase the photodegradation potential. To

achieve this goal, they initially synthesised poly(1,4,8-trioxaspiro[4.6]-9-undecanone)

(PTOSUO) and subsequently deprotected the ketone to yield POPD (Scheme 1.12).

The UV-visible spectra of POPD revealed a broad absorption band ranging from 230

to 300 nm due to the ketone whereas PCL was characterized by a narrow band around

240 nm assigned to the ester functional group. Latere et al.143 subsequently

demonstrated that OPD, when copolymerized with ɛ-caprolactone at 30 mol%,

accelerated the hydrolytic degradation compared to neat PCL due to higher

hydrophilicity (Scheme 1.13). However, in their work, no UV ageing studies were

undertaken to evaluate the prodegradant potential of OPD or POPD.


Scheme 1.12. Synthesis of poly(2-oxepane-1,5-dione) via the ROP of TOSUO.

Conditions and reagents: a. Al(OiPr)3, toluene, 25 °C, H3O+; b. (C6H5)3CBF4,

dichloromethane, 25 °C, 1 hour.

Scheme 1.13. Synthesis of poly(ɛ-caprolactone-co-2-oxepane-1,5-dione). Conditions

and reagents: a. tin (II) octanoate, toluene, 90 °C.


1.6 PROJECT PROPOSAL

Adding chromophores to the polymer structure effectively enhances the

photodegradation of a polymer by undergoing cleavage and release of radicals that

attack the polymer chains. This PhD project focusses on using ketones as efficient

chromophores to accelerate the photodegradation of PLLA. This form of PLA features

appropriate crystallinity and mechanical properties for film production and packaging

applications. Following the work of Tian and coworkers, 2-oxepane-1,5-dione (OPD)

was chosen as the chromophoric compound to modify poly(L-lactide). OPD not only

features a ketone in its structure, it also presents an ester linkage which can react with

PLLA via intermolecular transesterification in the melt, e.g. via reactive extrusion.

Alternatively, it can be incorporated by copolymerisation with L-lactide via a ring-

opening mechanism (refer to section 1.5). Therefore, the structure of OPD broadens

the possible strategies for modification of PLLA to potentially accelerate its

photodegradation. The thesis will be structured as follows.

Chapter 2 of this thesis presents an investigation into the prodegradant potential of 2-

oxepane-1,4-dione on the photodegradation of commercial grade poly(L-lactide). This

compound was blended with poly(L-lactide) in chloroform in various concentrations

(0 - 10 wt%). The high molecular weight of PLLA impacted its solubility and thus

limited the choice of solvent. Different films were obtained by solvent-casting and

subsequently artificially aged under UV-A light using a QUV accelerated weathering

tester (Q-lab, Ohio). The visual aspects, molecular weights, thermal properties and

chemical structures of the different films were evaluated to determine the effect of

OPD on the properties of PLLA. The films were characterized using GPC, DSC,

Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy,

and UV-visible spectroscopy. The morphology and embrittlement behaviour of the

films were visually monitored during the ageing process while the evolution of

molecular weight and thermal properties were monitored via GPC and DSC

measurements. The chemical modifications were monitored over time by ATR-FTIR

and UV-visible spectroscopies to gain insight into the prodegradant behaviour of OPD.

A photodegradation mechanism was derived from the obtained observations.

Following the investigation of the prodegradant potential of OPD when used as an

additive, Chapter 3 explores the potential of OPD as a photoprodegradant when

incorporated into the poly(L-lactide) backbone. First attempts focussed on melt-


modification of commercially available PLLA based transesterification during

reactive extrusion. The OPD initial feed, the residence time and the transesterification

catalyst were investigated to achieve the incorporation of OPD into the polymer

backbone. The extrudates were characterized by 1H NMR, ATR-FTIR spectroscopies,

as well as GPC and DSC. The characterization data revealed unsuccessful

incorporation of OPD, while identifying products of thermo-oxidative degradation.

Chapter 4 presents a second strategy involving the incorporation of OPD into PLLA

using milder conditions than during reactive extrusions via copolymerisation of L-

lactide and OPD. Ring-opening polymerisations were performed using two sets of

conditions (Scheme 1.14).

Scheme 1.14. Ring-opening polymerisation of L-lactide and 2-oxepane-1,5-dione to

afford poly(L-lactide-co-2-oxepane-1,5-dione). Conditions and reagents: a. tin (II)

octanoate, 110-160 °C, in the bulk; b. DBU, benzyl alcohol, DCM, room

temperature.

Firstly, copolymerisations were performed in the bulk at 110 ºC using tin (II) octanoate

as the catalyst. The chemical structures of the resulting polymers were characterized


by 1H NMR and ATR-FTIR spectroscopies, while their molecular weights and thermal

properties were assessed by GPC and DSC, respectively. Poly(L-lactide-co-OPD)

copolymers were successfully synthesised but only featured very low amounts of OPD

segments. Another set of conditions was then selected to increase the level of OPD

incorporation into the copolymer, in solution at room temperature using 1,8-

diazabicyclo[5.4.0]undec-7-ene (DBU) and benzyl alcohol, as the catalyst and the

initiator, respectively. In both cases, a competing action of the ketone moiety of OPD

with the ester groups of L-lactide and OPD towards the catalysts were observed,

accounting for the low level of OPD incorporation achieved.

Chapter 5 further explores the synthesis of poly(L-lactide-co-OPD) with increased

OPD concentration within the copolymer. To achieve such goal, a modified OPD

featuring an ethylene ketal protecting group was copolymerised with L-lactide, in the

bulk at 110 ºC using tin (II) octanoate and benzyl alcohol as the catalyst and the

initiator, respectively. The acetal protecting groups in the TOSUO segments were

subsequently removed to reveal the ketone of the OPD (Scheme 1.15).


Scheme 1.15. Synthesis of poly(L-lactide-co-OPD) via two steps: a. Ring-opening

polymerisation of L-lactide and TOSUO to afford poly(L-lactide-co-TOSUO). b.

Deprotection of the acetal groups of poly(L-lactide-co-TOSUO) to afford poly(L-

lactide-co-OPD).

Copolymers were synthesised with variable incorporation of OPD ranging from 4.8 to

12.7 mol%. The success of the copolymerisation and the deprotection step were

assessed by 1H NMR and ATR-FTIR spectroscopies, while the molecular weights and

thermal properties were analysed by GPC and DSC. Photodegradation studies were

undertaken of two poly(L-lactide-co-OPD)s with different OPD concentration to

evaluate the prodegradant behaviour of the incorporated OPD.

Chapter 6 summarises the project and provides recommendations for future work.

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Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 47

Chapter 2: 2-Oxepane-1,5-Dione: an Efficient

Photosensitizer for Poly(L-lactide)

2.1 BACKGROUND

Poly(L-lactide) is a biodegradable and biocompostable polyester with high mechanical

performance and low toxicity. The versatile properties of PLLA have attracted

significant research interest with broad applications including biomedical,

pharmaceutical and packaging.1-3 Although PLLA readily degrades under commercial

composting conditions (~ 60 °C), the biodegradation rate under less controlled

conditions remains slow because of variations in temperature, humidity, pH, types of

microorganisms. The combination of increased in applications of PLLA in conjunction

with its slow degradation rate may lead to the accumulation of plastic fragments in the

environment.4-6 To minimize such accumulation in the environment, it is desirable to

design new poly(L-lactide)-based polymers with accelerated degradation rates.

Photodegradation provides an attractive method to achieve such degradation.7 For

instance, previous studies on the photodegradation of neat PLLA have demonstrated

significant chain scissions leading to film embrittlement.8, 9

Enhancing the photodegradation rate of PLLA can be achieved by two methods: the

incorporation of chromophores into the polymer backbone, or blending the polymer

with photoinitiators.10 Previous studies on accelerating the photodegradation rate of

PLLA focused on using additives. Photosensitizers such as N,N,N,N-tetramethyl-1,4-

phenylenediamine have been extensively used and shown to enhance the

photodegradation rate of PLLA by producing radicals that subsequently attacked the

polymer backbone.11, 12 Inorganic additives, such as montmorillonite, sepiolite or

calcium sulfate have been successfully employed as nanoreinforcement materials and

were revealed to accelerate PLLA photodegradation by acting as chromophoric fillers,

which initiated the degradation.13-15 Although nanofillers were readily incorporated

into the PLLA matrix, their dispersion remained challenging. The possible formation

of aggregates could decrease the interfacial area between the nanofillers and PLLA,

altering the prodegradant potential.16, 17 Further treatment via surface modification of

48 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)

the nanoparticles was investigated to overcome the dispersion issue, which involved

extra steps in the preparation of the materials.18, 19

To overcome those limitations, our approach here was to use a molecule that

dissociates to give free-radicals upon UV irradiation and could be either physically

mixed with or chemically incorporated into the PLLA backbone. Among

chromophores, ketones are of interest for photochemistry due to their n-π* transition

and have been extensively reported to initiate and propagate the photooxidation

processes when incorporated in polyolefins.20-23 Here, 2-oxepane-1,5-dione (OPD,

Figure 2.1), a lactone-type molecule with a ketone functional group, was chosen and

investigated as a photosensitizer for PLLA. Previous work using OPD has focused on

the versatility of the ketone moiety to add functionality to biodegradable polyesters

through copolymerisation to achieve controlled architectures for biomaterials

applications such as tissue engineering and drug delivery. The hydrophilicity of OPD

was found to accelerate the hydrolytic degradation of the resulting copolymers and the

reduction of the ketone to hydroxyl moieties facilitated the binding with biological

molecules such as peptides.24-30 In contrast, the strategy described in this Chapter is an

investigation of the photodegradability of the OPD controlled by reactions of the

ketone that accelerate the photodegradation of poly(L-lactide) film.

Figure 2.1. Structure of 2-oxepane-1,5-dione.

In the work undertaken here, PLLA and OPD were physically blended, with OPD

concentrations varying from 2 to 10 wt%. The obtained films were subsequently aged

in the laboratory using UV lamps that mimicked natural outdoor conditions. Visual,

chemical and physical changes were monitored using various spectroscopic

techniques, gel permeation chromatography and differential scanning calorimetry. The

OPD-containing films were compared to neat PLLA films to investigate the effect of

OPD on the rate and mechanism of the photodegradation of PLLA.


2.2 RESULTS AND DISCUSSION

2.2.1 2-Oxepane-1,5-Dione as a Photosensitizer

In the study undertaken here, OPD was synthesised following procedures previously

reported (Scheme 2.1).27, 29, 31 OPD was recovered as white crystals after

recrystallization from cyclohexane and ethyl acetate at 80 °C in 40 % yield in

agreement with the literature.

Scheme 2.1. Synthesis of 2-oxepane-1,5-dione. Conditions and reagents: 1,4-

cyclohexanedione, mCPBA, DCM, 40 °C, 4 h, 45 % yield.

The 1H NMR spectra matched the spectra reported in the literature, and so did the

melting point (110-113 °C for a reported range of 110-112 °C) (refer to appendices).31

The Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectra

revealed a strong C=O stretching band at 1703 cm-1 with a shoulder at 1717 cm-1

supporting the presence of the ketone and the formation of the ester group (Figure

2.2). UV-Visible spectroscopic analysis of OPD revealed a broad absorption band with

a maximum at 273 nm due to the n-π* transition of the ketone (Figure 2.3). The

purified OPD was stored at 10 °C under inert atmosphere, in the dark, to avoid

degradation.

OPD was thus synthesised via a single step from commercially available reagents with

reproducible yields and purity. The ketone on the lactone ring conferred its

photodegradability potential (refer to section 2.1). Once fully characterized, OPD was

subsequently blended to PLLA and some films were obtained. They were

characterized to evaluate the influence of OPD on the polymer performance before

ageing.


Figure 2.2. ATR-FTIR average spectrum of 2-oxepane-1,5-dione (average of 9

spectra after baseline correction).


Figure 2.3. UV-Visible spectrum of OPD showing an absorbance maximum at 273

nm (measured in methanol at 1 mmol·L-1; a baseline spectrum was measured in

methanol).

2.2.2 Initial Characteristics of the Films of Poly(L-lactide) and 2-

Oxepane-1,5-Dione

2.2.2.1. Morphology of the Films

Blending PLLA with molecules or polymers represents a cost-effective processing

technique to successfully improve thermal and mechanical properties as well as to

accelerate the degradation rate.32-35 PLLA blend films have been commonly obtained

by solvent-casting, which involves the dissolution of the polymer in a solvent, the

casting of the solution onto a substrate and solvent evaporation.36

Following this procedure, poly(L-lactide) and 2-oxepane-1,5-dione were dissolved in

chloroform and cast into film. The choice of solvent matters as it can affect the chain

scission process by promoting hydrogen abstraction from the macromolecule chains.7

However, the high molecular weight of PLLA limited the choice of solvent and the


effect of solvent could be taken out of consideration because the films were obtained

using the same procedure for each OPD concentration used.

The OPD concentration was limited to the range 0 - 10 wt% in order to preserve the

homogeneity and mechanical performance of the resulting PLLA - OPD films. The

solutions were homogeneous, clear and transparent suggesting a complete dissolution

of both compounds. Films of PLLA - OPD blends were subsequently solvent-casted

and dried under vacuum to remove traces of solvent. The visual aspects of the different

films before degradation are shown in Figure 2.4.

Figure 2.4. Visual aspects of the films of PLLA - OPD blends before accelerated

ageing. a: 0 wt%; b: 2 wt%; c: 4 wt%; d: 6 wt%; e: 8 wt%; f: 10 wt% OPD.

The films looked homogeneous and transparent except for the formulation with 10

wt% of OPD. This film showed randomly dispersed white spherulites, possibly

attributed to OPD crystals, which will be further discussed in section 2.2.2.3. The

morphology differences were repeatedly observed for every batch of film produced.

The heterogeneity of the PLLA - OPD 10 wt% film confirmed the choice of 10 wt%

as the upper concentration limit.

The thicknesses of the films ranged from 41 to 46 μm. Film thickness has been reported

to influence the photodegradation process by enabling or limiting the oxygen diffusion

within the film.37 For instance, the photodegradation of LDPE films of thickness

greater than 200 μm was dependent on the rate of oxygen diffusion.38 In the work

undertaken here, the films were thin enough to ensure an homogeneous photooxidation

throughout the film thickness.39 No translation study to bulk materials was carried out


in this work (in this case, oxygen starvation could occur, impacting the degradation

process).

In order to further investigate the influence of OPD on the properties of the blends

before ageing, the molecular weights of the different films were analysed by GPC.

2.2.2.2. Initial Molecular Weight of the PLLA - OPD Blend Films

OPD was used as an additive with the aim of accelerating the photodegradation rate of

PLLA without altering its initial properties. The blends should still meet the property

requirements for packaging applications, such as mechanical stability. Such properties

are directly linked to the molecular weight of the polymer. Figure 2.5 shows the GPC

traces for PLLA - OPD films, measured in chloroform. The neat PLLA was

characterized by a broad unimodal distribution. The PLLA - OPD blends (2 - 10 wt%

OPD) presented similar distributions with no significant shift towards low molecular

weight. The average values of number, weight average molecular weights and

polydispersities for three batches of films before UV exposure are shown in Table 2.1

(values were calibrated against polystyrene narrow-molecular-weight-distribution

standards). The 𝑀𝑛 values for the six films ranged from 104,300 ± 10,900 Da to

125,900 ± 11,100 Da with polydispersities ranging from 1.5 to 2.5. No trend between

the OPD amount and the polydispersity could be observed.

Perego et al.40 measured the inherent viscosities in chloroform of PLLA and converted

the results into molecular weights with the Mark-Houwink equations in the following

form:

𝜂 = 5.45 × 10−4𝑀𝜈 0.73

They reported 55,000 g·mol-1 as the molecular weight for which mechanical properties

reached a plateau, with tensile strength of 58,000 MPa and modulus of elasticity of

3,750 MPa. As the PLLA used here was of commercial grade, the initial molecular

weights of the different films were unsurprisingly above this critical value suggesting

sufficient mechanical properties for film and packaging applications.


Figure 2.5. GPC traces of PLLA - OPD 0 - 10 wt% films before UV degradation,

measured in chloroform (the percentage values correspond to the concentration of

OPD in the blends).

Table 2.1. Average values of 𝑀𝑛 , 𝑀𝑤

and polydispersity of three batches of PLLA -

OPD blend films (OPD: 0 - 10 wt%) before accelerated ageing, measured by GPC in

chloroform.

OPD content (wt%) 𝑀𝑛 (Da) 𝑀𝑤

(Da) Ð

0 124,200 ± 5,600 237,100 ± 8,600 1.94 ± 0.04

2 108,800 ± 7,300 212,700 ± 2,400 2.46 ± 0.11

4 125,900 ± 11,100 217,400 ± 12,500 1.51 ± 0.15

6 104,600 ± 22,000 196,100 ± 13,400 2.33 ± 0.31

8 104,300 ± 10,900 196,000 ± 4,900 2.12 ± 0.15


10 102,200 ± 4,100 190,600 ± 1,400 1.99 ± 0.09

Once the molecular weights of the films were determined, the influence of OPD on the

thermal properties and degrees of crystallinity were subsequently investigated.

2.2.2.3. Initial Thermal Properties and Crystallinity

The initial thermal properties and degrees of crystallinity of the PLLA - OPD films

were determined using DSC on a second heating run to erase the thermal history of the

samples. Figure 2.6 shows the DSC thermograms.

Figure 2.6. DSC thermograms from the second heating cycle for PLLA - OPD blend

films before accelerated ageing (the percentage values correspond to the

concentration of OPD in the blends).

All films were characterized by a glass transition (Tg), a small exothermic peak

assigned to cold crystallization (Tcc) and a single endothermic peak corresponding to

a melting temperature (Tm). This unique melting peak suggested that OPD was

completely miscible in the PLLA matrix. The addition of OPD did not significantly


modify the melting peak. However, the glass transition shifted to slightly lower

temperatures with increasing OPD content.

Table 2.2 shows the thermal properties of the different films. The degree of

crystallinity was calculated according to the following equation:

𝜒𝑐(%) = ∆𝐻𝑚 + ∆𝐻𝑐𝑐

∆𝐻𝑚0 × 100

With ∆𝐻𝑚 the melting enthalpy, ∆𝐻𝑐𝑐 the cold crystallization enthalpy and ∆𝐻𝑚0 the

melting enthalpy of 100 % crystalline PLA sample (93.7 J·g-1).41, 42

Neat PLLA film displayed a glass transition at 59.8 ± 0.8 °C and a subsequent melting

peak at 149.1 ± 1.1 °C in agreement with literature.43, 44 A small endothermic peak at

127.1 °C preceded the melting peak. That peak corresponded to cold crystallization

and was attributed to a recrystallization of imperfect crystals into the α form.43 The

films with OPD displayed modified thermal properties. The glass transition gradually

shifted to lower temperatures with the addition of OPD. The PLLA - OPD 10 wt%

film displayed the lowest glass transition (49.4 ± 3.3 °C). The shift in Tg was possibly

due to a plasticization effect from the OPD on the PLLA matrix. The Tg values were

higher than the temperature used during the ageing in the QUV (50 °C) except for the

PLLA - OPD 10 wt% film. Ageing at temperatures below Tg limits oxygen diffusion

within the polymer matrix because the chain mobility is much lower than can be

observed in a rubbery state above Tg. The melting temperature proportionally

decreased with the concentration of OPD, from 148.6 ± 0.9 to 144.3 ± 3 °C with 2 and

10 wt% OPD, respectively. The cold crystallization temperature decreased, while the

degree of crystallinity increased with increasing OPD concentration. Such behaviours

highlighted that the presence of OPD accelerated the crystallization rates because of

its nucleation effect.

As discussed in section 2.2.2.1, the film containing 10 wt% OPD displayed evidence

of heterogeneity with both transparent and opaque sections.

Table 2.3 presents the thermal properties of those two sections (refer to appendices

for the DSC thermograms). The transparent section was characterized by a Tg of 55.6

°C, which is lower than neat PLLA and suggests the presence of OPD as a plasticizer.

The opaque section displayed much lower values of Tg and Tm than neat PLLA, as

well as a significant higher degree of crystallinity compared to any of the values


reported in Table 2.2. These results suggested that 10 wt% OPD enhanced the

crystallization in a heterogeneous manner during the solvent-casting process.

Table 2.2. Evolution of the glass transition and melting temperature of the films with

OPD content obtained by DSC before ageing (the measurements were performed on

three different batches of films and the values were averaged).

OPD

(wt%)

Tg

(°C)

Tcc

(°C) ∆𝐻𝑐𝑐 (J.g-1)

Tm

(°C) ∆𝐻𝑚 (J·g-1)

𝜒𝑐 (%)

0 59.8 ± 0.8 127.1 ± 1.5 4.69 ± 0.6 149.1 ± 1.1 3.50 ± 0.8 8.7

2 58.4 ± 1.9 126.6 ± 0.9 4.01 ± 1.7 148.6 ± 0.9 2.81 ± 1.2 5.9

4 54.6 ± 2.1 126.4 ± 2.4 4.09 ± 2.2 147.3 ± 1.9 3.87 ± 1.8 8.5

6 53.4 ± 3.3 125.0 ± 2.9 5.88 ± 5.6 146.4 ± 1.7 2.67 ± 1.3 12.7

8 50.5 ± 5.8 124.6 ± 4.3 6.41 ± 1.6 145.5 ± 2.3 5.84 ± 1.3 13.1

10 49.4 ± 3.3 121.6 ± 6.3 8.76 ± 4.7 144.3 ± 2.9 7.76 ± 4.2 17.6

Table 2.3. Comparison of the thermal properties of the transparent and opaque

sections of the PLLA - OPD 10 wt% film.

Tg

(°C)

Tcc

(°C) ∆𝐻𝑐𝑐 (J.g-1)

Tm

(°C) ∆𝐻𝑚 (J·g-1)

𝜒𝑐 (%)

Transparent section 55.9 - - 147.6 4.6 4.9

Opaque section 46.4 114.3 14.0 141.0 12.4 28.2

The chemical structures of the films were subsequently investigated by ATR-FTIR and

UV-visible spectroscopies to study their chemical structures and identify the presence

of OPD in the blends.

2.2.2.4. Chemical Characterization of the Films

ATR-FTIR spectroscopy was used as a non-destructive surface analysis technique to

further characterize the different films and evaluate their homogeneity.45 For PLA and

diamond as the ATR crystal, the depth penetration of the IR beam calculated from the

Harrick equation was reported to be 0.30 µm.15 Here, the film thicknesses ranged from

41 to 46 µm, confirming that information on the surface composition films was

produced by this ATR-FTIR analysis in this section.


Nine spectra were collected for each film on three different positions. Figure 2.7

shows the average spectra of the six films after being baseline corrected and

normalized by comparison with the band at 1455 cm-1 (due to -CH3 bending) to

suppress any effect from differences in contact with the ATR crystal and depth of

penetration of the IR beam. The spectra all revealed similar characteristic bands: the

C=O stretching band at 1747 cm-1, the –C-O- stretching bands at 1180 and 1085 cm-1,

and the –CH3 bending at 1455 cm-1.

Figure 2.7. ATR-FTIR average spectra of PLLA - OPD 0-10 wt% films before

degradation (average of 9 spectra per film after baseline correction and normalization

with the -CH3 bending band at 1455 cm-1).

The sharp band at 754 cm-1 was assigned to residual chloroform.46 This band was

consistently observed for all the film batches. The characteristic bands for neat PLLA

are given in Table 2.4 based on reported literature.13, 43 Every band obtained in the

spectra matched those of neat PLLA. The addition of OPD did not lead to any shift of

wavelength of the different bands. However, the carbonyl region (1800 - 1675 cm-1)

noticeably differed for the formulations containing OPD. The ester band of the PLLA


presented an additional shoulder from 1725 to 1690 cm-1 compared to neat PLLA

(Figure 2.8). This shoulder was assigned to the ketone of OPD.45

Figure 2.8. Carbonyl band in the ATR-FTIR spectra of the PLLA - OPD (0 - 10

wt%) blend films before ageing revealing the shoulder from 1725 to 1690 cm-1 due

to the OPD ketone moiety (average of 9 spectra per film after baseline correction and

normalization with the -CH3 bending band at 1455 cm-1).

Table 2.4. ATR-FTIR band assignment of poly(L-lactide) based on reported

literature.13, 47

Band position (cm-1) Assignment

2996 -CH- stretch (asymmetric)

2945 -CH- stretch (symmetric)

1747 -C=O carbonyl stretch

1455 -CH3 bend


Films were also characterized by UV-Visible spectroscopy before ageing. Figure 2.9

presents the spectra of the six film types.

Figure 2.9. UV-Visible spectra of PLLA - OPD films before accelerated ageing,

showing an increase in absorbance in the range 250 - 300 nm due to the n-π*

transition of the ketone moiety of OPD.

The spectrum of PLLA presented the lowest absorbance, which was expected

considering that the PLLA structure does not contains a UV chromophore. The

addition of OPD led to a proportional increase in absorbance in the range 250 - 300

nm. This increase was attributed to the n-π* transition of the OPD ketone. The increase

1384; 1359 -CH- deformation (symmetric and

asymmetric bend)

1182; 1085 -C-O- stretch

1043 -OH bend

926, 868 -C-C- stretch


in OPD content led to higher absorbance at higher wavelength, with the highest

absorbance reached with 10 wt% OPD. As discussed in section 2.2.2.1, the neat PLLA

film was transparent whereas the PLLA - OPD 10 wt% film presented some white

spherulites due to a heterogeneous crystallization. This difference in morphology led

to light scattering and thus increase in the baseline absorbance.

Spectroscopic methods enabled the characterization of the films before degradation

with accurate identification of OPD. OPD’s presence in the blends was characterized

by a shoulder at 1725 - 1690 cm-1 and an increase in absorbance between 250 - 300

nm in ATR-FTIR and UV-Visible spectroscopies, respectively. These regions were

monitored during ageing to assess chemical changes to OPD during artificial ageing.

2.2.3 Photodegradation of PLLA - OPD Blends

The reason for adding OPD to the PLLA matrix was to accelerate the photodegradation

of the polyester. A secondary goal was to tune the photodegradation rate through

modification of OPD concentration. Exposing the polymer directly under natural

outdoor exposure represents the most straightforward ageing approach. All external

factors such as fluctuation in the temperature, humidity and external stress are thus

taken into consideration. However, this type of experiment also includes a long

observation time range, up to several months or more. Artificially accelerated ageing

enables the assessment of polymer lifetimes within shorter study times. Devices such

as Sepap (Atlas) and QUV accelerated weathering testers (Q-lab, Ohio) are suitable

for mimicking natural outdoor exposure by controlling parameters such as UV light

intensity and wavelengths, temperature, humidity and time of each cycle.39 The

degradation times can then be correlated to the time taken to degrade during natural

outdoor exposure.

In the present work, films of PLLA - OPD blends (0 - 10 wt% OPD range) were

artificially aged using a QUV device by irradiating samples with UV-A light relevant

for natural outdoor exposure. 24 hours cycles were performed at 50 °C. Changes in

visual aspects, molecular weight, thermal properties and chemical structures were

monitored throughout the QUV ageing process.

2.2.3.1. Visual Changes of the Films

The visual aspect of the different films was observed after every cycle to evaluate the

deterioration for each formulation. The six films were transparent and homogeneous


before degradation, except for the film with 10 wt% OPD as previously discussed

(refer to section 2.2.2.1). As the degradation proceeded, whitening, opacity and

brittleness of the films significantly increased for films with 6, 8 and 10 wt% OPD

(Figure 2.10). Films with 4 to 10 wt% OPD embrittled in the time range 8 - 12 days.

After 27 days in the QUV, the films of neat PLLA and PLLA - OPD 2 wt% hadn’t

embrittled yet. These visual observations suggested that OPD had a prodegradant

effect during photodegradation, especially when the OPD content was greater than 4

wt%.

Figure 2.10. Effect of UV exposure on the films of PLLA - OPD (0-10 wt%) before

(top) and after 14 days (bottom) of UV exposure using a QUV device (UV-A 340

lamps, 50 °C) with whitening and embrittlement observed.

Changes in the sample opacity, colour (yellowing, whitening due to the disorganisation

of the matrix structure and to the enhance of the scattered light) or brittleness are visual

signs of degradation.48 Whitening, here, is caused by an increase in crystallinity as a

result of degradation firstly occurring in the amorphous phase of the polymer. New

crystal segments are formed, increasing the degree of crystallinity and thus the film

opacity.49-51

The prodegradant potential of OPD was investigated by assessing the molecular

weight changes throughout the photodegradation study.

2.2.3.2. Evolution of Molecular Weight

Aged film samples were characterized by GPC to investigate the evolution of the

molecular weights of the different formulations throughout the ageing process. Figure

2.11 presents the GPC distributions of each film before and after twelve days of UV

exposure in the QUV after baseline correction. The PLLA only sample showed a broad

unimodal distribution with a small shift towards lower molecular weight after ten days

in the QUV. The distributions of the PLLA - OPD (2 - 10 wt%) films presented more


significant shifts towards lower molecular weight within ten days of UV exposure. The

increase in OPD concentration led to greater shifts, suggesting a greater decrease in

the molecular weight.

Figure 2.11. Evolution of the GPC distributions of each film before (plain line) and

after ten irradiation days (dashed line) in the QUV, revealing a shift towards low

molecular weight for films containing OPD.

This observation was confirmed by calculating values of 𝑀𝑛 (Figure 2.12). The initial

values of the films presented a large variation, possibly due to an initial degradation

during the samples preparation due to OPD. The 𝑀𝑛 values of neat PLLA decreased

after four days in the QUV but a trend that could describe the decrease was not

observed. Ageing of PLLA under similar conditions (UV-A light, 60 °C) led a rapid

decrease in molecular weight after 100 hours of irradiation (18 % loss of molecular

weight) followed by slower rates for longer irradiation times.14


Each of the OPD-containing films showed a drastic decrease in 𝑀𝑛 after two days in

the QUV (approximately 50 %). The 𝑀𝑛 continued to slightly decrease until four days.

The values reach a plateau after that time in the QUV.

Figure 2.12. Decrease in the number average molecular weight of the aged PLLA -

OPD films (0 - 10 wt% OPD) versus irradiation days, as measured by GPC in

chloroform (data were obtained from three different batches of films and averaged).

Concerning the polydispersities of the six films, they remained within the range 2 - 2.5

independently of the UV exposure days (Figure 2.13).


Figure 2.13. Evolution of the polydispersities of the PLLA - OPD blend films during

artificial ageing.

A comparative kinetic study was performed based on the molecular weight data

obtained by GPC. The photodegradation of PLLA has been associated with both first

and second order kinetic laws defined by the following equations, respectively:

ln 𝑀𝑛(𝑡) = ln 𝑀𝑛(0)

− 𝑘𝑡 (1)

1

𝑀𝑛(𝑡)

= 1

𝑀𝑛(0)

+ 𝑘𝑡 (2)

With 𝑀𝑛(𝑜) the intial molecular weight, 𝑀𝑛(𝑡)

the molecular weight at an irradiation

time t and k the rate constant of photodegradation.9, 50 However, the first order kinetic

law was found using UV-C light while the second order kinetic law used UV-A light,

relevant for mimicking natural outdoor conditions.

Based on those findings, the inverse of the measured 𝑀𝑛 values were plotted against

irradiation time and demonstrated linear relationships (r2 ranging from 0.80 to 0.98)

(Figure 2.14, Table 2.5). The photodegradation rates significantly increased for the


films containing from 4 to 10 wt% OPD (Table 2.5). Those results suggested that the

presence of OPD accelerated the reduction of molecular weight at early stages of the

photodegradation process without showing an induction time.

The photodegradation of PLLA was previously studied under artificial and natural

conditions and followed by GPC.9, 15, 50, 51 Both types of ageing studies revealed

reductions in molecular weight and broadening of the polydispersities. In particular,

accelerated ageing under similar conditions as the present work (UV-A light, 60 °C)

revealed a rapid decrease in molecular weight after 100 hours of irradiation (18 % loss

of molecular weight) followed by slower rates for longer irradiation times.15 These

results suggested that chain scission occurs under these conditions at random locations

along the macromolecular chains.22, 52 In the work presented here, the combined

decrease in molecular weight and relatively constant polydispersity strongly suggests

similar process.8, 53

Figure 2.14. 1/𝑀𝑛 vs irradiation days of the PLLA - OPD films (0 - 10 wt%).


Table 2.5. r2 values and rate constants k determined from the 𝑀𝑛 measured by GPC

for the six formulation films.

Film formulation r2 k (Da·days-1)

PLLA 0.80 1.79.10-7 ± 6.66.10-8

PLLA - OPD 2 wt% 0.95 1.19.10-6 ± 1.90.10-7

PLLA - OPD 4 wt% 0.98 3.02.10-6 ± 3.32.10-7

PLLA - OPD 6 wt% 0.97 4.24.10-6 ± 5.03.10-7

PLLA - OPD 8 wt% 0.95 3.25.10-6 ± 5.25.10-7

PLLA - OPD 10 wt% 0.93 3.05.10-6 ± 6.05.10-7

To determine the number of chain scissions s, the following equation was used:

𝑠 =𝑀𝑛(0)

𝑀𝑛(𝑡)

− 1

With 𝑀𝑛(0) and 𝑀𝑛(𝑡)

the number average molecular weights before UV exposure and

after a time, t, in the QUV, respectively.54 As shown in Figure 2.15, the films presented

different behaviours. Neat PLLA only showed a small number of chain scissions after

four days in the QUV and this value remained small. The film with OPD also started

to show the occurrence of chain scissions after four days in the QUV. The films with

2 wt% OPD demonstrated the lowest values of chain scission compared to other

formulation film with OPD. The number of chain scissions increased for those films

approximately with irradiation time. However, a plateau seems to be reached after 4

days, which is in agreement with the 𝑀𝑛 values previously reported. The formulation

with 4 wt% OPD seemed to be the ideal amount as adding more OPD did not

significantly increase the number of chain scissions. This suggested that 4 wt% could

be the solubility limit of OPD in PLLA, having a real effect on the PLLA

photodegradation. Adding more OPD tended to separate phases and produce OPD rich

phases, which was in agreement with visual observations made in section 2.2.3.1.

These random chain scissions led to embrittlement. From the visual observations

previously discussed (refer to section 2.2.3.1), embrittlement of PLLA - OPD at 4 - 10

wt% OPD occurred from 8 - 10 days.


Figure 2.15. Evolution of the number of chain scissions of the aged films of PLLA -

OPD (0 - 10 wt%) in the QUV, calculated from the 𝑀𝑛 measured by GPC in

chloroform.

The molecular weight of a polymer directly influences its mechanical properties, with

critical values that define the existence of mechanical properties on one hand (e.g.

entanglement molecular weight), and the degradation extent on the other hand.

Rasselet et al.52 investigated the relationship between the molecular weight and

mechanical properties changes during oxidative degradation of PLA. They linked the

embrittlement behaviour to the 𝑀𝑛 and defined a critical molecular weight for

complete embrittlement at 40,000 g·mol-1. The strain at break reached a plateau value

of 1 % at this critical molecular weight. The 𝑀𝑛 of the formulations containing OPD

reached 40,000 g·mol-1 at different times and continued to decrease with increasing

time of exposure. The PLLA - OPD film containing 4 to 10 wt% OPD reached a 𝑀𝑛

of < 40,000 g·mol-1 after four days of UV exposure. The PLLA and PLLA with 2 wt%

OPD films did not reach 40,000 g·mol-1 after ten days of UV exposure. The neat PLLA

film presented a 𝑀𝑛 higher than 80,000 g·mol-1 after ten days in the QUV.


The GPC measurements suggested that photodegradation led to random chain

scissions with accelerated rates with increasing OPD concentration. In addition to

affecting mechanical properties, the molecular weight of the samples are directly

linked to the thermal properties.

2.2.3.3. Thermal Properties and Crystallinity Modifications

The thermal properties and degrees of crystallinity were determined by DSC

throughout the course of the ageing studies. Figure 2.16 presents the evolution of the

melting temperatures of the PLLA - OPD films during their ageing. The films

containing OPD revealed a decrease in the Tm with bigger shifts for the films with 6,

8 and 10 wt% OPD. The decrease may be attributed to the reduction in molecular

weight that led to structural changes of the crystalline regions. For instance, the

occurrence of chain scission led to lattice disorder in the crystalline regions.55

Figure 2.16. Evolution of the melting temperature of the PLLA - OPD films as a

function of irradiation time in the QUV.

Figure 2.17 shows the evolution of the glass transitions of the PLLA - OPD blends as

a function of irradiation time.


Figure 2.17. Evolution of the glass transition of the PLLA - OPD films as a function

of irradiation time in the QUV.

The ageing of neat PLLA did not significantly change the glass transition which only

slightly increased from 60 to 60.5 °C after ten days in the QUV. This behaviour was

reported by Tsuji et al.11 for a degraded PLLA with glass transition increasing from

59.9 to 65.4 °C after 400 hours of photodegradation. They attributed this increase to a

low-temperature annealing affect that stabilized the chain packing in the amorphous

phase of the PLLA. In contrast, the glass transition of the films containing OPD shifted

to lower temperatures. This decrease was more noticeable for the films containing 4 to

10 wt% OPD. These observations were consistent with the chain scission events

revealed by the GPC measurements, since the glass transition temperature and the

number average molecular weight are linked according to the Fox-Flory relationship

as follows:56

𝑇𝑔 = 𝑇𝑔,∞ −𝑘

𝑀𝑛

With k as the Flory-Fox constant, 𝑇𝑔,∞ the glass transition of polylactide having an

infinite molecular weight (reported value of 55 °C for PLA52) and 𝑀𝑛 is the number

average molecular weight. The reduction in 𝑀𝑛 thus provoked a decrease in Tg.


Moreover, the chain scissions likely released low molecular weight photoproducts,

such as oligomers. Oligomers can have a plasticization effect on the polymer matrix

leading to the lowering of Tg.47

2.2.3.4. Chemical Structural Modifications

Chemical changes resulting from the ageing process of the films were monitored by

ATR-FTIR spectroscopy. Figure 2.18, Figure 2.19 and Figure 2.20 show the spectra

of PLLA, PLLA - OPD 2 wt% and PLLA - OPD 10 wt% respectively, before and after

UV exposure (one and ten days) in the QUV (refer to appendices for the spectra of the

other formulations).

Figure 2.18. ATR-FTIR average spectra of PLLA film before and after one and ten

irradiation days (average of 9 spectra after baseline correction and normalization

with the -CH3 bending band at 1452 cm-1).

Every spectrum was normalized with the band at 1454 cm-1 (due to –CH3 bending) to

eradicate any effect from differences in contact with the ATR crystal and depth of


penetration of the IR beam. The main noticeable difference between aged and unaged

spectra was the decrease in the OPD shoulder at 1725 - 1690 cm-1 during ageing.

For both PLLA films containing 2 and 10 wt% OPD, the shoulder of the OPD ketone

started to disappear after one day in the QUV. This shoulder continued to noticeably

disappear throughout the ageing process in the QUV. This suggested that the OPD

ketone reacted to UV light at early stages of the irradiation process, without any

induction period.

Figure 2.19. ATR-FTIR average spectra of PLLA - OPD 2 wt% film before and

after one and ten irradiation days (average of 9 spectra after baseline correction and



Figure 2.20. ATR-FTIR average spectra of PLLA - OPD 10 wt% film before and

after one and ten irradiation days (average of 9 spectra after baseline correction and


UV-visible spectroscopy was also used to monitor changes during the

photodegradation of the films. The PLLA film revealed a very weak absorption in the

region 250 - 400 nm before degradation that slightly decreased after one day in the

QUV, but was not further affected with increased ageing time (Figure 2.21). The

PLLA - OPD 10 wt% revealed an increase in the absorbance that could be attributed

to the increase in whitening of the film throughout ageing (Figure 2.22) (refer to

appendices for the spectra of the other formulations).


Figure 2.21. UV-visible spectra from the PLLA only film as a function of irradiation

time in the QUV.


Figure 2.22. UV-visible spectra of the PLLA - OPD 10 wt% film as a function of

irradiation time in the QUV.

All the data obtained above were collected under accelerated photo-oxidative

conditions, involving UV-A light and heat (50 °C). To isolate the effects of UV, the

subsequent section of this work was undertaken to evaluate the influence of the

temperature on the photodegradation mechanism.

2.2.4 Influence of Temperature on the Degradation Behaviour of the

Blends

In photodegradation, pure photochemical effect and pure thermal effect are intimately

imbricated. In this section, the effect of pure thermo-oxidation was investigated with

the same temperature conditions in which photodegradation effectively induced the

degradation of the blends. Duplicates of each film were produced and simultaneously

aged in the QUV. For each formulation, one film was exposed to UV while the other

was covered to be protected from the light. Samples were collected every two days in


the QUV and characterized by GPC. Figure 2.23 and Figure 2.24 respectively show

a comparison of the evolution of the GPC distributions before and after ten irradiation

days in the QUV for the PLLA and PLLA - OPD 10 wt% films.

Figure 2.23. Comparison of the GPC traces of neat PLLA when covered (C) and

uncovered (U) before and after ten days in the QUV.

The neat PLLA did not present any significant shift towards low molecular weight

after ten days in the QUV for neither the uncovered nor covered film. These

observations were expected from previous experiments, with no significant decrease

in the molecular weight for the PLLA film detected as described earlier (refer to

section 2.2.3.2).

However, the PLLA - OPD 10 wt% films revealed interesting results. Only the

distribution of the uncovered, UV-exposed film shifted towards lower molecular

weight. The covered film (without UV exposure, only heat exposure) showed very

little change in distribution before and after ten days in the QUV, suggesting that no

chain scission occurred during the thermooxidation process (Figure 2.24).


The number average molecular weights and polydispersities of both uncovered and

covered films were compared as a function of irradiation days (Table 2.6). The

uncovered and covered films presented two opposite behaviours. Concerning the

uncovered films, the 𝑀𝑛 continuously and drastically decreased for the films

containing OPD, which is consistent with previously discussed results (refer to section

2.2.3.2). The covered films on the other hand presented a much lower degree of

decrease in 𝑀𝑛 . Only the neat PLLA film demonstrated similar behaviour for the

uncovered and covered films with values of 𝑀𝑛 remaining approximately unchanged

for both. The evolution of polydispersity values with the irradiation time were too

random to observe a particular trend.

These results firstly demonstrated that the absence of light prevented the decrease in

molecular weight for OPD containing samples. The temperature alone did not lead to

any significant degradation. However, the combined action of UV and temperature led

to reduction of molecular weight, confirming that the OPD acts as a photoprodegradant

under these ageing conditions. So, degradation of the OPD-containing films require

both UV and heat to proceed. Based on these findings and those previously reported,

a photodegradation mechanism is proposed in the following section.


Figure 2.24. Comparison of the GPC traces of PLLA - OPD 10 wt% when covered

(C) and uncovered (U) before and after ten days in the QUV.

Table 2.6. Comparison of the 𝑀𝑛 and the polydispersity of the uncovered and

covered films of PLLA - OPD blends (0 - 10 wt%) as a function of irradiation days.

Sample

𝑀𝑛 (g·mol-1) Ð

Days Covered Uncovered Covered Uncovered

PLLA 0 128,400 126,300 1.86 1.63

2 121,400 118,900 1.86 1.93

4 116,700 113,200 1.94 1.90

6 125,300 116,500 1.87 1.80

8 137,800 121,000 1.72 1.81

10 131,700 93,000 1.78 2.13


PLLA - OPD 2 wt% 0 117,100 105,900 1.84 2.01

2 113,100 101,200 1.86 1.91

4 111,700 62,300 1.93 2.34

6 119,000 71,600 1.77 1.84

8 124,600 54,600 1.71 2.15

10 104,900 45,700 1.82 2.16

PLLA - OPD 4 wt% 0 117,500 121,600 1.78 1.73

2 102,800 56,100 1.68 2.20

4 98,800 39,900 1.80 2.11

6 98,400 42,200 1.82 1.84

8 98,500 35,300 1.79 2.03

10 84,600 32,000 1.91 1.95

PLLA - OPD 6 wt% 0 100,000 124,300 1.96 1.69

2 106,900 68,200 1.80 1.78

4 85,400 38,500 1.99 2.16

6 95,300 42,300 1.78 1.69

8 93,100 29,300 1.81 1.99

10 83,900 14,200 1.85 2.96

PLLA - OPD 8 wt% 0 104,400 115,200 1.89 1.74

2 88,400 59,700 1.83 1.93

4 100,900 41,520 1.86 1.95

6 99,300 37,410 1.77 2.03

8 104,100 28,730 1.74 2.21

10 78,600 29,980 1.99 1.81

PLLA - OPD 10 wt% 0 103,600 105,400 1.83 1.80

2 108,000 75,700 1.64 1.66


4 91,300 40,400 1.93 2.17

6 93,500 39,300 1.99 1.96

8 92,800 35,600 1.93 1.96

10 91,800 31,300 1.82 2.03

2.2.5 Mechanism of Photodegradation

Six film formulations based on PLLA and OPD were artificially aged using conditions

that mimicked natural outdoor conditions. GPC analysis of PLLA without additives

revealed little change in molecular weight after ten irradiation days (refer to section

2.2.3.2). The polydispersity did not significantly change as well with values remaining

around 2 before and after irradiation. However, the presence of OPD in PLLA led to

more significant modifications. For all the films containing OPD, the molecular

weights decreased and the Ð reached values around 2 at early stages of the ageing

process. Those results supported random chain scission process occurring during

ageing. The chain scissions led to a decrease in mechanical properties and

embrittlement of the films (refer to section 2.2.3.1). Along with the reduction in

molecular weight, there was the fast disappearance of the OPD shown by ATR-FTIR

spectroscopy (refer to section 2.2.3.4). The results suggested that the OPD ketone was

lost due to chain cleavage at early stages of the photodegradation.

Different mechanisms have been discussed for the photodegradation of poly(L-

lactide). The first mechanism was based on main chain scission according to Norrish

type II photocleavage occurring at the carbonyl group in the ester linkage via a n-π*

transition.57 An alternative mechanism was proposed by Bocchini and coworkers,

confirmed by Gardette et al., based on studies using UV-A, relevant to natural outdoor

conditions.13, 14 This mechanism included initiation of degradation by impurities,

which form radicals upon UV exposure. The radicals formed subsequently abstract a

tertiary hydrogen from the PLLA chains, thus forming macroradicals. The

macroradicals subsequently react with oxygen to form peroxide radicals that propagate

the degradation by abstracting other hydrogens and forming hydroperoxides. The

photolysis of hydroperoxides leads to anhydrides as the most stable photo-products.13,

14


The physical blending of OPD with PLLA was attempted in order to promote the

photodegradation rate of the blends. Indeed, ketones have proved to efficiently

accelerate the photodegradation of polyolefins when used as either in-chain or side-

chain groups.20 Such ketones undergo chain scission via Norrish type I and II processes

(Scheme 2.2).

Scheme 2.2. Norrish type I and II cleavages of ketones.

The Norrish type I process is a photochemical α-cleavage of the ketone into two free

radical intermediates, including an acyl radical. Acyl radicals can eliminate carbon

monoxide to give alkyl radicals. The Norrish type II process involves an

intramolecular hydrogen abstraction from the carbon in the γ position relative to the

carbonyl group. This abstraction is followed by the cleavage of the α-β C-C bond,


resulting in the formation of an enol and a terminal C=C double bond. The enol

subsequently tautomerizes to the more stable ketone.22

In the work described here, the OPD ketone was expected to undergo photocleavage

via Norrish type reactions. The absence of a hydrogen on the carbon in the γ position

relative to the ketone reduced the likelihood of a contribution of a Norrish type II

mechanism to the photodegradation.22 Thus OPD cleavage via a Norrish Type I

mechanism should lead to the formation of intermediate radicals followed by the

elimination of carbon monoxide. The loss of the ketone was confirmed by ATR-FTIR

spectroscopy with the disappearance of the shoulder between 1725 and 1690 cm-1

(refer to section 2.2.3.4). The resulting alkyl radicals could subsequently attack the

PLLA chains by abstracting a tertiary hydrogen in the α-position relative to the ester

group. The rest of the mechanism should correspond to the one reported in the

literature, resulting in PLLA chain scission (Scheme 2.3). Those chain scissions were

observed for the PLLA - OPD blends based on the GPC measurements (refer to section

2.2.3.2). However, no photoproducts were identified in the present work, probably

because of the little ageing time range needed to embrittle the PLLA - OPD films.


Scheme 2.3. Proposed photodegradation mechanism for PLLA - OPD blends

initiated by the Norrish type I cleavage of OPD, releasing radicals that attack the

PLLA backbone leading to hydrogen abstraction. The rest of the mechanism is based

on previous reports, leading to PLLA chain scission and anhydride formation.13, 14

2.3 SUMMARY

OPD was investigated as an additive to enhance the photodegradation rate of poly(L-

lactide). Blends of PLLA and OPD were produced by film-casting with an OPD

content ranging from 0 to 10 wt%. The films were artificially aged in a QUV device

using UV-A at 50 °C. Films containing OPD in the range 4 - 10 wt% rapidly embrittled

compared to neat PLLA and PLLA - OPD films with 2 wt% OPD. From the chain

scissions calculated based on GPC measurements, 4 wt% seemed to be the solubility

limit of OPD in the PLLA-OPD formulations, making this value ideal to accelerate the

photodegradation of PLLA. This behaviour was attributed to the drastic decrease in

the molecular weight of the films at early stages of the ageing process. Spectroscopic

techniques revealed the disappearance of OPD at early stages of the photodegradation

process. A mechanism was proposed based on these observations. The

photodegradation was suggested to be initiated by the cleavage of OPD via a Norrish

type I mechanism, leading to the formation of initiating radicals. These radicals may

then attack the PLLA backbone ultimately leading to chain scission and embrittlement

of the films. As a conclusion, OPD was revealed to be an efficient photosensitizer for

PLLA. Further investigations as described in the following chapters were subsequently

undertaken to incorporate OPD into the PLLA backbone to evaluate the

photodegradation potential of the copolymers.

2.4 EXPERIMENTAL

2.4.1. Materials

Poly(L-lactide) 4043D grade was purchased from NatureWorks LLC and dried 4 h at

80 °C under vacuum in a dried oven prior to use. Sigmacote®, 1,4-cyclohexanedione

(98 %) and 3-chloroperbenzoic acid (≤ 77 %) were purchased from Sigma-Aldrich

and used as received. Anhydrous sodium sulfate, dichloromethane, diethyl ether,

cyclochexane and ethyl acetate (AR grades) were purchased from ChemSupply and


used as received. Chloroform (HPLC grade) was purchased from Merck and filtered

prior to use.

2.4.2. Methods

2.4.2.1. Synthesis of 2-Oxepane-1,5-Dione

m-Chloroperbenzoic acid (5 g, 29.0 mmol, 1.6 equiv.) was dissolved in DCM (40 mL)

and dried over anhydrous Na2SO4. The solution was filtered into a 100 mL round

bottom flask. 1,4-Cyclohexanedione (2 g, 17.8 mmol) was slowly added to the mCPBA

solution and the reaction mixture was stirred at 40 °C for 3 h. The resulting mixture

was cooled to room temperature while stirring for 15 h and then concentrated under

reduced pressure. The crude product was washed with Et2O, filtered, and recrystallized

from cyclohexane and EtOAc (5/3 vol/vol). OPD was recovered as white crystals in

40% yield. Mp. 111-113 °C (Lit., 110-112 °C31). 1H NMR (CDCl3, 600 MHz), δ ppm

= 4.42 (t, 2 H, CH2O), 2.85 (t, 2 H, CH2-COO), 2.65 (m, 4 H, CH2CO).13C NMR

(CDCl3, 600 MHz), δ ppm = 204.9 (CO), 173.3 (COO), 63.4 (CH2O), 44.8 (CH2CO),

38.7 (CH2CO), 28.0 (CH2COO). ATR-FTIR: ʋ max= 2969 (w, -CH2), 1740 (s, -C=O

of lactone), 1702 (s, -C=O of ketone), 1455, 1437 and 1392 (-CH2) cm-1. All

characterization data obtained were consistent with literature.31

2.4.2.2. Film Casting of Poly(L-lactide) and OPD

Poly(L-lactide) grade 4043D was purchased from NatureWorks LLC and used as

received. Poly(L-lactide) and OPD (0, 2, 4, 6, 8 and 10% wt.) were dissolved in

chloroform (10 mL). 1.5 mL of each solution was cast into a Petri dish (5 cm diameter),

previously coated with SigmaCote (Sigma-Aldrich). The dishes were covered to

ensure slow evaporation and avoid an orange-peel effect. Films were then vacuum

dried until a constant weight was reached, and stored under nitrogen at -15 °C in the

dark.


2.4.2.3. Accelerated Photo Ageing

PLLA - OPD (0-10 wt%) films were mounted onto 35 mm aluminium slide holders

and exposed to UV-A 340 lamps at an irradiance of 0.68 W/m2 at 340 nm in a QUV

accelerated weathering tester (Q-lab, Ohio). Water was present in the QUV in order to

maintain maximum and consistent levels of humidty for the degradation study. The

QUV was operated at a black panel temperature of 50 °C and cycles lasted for 24 h.

The irradiance sensors were calibrated every 500 hours.

2.4.2.4. Accelerated Thermal Ageing

Films of each formulation were mounted onto 35 mm aluminium slide holders and

covered with aluminium foil. The films were aged in the dark in the QUV using the

same conditions as shown above (refer to section 2.4.2.3).

2.4.3. Measurements

2.4.3.1. Films Thickness

The films thickness was measured on a Teclock Upright Stand Type US-16B. Three

measurements were performed on each film and averaged.

2.4.3.2. Gel Permeation Chromatography

Molecular weights of synthesised polymers were studied by gel permeation

chromatography with a Waters gel permeation chromatography system equipped with

a Waters 1515 isocratic HPLC pump, Waters 2707 autosampler with a 100 µL

injection loop, column heater (30oC) and a Waters 2487 dual wavelength absorbance

detector (analysis at 254 and 273 nm, corresponding to the absorbance of OPD) in

series with a Waters 2414 refractive index detector (analysis temperature, 30 oC) was

used for GPC analysis. Three consecutive Waters Styragel columns (HR5, HR4, and

HR1, all 7.8x300 mm, 5 µm particle size), preceded by a Waters Styragel guard

column (WAT054405, 4.6x30 mm, 20 µm particle size) were used during analysis.

Chloroform was used as the eluent at a flow rate of 1 mL·min-1. The molecular weight

separation ranges for the columns, relative to polystyrene are: HR5 – 50,000-4,000,000

Da; HR4 – 5,000-600,000 Da; HR1 - 100-5,000 Da; Guard column - 100-10,000 Da.

Samples were typically prepared at a concentration of 2.5 - 5 mg·mL-1 and

determination of molecular weight performed by calibration against polystyrene

narrow-molecular-weight-distribution standards.


2.4.3.3. Differential Scanning Calorimetry

The thermal properties of the samples were recorded by Differential Scanning

Calorimetry on a TA DSC Q100. Heat / cool / heat runs were performed on

temperature ranges that depended on the samples run (0 - 160 °C for OPD; 0 - 230 °C

for PLA-based polymers). Heating and cooling rates were set at 10 °C·min-1. The

transition temperatures reported were from the second heating cycle. The cycles were

performed on samples of 2 - 3 mg under nitrogen atmosphere, at a flow rate of 50

mL·min-1.

2.4.3.4. Fourier Transform Infrared Spectroscopy

Fourier Transform Infrared spectroscopy was performed on a Thermo Nicolet 5700

FTIR spectrometer, using OMNIC 7.2a software. 64 scans were run with a 4 cm-1

resolution. 9 spectra were collected on three different pieces of the samples, baseline-

corrected, normnalized and averaged. Spectral analysis was undertaken using OMNIC

7.2a and GRAMS software.

2.4.3.5. UV-visible Spectroscopy

UV-visible spectra of 2-oxepane-1,5-dione were collected on a Varian Cary 50 UV-

Visible spectrometer using quartz cells. OPD was dissolved in methanol (liquid

chromatography grade) at 1 mmol·L-1. A baseline spectrum was measured using

methanol (liquid chromatography grade) for solutions analysis and a baseline

correction was applied to the spectra. For the analysis of PLLA-OPD blends, films

were mounted into film holders. The baseline spectrum was measured out of air, and

a baseline correction was applied to the spectra. For all analysis, the range was 700 -

200 nm and a scan rate of 600 nm·min-1 was applied.


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Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 93

Chapter 3: Reactive Extrusion of Poly(L-

lactide) With 2-Oxepane-1,5-

Dione

3.1 BACKGROUND

In the preceding chapter (Chapter 2), the photodegradation of poly(L-lactide) was

accelerated via blending with a ketone-containing molecule, 2-oxepane-1,5-dione.

Films of PLLA with 4 - 10 wt% OPD embrittled after eight to twelve days of artificial

ageing, as opposed to neat PLLA that underwent little decrease in molecular weight.

Once the photosensitizing potential of OPD was assessed as an additive, the next step

was to investigate its prodegradant effect when incorporated onto the PLLA backbone.

The strategy to reach such goal was to perform in-melt modification of PLLA via

reactive extrusion. Reactive extrusion is a continuous, solvent-free and cost-effective

technique used to blend, polymerize or graft polymers in the melt. Modified polymers

are obtained within short times and can easily be recovered. However, the high

processing temperature required can result in degradation via chain scissions or

crosslinking.1

Thermal degradation of PLLA mainly proceeds through transesterification reactions,

resulting in reduction of molecular weight and broadening of polydispersity.2, 3 While

transesterifications are usually considered a drawback, they have great potential for

modifying the polymer backbone. For instance, transesterifications were used to

synthesise copolymers of polylactide with small molecules such as ricinoleic acid, -

caprolactone (ε-CL) in solution and in the bulk 4, 5, poly(ethylene-co-vinylalcohol)

(EVOH), catalysed by tin (II) octanoate and 1,5,7-triazabicyclo[4.4.0]dec-5-ene

(TBD) 6, or poly(butylene adipate-co-terephtalate) (PBAT), catalysed by titanium (IV)

butoxide.7 From the similarity between ε-CL and OPD, similar experiments could

possibly be conducted and successfully incorporate OPD into PLLA. If it cannot be

done as a monomer, transesterification of polymerized OPD and PLLA could be tried,

as done in the PBAT / PLA work.7

94 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione

This chapter explored transesterification reactions between a commercial grade PLLA

and OPD in reactive extrusion experiments. Firstly, POPD was investigated to

potentially be incorporated into PLLA in the melt. Due to handling difficulties of

POPD, reactive extrusion experiments focused on OPD as a monomer. Reactive

extrusions were performed by considering the effect of the OPD initial feed, the

residence time and the transesterification catalyst. The incorporation of OPD was

determined by ATR-FTIR and proton NMR spectroscopies, while the extent of

thermo-oxidative degradation due to processing was monitored by GPC and DSC.


3.2.1 Melt-Modification of Poly(L-lactide) with 2-Oxepane-1,5-Dione

Prior to exploring the transesterification strategy, polymerisations of OPD were

conducted, with the aim to subsequently incorporate POPD into PLLA via reactive

extrusions.

3.2.1.1. Polymerisation of 2-Oxepane-1,5-Dione

2-Oxepane-1,5-dione was polymerised in toluene at 90 °C and in the bulk at 120 °C,

both catalysed by tin (II) octanoate. In solution, a yellow precipitate formed after 2

hours and was recovered by filtration. The precipitate was washed with methanol,

filtered, dried under vacuum and recovered as a white powder in 40.9 % yield. When

carried out in the bulk at 120 °C, an increase in viscosity of the mixture was observed

during the reaction. The reaction was thermally quenched after 5 h, followed by the

recovery of a yellow and viscous product in 26 % yield. The compounds were dried

under vacuum prior to characterization.

The products were insoluble in common organic solvents (including acetone,

methanol, chloroform and dimethyl sulfoxide), thus the recovery after the reaction was

difficult, lowering the yield. Moreover, the low solubility hindered the characterization

by solution NMR spectroscopy. The ATR-FTIR spectrum of a typical polymer sample

revealed both an intense absorption band at 1723 cm-1 assigned to the ester –O-C=O

bond stretch and a less intense vibrational band at 1179 cm-1 assigned to the ester bond

(Figure 3.1). The presence of the ketone moiety was evidenced by a strong band at

1700 cm-1. Thermal analysis of the product, measured by DSC, revealed a broad


endothermic peak from 25 to 125 °C, from which no melting temperature could be

deduced, possibly due to different crystalline forms (refer to appendices).

Polymerisation of OPD was previously carried out in solution. Latere et al.8

polymerised OPD in dry toluene with 1-phenyl-2-propanol and tin (II) octanoate as

initiator and catalyst, respectively. POPD was obtained as a semicrystalline polymer,

featuring a glass transition of 37 °C and a melting temperature of 147 ºC. Tian and

coworkers reported the 1H NMR spectrum of POPD in trifluoroacetic anhydride /

deuterated chloroform (1 / 5 vol / vol).9 However, the insolubility of POPD and P(ε-

CL – OPD) copolymers was reported in common organic solvents, especially for

polymers with OPD contents higher than 50 mol%.8, 10

Figure 3.1. ATR-FTIR average spectrum of POPD revealing the ester and ketone

bands at 1723 and 1700 cm-1, respectively (nine spectra were collected, baseline-

corrected and averaged).

Although reported procedure for the synthesis of POPD exists, the insolubility of this

compound hindered the handling and the purification steps. OPD as a monomer, on

the contrary, was soluble in commonly used organic solvents, enabling complete


characterization. With an ester functional group on its structure, transesterifications

could still be performed between the OPD monomer and PLLA. Therefore, further

modification experiments of PLLA were conducted using OPD as a monomer.

3.2.1.2. Incorporation of 2-Oxepane-1,5-Dione into Poly(L-Lactide) by

Reactive Extrusion

The incorporation of OPD into PLLA was attempted via reactive extrusion by

investigating the following factors: the OPD initial feed, the residence time and the

transesterification catalyst. For each factor, the extrudates were purified and

characterized by GPC and DSC to assess their molecular weights and their thermal

properties, respectively. The potential incorporation of OPD into PLLA was

investigated by ATR-FTIR and 1H NMR spectroscopies.

3.2.1.2.1. Influence of OPD Initial Feed

A series of reactive extrusions were carried out using a laboratory-scale Haake Minilab

extruder. A backflow channel enabled the recirculation of the extrudates, while

sampling and recovering of the material was performed through a die. In terms of

weight, the Minilab extruder allowed the extrusion of samples ranging from 3 to 6 g.

Samples of a constant weight of 4 g of PLLA were mixed with appropriate amounts of

OPD and tin (II) octanoate (Table 3.1).

Table 3.1. Formulations of the extrusions of PLLA and OPD 0 - 15 wt% using tin

(II) octanoate as the transesterification catalyst.

OPD feed

(wt%)

OPD

(wt%)

Sn(Oct)2

(wt%)

Recovered mass

(g)

PLLA - - 1.1716

0 - 1.85 1.1265

5 5.0 1.86 1.4958

10 10.2 1.86 1.8252

15 14.9 1.71 1.6450


In order to remove residual moisture and minimize degradation via hydrolysis, OPD

was dried and stored under vacuum while poly(L-lactide) was dried under vacuum at

80 °C for 4 h prior to extrusion. The temperature of extrusion was optimized to ensure

the complete melting of compounds without favouring thermal degradation. The

minimum temperature that satisfied those conditions was 190 °C. The rotation speed

was set to 20 rpm during the feeding / melting step and to 100 rpm during the test to

efficiently mix the compounds. A residence time of 10 minutes was selected for each

extrusion. Dried PLLA without added OPD was extruded prior to each reactive

extrusion to clean the extruder. After 10 minutes, extrudates were flushed out and

purified by dissolution - reprecipitation using chloroform and methanol as solvent and

non-solvent, respectively. Polymers were recovered as white materials and dried under

vacuum prior to characterization. Only a limited amount of extrudates could be flushed

out of the extruder, explaining the low recovered masses reported (Table 3.1).

3.2.1.2.1.1. Torque and Relative Viscosity

The evolution of torque and apparent viscosity were recorded over time. The torque

values remained equal to zero during 10 minutes for every extrusion experiment. The

evolution of torque is linked to the changes in the melt viscosity of the extrudates and

is an indication of the success of grafting reactions during reactive extrusion For

instance, the extrusion of physical blends of polylactide and poly(ɛ-caprolactone)

revealed a continuous decrease in torque, while their compatibilized blends via

transesterification using triphenyl phosphite resulted in an increase in torque.11 A

similar increase in torque was observed in reactive extrusion of polylactide with

glycidol, a chain extender.12 The results obtained in this section did not allow for

monitoring the efficiency of the transesterifications between PLLA and OPD, as values

remained equal to zero throughout the extrusions, which could be explained by the

detection limit of the extruder.

On the contrary, the apparent viscosity values evolved throughout the extrusions. For

instance, the apparent viscosity of extruded PLLA continuously decreased over the 10

minutes, as illustrated by the averaged values of six different extrusions in Figure 3.2.

The PLLA - tin (II) octanoate formulation exhibited lower values of apparent

viscosities than neat PLLA, while the addition of OPD 5 - 15 wt% resulted in values

equal to zero during the extrusions (Figure 3.3). These experiments were performed


solely once, but based on the error calculated from the extrusions of neat PLLA, the

values obtained here could be close to each other. Moreover, they were so low that

they were below the signal-to-noise limit.

Polylactide grades of 𝑀𝑤 ~ 100,000 g·mol-1 feature melt viscosities in the range 5 -

10 kPa·s at shear rates of 600 - 3,000 rpm.13 However, a few factors impact the melt

viscosity, such as the initial 𝑀𝑤 and the shear rate or the type of processing.14 The

decrease in apparent viscosity of PLLA was previously reported to be emphasized by

higher residence times and temperature.15, 16 However, even short residence time (5

minutes) at 180 ºC was enough to reduce the melt viscosity.16

Figure 3.2. Evolution of apparent viscosity during the extrusion of neat PLLA at 190

°C for 10 minutes (six extrusions were performed and values of apparent viscosity

were averaged).


Figure 3.3. Evolution of apparent viscosity during the extrusion of pure PLLA,

PLLA with tin (II) octanoate and PLLA - OPD with tin (II) octanoate formulations at

190 °C for 10 minutes (the percentages account for the OPD initial feed; 0 wt%

corresponds to the PLLA – tin (II) octanoate formulation without OPD; the

formulations with 5; 10 and 15 wt% of OPD initial feed gave values of apparent

viscosity equal to 0).

The continuous decrease observed during the extrusion of PLLA alone suggested that

chain scission occurred. Indeed, the apparent viscosity is linked to molecular weight,

and hence chain scission events, according to the following relationships:

𝜂 = 𝑘𝑀𝑤 3.4

𝑠 =𝑀𝑤0

𝑀𝑤

− 1

With 𝜂 the apparent viscosity, 𝑠 the number of chain scissions, 𝑀𝑤0 the initial weight

average molecular weight and 𝑀𝑤 the weight average molecular weight.17 During

processing, the polymer is subjected to shearing forces, high temperatures and residual

traces of oxygen or water. These conditions favour mechanical, thermal, or oxidative


degradation as well as hydrolysis of ester moieties. As a result, the relative viscosity

is lowered due to scission of polymer chains.18, 19 The risk of hydrolysis can be

minimized by drying the materials. For instance, NatureWorks recommends

processing PLLA with a moisture content less than 250 ppm to prevent viscosity

degradation.20 Drying of PLA at 70 °C for 4 hours under vacuum has been shown to

reduce the moisture content to less than 190 ppm, as determined by Karl Fisher

titration.21

3.2.1.2.1.2. Analysis of Extrudates by Spectroscopic Techniques

After reprecipitation, purified extrudates were analysed by spectroscopic techniques

to investigate the potential incorporation of OPD into the poly(L-lactide) backbone.

ATR-FTIR average spectra of extrudates after one purification step are shown in

Figure 3.4. All spectra were baseline corrected and normalized to the band at 1455

cm-1 (due to -CH3 bending) to suppress any effect from differences in contact with the

ATR crystal and depth of penetration of the IR beam. The spectra revealed the

characteristic bands of PLLA: the C=O stretching band at 1756 cm-1, the –C-O-

stretching bands at 1183 and 1088 cm-1, and the –CH3 bending at 1455 cm-1.14 The

carbonyl band (within the region 1800 - 1675 cm-1) displayed an extra shoulder around

1715 cm-1, that could potentially correspond to the ketone moity of OPD (characterized

by a shoulder in the range 1725 - 1690 cm-1, refer to Chapter 2).22 However, the

spectra of PLLA-tin (II) octanoate also featured that extra shoulder on the carbonyl

band. This shoulder could potentially be assigned to carboxylic acids, as thermo-

oxidative degradation products. To further investigate whether OPD was incorporated

into the backbone of the PLLA, or simply blended with the PLLA, more purification

steps were performed and the evolution of this shoulder was monitored by ATR-FTIR

spectroscopy. After three purification steps, the shoulder seemed to show a relative

increase (Figure 3.5). The broadening of the carbonyl band of PLLA during thermo-

oxidative degradation was previously reported at 150 ºC and attributed to new carbonyl

compounds.19


Figure 3.4. ATR-FTIR spectra of extrudates of PLLA - OPD - tin (II) octanoate at

190 °C for 10 minutes after one purification step (the weight percentages represent

the OPD initial feed; 9 spectra were measured per film, baseline corrected and

normalized to the -CH3 bending band at 1455 cm-1).


Figure 3.5. Evolution of the ketone stretching shoulder at 1717 cm-1 in the ATR-

FTIR spectra of extrudates of PLLA - OPD 15 wt% - tin (II) octanoate at 190 °C for

10 minutes with the number of purification steps (average of 9 spectra per film after

baseline correction and normalization to the -CH3 bending band at 1454 cm-1).

To further investigate the potential incorporation of OPD into the PLLA, the purified

products were examined using 1H NMR spectroscopy. As an example for all

compositions, the formulation of PLLA and 10 wt% OPD will be discussed. The

spectrum of the crude extrudate featured a quartet at 5.16 ppm and a doublet at 1.58

ppm assigned to the methine protons (-CH) and the methyl group protons (-CH3) of

poly(L-lactide).23 A multiplet at 2.75 ppm and a quartet at 4.37 ppm were observed as

well (Figure 3.6).


Figure 3.6. Evolution of the 1H NMR spectra of the extrudates resulting from the

extrusion of PLLA with OPD 15 wt% at 190 ºC for 10 minutes (top: crude extrudate;

middle: extrudate after two dissolution-reprecipitation steps; bottom: extrudate after

three dissolution-reprecipitation steps), measured in CDCl3.

Ring-opened OPD is characterized in 1H NMR spectroscopy by a multiplet at 2.6 ppm

from the protons adjacent to the carbonyl moiety (-COCH2), a doublet at 2.7 - 2.8 ppm

assigned to the protons adjacent to the ketone (-CH2COCH2) and a quartet at 4.3 - 4.4

ppm from protons next to the oxygen of the ester moiety (-O-CH2-).10 Therefore, the

peaks observed at 2.75 and 4.37 ppm could be assigned to ring-opened OPD. However,

the spectra of the products from different purification steps revealed the disappearance

of those peaks, to solely display the quartet at 5.16 ppm and the doublet at 1.58 ppm

of PLLA. The 1H NMR spectra of other formulations revealed the same results. The


OPD could either not have reacted via transesterification reactions with PLLA, or only

in very low concentration that could not be detected by NMR spectroscopy.

3.2.1.2.1.3. Molecular Weights of Extrudates

Three-times-purified extrudates were analysed by GPC in chloroform to investigate

the extent of degradation during the extrusions. The GPC traces of the extrudates are

shown in Figure 3.7. After extrusion, PLLA alone featured a broad unimodal

distribution. The extrudates of the other formulations displayed broad unimodal

distributions with a shift towards low molecular weight, suggesting a decrease in the

molecular weight.

Figure 3.7. GPC traces of purified extrudates of PLLA and OPD 0 - 15 wt% at 190

°C for 10 minutes measured in chloroform (the traces were baseline-corrected and

normalized).

This observation was confirmed by the measured values of 𝑀𝑛 (Table 3.2). The

extruded PLLA displayed a 𝑀𝑛 of 38,800 ± 1,000 g∙mol-1 and a polydispersity of 1.80


± 0.05. The extrusion of PLLA with tin (II) octanoate at 190 °C for 10 minutes resulted

in a decrease in 𝑀𝑛 and a simultaneous increase in polydispersity from 1.80 ± 0.05 to

2.57 ± 0.23. The decrease in 𝑀𝑛 was greater when OPD was added to the PLLA with

tin (II) octanoate. The number of chain scissions increased with the initial OPD feed,

which is in agreement with the constant decrease in the relative viscosity described

earlier (refer to section 3.2.1.2.1.1).

Table 3.2. Molecular weight of three-times-purified extrudates of PLLA with OPD

(processed via reactive extrusion for 10 minutes at 190 °C, measured by GPC in

chloroform (three measurements were performed and values were averaged; the

percentages account for the OPD initial feed; 0 wt% corresponds to the PLLA – tin

(II) octanoate formulation without OPD).

Compound 𝑀𝑛 (g·mol-1) 𝑀𝑤

(g·mol-1) Ð Chain

scissions

Virgin PLLA 107,300 ± 4,900 209,300 ± 800 1.96 ± 0.09 -

PLLA 38,800 ± 1,000 69,900 ± 600 1.80 ± 0.05 2

0 9,500 ± 900 24,100 ± 100 2.57 ± 0.23 10

5 5,500 ± 100 12,100 ± 100 2.20 ± 0.01 19

10 3,100 ± 100 6,400 ± 0 2.12 ± 0.01 34

15 5,500 ± 100 11,400 ± 100 2.07 ± 0.04 19

3.2.1.2.1.4. Thermal Properties of Extrudates

The thermal properties and degrees of crystallinity of the three-times-purified

extrudates were determined using DSC with a heat / cool / heat cycle. When cooled

down from the melt at 10 ºC·min-1, the extrudates only exhibited vitrification due to

Tg and no crystallization (refer to appendices for the thermograms). The absence of

crystallization is usually observed for commercial PLLAs, whose crystallization rates

are slower than the cooling rate used in this work.24 After a second heating step (where

the thermal history of the samples had been previously erased), all extrudates were

characterized by a glass transition (Tg), an exothermic peak assigned to cold


crystallization (Tcc) and an endothermic peak corresponding to melting temperature

(Tm) (Figure 3.8).

The glass transition values of extruded formulations containing PLLA, OPD and tin

(II) octanoate shifted towards lower values compared to extruded PLLA (Table 3.3).

The differences in glass transition values were expected based on the GPC

measurements since Tg is linked to 𝑀𝑛 according to the Flory-Fox relationship:


𝑀𝑛

Where k is the Flory-Fox constant, 𝑇𝑔,∞ the glass transition of polylactide having an


average molecular weight.25 The thermo-oxidative degradation caused a reduction in

molecular weight due to chain scissions, thus inducing a drop in glass transition

temperature. Moreover, much lower values of 𝑀𝑛 were observed for extrudates of

PLLA - OPD with tin (II) octanoate compared to neat PLLA, accounting for the

difference in Tg values.

Regarding the melting peak, extruded PLLA featured a small exothermic peak before

the melting transition. This small exothermic peak was reported to arise from the

transition of the disorded α’ and to the ordered α phase (refer to Chapter 1).26 Products

from extrusion of PLLA - OPD (0 - 15 wt%) with tin (II) octanoate shifted to lower

values compared to the starting material and displayed a double melting peak.

Regarding the shift to lower values, processing induced thermo-mechanical

degradation, yielding chain scissions. The enhanced mobility due to shorter

macromolecular chains accounted for the decrease in melting temperatures.27 The

double melting behaviour was previously reported for polylactide and was attributed

to several events. It could be caused by a melt-recrystallization process, involving the

melting of original crystals, recrystallization and subsequent melting of recrystallized

crystals.28, 29 Melting of crystals featuring different lamellar thicknesses or melting of

the crystalline phases α and α’ are also reported explanations.30


Figure 3.8. DSC thermograms from the second heating cycle of purified extrudates

of PLLA - OPD (the percentages account for the OPD initial feed; 0 wt%

corresponds to the PLLA – tin (II) octanoate formulation without OPD).

Eventually, the degree of crystallinity was calculated according to the following

equation:

𝜒𝑐(%) = ∆𝐻𝑚 + ∆𝐻𝑐𝑐

∆𝐻𝑚0 × 100

With ∆𝐻𝑚 being the melting enthalpy, ∆𝐻𝑐𝑐 the cold crystallization enthalpy and ∆𝐻𝑚0

the melting enthalpy of 100 % crystalline PLLA sample (93.7 J∙g-1).31, 32 The degrees

of crystallinity did not significantly differ between each extrudate. Although the

addition of OPD and tin (II) octanoate led to a higher number of chain scissions, as

revealed by GPC measurements, no increase in the 𝜒𝑐 was obtained. Crystallinity is

usually enhanced by degradation occurring during processing. Indeed, chain scissions

release new chain segments that crystallize due to an enhanced chain mobility above

the Tg. This phenomenon is known as chemi-crystallization, and was previously

observed for PLLA subjected to thermo-oxidative degradation.19, 24


Table 3.3. Thermal properties of purified extrudates obtained by DSC on a second

heating cycle (three measurements were performed and values were averaged).

OPD

feed

(wt%)

Tg

(°C)

Tcc

(°C)

∆𝐻𝑐𝑐

(J·g-1)

Tm1

(°C)

Tm2

(°C

)

∆𝐻𝑚

(J·g-1)

𝜒𝑐

(%)

PLLA 60.8 ±

0.2

98.4 ±

0.4

18.5 ±

0.5

- 171.4

± 0.3

48.7 ±

0.3

71.6 ±

0.2

0 52.5 ±

2.1

94.8 ±

1.6

27.5 ±

2.4

- 164.6

± 1.1

39.2 ±

3.2

71.2 ±

5.9

5 49.3 ±

2.6

93.7 ±

2.1

30.5 ±

2.3

146.3 ±

1.0

159.3

± 1.9

38.9 ±

5.3

74.1 ±

8.0

10 50.1 ±

1.6

94.5 ±

1.5

30.8 ±

0.5

147.3 ±

2.6

158.1

± 1.7

39.1 ±

2.1

74.5 ±

1.7

15 51.3 ±

2.4

95.7 ±

2.9

27.2 ±

1.1

- 161.4

± 1.9

34.6 ±

3.3

65.8 ±

2.4

The absence of incorporation of OPD into the PLLA backbone could be due to the

short residence time (10 minutes). Subsequent reactive extrusions were performed for

longer residence times.

3.2.1.2.2. Influence of Residence Time

The residence time was increased to investigate its influence on transesterification

during reactive extrusion of PLLA and OPD. A first extrusion was performed at 190

°C for 2 hours with PLLA, 15.1 wt% OPD and 1.98 wt% tin (II) octanoate. Samples

were collected every 30 minutes to assess the extent of degradation. Increasing the

residence time from 30 minutes to 1 hour at 190 °C resulted in polymer degradation,

as characterized by a change of colour from yellow to brown. The colour of the

subsequent samples evolved from brown to almost black, indicating further

degradation over time (Figure 3.8).


A second experiment was performed for 1 h 20 min in which samples were collected

every 20 minutes to limit the extent of degradation. Sampling lowered the mass of

extruded sample inside the extruder, thus impacting the apparent viscosity

measurements. Therefore, no apparent viscosity data was reported here. The four

extrudates were purified twice using the dissolution - reprecipitation technique.

Purified extrudates with 20 and 40 minutes residence time were recovered as white

powders. However, the ones after 60 and 80 minutes were orange powders, suggesting

thermal degradation. Extrudates were dried under vacuum prior to characterization.

Figure 3.9. Photographs of extrudates collected every 30 minutes of a reactive

extrusion experiment of PLLA - OPD (15.1 wt%) catalysed by tin (II) octanoate,

revealing the change of colour over time.

Thermal degradation was confirmed by ATR-FTIR spectroscopy, as revealed by the

broadening of the carbonyl band (1820 - 1660 cm-1) as well as the appearance of a

broad band from 3700 to 2700 cm-1 after 40 minutes of extrusion (Figure 3.10). These

changes suggested the formation of carboxylic acid groups. Carboxylic acid was

previously reported as a product of the pyrolysis of the ester moieties of poly(ε-

caprolactone).33 The formation of alkene end-groups was also observed during the

thermal degradation of hydrogen β-substituted esters, including poly(ε-

caprolactone).33, 34 Pyrolysis of random copolymers of -caprolactone and OPD

revealed that the ketone moiety of OPD at 150 °C accelerated the pyrolysis compared

to PCL, resulting in chain scission and release of alkene end-groups.34

30 min 1 h 1 h 30 2 h


1H NMR spectroscopy revealed the presence of the quartet at 5.16 ppm and a doublet

at 1.58 ppm assigned to the methine and methyl protons of PLLA, respectively.23 A

doublet at 1.50 ppm was present after 20 minutes, featuring an increased in integration

with residence time. All four samples displayed both a multiplet at 4.37 ppm, and

peaks in the region between 2.88 - 2.55 ppm (Figure 3.11). The integration of the

multiplet at 4.37 ppm increased with residence time. The doublet at 1.50 ppm

corresponded to oligomers of polylactide.35 Oligomers are among the thermo-

oxidative degradation products reported for polylactide exposed to temperatures above

200 ºC.36-38 The multiplet at 4.36 ppm suggested the presence of –CH(CH3)OH end-

groups, as reported in the literature.39 As revealed by the GPC measurements, the

increase in residence time resulted in chain scissions, accounting for the increase in

end-groups.

Figure 3.10. ATR-FTIR spectra of double-purified extrudates after various residence

times revealing the broadening of the carbonyl band (1820 - 1660 cm-1) and the


appearance of a broad band between 3700 to 2700 cm-1 (average of 9 spectra per film

after baseline correction and normalization to the -CH3 bending band at 1454 cm-1).

Therefore, the increased residence time led to chain scissions of the PLLA matrix,

resulting in oligomers as thermo-oxidative degradation products. OPD may have also

undergone degradation, as revealed by the formation of carboxylic acids. Thermo-

oxidative degradation of the materials could account for the absence of incorporation

of OPD into PLLA.

Figure 3.11. 1H NMR spectra of double-purified extrudates of PLLA - OPD - tin (II)

octanoate after various residence times at 190 ºC, measured in CDCl3.

Samples were analysed by GPC in chloroform to assess the evolution of molecular

weights and polydispersities. The evolution of the GPC traces of the purified

extrudates collected every 20 minutes, after baseline correction and normalization, are

shown in Figure 3.12. The extrudates exhibited broad unimodal distributions, with a

shift towards lower molecular weight with increased residence time. The shift was

greater with longer residence times, suggesting a greater decrease in molecular

weights. Values of 𝑀𝑛 confirmed that observation (Table 3.4). The 𝑀𝑛

decreased from

107,300 ± 4,900 to 2,900 ± 100 g·mol-1 between the starting material and after 20


minutes of extrusion, respectively. The 𝑀𝑛 continuously decreased with increased

residence time. As a result, the chain scissions increased from 36 to 97 events between

20 and 80 minutes of extrusion.

Figure 3.12. GPC traces of purified extrudates of PLLA - OPD 15 wt%, collected

every 20 minutes at 190 °C, measured in chloroform (the traces were baseline-

corrected and normalized).

Table 3.4. Molecular weight of purified extrudates of PLLA - OPD 15 wt% at 190

°C collected every 20 minutes, measured by GPC in chloroform (three measurements

were performed and values were averaged).

Residence time

(min)

𝑀𝑛 (g·mol-1) 𝑀𝑤

(g·mol-1) Ð Chain

scissions

Virgin PLLA 107,300 ± 4,900 209,300 ± 800 1.96 ± 0.09 -


20 2,900 ± 100 6,600 ± 100 2.29 ± 0.02 36

40 2,000 ± 100 4,100 ± 0 2.03 ± 0.10 53

60 1,200 ± 100 2,400 ± 0 2.11 ± 0.20 88

80 1,100 ± 0 2,100 ± 100 1.89 ± 0.01 97

The decrease in molecular weight was expected to affect the thermal properties of the

double-purified extrudates. These properties were assessed by DSC using a heating /

cooling / heating cycle. All extrudates only exhibited vitrification when cooled down

from the melt at 10 ºC·min-1, the absence of crystallization being explained in Section

3.2.1.2.1.4 (refer to appendices for thermograms). After a second heating run, all

extrudates featured a glass transition, a cold crystallization and a double melting peak

(Figure 3.13).

Figure 3.13. DSC thermograms from the second heating cycle of double-purified

extrudates after various residence times.


Both glass transition and melting temperatures shifted towards lower temperatures

with increased residence time (Table 3.5). These decreases indicated reductions in

molecular weights, resulting from chain scissions due to thermal degradation.

Table 3.5. Evolution of the thermal properties of purified extrudates collected every

20 minutes, as measured by DSC on a second heating run (three measurements were

performed and values were averaged).

Time

(min)

Tg

(°C)

Tcc

(°C)

∆𝐻𝑐𝑐

(J·g-1)

Tm1

(°C)

Tm2

(°C)

∆𝐻𝑚

(J·g-1)

𝜒𝑐

(%)

20 49.2 ±

1.7

89.2 ±

1.4

8.7 ± 0.5 147.9 ±

0.3

153.5 ±

1.0

49.6 ±

2.2

62.2 ±

1.9

40 50.3 ±

0.3

95.8 ±

0.5

28.9 ±

1.8

144.6 ±

0.2

148.9 ±

0.2

48.4 ±

4.0

82.4 ±

6.0

60 39.1 ±

2.1

101.2 ±

1.3

26.6 ±

2.0

122.8 ±

1.7

136.3 ±

1.1

26.5 ±

1.7

56.7 ±

3.9

80 39.3 ±

1.5

104.9 ±

0.6

18.4 ±

1.4

122.9 ±

0.3

135.1 ±

1.0

19.0 ±

0.8

39.9 ±

2.4

Increasing the residence time did not result in successful OPD incorporation. Instead,

thermo-oxidative degradation seemed to occur, as revealed by an increase in chain

scissions and the formation of oligomers and carboxylic acids. The next extrusions

employed another transesterification catalyst.

3.2.1.2.3. Influence of Transesterification Catalyst

Incorporation of OPD onto PLLA was attempted by varying the OPD initial feed and

the residence time, which did not result in successful modification of PLLA. Regarding

experimental factors, both temperature and transesterification catalyst remained

unchanged in the previous extrusions. However, the temperature was previously

optimized and set at 190 ºC to ensure the melting of PLLA and to limit the extent of

thermal degradation. The final attempts to perform transesterification reactions

between PLLA and OPD investigated the role of the catalyst. Several catalysts, such

as zinc or titanium-based catalysts, have been found to improve blend


compatibilization of polyesters via transesterification. Comparative research to

compatibilize polyethylene terephthalate with functionalized polyethylene using zinc

acetate and titanium (IV) tetrabutoxide improved the transesterification yield.40

Titanium (IV) tetrabutoxide was also successfully used to compatibilize blends of

PLLA and poly(butylene adipate-co-terephthalate) (PBAT) in the melt at 200 ºC.7

Therefore, this catalyst was selected and extrusions of PLLA and 10 wt% OPD were

performed at 100 rpm and 190 ºC for 10 minutes with 0.07 wt% of catalyst (Table

3.6).

The torque and the relative viscosity were recorded over time. A control experiment

was carried out using the same conditions without transesterification catalyst. The

extrudates were flushed out from the extruder, purified twice as previously explained

and were obtained as white polymers. They were dried under vacuum prior to

characterization.

Table 3.6. Formulations of extrudates featuring titanium (IV) tetrabutoxide as the

transesterification catalyst.

Catalyst PLLA

(g)

OPD

(mg)

Catalyst

(wt%)

Mass

recovered (g)

None 4.0130 415.0 - 0.7918

Ti(OBu)4 4.0084 390.99 0.21 1.2869

The torque values remained equal to zero throughout both extrusions, potentially due

to the detection limit as reported for the previous extrusions. The apparent viscosity

values remained higher in the presence of titanium (IV) tetrabutoxide than for the

control extrusion (Figure 3.14). In both cases, the values decreased over time, with a

faster decrease in the extrusion containing the catalyst. However, based on the error

calculated from the extrusion of PLLA alone, the values obtained here could more

relatively close to each other (refer to section 3.2.1.2.1.1). Higher apparent viscosities

were also observed for PLLA with 0.07 wt% Ti(OBu)4 compared to neat PLLA in the

case of PLLA-PBAT compatibilization study cited earlier.7 The decrease in relative

viscosity suggested the occurrence of chain scissions, as explained in section

3.2.1.2.1.3.


Figure 3.14. Evolution of the apparent viscosity vs residence time for extrudates of

PLLA - OPD 10 wt% without and with titanium (IV) tetrabutoxide as the

transesterification catalyst.

The potential incorporation of OPD into PLLA was investigated by ATR-FTIR and

1H NMR spectroscopies. The ATR-FTIR spectra of extrudates in both cases displayed

the characteristic bands of PLLA (Figure 3.15). No difference was observed between

the two extrudates. An additional shoulder on the carbonyl band could demonstrate

either the presence of incorporated OPD or new carbonyl-containing products resulting

from the thermo-oxidative degradation. The 1H NMR spectra of both extrudates solely

showed the characteristics peaks of PLLA, without any trace of ring-opened OPD

(Figure 3.16), suggesting that no incorporation of OPD onto PLLA occurred during

the extrusion experiments.


Figure 3.15. ATR-FTIR spectra of twice-purified extrudates without and with

titanium (IV) tetrabutoxide as transesterification catalyst (average of nine spectra

after baseline-correction and normalization with the –CH stretching band at 1454 cm-

1).


Figure 3.16. 1H NMR spectra of twice-purified extrudates: using titanium (IV)

tetrabutoxide (top) and without any transesterification catalyst (bottom), measured in

CDCl3.

The molecular weight of the extrudates was analysed by GPC using chloroform as the

eluent (Figure 3.17). No significant shift was observed towards the low molecular

weight when titanium (IV) tetrabutoxide was employed during the extrusion. Values

of 𝑀𝑛 and 𝑀𝑤

that did not significantly differ with or without transesterification

catalyst (Table 3.7). The distribution of PLLA – Ti(OBu)4 solely broadened,

suggesting an increase in polydispersity. Indeed, the polydispersity evolved from 1.84

± 0.01 to 1.94 ± 0.03 without and with catalyst, respectively (Table 3.7).


Figure 3.17. GPC traces of PLLA - OPD 10 wt% with or without transesterification

catalyst, measured in chloroform (the traces were baseline-corrected and

normalized).

Table 3.7. Molecular weight of twice-purified extrudates of PLLA - OPD 10 wt%

for 10 minutes at 190 °C with or without titanium (IV) tetrabutoxide measured by

GPC in chloroform (three measurements were performed and values were averaged).

Compound 𝑀𝑛 (g·mol-1) 𝑀𝑤

(g·mol-1) Ð

Virgin PLLA 107,300 ± 4,900 209,300 ± 800 1.96 ± 0.09

None 29,000 ± 200 53,700 ± 50 1.84 ± 0.01

Ti(OBu)4 27,600 ± 700 53,600 ± 500 1.94 ± 0.03


Thermal properties of extrudates were analysed by DSC in the second heating cycle,

after erasing the thermal history of the samples. Both formulations were semi-

crystalline, as demonstrated by the presence of a glass transition, an exothermic peak

assigned to cold crystallization, and a small exothermic peak prior to an endothermic

peak corresponding to the melting temperature (Figure 3.18). This small exothermic

peak was reported to arise from the transition of the disorded α’ and to the ordered α

phase. Indeed, at relevant crystallization temperatures, the α’ phase is preferentially

formed, as reviewed in Chapter 1.26

As a conclusion, similar results to those previously obtained were observed by 1H

NMR and FTIR spectroscopic analyses. Indeed, only the typical peaks in NMR and

absorption bands in FTIR spectroscopies of PLLA were observed for the crude and

purified extrudates. The nature of transesterification catalyst did not seem to influence

a successful incorporation of OPD into the PLLA backbone, under the set of conditions

used.


Figure 3.18. DSC thermograms of purified extrudates of PLLA - OPD 10 wt% with

and without titanium (IV) tetrabutoxide as the transesterification catalyst.

Table 3.8. Thermal properties of extrudates of PLLA - OPD 10 wt% with and

without titanium (IV) tetrabutoxide, as measured by DSC in the second heating run

(three measurements were performed and values were averaged).

Catalyst Tg

(°C)

Tcc

(°C)

∆𝐻𝑐𝑐

(J·g-1)

Tm

(°C)

∆𝐻𝑚

(J·g-1)

𝜒𝑐

(%)

None 59.8 ± 1.2 99.2 ± 1.2 15.2 ± 2.8 170.2 ± 0.2 51.1 ± 0.6 70.7 ± 2.8

Ti(OBu)4 60.4 ± 0.2 97.5 ± 0.2 13.1 ± 1.1 170.1 ± 0.2 51.6 ± 0.8 69.1 ± 1.7


3.2.2 Thermal Stability of 2-Oxepane-1,5-Dione

Attempts to incorporate OPD into PLLA was investigated through varying the

extrusion factors (OPD initial feed, residence time, transesterification catalyst).

However, none of the extrusions performed resulted in successful incorporation of

OPD. The thermal stability of OPD could possibly explain the lack of incorporation.

Therefore, the thermal degradation of OPD was assessed by Thermal Gravimetric

Analysis (TGA). One decomposition step occurred around 160 °C, with an inflection

point at 184.1 ºC, accounting for a mass loss of 96.3 % (Figure 3.19).

A nonisothermal DSC run of OPD up to 250 ºC confirmed the decomposition of OPD

at high temperature. Indeed, the DSC thermogram exhibited both an endothermic peak

at 113.6 ± 0.65 ºC and an exothermic peak starting from 175 ºC with a maximum at

196.9 ± 0.55 ºC (Figure 3.20). The transition at 113 ºC was identified as the melting

of OPD crystals, while the exothermic peak corresponded to decomposition.

Figure 3.19. TGA trace of 2-oxepane-1,5-dione measured from 0 to 1000 °C under

nitrogen showing a decomposition step around 160 ºC with an inflection point at

184.1 °C.


The combined TGA and DSC measurements confirmed the hypothesis of the OPD

degradation with increased temperatures. As the reactive extrusions were performed

at 190 °C, OPD probably underwent thermal degradation, from which carboxylic acids

and alkene end-groups were formed, as revealed by spectroscopic techniques (refer to

section Error! Reference source not found.). Regarding the thermal stability of p

olylactide, the temperature at which 5 wt% of the total mass of unprocessed PLA was

volatilized was determined to be 331 ºC.41, 42

The degradation of OPD over time would inhibit the occurrence of transesterification

reactions with PLLA, explaining the absence of ring-opened OPD on the PLLA

backbone. As the PLLA employed in this work did not allow lower extrusion

temperatures (refer to section 3.2.1.2.1), thermal degradation of OPD could not be

avoided.

Figure 3.20. DSC thermogram of OPD on a nonisothermal run at a heating rate of 10

ºC·min-1 under nitrogen showing both the melting (Tm) and decomposition

(Tdecomposition) phases.


3.3 SUMMARY

In this chapter, a reactive extrusion route was explored for incorporating ketone

moieties into poly(L-lactide) via transesterification reactions using tin (II) octanoate as

the catalyst by considering the effects of the OPD initial feed and the residence time.

ATR-FTIR and 1H NMR spectroscopies of purified extrudates revealed the absence of

ring-opened OPD, while demonstrating carboxylic acids and alkene-end groups as

proof of thermo-oxidative degradation. Changing the transesterification catalyst did

not enable to successfully incorporate OPD onto PLLA with the set of conditions used.

Eventually, TGA and DSC measurements of OPD demonstrated its thermal instability

at the high temperatures required for PLLA processing. In order to limit the extent of

thermal degradation, the next chapter focused on copolymerisation experiments of L-

lactide and OPD, using milder conditions.

3.4 EXPERIMENTAL

3.4.1. Materials

Poly(L-lactide) 4003D grade was purchased from NatureWorks LLC and stored under

nitrogen. OPD was synthesised as previously reported (refer to Chapter 2) and dried

under vacuum prior to use. Methanol and chloroform, AR grades, were purchased from

ChemSupply and used as received. Chloroform (HPLC grade) used for GPC analysis

was purchased from Merck and filtered prior to use. Deuterium chloroform used for

NMR spectroscopy analysis was also purchased from Merck and stored at 10 °C.

3.4.2. Methods

3.4.2.1. Haake Minilab Extruder

Laboratory-scale extrusions were performed on a Thermo-Haake Minilab Rheomex

CTW5 laboratory-scale extruder (Thermo-Electron). The extruder was equipped with

conical twin screw of diameters of 5 - 14 mm and a length of 109.5 mm and co-rotation

was applied in every experiment. The extrudates recirculated thanks to a backflow

channel. The apparent viscosity was determined from the pressure difference between

two pressure sensors located in the backflow channel. A die enabled the sampling and

flushing out of the material.

3.4.2.2. Reactive Extrusion of Poly(L-Lactide) and OPD


Poly(L-lactide) 4003D grade was dried under vacuum for 4 hours while OPD was

vacuum-dried for 24 hours prior to extrusion. In a typical reactive extrusion

experiment, poly(L-lactide) 4003D grade, OPD and the transesterification catalyst

were mixed together. The Haake Minilab extruder was pre-heated at 190 °C and the

torque and screw speed were calibrated. The reactants were introduced into the Haake

extruder via the feeding arm, and melted at 190 °C with a screw speed at 20 rpm. Once

the reactants were completed melted, the speed screws automatically increased to 100

rpm, marking the start of the extrusion. After a determined residence time, the

extrudate was flushed out of the Haake, and purified by reprecipitation using

chloroform and cold methanol (0 - 1 °C; vol:vol 1:10) as the solvent and non-solvent,

respectively.

Table 3.9. Formulations of the different extrudates.

PLLA

(g)

OPD

(g)

Sn(Oct)2

(mg)

Ti(OBu)4

(mg)

OPD initial feed (wt%)

PLLA alone 4.0125 - - -

0 4.0054 - 74.1 -

5 4.0032 200.8 74.6 -

10 4.0057 407.2 74.5 -

15 4.0086 597.1 68.6 -

Residence time

2 h 4.0047 600.7 68.1 -

1 h 20 4.0064 605.2 71.6 -

Transesterification catalyst

None 4.0130 415.0 - -

Ti(OBu)4 4.0084 391.0 - 8.49

OPD initial feed


PLLA alone, after three reprecipitation steps:

White polymer. 1H NMR (CDCl3, δ ppm): 5.16 (q, H, CHL-LA), 1.58 (d, 3H, CH3L-LA).

ATR-FTIR: ʋ max = 2995 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,

symmetric), 1753 (s, -C=O carbonyl stretching), 1454 (-CH3 bending), 1383 and 1359

(-CH- deformation, symmetric and asymmetric), 1182, 1130 and 1085 (-C-O-

stretching), 1044 (-OH bending), 871 (w, -C-C- stretching). GPC (Refractive Index

detector): 𝑀𝑛 (Ð) = 38,800 ± 1,000 g·mol-1 (1.80 ± 0.05). DSC (second heating cycle):

Tg = 60.8 ± 0.2 ºC, Tcc = 98.4 ± 0.4 ºC (∆𝐻𝑐𝑐 = 18.5 ± 0.5 J·g-1), Tm = 171.4 ± 0.3 ºC

(∆𝐻𝑚 = 48.7 ± 0.3 J·g-1).

PLLA - OPD 0 wt% - tin (II) octanoate, after three reprecipitation steps:

White polymer. 1H NMR (CDCl3, δ ppm, subscripts L-LA and OPD denote each

repeating units): 5.17 (q, H, CHL-LA), 4.44 (t, 2H, -CH2OOPD), 4.35 (m, H, -

CH(CH3)OHL-LA), 2.74 (m, 2H, CH2C=OOPD), 2.66 (m, 2H, C=OCH2OPD), 1.59 (d, 3H,

CH3L-LA). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2

stretching, symmetric), 1755 (s, -C=O carbonyl stretching) with a shoulder from 1730

to 1695 cm-1, 1455 (-CH3 bending), 1383 and 1359 (-CH- deformation, symmetric and

asymmetric), 1181, 1130 and 1085 (-C-O- stretching), 1044 (-OH bending), 871 (w, -

C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) = 9,500 ± 900 g·mol-1 (2.57

± 0.23). DSC (second heating cycle): Tg = 52.5 ± 2.1 ºC, Tcc = 94.8 ± 1.6 ºC (∆𝐻𝑐𝑐 =

27.5 ± 2.4 J·g-1), Tm = 164.6 ± 1.1 ºC (∆𝐻𝑚 = 39.2 ± 3.2 J·g-1).











30.5 ± 2.3 J·g-1), Tm1 = 146.3 ± 1.0 ºC, Tm2 = 159.5 ± 1.9 ºC (∆𝐻𝑚 = 38.9 ± 5.3 J·g-1).












30.8 ± 0.5 J·g-1), Tm1 = 147.3 ± 2.6 ºC, Tm2 = 158.1 ± 1.7 ºC (∆𝐻𝑚 = 39.1 ± 2.1 J·g-1).











27.2 ± 1.1 J·g-1), Tm = 161.4 ± 1.9 ºC (∆𝐻𝑚 = 34.6 ± 3.3 J·g-1).

Residence time

20 minutes, after two reprecipitation steps:

White powder. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CHL-LA), 4.36 (m, H, -

CH(CH3)OHL-LA), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 2997 (w, -CH2

stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1752 (s, -C=O

carbonyl stretching) with a shoulder from 1730 to 1695 cm-1, 1454 (-CH3 bending),

1384 and 1359 (-CH- deformation, symmetric and asymmetric), 1181, 1129 and 1085

(-C-O- stretching), 1044 (-OH bending), 871 (w, -C-C- stretching). GPC (Refractive

Index detector): 𝑀𝑛 (Ð) = 2,900 ± 0 g·mol-1 (2.29 ± 0.02). DSC (second heating cycle):

Tg = 49.2 ± 1.7 ºC, Tcc = 89.2 ± 1.4 ºC (∆𝐻𝑐𝑐 = 8.7 ± 0.5 J·g-1), Tm1 = 147.9 ± 0.3 ºC,

Tm2 = 153.5 ± 1.0 ºC (∆𝐻𝑚 = 49.6 ± 2.2 J·g-1).



White powder. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CHL-LA), 4.36 (m, H, -

CH(CH3)OHL-LA), 1.57 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 3500 (b, w, -COOH

stretching), 2997 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,

symmetric), 1755 (s, -C=O carbonyl stretching) with a shoulder from 1730 to 1695

cm-1, 1455 (-CH3 bending), 1384 and 1359 (-CH- deformation, symmetric and


C-C-stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) = 2,000 ± 100 g·mol-1 (2.03


28.9 ± 1.8 J·g-1), Tm1 = 144.6 ± 0.2 ºC, Tm2 = 148.9 ± 0.2 ºC (∆𝐻𝑚 = 48.4 ± 4.0 J·g-1).


Yellow powder. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CHL-LA), 4.37 (m, H, -


stretching ), 2997 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,






26.6 ± 2.0 J·g-1), Tm1 = 122.8 ± 1.7 ºC, Tm2 = 136.3 ± 1.1 ºC (∆𝐻𝑚 = 26.5 ± 1.7 J·g-1).


Orange powder. 1H NMR (CDCl3, δ ppm): 5.16 (q, H, CHL-LA), 4.37 (m, H, -


stretching), 2997 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,






18.4 ± 1.4 J·g-1), Tm1 = 122.9 ± 0.3 ºC, Tm2 = 135.1 ± 1.0 ºC (∆𝐻𝑚 = 19.0 ± 0.8 J·g-1).

Transesterification catalyst


PLLA - OPD 10 wt% - no transesterification catalyst, after two reprecipitation steps:

Orange powder. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CHL-LA), 1.58 (d, 3H, CH3L-LA).




asymmetric), 1181 and 1085 (-C-O- stretching). GPC (Refractive Index detector): 𝑀𝑛

(Ð) = 29,100 ± 200 g·mol-1 (1.84 ± 0.01). DSC (second heating cycle): Tg = 58.8 ± 1.2

ºC, Tcc = 99.2 ± 1.2 ºC (∆𝐻𝑐𝑐 = 15.2 ± 2.8 J·g-1), Tm1 = 158.6 ± 0.9 ºC, Tm2 = 170.2 ±

0.2 ºC (∆𝐻𝑚 = 51.1 ± 0.6 J·g-1).

PLLA - OPD 10 wt% - titanium (IV) tetrabutoxide, after two reprecipitation steps:

Orange powder. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CHL-LA), 1.59 (d, 3H, CH3L-LA).




asymmetric), 1181 and 1084 (-C-O- stretching). GPC (Refractive Index detector): 𝑀𝑛



0.2 ºC (∆𝐻𝑚 = 51.6 ± 0.8 J·g-1).

3.4.2.3. Gel Permeation Chromatography

Please refer to section 2.4.3.2 (Chapter 2).

3.4.2.4. Differential Scanning Calorimetry


3.4.2.5. Fourier Transform Infrared Spectroscopy


3.4.2.6. Proton Nuclear Magnetic Resonance Spectroscopy

Proton Nuclear Magnetic Resonance spectra were recorded on a 600 MHz Bruker

spectrometer with 32 scans. 1 mg.mL-1 solutions in deuterated chloroform were used

for NMR analyses. The spectra were calibrated with the CDCl3 peak at 7.26 ppm.

3.4.2.7. Thermogravimetric Analysis


Thermogravimetric analysis was conducted on a TA instruments Q500

thermogravimetric analyser. Approximately 30 mg of sample were heated in a

platinum crucible in the temperature range 25 - 1000 °C at 5 °C·min-1, under nitrogen.


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Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 133

Chapter 4: Functionalization of Poly(L-

lactide) with 2-Oxepane-1,5-Dione

4.1 BACKGROUND

In the preceding chapter (Chapter 3), in-melt modification of poly(L-lactide) was

undertaken to incorporate 2-oxepane-1,5-dione in order to increase the functionality.

However, such attempts remained unsuccessful due to the thermal instability of 2-

oxepane-1,5-dione. Another modification strategy under milder conditions relies on

the polymerisation of L-lactide with functionalized monomers. Such reactions enable

the design of copolymers with predictable molecular weights and controlled

architectures with well-defined end-groups. 2-Oxepane-1,5-dione features the same

structure as ε-caprolactone with an additional ketone. Copolymerisations of ε-

caprolactone and OPD were carried out in solution and in the bulk. The

copolymerisations were catalysed by various metal derivatives including aluminium

isopropoxide, dimethyl tin dimethoxide and tin (II) octanoate.1 Random copolymers

were obtained with up to 30 mol% incorporation of OPD.2 Regarding

copolymerisation with L-lactide, Prime et al.3 first synthesized a poly(L-lactide-co-

OPD) in the bulk, using tin (II) octanoate as the catalyst and butanol as the initiator.

They carried out a polymerisation with only one OPD initial feed ratio, 24.6 mol%,

which resulted in an incorporation level of 4 mol%. Dai and coworkers also employed

OPD to afford comb-type copolymers of poly(4-hydroxyl-ε-caprolactone-co-ε-

caprolactone)-g-poly(L-lactide). They first copolymerised poly(ε-caprolactone-co-

OPD) (OPD initial feed 25 mol%) in toluene at 90 ºC with tin (II) octanoate as catalyst.

Subsequent reduction of the ketone moieties to the corresponding pendent hydroxyl

groups enabled the initiation of the graft polymerisation of L-lactide, in the bulk with

tin (II) octanoate catalyst.4

In terms of catalyst, ring-opening polymerization of lactide and lactone using metal

alkoxides as catalyst has been extensively investigated.5-7 The polymerisation is

initiated by protic reagents such as alcohols or residual impurities (lactic acid, water)

that react with tin (II) octanoate to yield tin (II) alkoxide (Scheme 4.1).8

134 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione

Scheme 4.1. Conversion of tin (II) octanoate into tin (II) alkoxide via reaction with

alcohol or residual protic impurities.

The reaction proceeds via a coordination-insertion mechanism. The monomer is

coordinated to the Lewis-acidic metal centre, and subsequently inserted into one of the

tin (II) alkoxide bonds via nucleophilic addition of the alkoxy group on the carbonyl

carbon. The acyl-oxygen cleavage results in the opening of the ring and the insertion

of the monomer. Eventually the polymerisation is terminated by hydrolysis of the

active propagation chain. 9, 10 This catalyst affords high molecular weight polylactide

with values up to 105 - 106 Da.5

With the aim of designing polymers with predictable molecular weights, low

polydispersities and end group fidelity, extensive research has been focused on the use

of guanidines and amidines to catalyse the ROP of polyesters. Lohmeijer et al.6

demonstrated the efficiency of organocatalytic polymerisation of L-lactide with 1,8-

diaza[5.4.0]bicycloundec-7-ene (DBU) in solution at room temperature. They

suggested a mechanism based on the activation of the alcohol by DBU through

hydrogen bonding. The polymerisation proceeds through nucleophilic attack of lactide

by the activated catalyst, resulting into polylactides with predictable molecular weights

and high end group fidelity within short reaction times (Scheme 4.2). Few

transesterifications were observed as supported by their narrow polydispersities (Ð ≤

1.1). As a pseudo-living polymerisation, none or few terminating chains occur and

quenching is necessary to terminate the polymerisation. Adding an acid in the

polymeric medium results in deactivating DBU, thus quenching the reaction.7

Sn(Oct)2 + ROH Oct-Sn-OR + OctH

Oct-Sn-OR + ROH Sn(OR)2 OctH+


Scheme 4.2. Ring-opening polymerisation of lactide using DBU and an alcohol as

catalyst and initiator, respectively.

This chapter extends the copolymerisation work of L-lactide and OPD in the bulk,

using tin (II) octanoate as the catalyst. Several OPD initial feeds were selected with

the aim to obtain higher level of incorporation than previously achieved by other

studies reported in the literature. Due to limited OPD incorporation, the temperature

was also investigated as a factor to increase the OPD incorporation resulting into the

formation of red precipitates. GPC, DSC and spectroscopic techniques were used to

characterize the compounds and explain the low incorporation level of OPD achieved.

In order to increase the concentration of OPD into the copolymers, another set of

conditions was employed – that uses DBU as catalyst and benzyl alcohol as the

initiator. Polymerisations were performed in dichloromethane at room temperature.



4.2.1. Transition Metal-Catalysed Copolymerisation of L-lactide and 2-

Oxepane-1,5-Dione in the Bulk

The copolymerisation strategy was applied to modify poly(L-lactide). Ring-opening

polymerisations of L-lactide and OPD were performed in the bulk to mimic the reactive

extrusion conditions used in Chapter 3, while avoiding thermal degradation.

4.2.1.1. Ring-Opening Polymerisation of L-Lactide and OPD 5 - 20 wt%

A series of poly(L-lactide-co-OPD) polymers were synthesized in the bulk by the

transition-metal catalysed ring-opening polymerisation with various feed monomer

ratios using tin (II) octanoate catalyst (Scheme 4.3). Prior to the reaction, both

monomers were purified by recrystallization and dried under vacuum for several days

to limit any initiation processes from residual impurities. Polymerisations were carried

out under inert atmosphere at 110 °C to ensure complete melting of both monomers

(melting temperatures of L-lactide and OPD: 95 and 110 C respectively) and lasted

until high monomer conversion (supported by an increase in viscosity of the polymeric

mixture which stops the stirring process). The copolymers were purified by

reprecipitation using chloroform as solvent and methanol as non-solvent. Depending

on the polymerisation, several precipitations were required to remove unreacted

monomers, as monitored by 1H NMR spectroscopy. The conditions, time, yields, and

molecular weights for each copolymerisation are summarized in Table 4.1.


Scheme 4.3. Tin (II) octanoate-catalysed ROP of L-lactide and OPD at 110 °C in the

bulk to afford poly(L-lactide-co-OPD).

Table 4.1. Conditions and results of the ring-opening polymerisations of L-lactide

and OPD 0 – 20 mol% at 110 ºC catalysed by tin (II) octanoate in the bulk.

a Calculated from integration ratios in 1H NMR spectra of the purified polymers; b

Purification yield.

Regarding the ROP of L-lactide alone, the stirring stopped after 4 hours due to high

viscosity. The addition of OPD slowed down the kinetics of L-lactide, as revealed by

the longer reaction times needed to increase the viscosity of the polymeric mixture.

After four purification steps, the spectra of poly(L-lactide-co-OPD)s revealed a quartet

at 5.17 ppm and a doublet at 1.57 ppm which are respectively assigned to the methine

and methyl protons of poly(L-lactide).10 Two multiplets at 4.40 ppm and in the range

2.80 - 2.60 ppm correspond to protons next to the ester oxygen and the protons adjacent

to the carbonyl and ketone moieties of ring-opened OPD respectively (Figure 4.1).1,

Entry Initial feed molar

ratio (L-LA / OPD)

Time (h) Copolymer molar

ratio (PLLA / OPD) a

Yield (%) b

1 1 / 0 4 1 / 0 29.2

2 1 / 0.058 46.5 1 / 0.048 24.6

3 1 / 0.11 46 1 / 0.052 35.4

4 1 / 0.23 72 1 / 0.070 61.0


11 The multiplet at 4.36 ppm represents a characteristic –CH(CH3)OH end-group.12

The molar ratio between the converted monomers was calculated based on the

integration ratios of the peaks at 5.17 and in the range 2.80 - 2.60 ppm. An

incorporation of 4.8 to 7 mol% of OPD was found for the copolymers.

Figure 4.1. 1H NMR spectra of various poly(L-lactide-co-OPD) with initial OPD

concentration of 5 (bottom); 10 (middle) and 20 mol% (top), measured in CDCl3.

The chemical shifts of OPD as a monomer or when copolymerized do not differ in the

1H NMR spectra. The only noticeable difference is the two triplets, at 4.40 and 2.80

ppm in the monomer, changing to multiplets in the ring-opened OPD.11, 13 The

presence of OPD segments in the copolymer was confirmed by Pulsed Field Gradient

Spin-Echo (PGSE) NMR spectroscopy. This technique relies on the fact that molecules

move via rotational and translational motions in solution. Translational motions are

characterized by self-diffusion coefficients which are related to molecular dimensions,


specifically the hydrodynamic radius (RH). RH corresponds to the radius of the

hypothetical size of the spherical particle of a molecule in solution. Molecules can be

separated based on their diffusion coefficients by applying two different magnetic field

gradient pulses.14 This technique enables to determine polymer compositions among

others.15, 16 Poly(L-lactide-co-OPD) was dissolved in CDCl3 and subjected to two

different magnetic field gradient pulses: 3 and 95 %. At 3 % gradient, molecules of all

diffusion coefficients appear on the spectrum from short to long hydrodynamic sizes.

Both small molecules, such as unreacted monomers, and slowly diffusing polymers

will appear on the spectrum. On the contrary, only peaks for slowly diffusing polymers

can be observed when the 95 % magnetic field gradient is applied. From this spectrum,

the chemical composition of the macromolecule can be determined, with the assurance

that every peak is originated from protons in the polymer.

The spectrum at 3 % gradient shows a quartet at 5.17 ppm and a doublet at 1.58 ppm

which are assigned to the methine and methyl protons of poly(L-lactide) respectively

(Figure 4.2). The multiplets at 4.36 ppm and in the 2.80 - 2.65 ppm range are assigned

to OPD, either in its ring-opened or cyclic form, accounting for 1.2 mol% based on

integration ratios.1 The spectrum at 95 % displays the peaks assigned to PLLA as well

as smaller peaks corresponding to ring-opened OPD. The OPD segments account for

1 mol% based on integration ratios, thus suggesting a small amount of unreacted OPD

was still present.


Figure 4.2. (a) 1H NMR spectrum of poly(L-lactide-co-OPD); (b) PGSE NMR

spectra of poly(L-lactide-co-OPD) using 3 % magnetic field gradient pulse; (c) PGSE

NMR spectra of poly(L-lactide-co-OPD) using 95 % magnetic field gradient pulse,

measured in CDCl3.

The chemical structures of the purified copolymers were analysed by ATR-FTIR

spectroscopy. Nine spectra were collected on three different samples of each polymer

- each baseline corrected, averaged and normalized to the band at 1453 cm-1 (assigned

to -CH3 bending) to suppress any effect from contact differences with the ATR crystal

and depth of penetration of the IR beam (Figure 4.3). The spectra all revealed similar

characteristic bands: the C=O stretching band of the ester groups at 1750 cm-1, the -C-

O- stretching band of the ester deformation bands at 1181 and 1083 cm-1, and the -CH3

bending band at 1453 cm-1. All the bands obtained in the spectra matched those of neat

PLLA. 17, 18 The addition of OPD neither led to any shift of wavelength of the different


bands nor to splitting of the ester band into two distinct bands. However, an additional

shoulder appeared on the band from 1725 to 1700 cm-1 which are assigned to a ketone

functionality (Figure 4.4).19 This ketone could either be assigned to the OPD monomer

or the ring-opened comonomer in the polymer as no wavenumber shift was noticeable

between them. Besides the copolymer with 7 mol% OPD, the single ester band

observed for the copolymers indicates the randomness of the copolymers. The two

maxima observed around 1750 cm-1 with 7 mol% OPD is likely due to noise resulting

from the resolution (4 cm-1) used to run the spectra and the normalization process. Qian

et al.20 reported the ATR-FTIR spectra of random copolymers of L-lactide and ε-

caprolactone which exhibits a single absorption band at 1756 cm-1 whereas two distinct

bands at 1761 and 1732 cm-1 were observed for the block structures.

Figure 4.3. ATR-FTIR spectra of the different poly(L-lactide-co-OPD)s revealing

the characteristic bands of poly(L-lactide) (the mol% represents the concentration of

incorporated OPD within the copolymer).


Figure 4.4. Enlarged view of the carbonyl region of the ATR-FTIR spectra of the

different poly(L-lactide-co-OPD)s revealing the OPD shoulder at 1725 - 1700 cm-1

(the two maxima observed around 1750 cm-1 for 7 mol% OPD was due to noise

resulting from the resolution (4 cm-1) used to run the spectra and the normalization

process to the band at 1453 cm-1 assigned to -CH3 bending).

Latere and coworkers synthesized random copolymers of ε-caprolactone and OPD in

dry toluene at 90 ºC using tin (II) octanoate catalyst. The concentration of OPD

segments in the copolymers closely matched the initial feed. This was confirmed by

1H NMR spectroscopy analysis. Copolymers with OPD concentrations up to 77 mol%

were obtained.1 However, Prime et al.11 first reported the copolymerisation of L-lactide

and OPD in the bulk at 110 °C initiated by butanol in the presence of tin (II) octanoate

catalyst. The resulting copolymer contained 4 mol% of incorporated OPD for an initial

feed of 24.6 mol%. They justified this low incorporation level to the interference of

the ketone moiety with tin (II) octanoate. No other initial OPD feed was tested to reach

higher incorporation levels. A difference in the reactivity ratios could also explain the


low incorporation level of OPD when copolymerized with L-lactide compared to ε-

caprolactone. Extensive work was performed on synthesize random or block

copolymers of L-lactide and ε-caprolactone using various catalysts such as aluminium

isopropoxide and tin (II) octanoate.20-22 These studies demonstrated the preferential

conversion of L-lactide compared to ε-caprolactone leading to a lower incorporation

level of the latter than expected. The higher polymerisation enthalpy of L-lactide was

suggested to arise from the ring strain of the bond oppositions of the six-membered

ring.23, 24 By analogy with PLLA - PCL copolymers, OPD could be characterized by a

lower polymerisation enthalpy compared to L-lactide when catalysed by tin (II)

octanoate, resulting in slower conversions.

The molecular weights and polydispersities of the polymers were assessed by GPC in

chloroform. Each polymer exhibited sharp monodal distributions. A shift towards the

lower molecular weights was observed with increase in the initial OPD feed as well as

broadening of the distributions compared to neat PLLA 1 (Figure 4.5). Calculated

values of 𝑀𝑛 and polydispersities confirmed the decrease in molecular weight with

increase in the initial OPD feed as well as an increase of the polydispersity (Table

4.2). Both long reaction times and the presence of tin (II) octanoate favour thermal

degradation. This predominantly proceeds through backbiting transesterifications and

results into chain scission and increased polydispersities.25, 26 As the addition of OPD

reduced the conversion of L-lactide resulting into longer reaction times, such thermal

degradation could occur which thus explains the GPC result obtained herein. Apart

from neat PLLA 1, only copolymer 2 featured a molecular weight above the

entanglement molecular weight (𝑀𝑒 = 8 - 10 kDa).27 Attempts to prepare a film with

the solvent-casting technique resulted into recrystallization of the copolymer instead

of producing a film. The GPC calibration, performed against narrow polystyrene

standards, is known to overestimate molecular weight values for PLLA and PCL

(correction factor in DCM ~ 0.68).28, 29 Thus copolymer 2 probably displayed a lower

molecular weight than 𝑀𝑒 , thus hindering film formation.


Figure 4.5. GPC traces of synthesized poly(L-lactide) and poly(L-lactide-co-OPD)s

measured in chloroform (the traces were baseline-corrected and normalized).

Table 4.2. Molecular weight and polydispersities of PLLA and poly(L-lactide-co-

OPD)s measured in chloroform (two measurements were performed and values were

averaged).

Entry 𝑀𝑛 (g·mol-1) 𝑀𝑤

(g·mol-1) Ð

1 43,000 ± 900 53,700 ± 1,500 1.25 ± 0.06

2 12,700 ± 100 16,200 ± 100 1.28 ± 0.02

3 6,900 ± 600 11,400 ± 2,200 1.64 ± 0.18

4 3,900 ± 100 5,900 ± 100 1.51 ± 0.02


The OPD incorporation into the PLLA backbone was expected to modify the thermal

properties of the resulting copolymers. Such properties were determined by DSC using

a heat / cool / heat cycle. The DSC thermograms of polymers measured on a second

heating run are shown in Figure 4.6.

Figure 4.6. DSC thermograms of purified poly(L-lactide) 1 and poly(L-lactide-co-

OPD) 2, 3, 4 under nitrogen on a second heating run.

Poly(L-lactide) 1 displayed a glass transition and an endothermic peak corresponding

to the melting temperature. Poly(L-lactide-co-OPD) 2 and 3 exhibited a glass transition

and a double endothermic peak. Poly(L-lactide-co-OPD) 4 featured a glass transition,

an exothermic peak due to cold crystallization and a broad endothermic peak. The

single glass transition confirmed the random structure of the copolymers, even though

the glass transitions were weak and difficult to observe. For random copolymers, the

Tg can be predicted from the Fox relationship:


1

𝑇𝑔=

𝑤1

𝑇𝑔1+

𝑤2

𝑇𝑔2

With 𝑤1, 𝑤2 the weight fraction and 𝑇𝑔1, 𝑇𝑔2 the glass transitions of PLLA and POPD

respectively.30 As POPD was reported to exhibit a Tg of 37 ºC, lower values of Tg for

the copolymers compared to neat PLLA were expected.13 Noticeable changes observed

include the appearance of a double melting behaviour as well as the shift of the melting

temperature towards the lower temperatures. The double melting behaviour of PLA

was attributed to either a melt-recrystallization process, the melting of different

lamellar thicknesses or melting of the α and α’ crystals forms.31-33 However, the double

endotherm could also arise from the presence of different crystalline structures as per

the incorporation of ring-opened OPD reported to be semi-crystalline.13 The decrease

in melting temperature was in agreement with the Fox-Flory theory. The molar mass

decrease induced structural changes in the crystalline regions. The enhanced chain

mobility facilitated crystallization resulting in the cold crystallization transition

observed for copolymer 4.

Table 4.3. Thermal properties of purified poly(L-lactide-co-OPD) measured by DSC

on a second heating cycle (two measurements were performed and values were

averaged).

Entry Tg

(°C)

Tcc

(°C)

∆𝐻𝑐𝑐

(J·g-1)

Tm1

(°C)

Tm2

(°C)

∆𝐻𝑚

(J·g-1)

𝜒𝑐

(%)

1 56.2±2.9 - - - 174.3±0.1 51.0±0.7 54.4±0.7

2 42.0±2.5 - - - 162.8±0.3 55.2±0.7 58.9±0.7

3 51.9±0.5 - - 153.3±0.2 159.5±0.4 47.9±3.4 51.1±3.6

4 39.8±2.0 88.1±1.7 7.0±0.7 137.4±0.8 - 35.1±1.2 44.8±2.0

4.2.1.2. Attempted Synthesis of Poly(L-Lactide-co-OPD) with Increased

OPD Initial Feed

Other polymerisations were performed with higher initial OPD feeds with the aim to

increase its incorporation into the copolymers. Initial OPD feeds of 50 and 75 wt%

were selected. Polymerisations were carried out for 7 hours at 100 °C in the bulk,

catalysed by tin (II) octanoate. A single purification of the products was achieved by


reprecipitation using chloroform and cold hexane (0 - 1 ºC; 1:10 vol:vol). The

products, obtained as wax materials, were dried under vacuum prior to

characterization. Compounds 5 and 6 were obtained in 50 % and 48.4 % yields

respectively.

The chemical structure of the resulting polymers were analysed by ATR-FTIR

spectroscopy. Nine spectra were collected on three different samples of each polymer,

baseline corrected, averaged, and normalized to the band at 1456 cm-1. Both average

spectra displayed sharp and quite intense bands at 1241 and 934 cm-1 which were

absent on the spectrum of poly(L-lactide) (Figure 4.7 and Figure 4.8). These bands,

which are characteristics of monomeric L-lactide, confirm the conversion did not

proceed to completion in either case (Table 4.4).34

Figure 4.7. ATR-FTIR average spectrum of the product 5, resulting from the ROP of

L-lactide with an initial OPD feed of 50 mol% (average of 9 spectra per film after

baseline correction and normalization with the -CH3 bending band at 1456 cm-1).


Figure 4.8. ATR-FTIR average spectrum of the product 6, resulting from the ROP of

L-lactide with an initial OPD feed of 75 mol% (average of 9 spectra per film after

baseline correction and normalization with the -CH3 bending band at 1456 cm-1).

Table 4.4. Comparison of characteristic bands in the ATR-FTIR spectra of L-lactide

and poly(L-lactide) with their assignments based on reported literature.34, 35

Band position (cm-1) Assignment

L-lactide Poly( L-lactide)

3000 2996 -CH- stretch (asymmetric)

2931 2945 -CH- stretch (symmetric)

1752 1747 -C=O carbonyl stretch

1456 1455 -CH3 bend

1355; 1325 1384; 1359 -CH- deformation (symmetric and

asymmetric bend)

1241 - -CO-O-C deformation

1145; 1094 1182; 1085 -C-O- deformation

1054 1043 -OH deformation


The molecular weights of purified product were measured by GPC in chloroform. Both

traces featured a broad distribution assigned to an oligomer, and a second sharp

distribution assigned to unconverted L-lactide (Figure 4.9).

Figure 4.9. GPC trace of products 5 and 6, resulting from the ROP of L-lactide with

an OPD initial feed of 50 and 75 mol%, respectively, measured in chloroform (the

traces were baseline-corrected and normalized).

The thermal properties of the purified products were determined using DSC on a

second heating run to erase the thermal history of the samples (refer to appendices for

thermograms). Compound 5 featured a broad endothermic peak from which no real

conclusion could be made. Compound 6 featured a broad endothermic peak with a

maximum at 55.4 C corresponding to the melting temperature. Poly(L-lactide) is

932 - -CO-O- deformation

823 868 -C-C- deformation


typically characterized by a glass transition ranging from 50 to 70 C and a melting

temperature from 170 to 190 C.36 However, neither of the samples featured a glass

transition. This, along with the lower melting point observed, confirmed an incomplete

conversion of L-lactide to poly(L-lactide). However, the 55.4 C melting point

corresponds to the melting temperature of meso-lactide and suggests that racemization

occurred during the reaction. Such racemisation is favoured under longer reaction

conditions.37

4.2.1.3. Attempted Synthesis of Poly(L-Lactide-co-OPD) at Higher

Temperatures

Final efforts to increase the OPD segments within the copolymers involved

investigating the effect of temperature (increasing the temperature). Polymerisations

were carried out at 150 and 170 °C with a constant OPD concentration (20 mol%) in

the bulk and catalysed by tin (II) octanoate. Some red precipitates formed during the

polymerisation. The polymeric mixtures turned yellow, orange to brown when the

polymerisation was carried out at 170 ºC. Once thermally quenched, the reaction

mixtures were reprecipitated in cold hexane to give a slightly yellow powder (7) and

an orange wax (8) in 55.3 and 49.2 % conversions respectively. Hexane was selected

as a non-solvent for the reprecipitation because the colour suggested degradation and

thus, low molecular weight polymers – known to readily reprecipitate in hexane.

The 1H NMR spectra of the purified compounds are illustrated in Figure 4.10. Along

with the methine and methyl protons of PLLA at 5.16 and 1.58 ppm respectively,

various diagnostic peaks were observed in the range 2.25 - 0.80 ppm. The singlet at

1.25 and doublet at 1.49 ppm could be assigned to residual tin (II) octanoate. The broad

peak in the range 2.00 - 1.50 ppm could arise from the coordination of the

paramagnetic tin ion.


Figure 4.10. 1H NMR spectra of (a) purified compound 7; (b) purified compound 8,

measured in CDCl3.

Increasing the temperature induced the precipitation of red crystals insoluble in

common organic solvents, including dimethyl sulfoxide. The DSC thermogram of the

red precipitate revealed a broad endothermic peak, possibly due to different kind of

crystal structures (refer to appendices). The ATR-FTIR spectra showed a strong band

at 1700 cm-1 corresponding to ketone moiety, with the shoulder at 1727 cm-1 assigned

to the -C=O bond of an ester group.

The chemical composition of the product was further analysed by inductively coupled

plasma optical emission spectroscopy (ICP-OES). This technique converts a sample

in solution into an aerosol, which is subsequently vaporized within a plasma. The

atoms reach an excited state due to collisions, then relax to their ground state via the

emission of light. Elements can then be identified and quantified due to the

characteristic wavelengths and their intensities respectively.38 The red crystals were

dissolved in a mixture of hydrofluoric acid / hydrogen chloride and analysed by this

technique. The results indicate the compounds were predominantly composed of tin,

with a small percentage of iron for 11 (Figure 4.11). A tiny amount of silicon was

observed as well. However, silicon is a common contamination for solution ICP-OES

due to the handling of samples, particularly when using hydrofluoric acid. As much as


a tiny amount of silicon could arise from contamination, some of it possibly originated

from the sample handling as well. Based on the ATR-FTIR and ICP-OE spectroscopic

results, these compounds seemed to be complexes formed between OPD and tin (II)

octanoate during the polymerisations. The ketone moiety of OPD possibly reacted with

tin (II) octanoate, thus hindering the conversion of both L-lactide and OPD. Latere and

coworkers reported a competition between the ketone moieties of OPD and the ester

groups of ɛ-caprolactone in the coordination process with tin (II) octanoate. Such

interaction was shown to slow down the kinetics of the polymerisation. ROP control

experiments performed with cyclohexanone and tin (II) octanoate resulted in a

decrease of the conversion of ε-caprolactone from 67 to 25 % for a concentration of

cyclohexanone of 0 to 58 mol% respectively. They concluded a hindering effect by the

ketone moiety on the polymerisation.1

Figure 4.11. Concentrations of analytes in red complexes measured by ICP-OES,

revealing tin and silicon as the main components.

In conclusion, the metal-catalysed polymerisations of L-lactide and OPD, with an

initial concentration range 0 - 20 mol%, afforded copolymers with only very low

amounts of incorporated OPD. To increase the concentration of OPD segments within


the copolymer backbone, reactions were performed with higher initial OPD

concentrations and higher temperatures (150; 170 ºC). Higher OPD initial

concentrations (50; 75 mol%) seemed to block the copolymerisation by inhibiting the

conversion of L-lactide. Increasing the temperature resulted in the precipitation of a

red complex in the polymerisation medium. The precipitate was mainly composed of

tin and OPD. The ketone moiety of OPD possibly hindered the conversion of both

monomers by reacting with the tin (II) catalyst to form complexes instead of

facilitating ring-opening.

4.2.2. Organocatalysed Copolymerisation of L-Lactide and OPD in Solution

In order to increase the level of OPD incorporated into the PLLA backbone,

polymerisations were performed in solution using a different catalytic system: 1,8-

diaza[5.4.0]bicycloundec-7-ene (DBU) as catalyst and benzyl alcohol as the initiator.

4.2.2.1. Homopolymerisation of L-Lactide

The kinetics of the ROP of L-lactide were investigated using DBU and benzyl alcohol

as catalyst and initiator respectively. A molecular weight of 12,500 g·mol-1 was

targeted (degree of polymerisation (DPn) 87) to obtain polymers featuring molecular

weights above the entanglement molecular weight.39

Efforts were made to remove residual traces of oxygen and moisture. The

polymerisations were performed under nitrogen in a glovebox. Reagents and the

solvent were prepared as followed: dry dichloromethane was degassed via five freeze-

pump-thaw cycles and stored over activated molecular sieves (4 Å); DBU and benzyl

alcohol were distilled over calcium hydroxide under vacuum and degassed; L-lactide

was dissolved in dry DCM and dried over activated molecular sieves (4 Å) for at least

24 hours prior to the reaction. A solution of appropriate volumes of L-lactide, benzyl

alcohol and DBU was stirred at room temperature with samples collected over time

and quenched with excess glacial acetic acid.

The conversion of L-lactide was determined from the integration of the methylene

protons of L-lactide and PLLA at 5.02 and 5.16 ppm in the 1H NMR spectrum

respectively. A rapid consumption of L-lactide was observed with up to 98 - 99 %

conversion after 1.5 minutes of polymerisation. The presence of peaks at 7.33 ppm,

corresponding to protons of benzyl alcohol, confirmed its role as the initiator.


The evolutions of molecular weight and polydispersity were assessed by GPC in

chloroform (refer to appendices). After 1.5 minutes, a number average molecular

weight of 18,500 ± 800 g∙mol-1 was obtained with a polydispersity of 1.21 ± 0.02

(Table 4.5). The calibration against narrow polystyrene standards, known to

overestimate molecular weight values for polyesters, accounts for the difference

between the targeted and measured values.28, 29 In comparison, 𝑀𝑛 were calculated

from the integration ratio of methylene protons of PLLA and protons from the benzyl

alcohol in the 1H NMR spectra, as followed:

𝐷𝑃𝑛 =𝐼(𝐶𝐻𝑃𝐿𝐿𝐴) × 5

𝐼(𝑃ℎ) × 1

The obtained values for 𝑀𝑛 after 1.5 minutes matched the expected values closely. The

small difference could be due to the resolution of the peaks in the 1H NMR spectra and

the accuracy of their integration. In both cases, however, values remained relatively

unchanged throughout the polymerisation time, suggesting few transesterification

reactions.

Table 4.5. Evolution of the conversion and number average molecular weight over

time of the ROP of L-lactide using DBU and benzyl alcohol as catalyst and initiator,

respectively, DPn 87.

Time Conversion (%) a 𝑀𝑛 (g·mol-1) a 𝑀𝑛

(Da) b Ð b

Time 0 0 - - -

1.5 min 98.5 ± 0.7 13,700 ± 100 18,500 ± 800 1.21 ± 0.02

3 min 98.5 ± 0.7 12,400 ± 2,900 19,500 ± 800 1.18 ± 0.02

4.5 min 98.5 ± 0.7 14,400 ± 0 17,900 ± 1,100 1.26 ± 0.01

6 min 98 ± 0 12,000 ± 0 17,700 ± 200 1.26 ± 0.02

7.5 min 98 ± 0 12,000 ± 0 17,100 ± 900 1.29 ± 0.01

a Determined by 1H NMR spectroscopy in CDCl3; b Measured by GPC analysis in

chloroform.


Based on the conversion determined by 1H NMR spectroscopy, a kinetic study of the

polymerisation was performed. The ROP of L-lactide catalysed by DBU was reported

to follow a pseudo first-order kinetic model, according to the relationship:

ln (1

1 − 𝑥) = 𝑘𝑎𝑝𝑝𝑡

With 𝑥 the conversion of L-lactide, calculated from the 1H NMR spectra and 𝑘𝑎𝑝𝑝 the

apparent kinetic constant for given initial conditions.7, 11 This relationship was used

with the experimental values reported in Table 4.5. For experimental reasons, the first

collected sample was after 1.5 min when the conversion was complete. This explains

the shape of the curve illustrated in Figure 4.12.

Figure 4.12. Homopolymerisation of L-lactide in DCM at room temperature

(monomer conversion calculated from 1H NMR spectroscopy) under the following

conditions: [LLA]0 = 2.072 mol·L-1, LLA / BDU = 15, LLA / benzyl alcohol = 87.

In addition to 1H NMR spectroscopy, the chemical structure of the synthesised PLLA

was investigated by ATR-FTIR spectroscopy. The spectra presented similar

characteristic bands of poly(L-lactide), including the -C=O stretching band of the ester


groups at 1748 cm-1, the -C-O- deformation bands of the ester at 1180 and 1082 cm-1,

and the -CH3 bending band at 1454 cm-1 (Figure 4.13).17, 18

Figure 4.13. Representative ATR-FTIR average spectrum of synthesized poly(L-

lactide) using DBU and benzyl alcohol as catalyst and initiator, respectively.

Benzyl alcohol and DBU efficiently initiated and catalysed, respectively, the ROP of

L-lactide in DCM at room temperature. Optimisation of the experimental conditions

enabled to obtain poly(L-lactide) with known end-groups and controlled molecular

weights.

4.2.2.2. Batch Polymerisations of L-Lactide and OPD

Once the experimental conditions were optimized, a series of batch polymerisations of

L-lactide and OPD were performed with an OPD initial concentration in the range 10

- 75 wt%. A reaction with pure L-lactide was simultaneously carried out as a control.

The DPn were calculated to reach a molecular weight at least equal to the entanglement

molecular weight of PLLA (Table 4.6). The polymerisations were quenched by glacial


acetic acid, then concentrated under reduced pressure and purified by reprecipitation

using chloroform and hexane (1:10 vol:vol) to furnish white powders.

Table 4.6. Conditions and results of the batch polymerisations of L-lactide and OPD

in solution at room temperature with DBU and benzyl alcohol as catalyst and initiator,

respectively.

a Initial OPD feed (mol%); b Calculated from the integration ratios in the 1H NMR

spectra of crude polymers.

The conversion of L-lactide was calculated from the integrations ratio of the respective

methine protons at 5.02 ppm and at 5.16 ppm for L-lactide and PLLA in the 1H NMR

spectra. The L-lactide conversion varied depending on the initial feed of OPD (Table

4.6). When polymerized without OPD, L-lactide was completely polymerized after 20

minutes. The addition of 11 mol% of OPD to the polymerisation medium resulted in

slowing down the kinetics of L-lactide, reaching 99 % conversion only after 17 hours

instead of 20 minutes as observed without OPD. Increasing the OPD feed by > 50

mol% completely inhibited the conversion of L-lactide even after 50 hours of reaction.

The chemical structures of purified polymers were further analysed by ATR-FTIR

spectroscopy. Nine spectra were collected on three different samples of each polymer,

baseline-corrected, averaged and normalized with the -CH3 bending band at 1454 cm-

1. The spectra of polymers 13, 14 and 15 displayed similar characteristic bands: the -

C=O stretching band of ester at 1747 cm-1, the -C-O- deformation band at 1181 and

1084 cm-1 and the -CH3 bending band at 1454 cm-1 (Figure 4.14). The presence of a

Polyme

r

OPD0

a

LLA/I

n

LLA/Ca

t

OPD/I

n

OPD/Ca

t

Tim

e

Conversio

n of LLAb

Yiel

d

(%)

13 0 72 13 - - 20

min

99 18.5

14 11 70 13 8 2 17 h 99 21.6

15 23 70 13 16 3 18 h 85 21.8

16 55 72 13 44 8 50 h 0 12.4

17 77 72 13 62 12 50 h 0 10.8


small band at 1647 cm-1 in the spectra of polymers 16 and 17 was assigned to the -

C=N stretching band of residual DBU (Figure 4.15).40 The carbonyl band of the

polymer 17 revealed two strong -C=O stretching bands, at 1722 and 1703 cm-1,

corresponding to the ester and ketone of unreacted OPD (refer to Chapter 2).

Figure 4.14. ATR-FTIR averaged spectra of purified polymers with OPD initial feed

ranging from 0 to 77 mol% (average of 9 spectra after baseline correction and


The incomplete conversion of monomers was expected to affect the molecular weights

of the products. The GPC traces measured in chloroform are illustrated in Figure 4.16.

As no conversion was observed for the OPD initial feeds of 55 and 77 mol%, no GPC

measurement was performed on compounds 16 and 17. Compounds 13 and 14 were

characterized by narrow monomodal distributions as expected from a full conversion

of L-lactide. On the contrary, compound 15 displayed two narrow distributions, at

about 25 and 31 minutes retention time. They correspond to the oligomer and residual

L-lactide respectively. The values of 𝑀𝑛 for 13 and 14 were in agreement with the


predicted ones. 𝑀𝑛 of 15 was lower because of the low conversion of L-lactide (Table

4.7).

Figure 4.15. Enlarged view of the carbonyl region in the ATR-FTIR spectra of

purified polymers with an OPD initial feed of 0 to 77 mol%.


Figure 4.16. GPC traces of copolymers of L-lactide and OPD with an initial OPD

concentration of 0 to 23 mol% measured in chloroform (the traces were baseline-


Table 4.7. Molecular weight and polydispersities of the purified products of the

batch polymerisations of L-lactide and OPD using DBU as the catalyst.

Entry

OPD/LLA (mol%) 𝑀𝑛 (g·mol-1)a

𝑀𝑛 (Da) b

𝑀𝑤 (Da) b Ð b

13 0 10,316 9,400 ± 100 12,300 ±

100

1.31 ±

0

14 11 11,141 10,400 ±

100

13,300 ±

100

1.28 ±

0

15 23 12,166 5,800 ± 100 7,000 ± 100 1.22 ±

0


a Theoretical molecular weight assuming full conversion of both monomers. b

Measured by GPC in chloroform.

Adding OPD to the polymerisation of L-lactide resulted in either slowing down or

completely hindering the conversion of L-lactide. Lohmeijer and coworkers attempted

to polymerise δ-valerolactone and ε-caprolactone in solution at room temperature

using DBU as catalyst and 4-pyrenobutanole as initiator. Although they used DBU

loadings up to 20 mol% relative to the monomer, neither of the lactones got converted.

They suggested the catalyst was not nucleophilic enough to activate the ring-opening

polymerisation of both lactones. The addition of a thiourea-based catalyst was

necessary as a dual-activation of the monomer.7

The ring-opening polymerisation of OPD by DBU could also be hindered due to the

ketone moiety, as demonstrated when tin (II) octanoate was used as catalyst. Protecting

the ketone moiety and performing a batch polymerisation with L-lactide and the

protected version of OPD could enable the investigation of the role of the ketone

moiety on the polymerisation process. In this work, this strategy was investigated due

to its similarity with the polymerizations already carried out.

4.2.2.3. Investigation on the Role of the OPD Ketone Moiety on the

Polymerisation

The potential inhibiting role of OPD on the conversion of L-lactide during the

polymerisation was subsequently studied by protecting the ketone moiety of OPD with

acetal group (Figure 4.17).

Figure 4.17. Structure of 1,4,8-trioxaspiro[4.6]-9-undecanone.

Further information on the synthesis and characterization of this protected monomer

will be discussed in Chapter 5. L-lactide and the protected OPD (25 mol%) were


stirred together at room temperature. DBU and benzyl alcohol were added to the

polymeric mixture. After 10 minutes of reaction, 97 % of L-lactide was converted, as

revealed by the integration ratio of the methine protons of PLLA and residual L-lactide

at 5.16 and 5.02 ppm respectively in 1H NMR spectra. After quenching with glacial

acetic acid, the polymerisation mixture was concentrated under reduced pressure and

the crude residues purified twice by reprecipitation, using chloroform and cold hexane

(0 - 1 °C) as solvent and non-solvent respectively. The desired product was obtained

as a white powder in 39 % yield. The 1H NMR spectrum of the crude polymer revealed

a quadruplet at 5.16 ppm and a doublet at 1.57 ppm which were assigned to the methine

and methyl protons of the L-lactide repeating unit. A quadruplet at 5.02 ppm and a

doublet at 1.69 ppm provided support for residual L-lactide. A multiplet at 7.35 ppm

of benzyl alcohol end-groups confirmed its initiating role. However, the triplets at

4.29, 2.01 and 1.90 ppm, as well as the singlet at 3.99 ppm were all characteristic of

TOSUO monomer (Figure 4.18).41 This suggested that DBU solely catalysed the

polymerisation of L-lactide and remained inactive towards the protected OPD. When

L-lactide and OPD (23 mol%) were copolymerised using the same conditions, the

kinetics of L-lactide conversion was much slower (85 % after 18 hours, refer to section

4.2.2.2). The protection of the ketone moiety of OPD with acetal groups enabled to

polymerise L-lactide within 10 minutes. This result suggests the ketone inhibited the

polymerisation. The next chapter focuses on investigating the protected version of

OPD to increase the level of incorporation and study the photodegradation of the

resulting copolymers, once the ketone would be deprotected.


Figure 4.18. 1H NMR spectrum of the crude product of the ROP of L-lactide and

protected OPD revealing the conversion of L-lactide only, measured in CDCl3.

4.3 SUMMARY

Following the reactive extrusions of PLLA and OPD in Chapter 3, the strategy

employed in this chapter relied on the copolymerisation of L-lactide and OPD in the

bulk in the presence of the versatile tin (II) octanoate catalyst. Several monomer feed

ratios and conditions were investigated. Although the desired copolymers were

successfully obtained, PGSE spectroscopy analysis confirmed the incorporation of

only low amounts of OPD. Another catalytic system, comprising DBU and benzyl

alcohol as catalyst and initiator, respectively, was investigated in order to increase the

incorporation level. Poly(L-lactide) was obtained within a few minutes. A good control


over the molecular weight was obtained as well as narrow polydispersities (suggesting

few transesterification reactions) and chain-end fidelity. However, DBU, an

insufficient nucleophile, no OPD ring-opening was observed. Hence, none of the

desired copolymers were obtained with the DBU-benzyl alcohol system. In both

catalytic systems, the ketone moiety hindered the polymerisation to occur. Concerning

tin (II) octanoate, the ketone moiety underwent competition with the ester functions of

both monomers in the coordination process with tin. Employing DBU on the acetal

protected OPD resulted into a complete conversion of L-lactide into PLLA. The result

supported the non-nucleophilicity of DBU as well as its effect on the ketone moiety.

The next chapter further expands the synthesis of random copolymers of poly(L-

lactide-co-OPD) with the protected-OPD so as to increase the incorporation level.

Artificially ageing was subsequently undertaken on the modified PLLAs.

4.4 EXPERIMENTAL

4.4.1. Materials

L-lactide ((3s)-cis-3.6-dimethyl-1,4-dioxane-2,5-dione) (98 %) was purchased from

Sigma-Aldrich, recrystallized twice from toluene and dried under vacuum prior to use.

OPD was synthesized as previously reported (refer to Chapter 2) and dried under

vacuum prior to use. Methanol, hexane fraction and chloroform were all AR grade,

purchased from ChemSupply and used as received. Chloroform (HPLC grade) used

for GPC analysis was purchased from Merck and filtered prior to use. Deuterium

chloroform used for NMR spectroscopy analysis was also purchased from Merck and

stored at 10 °C.

4.4.2. Methods

4.4.2.1. Ring-Opening Polymerisation of L-lactide and OPD 5 - 20 wt%

L-Lactide and OPD were introduced into a flame-dried and degassed Schlenk vessel

under nitrogen atmosphere. The Schlenk vessel was then sealed under nitrogen

atmosphere and immersed into a silicone oil bath preheated at 110 °C. Once the

monomers were completely melted, tin (II) octanoate was added to the Schlenk vessel

under nitrogen atmosphere. After being thermally quenched, the polymeric mixture

was purified by dissolution - reprecipitation using chloroform and cold methanol (0 -


1 ºC; 1:10 vol:vol) as solvent and non-solvent, respectively. The polymer was

recovered by filtration.

Compound 1: L-lactide (2.0467 g, 14.20 mmol), tin (II) octanoate (5 mg, 12 µmol).

White polymer. Purification yield: 29.2 %. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CH),

1.57 (d, 3H, CH3). ATR-FTIR: ʋ max = 2995 (w, -CH2 stretching, asymmetric), 2948

(w, -CH2 stretching, symmetric), 1750 (s, -C=O carbonyl stretching), 1453 (-CH3

bending), 1382 and 1359 (-CH- deformation, symmetric and asymmetric), 1181, 1127

and 1083 (-C-O- stretching), 1043 (-OH bending), 869 (w, -C-C- stretching). GPC

(Refractive Index detector): 𝑀𝑛 (Ð) = 43,000 ± 900 g·mol-1 (1.25 ± 0.06). DSC (second

heating cycle): Tg = 56.2 ± 2.9 ºC, Tm = 174.3 ± 0.1 ºC (∆𝐻𝑚 = 51.0 ± 0.7 J·g-1).

Compound 2: L-lactide (2.0014 g, 13.89 mmol), OPD (104.0 mg, 812.5 µmol), tin (II)

octanoate (5 mg, 12 µmol). White powder. Purification yield: 24.6 %. 1H NMR

(CDCl3, δ ppm, subscripts L-LA and OPD denote each repeating units): 5.17 (q, H,

CHL-LA), 4.40 (t, 2H, -CH2OOPD), 4.34 (m, H, -CH(CH3)OHL-LA), 2.81 (m, 2H,

CH2C=OOPD), 2.66 (m, 2H, C=OCH2OPD), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max =

2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1750

(s, -C=O carbonyl stretching) with a shoulder from 1725 to 1700 cm-1, 1454 (-CH3




heating cycle): Tg = 42.0 ± 2.5 ºC, Tm = 162.8 ± 0.3 ºC (∆𝐻𝑚 = 55.2 ± 0.7 J·g-1).

Compound 3: L-lactide (0.9937 g, 6.894 mmol), OPD (98.9 mg, 772.7 µmol), tin (II)

octanoate (5 mg, 12 µmol). White powder. Yield: 43.5 %. 1H NMR (CDCl3, δ ppm,

subscripts L-LA and OPD denote each repeating units): 5.17 (q, H, CHL-LA), 4.41 (t,

2H, -CH2OOPD), 4.36 (m, H, -CH(CH3)OHL-LA), 2.79 (m, 2H, CH2C=OOPD), 2.66 (m,

2H, C=OCH2OPD), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 2996 (w, -CH2

stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1750 (s, -C=O

carbonyl stretching) with a shoulder from 1725 to 1700 cm-1, 1453 (-CH3 bending),


(-C-O- stretching), 1043 (-OH bending), 870 (w, -C-C- stretching). GPC (Refractive

Index detector): 𝑀𝑛 (Ð) = 6,900 ± 600 g·mol-1 (1.64 ± 0.18). DSC (second heating

cycle): Tg = 51.9 ± 0.5 ºC, Tm1 = 153.3 ± 0.2 ºC, Tm2 = 159.5 ± 0.4 ºC (∆𝐻𝑚 = 47.9 ±

3.4 J·g-1).


Compound 4: L-lactide (2.0422 g, 14.17 mmol), OPD (418.5 mg, 3.270 mmol), tin

(II) octanoate (5 mg, 12 µmol). Yellow powder. Purification yield: 61.0 %. 1H NMR

(CDCl3, δ ppm, subscripts L-LA and OPD denote each repeating units): 5.17 (q, H,

CHL-LA), 4.42 (t, 2H, -CH2OOPD), 4.35 (m, H, -CH(CH3)OHL-LA), 2.80 (m, 2H,



(s, -C=O carbonyl stretching) with a shoulder from 1730 to 1700 cm-1, 1455 (-CH3




heating cycle): Tg = 39.8 ± 2.0 ºC, Tcc = 88.1 ± 1.7 ºC (∆𝐻𝑐𝑐= 7.0 ± 0.7 J·g-1), Tm =

137.4 ± 0.8 ºC (∆𝐻𝑚 = 35.1 ± 1.2 J·g-1).

4.4.2.2. Attempted Synthesis of Poly(L-Lactide-co-OPD) with Increased

OPD Initial Feed

L-lactide and OPD were introduced in a degassed Schlenk vessel under nitrogen

atmosphere. The Schlenk vessel was then sealed under nitrogen atmosphere and

immersed into a silicone oil bath preheated at 110 °C. Once the monomers were

completely melted, tin (II) octanoate was added to the Schlenk vessel under nitrogen

atmosphere. After 7 hours of reaction, the polymerisation was thermally quenched.

The polymeric mixture was purified by dissolution - reprecipitation using chloroform

and cold methanol (0 - 1 ºC; 1:10 vol:vol) as solvent and non-solvent, respectively.

The polymer was recovered by filtration


(II) octanoate (5 mg, 12 µmol). Yellow wax. Yield: 50.0 %. 1H NMR (CDCl3, δ ppm):

5.17 (q, H, CHPLLA), 5.10 (q, H, CHL-lactide), 4.43 (m, 2H, -CH2OOPD), 4.36 (m, H, -


CH3L-LA), 1.50 (d, 3H, CH3L-lactide). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching,

asymmetric), 2932 (w, -CH2 stretching, symmetric), 1752 (s, -C=O carbonyl

stretching) with a shoulder from 1730 to 1700 cm-1, 1456 (-CH3 bending), 1355 (-CH-

deformation, asymmetric), 1094 (-C-O- stretching), 1053 (-OH bending), 934 (s, -CO-

O- deformation of L-lactide).



(II) octanoate (5 mg, 12 µmol). Yellow wax. Yield: 48.4 %. 1H NMR (CDCl3, δ ppm):

5.19 (q, H, CHPLLA), 5.09 (q, H, CHL-lactide), 4.45 (m, 2H, -CH2OOPD), 4.37 (m, H, -

CH(CH3)OHL-LA), 2.81 (m, 2H, CH2C=OOPD), 2.687 (m, 2H, C=OCH2OPD), 1.59 (d,

3H, CH3L-LA), 1.49 (d, 3H, CH3L-lactide). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching,


stretching) with a shoulder from 1730 to 1700 cm-1, 1456 (-CH3 bending), 1355 (-CH-

deformation, asymmetric), 1094 (-C-O- stretching), 1052 (-OH bending), 934 (s, -CO-

O- deformation of L-lactide).

4.4.2.3. Attempted Synthesis of Poly(L-Lactide-co-OPD) at Higher

Temperatures

L-Lactide and OPD were introduced in a degassed Schlenk vessel under nitrogen

atmosphere. The Schlenk vessel was then sealed under nitrogen atmosphere and

immersed into a silicone oil bath preheated at the relevant temperature. Once the

monomers were completely melted, tin (II) octanoate was added to the Schlenk vessel

under nitrogen atmosphere. After 7 h of reaction, the polymerisation was thermally

quenched. The polymeric mixture was purified by dissolution - reprecipitation using

two different solvent systems: chloroform and cold methanol (0 - 1 ºC; 1:10 vol:vol)

as solvent and non-solvent, respectively and chloroform and cold hexane (0 - 1 ºC;

1:10 vol:vol) as solvent and non-solvent, respectively. The polymer was recovered by

filtration as yellow waxes.


(II) octanoate (5 mg, 12 µmol). Purification yield: 67 %. ATR-FTIR: ʋ max = 2997

(w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1754 (s, -

C=O carbonyl stretching) with a shoulder from 1730 to 1666 cm-1, 1455 (-CH3


and 1086 (-C-O- stretching), 1043 (-OH bending), 934 (-CO-O- deformation of L-

lactide), 871 (w, -C-C- stretching).


(II) octanoate (5 mg, 12 µmol). Purification yield: 74 %. ATR-FTIR: ʋ max = 2997

(w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1749 (s, -

C=O carbonyl stretching) with a shoulder from 1730 to 1666 cm-1, 1454 (-CH3



and 1085 (-C-O- stretching), 1043 (-OH bending), 871 (w, -C-C- stretching).

4.4.2.4. Organocatalysed Copolymerisation of L-Lactide and OPD in

Solution

All the following experiments of this section were performed under an argon

atmosphere (Ultra High Purity) using a glovebox (Labconco). All the glassware was

oven dried at 60 °C, before being introduced into the glovebox. DCM, dried over a

purification system, was degassed 5 times and stored over molecular sieves (4 Å) under

argon atmosphere. L-lactide and OPD were both dissolved in DCM and dried over

molecular sieves (4 Å) at least 24 h prior to the reaction.

4.4.2.4.1. Homopolymerisation of L-lactide

In a typical experiment, L-lactide (298.6 mg, 2.072 mmol) was added to a glass vial.

Benzyl alcohol (2.575 mg, 23.81 µmol) and DBU (20.45 mg, 134.3 mmol) were added

to the monomer solution. Samples were collected at different times of the reaction to

evaluate the monomer conversion. The polymerisation was quenched with glacial

acetic acid after 7 minutes 30 before being concentrated under reduced pressure. The

polymeric mixture was purified by dissolution / reprecipitation using chloroform and

cold hexane (0 - 1 ºC) as the solvent and non-solvent respectively (1:10, vol:vol). The

purified product was recovered by filtration and dried under vacuum. 1H NMR (CDCl3,

δ ppm): 7.33 (m, 6H, CHBnO), 5.16 (q, H, CH), 1.57 (d, 3H, CH3). ATR-FTIR: ʋ max

= 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1749

(s, -C=O carbonyl stretching), 1454 (-CH3 bending), 1383 and 1359 (-CH-

deformation, symmetric and asymmetric), 1182, 1129 and 1084 (-C-O- stretching),

1044 (-OH bending), 871 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛

(Ð) = 12,000 ± 0 g·mol-1 (1.29 ± 0.01).

4.4.2.4.2. Batch Polymerisations of L-lactide and OPD

In a typical experiment, volumes of L-lactide and OPD solutions were added to a glass

vial. Benzyl alcohol and DBU were both dissolved in 2 mL of DCM and an aliquot of

each solution was added to the monomer solution (the volumes and masses are

reported in Table 4.8). The reaction was performed at room temperature and protected

from the light. Samples were collected at different times of the reaction to evaluate the


monomer conversions. The polymerisation was quenched with glacial acetic acid

before being concentrated under reduced pressure. The polymeric mixture was purified

by dissolution/reprecipitation using chloroform and cold hexane (0 - 1 ºC) as the

solvent and non-solvent respectively (1:10, vol:vol). A crude fraction was collected

for conversion calculation. The purified product was recovered by filtration and dried

under vacuum. A control polymerisation of L-lactide alone was carried out using the

same procedure.

Table 4.8. Conditions of batch copolymerisations in DCM at room temperature using

DBU and benzyl alcohol as catalyst and initiator, respectively.

Entry L-lactide OPD Benzyl alcohol DBU

mg μmol mg μmol mg μmol mg μmol

13 99.50 690.30 - - 1.04 9.65 7.84 51.50

14 99.50 690.30 9.97 77.85 1.06 9.83 7.85 51.56

15 99.50 690.30 20.04 156.66 1.06 9.83 7.85 51.56

16 99.50 690.30 54.75 427.73 1.04 9.65 7.84 51.50

17 99.50 690.30 76.65 598.83 1.04 9.65 7.84 51.50

Compound 13: white powder. Purification yield: 21.6 %. 1H NMR (CDCl3, δ ppm):

7.33 (m, 6H, CHBnO), 5.17 (q, H, CHL-LA), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max =





(Ð) = 9,400 ± 0 g·mol-1 (1.31 ± 0).

Compound 14: white powder. Purification yield: 27.2 %. 1H NMR (CDCl3, δ ppm,

subscripts L-LA and OPD denote each repeating units): 7.34 (m, 6H, CHBnO), 5.17 (q,

H, CHL-LA), 4.42 (t, 2H, -CH2OOPD), 4.35 (m, H, -CH(CH3)OHL-LA), 2.80 (m, 2H,







(Ð) =10,400 ± 0 g·mol-1 (1.28 ± 0).


subscripts L-LA and OPD denote each repeating units): 7.34 (m, 6H, CHBnO), 5.17 (q,

H, CHL-LA), 4.42 (t, 2H, -CH2OOPD), 4.35 (m, H, -CH(CH3)OHL-LA), 2.80 (m, 2H,






(Ð) = 5,800 ± 0 g·mol-1 (1.22 ± 0).


subscripts L-LA and OPD denote each monomers): 5.01 (q, H, CHL-LA), 4.42 (t, 2H, -

CH2OOPD), 2.80 (m, 2H, CH2C=OOPD), 2.67 (m, 2H, C=OCH2OPD), 1.70 (d, 3H, CH3L-

LA). ATR-FTIR: ʋ max = 3304 (b, w, -COOH stretching), 2984 (w, -CH2 stretching,


stretching), 1704 (s, -C=O carbonyl stretching of OPD), 1454 (-CH3 bending), 1374

and 1346 (-CH- deformation, symmetric and asymmetric), 1123 and 1082 (-C-O-

stretching), 1042 (-OH bending), 870 (w, -C-C- stretching).

Compound 17: white wax. Purification yield: 6.31 %. 1H NMR (CDCl3, δ ppm,

subscripts L-LA and OPD denote each monomers): 5.01 (q, H, CHL-LA), 4.42 (t, 2H, -

CH2OOPD), 2.80 (m, 2H, CH2C=OOPD), 2.67 (m, 2H, C=OCH2OPD), 1.70 (d, 3H, CH3L-

LA). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2

stretching, symmetric), 1749 (s, -C=O carbonyl stretching), 1453 (-CH3 bending),


(-C-O- stretching), 1044 (-OH bending), 940 (-CO-O- deformation of L-lactide), 867

(w, -C-C- stretching).

4.4.2.5. Investigation on the Role of the OPD Ketone Moity on the

Polymerisation

L-Lactide (202.0 mg, 1.402 mmol) and TOSUO (60.00 mg, 348.8 µmol) and were

dissolved in 1 and 1.2 mL DCM respectively, dried over molecular sieves, and mixed


together. Benzyl alcohol (2.268 mg, 20.97 μmol) and DBU (14.5 mg, 95.27 µmol)

were both added to the monomers solution. After 10 minutes, a fraction was collected

for conversion calculation by 1H NMR spectroscopy. The polymerisation was

quenched with glacial acetic acid before being concentrated under reduced pressure.

The polymeric mixture was purified by dissolution/reprecipitation using chloroform

and cold hexane (0 - 1 ºC) as the solvent and non-solvent respectively (1:10, vol:vol).

1H NMR (CDCl3, δ ppm, subscript L-LA denotes the PLLA repeating units, subscript

TOSUO denotes the TOSUO monomer): 7.34 (m, 6H, CHBnO), 5.17 (q, H, CHL-LA),

4.29 (t, 2H, C=OOCH2TOSUO), 3.99 (s, 4H, C-O-CH2CH2OTOSUO), 2.70 (t, 2H,

CH2C=OOCH2TOSUO), 2.01 (t, 2H, C-OCH2CH2O-CH2CH2TOSUO), 1.90 (t, 2H, C-

OOCH2CH2C-OTOSUO), 1.58 (d, 3H, CH3L-LA).



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Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 175

Chapter 5: Photodegradation of

Functionalized Poly(L-Lactide)

with 2-Oxepane-1,5-Dione

5.1 BACKGROUND

In the preceding chapter (Chapter 4), modification of poly(L-lactide) was achieved

through bulk copolymerisation of L-lactide and 2-oxepane-1,5-dione using tin (II)

octanoate as the catalyst. However, only low levels of incorporated OPD were

achievable because of a competition between the ketone moiety in OPD and the ester

groups of the monomers for the coordination process with the tin catalyst. In line with

these findings, the synthesis of copolymers containing higher concentrations of

chromophores by employing a modified OPD monomer is explored in this chapter.

This modified OPD monomer, 1,4,8-trioxaspiro[4.6]-9-undecanone (TOSUO)

features an ethylene ketal protecting group (Figure 5.1).1 The first reported synthesis

of OPD was achieved via a Baeyer-Villiger oxidation of 1,4-cyclohexane

monoethylene acetal with m-chloroperoxybenzoic acid (mCPBA) to afford TOSUO.

Subsequent deprotection of the ketone moiety with triphenylcarbenium

tetrafluoroborate furnished OPD.2

Figure 5.1. Structure of 1,4,8-trioxaspiro[4.6]-9-undecanone.

TOSUO has been used by others to generate a platform of polyesters bearing additional

functional groups. For instance, TOSUO was successfully polymerized with ε-

caprolactone (ε-CL) to yield random and block copolymers in solution. In this case,

176 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione

aluminium isopropoxide was employed as the initiator. Close agreement was found

between the initial TOSUO feed and the concentration of repeating units with values

up to 90 mol%.3-5 Tin (II) octanoate, as a catalyst, also proved to efficiently

copolymerize ε-CL and TOSUO when combined with poly(ethylene glycol) or benzyl

alcohol as initiators.6-9 Prime et al.10 first reported the synthesis of poly(L-lactide-co-

TOSUO) in the bulk using tin (II) octanoate and butanol as the catalyst and initiator

respectively. A level of incorporated TOSUO of 3.8 mol% was achieved for a 7.4

mol% initial feed. Babasola and coworkers synthesized low molecular weight

copolymers of poly(TOSUO-co-ε-caprolactone) and D,L-lactide in the bulk at 110 ºC,

using tin (II) octanoate. They used both octan-1-ol and methoxy poly(ethylene glycol)

(350 Da) as initiators to tune the viscosity. They obtained copolymers featuring

number average molecular weights ranging from 2,200 to 2,900 Da.11

In this chapter, L-lactide was copolymerized with TOSUO in the bulk at 110 °C, via

initiation with benzyl alcohol and tin (II) octanoate to afford copolymers with various

compositions (Scheme 5.1.a). The ketones were subsequently deprotected to afford

poly(L-lactide-co-OPD) (Scheme 5.1.b). An ageing study was undertaken for two

PLLA-co-OPD to assess the influence of the ketone moieties and their concentration

on the photodegradation of the copolymers.


Scheme 5.1. Two step synthesis of poly(L-lactide-co-OPD): Conditions and reagents:

a. tin (II) octanoate, in the bulk, 110 ºC; b. triphenylcarbenium tetrafluoroborate

(TPFB), DCM, room temperature, 2 hours.


1,4,8-Trioxaspiro[4.6]-9-undecanone (TOSUO) was first synthesized and

characterized. Following the synthesis of TOSUO, the monomer was copolymerized

with L-lactide in the bulk at 110 ºC, using tin (II) octanoate and benzyl alcohol as the

catalyst and initiator respectively.

5.2.1 Synthesis of 1,4,8-Trioxaspiro[4.6]-9-Undecanone

1,4,8-Trioxaspiro[4.6]-9-undecanone was synthesized via a Baeyer-Villiger oxidation

of 1,4-cyclohexane monoethylene acetal with m-chloroperoxybenzoic acid (mCPBA)

according to reported procedures (Scheme 5.2).1, 11 TOSUO and the mCPBA by-

product, 3-chlorobenzoic acid, were separated via column chromatography

(hexane/ethyl acetate 65/35), with TOSUO obtained as an oil in 40% yield. TOSUO


remained as an oil (no solid obtained as reported in the literature) even after prolonged

drying under vacuum.

Scheme 5.2. Baeyer-Villiger oxidation of 1,4-cyclohexane monoethylene acetal by

mCPBA to afford TOSUO and the mCPBA by-product, 3-chlorobenzoic acid.

1H NMR spectroscopy confirmed the presence of the ethylene ketal protons at 3.99

ppm, the methylene protons α to the ethylene ketal groups at 2.01 and 1.90 ppm, as

well as the methylene protons adjacent to the ester group at 4.29 and 2.70 ppm (Figure

5.2).1 The obtained 13C NMR spectrum of TOSUO was also consistent with data

reported in the literature (Figure 5.3)12. The measured melting point was in the range

44 – 47 ºC. The Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR)

spectra revealed a strong ester stretching band at 1724 cm-1 supporting the formation

of the ester group.

Once prepared, the TOSUO monomer was subsequently used to modify the backbone

of PLLA. Previous report stated the thermal instability of TOSUO for temperatures

above 120 ºC, leading to the deprotection of the ketone acetal groups to afford 2-

oxepane-1,5-dione.11 Based on both the literature and the findings in Chapter 3, no

reactive extrusion was performed between PLLA and TOSUO. Instead, the TOSUO

monomer prepared here was subsequently copolymerised with L-lactide in the bulk.


Figure 5.2. 1H NMR spectrum of TOSUO, measured in CDCl3.

Figure 5.3. 13C NMR spectrum of TOSUO, measured in CDCl3.


5.2.2 Synthesis of Poly(L-Lactide-co-TOSUO)

5.2.2.1. Ring-Opening Polymerisation of L-Lactide and TOSUO

Poly(L-lactide-co-TOSUO) was synthesized by a transition-metal catalysed ring-

opening polymerisation of L-lactide and TOSUO using tin (II) octanoate and benzyl

alcohol as the co-initiator (Scheme 5.3). A typical polymerisation involved the

introduction of both monomers and benzyl alcohol in a flame-dried Schlenk vessel

under an inert atmosphere in a glovebox, followed by the melting of the monomers

and subsequent addition of tin (II) octanoate. The initial molar fraction of TOSUO

ranged from 5 to 25 mol%. This range was selected because higher OPD incorporation

within the copolymer would lead to insolubility of the compounds.13, 14 Moreover, this

range is suitable enough to check whether the incorporated OPD has any effect on the

photooxidation process.

Scheme 5.3. ROP of L-lactide and TOSUO in the bulk at 110 °C to afford poly(L-

lactide-co-TOSUO) using tin (II) octanoate and benzyl alcohol as the catalyst and the

initiator respectively.

Efforts were made to remove traces of moisture from the reactants and atmosphere

used during polymerisation. This involved drying the monomers under vacuum prior

to each polymerisation, and distilling benzyl alcohol over calcium hydride and storing

it under an inert atmosphere. The polymerisations were carried out under an inert


atmosphere in the bulk at 110 °C to ensure the complete melting of both monomers

and yet limit the risk of thermal degradation via backbiting transesterifications as well

as epimerization.13, 14 Deprotection of TOSUO was also reported to occur at 120 ºC

after twenty four hours, revealing the ketone pendent moieties.11 Benzyl alcohol was

used as an initiator in order to control the architecture of the synthesized copolymers

to deliver a known end-group. The high boiling point of benzyl alcohol allowed its use

at 110 °C. The reactions were performed until no stirring of the polymeric mixture was

possible anymore due to an increase in viscosity. At this stage they were thermally

quenched. The polymers were purified by reprecipitation using tetrahydrofuran and

cyclohexane as solvent and non-solvent respectively to remove any unreacted

monomer and tin (II) octanoate residues. In comparison to Soxhlet extraction, the

dissolution / reprecipitation technique is efficient at removing higher quantities of

residual tin.15

5.2.2.1.1. Chemical Structures of Synthesized Poly(L-Lactide-co-TOSUO)

The incorporation of TOSUO onto PLLA was confirmed by 1H NMR spectroscopy

analysis. The quadruplet at 5.16 ppm and the doublet at 1.57 ppm were respectively

assigned to the methine and methyl protons of the L-lactide repeating unit (Figure 5.4).

The doublet at 1.50 ppm and the multiplet at 4.35 ppm correspond to the methyl and

methine of the -CH(CH3)-OH end group respectively.16 The TOSUO repeating unit

was confirmed by the presence of ethylene ketal protons at 3.92 ppm, the methylene

protons adjacent to the ethylene ketal groups at 1.98 ppm and the methylene protons

adjacent to the ester moiety at 2.45 and 4.23 ppm.1 The multiplet at 1.98 ppm supports

successful ring-opening of TOSUO as reported in the literature.1, 12 In comparison, this

multiplet resonates at 1.90 ppm in the monomer. The multiplet at 7.35 ppm shows the

presence of benzyl alcohol end-groups, with a small multiplet observed next to the

methine protons of the L-lactide repeat unit (at 5.23 – 5. 19 ppm) being attributed to

stereosequence combinations including D-lactide units.17, 18 This suggests that

epimerization occurred during polymerisation, which can be caused by both inter- and

intramolecular transesterifications that are favoured by prolonged reactions as well as

the use of tin (II) octanoate as the catalyst.15


Figure 5.4. Representative 1H NMR spectrum of poly(L-lactide-co-TOSUO),

measured in CDCl3.

The concentration of TOSUO repeat units was determined, after several purification

steps to ensure complete removal of any residual unreacted TOSUO, by calculations

based on the integrations of the methylene and alkyl protons of PLLA and PTOSUO

respectively. The composition of each copolymer is summarized in Table 5.1.

Although the polymerisations were thermally quenched after stirring was ceased (due

to high conversion of monomers), the reaction times varied from 19 to 44 hours. This

suggests some instability in the process which could arise from impurities that

potentially acted as initiating species.


Table 5.1. Copolymerisation of L-lactide and TOSUO in the bulk at 110 °C catalysed

by tin (II) octanoate.

a Calculated from integration ratios in 1H NMR spectra of purified copolymers.

The chemical structure of the product was further analysed by ATR-FTIR

spectroscopy. The ATR-FTIR spectra of the different poly(L-lactide-co-TOSUO)s are

shown in Figure 5.5. All the absorption bands matched the characteristic bands of a

neat PLLA: including a -C=O stretching band at 1755 cm-1, -C-O- stretching bands at

1182 and 1087 cm-1, and a -CH3 bending at 1455 cm-1.19, 20 Closer observation of the

carbonyl region (1850 – 1650 cm-1) indicates two shoulders around 1775 and 1715 cm-

1 (Figure 5.6). The shoulder at 1775 cm-1 was assigned to a butyrolactone carbonyl

group arising from the transesterifications between an ester group and a hydroxyl

group;21, 22 while that at 1715 cm-1 represented the ketone stretching band, suggesting

that deprotection of the ketone occurred to a small degree during the polymerisation

(favoured by relatively high temperatures and long reaction times, as discussed in

Section 5.2.1). The 1H NMR spectra did not reveal any resonance at 2.9 and 2.7 ppm,

assigned to ketone pendent moieties.2, 23 The concentration of such groups could be

below the detection limit of NMR spectroscopy. The two maxima observed around

1755 cm-1 were due to noise resulting from the resolution (4 cm-1) used to run the

spectra and the normalization process.

Entry Initial feed ratio

(mol%)

(LLA / TOSUO)

Reaction time Copolymer ratio

(mol%) a

(PLLA / PTOSUO)

18 100 / 4.4 19 h 100 / 4.8

19 100 / 9.6 43 h 100 / 4.8

20 100 / 15 44 h 100 / 8.3

21 100 / 25 20 h 100 / 12.7


Figure 5.5. ATR-FTIR spectra of different poly(L-lactide-co-TOSUO)s (average of 9

spectra per film after baseline correction and normalization with the -CH3 bending

band at 1455 cm-1).


Figure 5.6. Enlarged view of the carbonyl region (1850 – 1650 cm-1) in the ATR-

FTIR spectra of the different poly(L-lactide-co-TOSUO)s (the two maxima observed

around 1755 cm-1 were due to noise resulting from the resolution (4 cm-1) used to run

the spectra and the normalization process).

5.2.2.1.2. Molecular Weights of Synthesized Poly(L-lactide-co-TOSUO)

The molecular weights and polydispersities of the copolymers were evaluated by GPC

using chloroform as eluent (Figure 5.7).


Figure 5.7. GPC traces of purified poly(L-lactide-co-TOSUO) measured in

chloroform (the traces were baseline-corrected and normalized).

Table 5.2. Comparison of the theoretical 𝑀𝑛 and the measured values for the

different poly(L-lactide-co-TOSUO) copolymers.

Copolymer Theoretical

𝑀𝑛 a

𝑀𝑛

(Da)b

𝑀𝑤

(Da)b

Ð b 𝑀𝑛

(g·mol-1)c

18 16,250 5,600 ± 200 7,300 ± 100 1.32 ± 0.05 2,600

20 18,300 6,100 ± 500 8,900 ± 100 1.47 ± 0.10 2,220

21 13,630 4,700 ± 900 6,000 ± 1,300 1.28 ± 0.06 2,820

a Theoretical value of 𝑀𝑛 in the case of full conversion of both monomers; b Measured

by GPC in chloroform; c Calculated from integration ratios of benzyl alcohol end

groups at 7.35 ppm with both methine protons of PLLA at 5.16 ppm and the methylene

protons adjacent to the ethylene ketal groups at 1.98 ppm, in 1H NMR spectroscopy.


The initial ratios of monomer to initiator were calculated to reach a 𝑀𝑛 that was greater

than the entanglement molecular weight of PLLA (8-10 kg·mol-1), assuming living

polymerisation conditions were achieved.24 The 𝑀𝑛 values measured by GPC were

significantly lower than the expected values. However, these values were relative to

polystyrene. Compound 19 featured the lowest values measured by GPC with a 𝑀𝑛 of

800 ± 0 Da, a 𝑀𝑤 of 1,200 ± 0 Da and a polydispersity of 1.52 ± 0.01. However,

compound 19 was analysed by GPC six months after being synthesized. It was stored

in the dark in the meantime. These low values suggested some degradation upon

storage for several months. Regarding the other compounds, the lower molecular

weights obtained suggest the potential presence of other initiators such as solvent

impurities or residual moisture that could react with tin (II) octanoate. The

polymerisations follow a coordination-insertion mechanism in which tin (II) octanoate

reacts with benzyl alcohol or residual protic impurities to afford tin (II) alkoxide as the

actual initiator (Scheme 5.4).25 The polymerisation then proceeds via the cleavage of

the acyl-oxygen bond of the monomer, followed by its subsequent insertion into the

chain.26, 27 Adding benzyl alcohol to the initial reacting mixture is aimed to control the

resulting molecular weight from the monomer / initiator molar ratio.25 However, other

impurities may have initiated tin (II) octanoate, as revealed by the GPC measurements.

Scheme 5.4. Conversion of tin (II) octanoate into tin (II) alkoxide via reaction with

alcohol or residual protic impurities.

Moreover, the results were obtained following a calibration with narrow polystyrene

standards, which can also explain the observed differences. The molecular weights

were determined by 1H NMR spectroscopy as well. The experimental 𝑀𝑛 was deduced

from the integration ratios of the multiplet at 7.35 ppm assigned to benzyl end-group

protons with the methine proton of PLLA at 5.16 ppm and the methylene protons of

PTOSUO at 1.98 ppm. Similar to the values measured by GPC, the calculated 𝑀𝑛 were

lower than the theoretical values. However, only the protons from the benzyl end-

group could be observed in the 1H NMR spectra.

Sn(Oct)2 + ROH Oct-Sn-OR + OctH

Oct-Sn-OR + ROH Sn(OR)2 OctH+


5.2.2.1.3. Thermal Properties of Synthesized Poly(L-lactide-co-TOSUO)

The thermal properties of the copolymers were analysed by DSC on the second heating

run after erasing the thermal history of the copolymers. All copolymers were

semicrystalline, exhibiting a glass transition (Tg), an exothermic peak assigned to cold

crystallization (Tcc) and an endothermic peak corresponding to melting (Tm) (Figure

5.8). The unique glass transition indicates the amorphous phases of both PLLA and

PTOSUO were miscible and suggests the random distribution of TOSUO segments

along the PLLA chains. The values of Tg randomly varied with the concentration of

TOSUO segments (Table 5.3). For random copolymers, the Tg can be predicted based

on the Fox relationship:

1

𝑇𝑔=

𝑤1

𝑇𝑔1+

𝑤2

𝑇𝑔2

Where 𝑤1, 𝑤2 represent the weight fraction and 𝑇𝑔1, 𝑇𝑔2 are the glass transitions of

PLLA and POPD respectively.28 PTOSUO was reported to exhibit a Tg ranging from

-35 to -14 ºC (for number average molecular weights ranging from 2,600 to 12,400

g·mol-1 respectively).4 Therefore lower values of Tg for the copolymers compared to

neat PLLA were expected. The endothermic peak featured a shoulder, referred to as

Tm1 in Table 5.3.

Tian and coworkers synthesized random copolymers of -caprolactone and TOSUO

and reported a single glass transition suggesting the homogeneity of amorphous phases

and their randomness while block copolymers were characterized by two glass

transitions.3, 4 An increase in the glass transition of poly(ε-caprolactone-co-TOSUO)

was observed with the TOSUO content (-60 to -40 ºC with a TOSUO content of 0 to

1 mol% respectively).3 On the contrary, the melting temperatures decreased until a

TOSUO content of 15 mol%, above which the copolymers became amorphous.3 The

double melting behaviour of PLLA was explained by the melting-recrystallization

model, with the melting of original crystals and the melting of crystals formed during

the heating cycle of the DSC measurement.29


Figure 5.8. DSC thermograms from the second heating cycle of purified poly(L-

lactide-co-TOSUO).

Table 5.3. Thermal properties of the different poly(L-lactide-co-TOSUO) (three

measurements were performed and the values were averaged).

Entry Tg (°C) Tcc (°C) ∆𝐻𝑐𝑐

(J.g-1)

Tm1 (°C) Tm2 (°C) ∆𝐻𝑚(J.g-

1)

𝜒𝑐 (%)

18 49.5 ±

0.9

88.8 ±

0.4

14.3 ±

4.7

- 148.2 ±

1.7

49.1 ±

5.2

67.6 ±

9.8

19 35.3 ±

2.0

100 ±

2.0

1.1 ±

0.4

- 113.9 ±

1.3

1.6 ± 0.8 2.5 ±

1.4

20 43.7 ±

1.7

99.8 ±

2.1

27.9 ±

3.1

125.5 ±

1.4

139.9 ±

0.7

40.9 ±

0.9

73.4 ±

4.3

21 40.1 ±

0.2

88.8 ±

0.3

15.2 ±

0.9

128.1 ± 0 143.2 ±

0

43.9 ± 0 42.0


5.2.3 Synthesis of Poly(L-Lactide-co-2-Oxepane-1,5-Dione)

5.2.3.1. Deprotection of the Ketone Acetal Groups

As shown above, poly(L-lactide-co-TOSUO) was successfully synthesized with

various levels of TOSUO incorporated into the structure. Copolymers 18 and 20, with

4.8 and 8 mol% incorporated TOSUO and featuring relatively the same molecular

weight, were selected to be deprotected for subsequent artificial ageing. Deprotection

of the ethylene ketal groups afforded the corresponding ketone-containing

copolymers. Deprotection of acetal groups is usually performed under acidic

conditions. However, such conditions tend to hydrolyse the ester linkages of the L-

lactide repeating units which leads to reduction of the molecular weight.3

Triphenylcarbenium tetrafluoroborate (TPFB), on the other hand, efficiently

deprotected the ethylene ketal groups without altering the polymer backbone (Scheme

5.5).3, 10 The mechanism involves hydride transfer from the ethylene acetal to

triphenylcarbenium tetrafluoroborate, resulting in the formation of an oxonium ion that

is subsequently hydrolysed during aqueous work-up (Scheme 5.6).30, 31

In a typical deprotection reaction, poly(L-lactide-co-TOSUO) and TPFB were

dissolved in DCM and stirred at room temperature for 2 hours (Scheme 5.7).

Following the re-precipitation of the polymeric mixture from cold methanol, the

precipitate was isolated by filtration to give a white powder.

Scheme 5.5. Chemical structure of triphenylcarbenium tetrafluoroborate (TPFB).


Scheme 5.6. Mechanism of the deprotection using TPFB involving a hydride

abstraction from the ethylene acetal that affords an oxonium ion that is subsequently

quenched by aqueous work-up.30, 31

Scheme 5.7. Deprotection of the ketone acetal groups of poly(L-lactide-co-TOSUO)

using TPFB in DCM at room temperature to afford poly(L-lactide-co-OPD).

Conditions and reagents: a. poly(L-lactide-co-OPD), TPFB (1.5 equivalents of

ethylene ketal groups), DCM, 2 hours, room temperature, 80 - 85 %.

5.2.3.2. Chemical Structures of Copolymers After Deprotection

The conversion to ketone groups after deprotection was monitored using 1H NMR

spectroscopy. A representative spectrum of the copolymer before and after the

deprotection step is depicted in Figure 5.9. The acetal protons at 3.99 ppm in the


poly(L-lactide-co-TOSUO) spectrum were not visible in the poly(L-lactide-co-OPD)

spectrum. Moreover, the multiplets at 1.99 and 2.47 ppm (referred to as e and d

respectively in the starting material) shifted to 2.80 and 2.66 ppm respectively in the

product, thus supporting the completion of the deprotection reaction. The ratio a:b

between the resonances of the protons of benzyl chain-ends and of the methine protons

of the PLLA segments slightly decreased after the deprotection step. This either

suggests an increase in molecular weight or cleavage of benzyl groups.

Figure 5.9. Comparison of the 1H NMR spectra of copolymer before (referred to as

A) and after deprotection of the ketones in DCM at room temperature (referred to as

B).


ATR-FTIR spectroscopy confirmed the recovery of the ketone. The carbonyl region

(1850 – 1650 cm-1) of the ATR-FTIR spectra of the copolymer before and after the

deprotection of the ketone acetal is shown in Figure 5.10. The spectra shows two

distinct bands with maximum at 1745 and 1755 cm-1 respectively. However, these two

bands result from noise due to the resolution (4 cm-1) and the normalization process.

The ester band of poly(L-lactide-co-OPD) shows additional shoulders compared to

poly(L-lactide-co-TOSUO). The shoulder from 1730 to 1700 cm-1 corresponds to the

ketone stretching band of OPD repeating units.32 The shoulder around 1770 cm-1 could

be due to the carbonyl stretching of a butyrolactone-type product resulting from

intramolecular transesterification during the deprotection step. The shoulder observed

after the deprotection step remained small, suggesting a low extent of

transesterification. Such unwanted intramolecular transesterification was reported

before for crosslinked star-poly(OPD-co--CL) with terminal hydroxyl groups

attacking an internal ester function during ketone reduction step.21, 33

Figure 5.10. ATR-FTIR spectra of poly(LLA-co-TOSUO) and poly(LLA-co-OPD)

with an enlarged view of the carbonyl region (1850 – 1650 cm-1) revealing a


shoulder at 1715 cm-1 corresponding to the C=O stretching band of the ketone moity

of OPD, a shoulder at 1775 cm-1 corresponding to the carbonyl stretching of a

lactone-type product resulting from transesterification during the deprotection step.

5.2.3.3. Molecular Weights of Poly(L-Lactide-co-OPD)

The extent of degradation during the deprotection step was further investigated by

analysing the molecular weights of the different poly(L-lactide-co-OPD) polymers by

GPC in chloroform with a refractive index detector. The deprotection step did not

significantly alter the molecular weights as shown by the negligible difference between

the GPC traces (Figure 5.11).

Figure 5.11. Comparison of the GPC traces of the copolymer before and after

deprotection of the ketones, measured in chloroform (the traces were baseline-



Table 5.4. Molecular weights of copolymers before and after deprotection, as

measured by GPC in chloroform.

𝑀𝑛

(g·mol-1)

𝑀𝑤

(g·mol-1)

Đ

18 Protected 5,600 ± 800 6,500 ± 1,200 1.28 ± 0.06

18’ Deprotected 5,400 ± 400 6,710± 100 1.32 ± 0.01

20 Protected 6,100 ± 500 8,900 ± 100 1.47 ± 0.10

20’ Deprotected 5,500 ± 100 7,500 ± 100 1.37 ± 0.03

5.2.3.4. Thermal Properties of Poly(L-Lactide-co-OPD)

Thermal properties of the copolymers were analysed by DSC using the second heating

run after erasing the thermal history of the samples. Similar to the protected

copolymers (refer to Section 5.2.2.1.3.), the deprotected copolymers were

semicrystalline, featuring a glass transition, an exothermic peak assigned to cold

crystallization (Tcc), and an endothermic peak corresponding to melting (Tm). The

variations in glass transition were within the measurement error, while the other

thermal transitions appeared randomly with no specific trend (Table 5.5).

Table 5.5. Thermal transitions of poly(L-lactide-co-OPD) after the deprotection of

the ketone acetal groups (three measurements were performed and values were

averaged).

Entry Tg

(°C)

Tcc

(°C)

∆𝐻𝑐𝑐

(J·g-1)

Tm1

(°C)

Tm2

(°C)

∆𝐻𝑚

(J·g-1)

𝜒𝑐

(%)

18 49.5 ±

0.9

88.8 ±

0.4

14.3 ±

4.7

- 148.2 ±

1.7

49.1 ±

5.2

67.6 ±

9.8

18’ 49.6 ±

5.7

- - - 149.4 ±

0.4

69.1 ±

6.5

73.7 ±

7.0

20 43.7 ±

1.7

99.8 ±

2.1

27.9 ±

3.1

125.5 ±

1.4

139.9 ±

0.7

40.9 ±

0.9

73.4 ±

4.3

20’ 44.3 ±

1.5

94.9 ±

1.0

10.8 ±

2.8

131.0 ±

1.7

142.2 ±

0.9

39.7 ±

2.6

53.9 ±

4.9


Previous studies have shown that removal of the ethylene ketal protecting groups of

poly(ε-caprolactone-co-TOSUO) random copolymers induced a marked increase in

glass transition, melting temperature and melting enthalpies (-53 to -43 ºC; 46 to 64

ºC and 55.5 to 87.5 J·g-1, respectively).3, 4 The variations of glass transitions observed

in the literature could be explained by the Fox-Flory relationship. For instance, the

glass transition of PTOSUO was determined to be in the range -35 to -14 ºC (depending

on the number average molecular weight) whereas POPD displayed a Tg of 37 ºC.2, 4

A series of poly(L-lactide-co-TOSUO) were synthesized with various initial feeds of

TOSUO. Several low molecular weight copolymers were obtained with a maximum

incorporation level of 8 mol%. The low molecular weights resulted from remaining

initiator-derived impurities in the TOSUO monomer, as well as the difficulty to handle

tin (II) octanoate due to its high viscosity. A deprotection step afforded poly(L-lactide-

co-OPD) as confirmed by spectroscopic techniques.

5.2.4 Photodegradation of Poly(L-lactide-co-OPD)

Following the synthesis of copolymers of L-lactide and OPD, the prodegradant effect

of the ketone moieties incorporated into the polymer backbone was investigated with

respect to the photodegradation rate. Copolymers were artificially aged using a QUV

accelerated weathering tester (Q-lab, Ohio) under UV-A light at 50 °C for 240 hours.

As discussed in Section 5.2.2.3., the molecular weights of the synthesized copolymers

range between 2,500 and 6,000 g·mol-1. The obtained molecular weights were thus

below the entanglement molecular weight of PLLA, reported to be 8,000 – 10,000

g·mol-1.34 Attempts to obtain films via a solvent-casting method resulted in recovering

the polymer as a powder and films could not be produced. Therefore, the copolymers

were aged as powders between quartz plates, which were then mounted onto

aluminium holders in the QUV. Two poly(L-lactide-co-OPD) copolymers were

artificially aged to assess the prodegradant role played by the ketone moieties on the

evolution of the chemical structure and molecular weight with UV-A irradiation. The

copolymers featured different concentrations of OPD segments, 5.2 and 8 mol%, to

study the effect of this concentration on the photodegradation rate.

As reviewed in Chapter 1, PLLA features a carbonyl group in its backbone. Such

group absorbs UV irradiation at 280 nm via the n-π* transition with the corresponding

extinction coefficient ɛ at that wavelength less than 100 L·mol-1·cm-1.35 The low value


of ɛ induces a relative stability towards UV light. Thus, the aim herein was to

investigate the extent by which incorporated ketone moieties accelerate the

photooxidation rate Two PLLA-co-OPD copolymers featuring various concentrations

of OPD segments (5.2 and 8 mol%) were artificially aged under UV-A light at 50 ºC.

Samples were collected every two days to monitor the photodegradation by visual

observations, to assess the evolution of molecular weight and investigate any chemical

changes. In terms of visual observations, the copolymers were white powders before

irradiation and remained the same throughout the ageing process.

5.2.4.1. Changes of Molecular Weight

The evolution of molecular weight and polydispersity of irradiated poly(L-lactide-co-

OPD) with both 5.2 mol% (referred to as 18’) and 8 mol% (referred to as 18’)

incorporated OPD was assessed by GPC using tetrahydrofuran (THF) as the eluent.

The GPC traces of the copolymer 18’ as a function of irradiation time are shown in

Figure 5.12.


Figure 5.12. Evolution of the GPC traces of poly(L-lactide-co-OPD) (5.2 mol% OPD

segments) as a function of irradiation time, measured in THF (the traces were

baseline-corrected and normalized).

Before UV exposure, the copolymer was characterized by a unimodal trace. After two

days of irradiation, a shoulder appeared towards higher molecular weight. This

shoulder continuously increased with increasing irradiation time. The SEC traces of

poly(L-lactide-co-OPD) with 8 mol% incorporated OPD exhibited similar changes

(Figure 5.13). A shoulder appeared after two days irradiation with the intensity

increasing until four days irradiation, after which it remained constant.


Figure 5.13. Evolution of the GPC traces of poly(L-lactide-co-OPD) (8 mol% OPD

segments) as a function of irradiation time, measured in THF (the traces were

baseline-corrected and normalized).

In terms of molecular weights, the weight average molecular weights increased more

than the number average for both copolymers (Table 5.6). Broadening of the GPC

traces was also observed and shown by polydispersity values which increased for both

copolymers throughout the ageing process.

These observations suggest poly(L-lactide-co-OPD) undergoes changes at early stages

of the degradation process, with the broadening of the distributions after only two days

of irradiation in the QUV. The stable values of number average molecular weight

combined with the increase in weight average molecular weight and polydispersity

suggest crosslinking as the dominant pathway in the photooxidation of poly(L-lactide-

co-OPD).36 Under UV-visible light irradiation and air, polymers can undergo various

photodegradation reactions, from photooxidation to photolysis. Absorption of UV-


visible light by chromophores, either within the polymer structure (e.g. ketones) or as

defects, generates radicals that induce the formation of macroradicals by hydrogen

abstraction. Those macroradicals then react with oxygen to yield peroxide radicals.

Depending on the type of polymers, those peroxide radicals can propagate the

photooxidation mechanism and result in photolysis with chain cleavages and

formation of various photodegradation products (including hydroperoxides,

anhydrides, esters).37, 38 On the other hand, the macroradicals can recombined and

results in crosslinking. An increase in the weight average molecular weight and

broadening of the molar mass are characteristics of crosslinking.36, 39 As discussed in

Chapter 1, poly(L-lactide) did not undergo drastic modification when artificially aged

in the QUV under UV-A light at 50 ºC. Studies have shown that when exposed to UV-

A light in a Sepap 12.24 at 60 ºC up to 670 hours, PLLA photooxidation solely results

into chain scission and no crosslinking.19, 20 The occurrence of crosslinking revealed

by GPC could be attributed to the OPD segments in the copolymer.

Table 5.6. Evolution of the number and weight averaged molecular weights,

polydispersity and the chain scission of poly(L-lactide-co-OPD) as a function of


Incorporated

OPD

Irradiation (Days) 𝑀𝑛 (g·mol-1) 𝑀𝑤

(g·mol-1) Đ

5.2 mol% 0 5,100 ± 900 6,100 ± 1,000 1.26 ± 0.04

2 5,200 ± 300 7,000 ± 100 1.35 ± 0.07

4 5,500 ± 250 7,100 ± 100 1.30 ± 0.05

6 5,500 ± 100 7,300 ± 100 1.34 ± 0.01

8 5,500 ± 400 7,100 ± 0 1.31 ± 0.09

8 mol% 0 5,500 ± 100 7,500 ± 100 1.37 ± 0.03

2 5,800 ± 0 7,500 ± 0 1.29 ± 0.01

4 5,800 ± 200 7,900 ± 400 1.36 ± 0.01

6 5,500 ± 100 7,600 ± 100 1.38 ± 0.02

8 6,000 ± 300 7,200 ± 800 1.38 ± 0.01

5.2.4.2. Thermal Properties and Crystallinity

The variation in molecular weight of the copolymers throughout ageing were expected

to impact their thermal properties. Aged samples were characterized by DSC using a


heat / cool / heat cycle. When cooling down from the melt at 10 ºC·min-1, copolymers

with 5.2 (18’) and 8 mol% (20’) OPD exhibit different behaviours (thermograms not

shown here). The thermograms of poly(L-lactide-co-OPD) 18’ show an exothermic

peak assigned to crystallization before and after ageing, whereas no such transition is

observed for the copolymer 20’. This difference could be explained by different

crystallization rates between the two copolymers, with 20’ showing a slower rate of

crystallization than 18’.

After a second heating run to erase the thermal history of the samples, copolymer 18’,

before and after ageing, featured a glass transition (Tg) and an endothermic peak

corresponding to its melting temperature (Tm) (Figure 5.14). The initial glass

transition, 45.7 ± 1.9 ºC, was below the temperature used during the ageing process in

the QUV (50 °C). This suggests the polymer is in a rubbery state and therefore

facilitates oxygen diffusion within the polymer matrix during the ageing process.

Overall, the glass transitions were weak and hardly visible (Figure 5.15). The values

did not significantly vary during the ageing process with differences within the error

of the measurements. This lack of variation was expected based on GPC measurements

which showed no changes in 𝑀𝑛 with ageing since Tg is linked to 𝑀𝑛

according to the

Fox-Flory relationship:


𝑀𝑛

Where k is the Flory-Fox constant, 𝑇𝑔,∞ the glass transition of polylactide having an


average molecular weight.41 The 𝑀𝑛 values did not decrease with irradiation time,

therefore the glass transitions remained constant as well. Similar to Tg, there was no

noticeable shift towards lower temperatures for the melting peak. The melting

temperature also depends on the molecular weight and follows the relationship:

𝑇𝑚 = 𝑇𝑚,∞ −𝐴

𝑀𝑛

With A a constant, and 𝑇𝑚,∞ the melting temperature of PLA having an infinite

molecular weight.42


Figure 5.14. DSC thermograms from the second heating cycle of poly(L-lactide-co-

OPD) with 5.2 mol% OPD with increasing irradiation time in the QUV.


Figure 5.15. Enlarged view of the glass transitions in the DSC thermograms from the

second heating cycle of poly(L-lactide-co-OPD) with 5.2 mol% OPD with increasing


On the other hand, copolymer 20’ exhibits a glass transition (Tg), an exothermic peak

assigned to cold crystallization (Tcc), and a double endothermic peak corresponding to

melting temperatures (Tm) before and after ageing (Figure 5.16). The difference

between the thermograms of 18’ and 20’ could arise from different crystalline states,

with 18’ being more crystalline than 20’.43 As for 18’, the initial glass transition was

below the temperature used during the ageing in the QUV (50 °C). Overall, the glass

transitions did not significantly vary during the ageing process, with differences within

the error of the measurements.

The temperature of cold crystallization increased from 95.6 ± 0.7 °C, before

irradiation, to 100.9 ± 1.1 °C after ten irradiation days, as well as their associated

crystallization enthalpies (12.7 ± 0.8 to 24.8 ± 0.9 J·g-1). Such an increase in Tcc was

previously observed during the photooxidative degradation of PLA under UV-A light,


from 115 to 118 ºC after 400 hours in a Sepap 12.24 device. The formation of

additional nuclei of crystallization was suggested to induce that increase.20

Concerning the melting behaviour, 20’ exhibits a double melting peak that did not shift

to lower temperatures with irradiation days. This double behaviour was previously

reported for polylactide and was attributed to several possible reasons. It could be

caused by a melt-recrystallization process involving the melting of original crystals,

recrystallization and subsequent melting of recrystallized crystals.44, 45 Melting of

crystals featuring different lamellar thicknesses or melting of the crystalline phases α

and α’ are also reported explanations.46

Figure 5.16. DSC thermograms from the second heating cycle of poly(L-lactide-co-

OPD) with 8 mol% OPD with irradiation days in the QUV.

As shown by GPC measurements, an increase in molecular weight, which was likely

due to crosslinking occurred during the irradiation process. The formation of

crosslinked networks induced changes in the thermal properties of the resulting


polymer. Contrary to the results observed here, glass transition usually increases when

crosslinking occurs due to a variation of conformational entropy.47 For instance, films

of crosslinked PLLA with 3 wt% of triallyl isocyanurate, a crosslinking agent, were

characterized by an increase in Tg with irradiation time (electron beam irradiation at

room temperature). A correlation was observed between the crosslinking density and

glass transition, with higher Tg for higher crosslinking densities.48 The appearance of

a cold crystallization peak for aged PLLA was reported by Gardette and coworkers

who suggested that additional crystallization nuclei were formed during the

photodegradation.20 Concerning the melting temperature, the combined increase in gel

fraction and decrease in melting temperature was previously reported in the literature

for crosslinked polylactide.49, 50 The crystallization was inhibited by the crosslinked

network that restrained the mobility of the macromolecular chains, resulting in a lower

degree of crystallization and a lower melting enthalpy.49, 50

Table 5.7. Thermal properties of aged poly(L-lactide-co-OPD) copolymers

determined by DSC for samples before irradiation and after UV irradiation for 2-10

days (two measurements were performed and values were averaged).

UV

irradiation

(days)

Tg

(°C)

Tcc

(°C)

∆𝐻𝑐𝑐

(J.g-

1)

Tm1

(°C)

Tm2

(°C)

∆𝐻𝑚

(J.g-1)

𝜒𝑐

(%)

18

’

0 45.7 ±

1.9

- - 149.2

± 0.2

- 73.4 ±

3.0

2 43.4 ±

3.7

- - 147.3

± 1.1

- 63.7 ±

7.9

4 44.1 ±

1.6

- - 147.7

± 1.5

- 63.8 ±

2.1

6 50.1 ±

3.6

- - 148.6

± 0.2

- 57.3 ±

2.1

8 - - -

10 - - -

20

’

0 45.2 ±

1.0

95.6

± 0.7

12.7

± 0.8

130.1

± 1.3

141.6 ±

0.3

40.2 ±

3.0

56.5 ±

4.1


2 41.7 ±

1.9

93.3

± 2.0

15.5

± 3.0

127.2

± 0.8

139.3 ±

0.1

42.2 ±

2.9

61.6 ±

6.3

4 43.0 ±

1.3

96.3

± 1.8

22.1

± 6.0

127.5

± 1.1

139.3 ±

0.1

38.8 ±

0.1

69.2 ±

1.5

6 45.5 ±

0.1

99.4

± 0.3

26.5

± 1.4

128.7

± 0

139.3 ±

0.1

38.4 ±

0.1

69.2 ±

1.6

8 44.9 ±

0.9

98.2±

0.1

12.7

±0.8

130.1

± 1.8

141.6 ±

0.3

40.2 ±

3.0

56.5 ± 0

10 42.9 ±

3.6

100.9

± 1.1

24.8

± 0.9

125.0

± 4.7

138.6 ±

2.1

37.4 ±

2.1

66.3 ±

1.9

5.2.4.3. Chemical Structure of the Films

The chemical structure of the irradiated samples of poly(L-lactide-co-OPD) were

further analysed by ATR-FTIR spectroscopy. Nine spectra were collected for each

sample after every two days in the QUV, baseline-corrected, averaged and normalized

to the band at 1454 cm-1 (–CH3 bending) to eradicate any effect from differences in

contact with the ATR crystal and depth of penetration of the IR beam.

The averaged spectra of poly(L-lactide-co-OPD) (5.2 mol% OPD), before and after ten

days of UV irradiation in the QUV, are shown in Figure 5.17. No particular trends

were observed as highlighted by the carbonyl region (Figure 5.18). The shoulder in

the range 1720 - 1710 cm-1 seems to undergo little change before and after eight

irradiation days, and disappears after ten days in the QUV. This suggests a

disappearance of the ketone moieties within the copolymer. However, no absorbance

band of potential products resulting from these chemical changes could be detected by

ATR-FTIR spectroscopy for this copolymer. In comparison, the evolution of the

averaged spectra of aged poly(L-lactide-co-OPD) (8 mol% OPD), before and after UV

exposure, are illustrated in Figure 5.19. The carbonyl band changed throughout the

ageing process, with variation in absorbance (Figure 5.20). Any trend was hardly

observed between 1700 and 1730 cm-1, assigned to the OPD ketone. However, a

shoulder appears throughout the ageing process between 1830 and 1860 cm-1, with a

maximum at 1845 cm-1. This shoulder increased with increasing irradiation time. Such

a band has been previously attributed to anhydrides as the major photodegradation

products for irradiated PLLA.19, 20


Figure 5.17. ATR-FTIR average spectra of poly(L-lactide-co-OPD) (5.2 mol% OPD)

before and after ten days of UV irradiation (average of 9 spectra after baseline

correction and normalization to the -CH3 bending band at 1454 cm-1).


Figure 5.18. Enlarged view of the carbonyl band in the ATR-FTIR spectra of poly(L-

lactide-co-OPD) (5.2 mol% OPD): anhydride region 1900 - 1810 cm-1 and ketone

region 1740 - 1690 cm-1.


Figure 5.19. ATR-FTIR average spectra of poly(L-lactide-co-OPD) (8 mol% OPD)

powder before and after irradiation (average of 9 spectra after baseline correction and



Figure 5.20. Enlarged view of the carbonyl band in the ATR-FTIR spectra of poly(L-

lactide-co-OPD) (8 mol% OPD): anhydride region 1900 - 1810 cm-1 and ketone

region 1740 - 1690 cm-1.

5.2.5 Mechanism of Photodegradation

The photooxidation of two poly(L-lactide-co-OPD) was assessed under accelerated

artificial conditions. Both copolymers were characterized by lower values of molecular

weights than the entanglement molecular weight, preventing from obtaining them as

films. The copolymers were then aged as white powders. No visual sign of degradation

(change of colour) was observed during the ten irradiation days. However, GPC

analysis revealed changes at early stages of the UV irradiation process, with the

apparition of a shoulder towards the high molecular weights. This shoulder increased

with irradiation days for both copolymers. Although number average molecular

weights remained stable, both weight average molecular weights and polydispersities

increased throughout ageing. These results suggest little or no chain scission occurred,

while crosslinking seemed to predominantly happen and increase with irradiation days.

Along with the GPC results, ATR-FTIR spectroscopy showed the disappearance of the


1730 – 1700 cm-1 shoulder, assigned to ketone moieties, for the copolymer with 5.2

mol% incorporated OPD, while the apparition of a shoulder at 1845 cm-1 for the

copolymer with 8 mol% incorporated OPD.

As reviewed in Chapters 1 and 2, ketone moieties undergo chain scissions via Norrish

type I and II processes. As reviewed in Chapter 2, the absence of a hydrogen on the

carbon γ to the ketone in the OPD monomer reduced the likelihood of a contribution

of a Norrish type II mechanism. However, incorporated OPD could potentially

undergo both pathways as shown in Scheme 5.8. If following the Norrish type I, ring-

opened OPD would undergo α-cleavage of the ketone to give two free radical

intermediates, including an acyl radical. Acyl radicals can eliminate carbon monoxide

to give alkyl radicals. On the other hand, the Norrish type II would involve an

intramolecular hydrogen abstraction from the carbon γ to the carbonyl group. This

abstraction is followed by the cleavage of the α-β C-C bond resulting in the formation

of an enol and a terminal C=C double bond. The enol subsequently tautomerizes to the

more stable ketone.53 Regarding poly(L-lactide), the photooxidation under UV-A light

resulted in chain scissions, reducing the molecular weight, and anhydrides as major

degradation products, as confirmed by the presence of a band at 1845 cm-1 in the IR

spectrum.21, 22

In the work described here, the ketone of ring-opened OPD was expected to undergo

photocleavage via Norrish type reactions, generating radicals. Such radicals could

recombine due to cage effect, resulting in the crosslinking events demonstrated by

GPC. For the copolymer with 8 mol% incorporated OPD, some radicals potentially

attacked the PLLA segments to initiate the photooxidation mechanism by abstracting

hydrogen and forming anhydrides as degradation products, as revealed by ATR-FTIR

spectroscopy.


Scheme 5.8. Norrish type I and II cleavages of the ketone of ring-opened OPD.

5.3 SUMMARY

OPD was investigated as a photoprodegradant to enhance the photodegradation rate of

random copolymers of poly(L-lactide-co-OPD). Following the findings presented in

Chapter 4, a modified OPD with ketal protecting groups was selected and

copolymerised with L-lactide. Synthesis of random copolymers of L-lactide and

modified OPD was performed in the bulk at 110 ºC, using tin (II) octanoate and benzyl


alcohol as catalyst and initiator respectively. Copolymers with TOSUO incorporation

up to 12.7 mol% were obtained. Additional initiating species resulted in lowering of

the expected molecular weights, with values lower than the entanglement molecular

weight obtained. A subsequent deprotection step of the TOSUO units enabled the

recovery of the ketone moieties without significantly lowering the molecular weights.

Subsequently, two poly(L-lactide-co-OPD), featuring similar molecular weight and

two concentrations of incorporated OPD (5.2 and 8 mol%), were artificially aged under

UV-A light in the QUV. GPC analysis revealed crosslinking events occurred at early

stages of the photooxidation process, and increased with irradiation time. ATR-FTIR

spectroscopy analysis revealed the apparition of anhydrides as degradation products.

The photooxidation was proposed to be initiated by the cleavage of the ketone moieties

via Norrish type reactions, generating radicals. Such radicals could either recombine

leading to crosslinking, or initiate the photooxidation pathway of the poly(L-lactide)

segments.

5.4 EXPERIMENTAL

5.4.1 Material

1,4-Cyclohexanedione monoethyleneketal (97 %), 3-chloroperbenzoic acid (≤ 77 %),

tin (II) octanoate (95 %), and triphenylcarbenium tetrafluoroborate were purchased

from Sigma-Aldrich and used as received. L-lactide ((3s)-cis-3.6-dimethyl-1,4-

dioxane-2,5-dione) (98 %) was purchased from Sigma-Aldrich, recrystallized twice

from toluene and dried under vacuum prior to use. Hexane, ethyl acetate, and

dichloromethane were all AR grade, purchased from ChemSupply and used as

received.

5.4.2 Methods

5.4.2.1. Synthesis of 1,4,8-Trioxaspiro[4.6]-9-Undecanone


m-Chloroperbenzoic acid (4.44 g, 25.7 mmol, 2.0 equiv.) was dissolved in 110 mL

DCM and dried over anhydrous Na2SO4. The solution was filtered into a 250 mL flask.

1,4-Cyclohexanedione monoethyleneketal (2 g, 12.8 mmol) was slowly added to the

mCPBA solution under stirring. The solution was refluxed at 40 °C for 24 h before

allowing to cool to ambient temperature. The solution was washed with brine. The

organic layer was collected, dried over anhydrous Na2SO4 and concentrated under

reduced pressure. The product was purified by column chromatography using hexane

/ ethyl acetate (65 / 35). TOSUO was recovered as an oil in 39 - 40 % yield. Mp. 44-

45 °C (Lit., 49-51 °C1). 1H NMR (CDCl3, 600 MHz), δ ppm = 4.28 (t, 2H, C=OOCH2),

3.98 (s, 4H, C-O-CH2CH2O), 2.69 (t, 2H, CH2C=OOCH2), 2.00 (t, 2H, C-OCH2CH2O-

CH2CH2), 1.89 (t, 2H, C-OOCH2CH2C-O).13C NMR (CDCl3, 600 MHz), δ ppm =

175.4 (C=O), 107.9 (C-OCH2CH2O-), 64.7 (C-OCH2CH2O-), 64.3 (C=OOCH2), 39.1

(C=OOCH2CH2), 32.7 (C-OCH2CH2O-CH2CH2), 28.8 (CH2C=OOCH2). ATR-FTIR:

ʋ max = 2990 (w, -CH2 stretching), 1724 (s, -C=O carbonyl stretching of the lactone),

1454 (-CH3 bending), 1121 and 1094 (s, -C-O- stretching).

5.4.2.2. Synthesis of Poly(L-lactide-co-TOSUO) in the bulk

All glassware was oven dried overnight at 80 °C. L-lactide, TOSUO and benzyl alcohol

were introduced into a Schlenk vessel under inert atmosphere in a glovebox. Tin (II)

octanoate was added to the Schlenk vessel under a nitrogen atmosphere (Table 5.8).

The Schlenk vessel was then sealed under an inert atmosphere and immersed into a

preheated silicone oil bath at 110 °C. The polymerisations lasted from one to two days

O

O

O

O


depending on the conditions. The polymerisations were thermally quenched when the

stirring was no longer working because of an increased viscosity. The polymeric

mixture was dissolved in THF. A crude fraction was kept for the determination of the

monomer conversion. The rest was reprecipitated into cold cyclohexane (10 fold

excess). The purification step was repeated until the unreacted monomer was

completely removed.

Table 5.8. Conditions of the ROP of L-lactide and TOSUO in the bulk at 110 °C.

Compound 18: White polymer. Purification yield: 55.3 %. 1H NMR (CDCl3, δ ppm,

subscripts L-LA and TOSUO denote each repeating units): 7.33 (m, 6H, CHBnO), 5.17

(q, H, CHL-LA), 4.36 (m, H, -CHCH3-OHL-LA-end group), 4.22 (t, 2H, CH2OTOSUO), 3.93

(s, 4H, OCH2CH2TOSUO), 2.46 (t, 2H, OC=OCH2TOSUO), 1.99 (m, 4H, CH2C-

OCH2CH2O-CH2TOSUO), 1.58 (d, 3H, CH3L-LA). 13C NMR (CDCl3, δ ppm): 170.0 (-

C=OL-LA, -C=OTOSUO), 128.6 (-C=CBnO), 69.4 (-C-CH3L-LA), 67.1 (-CO-CH2-CH2-

OCTOSUO), 57.5 (C=OOCH2CH2TOSUO), 27.3 (OC=OCH2CH2TOSUO), 17.0 (-C-CH3L-LA).



(-CH- deformation, symmetric and asymmetric), 1180 and 1084 (-C-O- stretching),



ºC, Tcc = 88.8 ± 0.4 ºC (∆𝐻𝑐𝑐 = 14.3 ± 4.7 J·g-1), Tm = 148.2 ± 1.7 ºC (∆𝐻𝑚 = 49.1 ±

5.2 J·g-1).




code Initial TOSUO

content

L-lactide TOSUO Benzyl

alcohol

mol% g mmol mg mmol μL μmol

18 4.42 1.484 10.30 78.30 0.4552 10 96.17

19 9.60 0.9947 6.901 113.9 0.6622 10 96.17

20 15.3 1.489 10.33 271.5 1.579 10 96.17

21 24.4 1.007 6.987 293.3 1.705 10 96.17










(Ð) = 800 ± 0 g·mol-1 (1.52 ± 0.01). DSC (second heating cycle): Tg = 35.3 ± 2.0 ºC,

Tcc = 100.0 ± 2.0 ºC (∆𝐻𝑐𝑐 = 1.1 ± 0.4 J·g-1), Tm = 113.9 ± 1.3 ºC (∆𝐻𝑚 = 1.6 ± 0.8

J·g-1).














0.7 ºC (∆𝐻𝑚 = 40.9 ± 0.9 J·g-1).



(q, H, CHL-LA), 4.28 (t, 2H, CH2OTOSUO), 3.94 (s, 4H, OCH2CH2TOSUO), 2.45 (t, 2H,

OC=OCH2TOSUO), 2.00 (m, 4H, CH2C-OCH2CH2O-CH2TOSUO), 1.58 (d, 3H, CH3L-LA).

13C NMR (CDCl3, δ ppm): 169.5 (-C=OL-LA, -C=OTOSUO), 128.2 (-C=CBnO), 67.2 (-C-

CH3L-LA), 65.0 (-CO-CH2-CH2-OCTOSUO), 63.4 (C=OOCH2CH2TOSUO), 35.8

(C=OOCH2CH2TOSUO), 26.9 (OC=OCH2CH2TOSUO), 16.6 (-C-CH3L-LA). ATR-FTIR: ʋ

max = 2994 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric),


1747 (s, -C=O carbonyl stretching), 1454 (-CH3 bending), 1384 and 1359 (-CH-

deformation, symmetric and asymmetric), 1182 and 1085 (-C-O- stretching), 1043 (-

OH bending), 871 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) =

4,700 ± 900 g·mol-1 (1.28 ± 0.06). DSC (second heating cycle): Tg = 40.1 ± 0.2 ºC, Tcc

= 88.8 ± 0.3 ºC (∆𝐻𝑐𝑐 = 15.2 ± 0.9 J·g-1), Tm1 = 128.1 ± 0 ºC, Tm2 = 143.2 ± 0 ºC (∆𝐻𝑚

= 43.9 ± 0 J·g-1).

5.4.2.3. Synthesis of Poly(L-lactide-co-OPD)

In a typical deprotection reaction, poly(L-lactide-co-TOSUO) (531.6 mg, 186.1 μmol,

TOSUO 6.701 μmol) was dissolved in 1 mL DCM. Trityl fluoroborate (2 equivalents,

4.4 mg, 13 μmol) was dissolved in 0.3 mL DCM and added to the copolymer solution.

The yellow solution was stirred for 2 hours at room temperature. The solution was then

reprecipitated in cold methanol (10 fold excess) as a white powder.

Compound 18’: White polymer. Yield: 84.6 %. 1H NMR (CDCl3, δ ppm, subscripts

L-LA and OPD denote each repeating units): 7.33 (m, 6H, CHBnO), 5.16 (q, H, CHL-

LA), 4.35 (t, 2H, -CH2OOPD), 2.80 (m, 2H, CH2C=OOPD), 2.66 (m, 2H, C=OCH2OPD),

1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric),

2948 (w, -CH2 stretching, symmetric), 1748 (s, -C=O carbonyl stretching) with a

shoulder from 1730 to 1700 cm-1, 1454 (-CH3 bending), 1383 and 1359 (-CH-



5,400 ± 400 g·mol-1 (1.32 ± 0.01). DSC (second heating cycle): Tg = 49.6 ± 5.7 ºC, Tm

= 149.4 ± 0.4 ºC (∆𝐻𝑚 = 69.1 ± 6.5 J·g-1).

Compound 20’: White polymer. Yield: 84.6 %. 1H NMR (CDCl3, δ ppm, subscripts

L-LA and OPD denote each repeating units): 7.33 (m, 6H, CHBnO), 5.16 (q, H, CHL-

LA), 4.35 (t, 2H, -CH2OOPD), 2.80 (m, 2H, CH2C=OOPD), 2.66 (m, 2H, C=OCH2OPD),

1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric),

2948 (w, -CH2 stretching, symmetric), 1749 (s, -C=O carbonyl stretching) with a

shoulder from 1730 to 1700 cm-1, 1453 (-CH3 bending), 1382 and 1359 (-CH-




5,500 ± 100 g·mol-1 (1.37 ± 0.03). DSC (second heating cycle): Tg = 44.3 ± 1.5 ºC, Tcc

= 94.9 ± 1.0 ºC (∆𝐻𝑐𝑐 = 10.8 ± 2.8 J·g-1), Tm1 = 131.0 ± 1.7 ºC, Tm2 = 142.2 ± 0.9 ºC

(∆𝐻𝑚 = 39.7 ± 2.6 J·g-1).

5.4.2.4. Accelerated Photo Ageing

The copolymer powders were placed between quartz plates and mounted onto 35 mm

aluminium slide holders and exposed to UV-A 340 lamps at an irradiance of 0.68

W/m2 at 340 nm in a QUV accelerated weathering tester (Q-lab, Ohio). Water was

present in a tray at the bottom of the QUV chamber in order to maintain maximum and

consistent levels of humidity for the degradation study. The QUV was operated at a

black panel temperature of 50 °C and cycles lasted for 24 h. The irradiance sensors

were calibrated every 500 hours.


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222 Chapter 6: Conclusions and Future Research Directions

Chapter 6: Conclusions and Future Research

Directions

6.1 CONCLUSIONS

As a wider strategy towards controlling the degradation of biodegradable polymers for

limiting and reducing pollution, the main aim of this PhD research project was to tune

the photodegradability of a biodegradable polyester by employing a photosensitizing

molecule, 2-oxepane-1,5-dione (OPD), as an additive mixed with commercial polymer

and as a monomer to be copolymerized..

Films of blended commercial grade poly(L-lactide) and OPD synthesised here were

produced via a solvent-casting process with an initial OPD concentration ranging from

0 to 10 wt%. The films were artificially aged in a QUV accelerated weathering device

using conditions relevant to outdoor natural conditions; UV-A light at 50 °C. OPD as

an additive led to accelerated photodegradation of the polymeric blends, as revealed

by drastic decreases in molecular weights as a result of chain scissions. The number of

chain scissions increased with the initial OPD concentration, with the

photodegradation proposed to be initiated by the cleavage of OPD via a Norrish type

I mechanism, resulting in the formation of initiating radicals that further attacked the

macromolecular chains.

Following these results, the photosensitizing effect of OPD was investigated when it

was incorporated into the PLLA backbone as a copolymer. In-melt modification

studies were carried out initially to try to prepare PLLA - OPD copolymers using

reactive extrusion of commercial grade PLLA and OPD. The incorporation was

expected to occur due to transesterification reactions, catalysed by tin (II) octanoate.

However, the purified extrudates were only composed of PLLA, demonstrating the

absence of transesterification with OPD, independent of the initial feeds, the residence

time or the transesterification catalyst. Subsequent thermal analysis of OPD revealed

that thermal degradation of the compound occurred at temperatures close to 160°C,

which is expected to have limited the success of the in-melt modification. In order to

avoid thermal degradation of OPD, direct copolymerisation of OPD with L-lactide

Chapter 6: Conclusions and Future Research Directions 223

monomer was investigated. Various conditions were trialled, and included

polymerisation in the bulk at 110 °C with tin (II) octanoate as the catalyst, and in

solution at room temperature with DBU and benzyl alcohol as the catalyst and initiator,

respectively. The tin-catalysed ring-opening polymerisations of L-lactide and OPD

resulted in only very low amounts of incorporated OPD within the PLLA structure. It

is proposed that the ketone moiety of OPD hindered the polymerisation of both

monomers by reacting with the tin (II) octanoate catalyst to form a complex. Regarding

the other catalytic system investigated (DBU and benzyl alcohol), it was found that the

nucleophilicity of DBU was high enough to efficiently polymerize L-lactide, while not

being nucleophilic enough to polymerize the lactone-type monomer, OPD.

In order to increase the amount of OPD incorporated into the PLLA backbone, further

work focussed on copolymerising L-lactide with a functionalised OPD monomer,

TOSUO, which contained ketone acetal groups to protect the ketone. L-lactide-

TOSUO copolymers were successfully synthesized, with TOSUO incorporation

ranging from 4.8 to 12.7 mol%. Subsequent deprotection of the ketone acetals using

triphenylcarbenium tetrafluoroborate afforded several poly(L-lactide-co-OPD)

copolymers. Photodegradation studies were carried out on deprotected copolymers in

the QUV using UV-A light at 50 °C and showed that cross-linking of the copolymers

occurred, which is in contrast to the scission mechanism found when OPD was used

as a non-polymerized additive.

It has been found here that the method of incorporation of 2-oxepane-1,5-dione, OPD,

into poly(L-lactide) has a significant effect on the photodegradation mechanism. When

used as an additive physically blended with poly(L-lactide) up to 10 wt%, OPD

drastically decreased the molecular weight via chain scissions, resulting in a

accelerated embrittlement of the blend films during irradiation compared to neat

PLLA. However, when incorporated into poly(L-lactide) as a copolymer at 5.2 and 8

mol%, UV irradiation resulted in crosslinking, as revealed by GPC measurements.

Based on its structure and the photochemistry of related compounds, OPD was

expected to undergo cleavage through a Norrish type I mechanism, to form carbon-

centred radicals that could attack the PLLA macromolecular chains, leading to chain

scissions. This was found to be the case where OPD was used as a blend, but the

contrary result found for poly(L-lactide-co-OPD) copolymers suggests that these

radicals, or their radical by-products recombined between copolymer molecules,

224 Chapter 6: Conclusions and Future Research Directions

resulting in crosslinking. In both cases, OPD was found to accelerate the rate of

photodegradation of PLLA, with the differing mechanisms of degradation found for

blends and copolymers providing scope for tuning the photodegradability of PLLA

polymers via alteration of the method of OPD incorporation.

6.2 FUTURE RESEARCH DIRECTIONS

Following the findings on the photosensitizing potential of 2-oxepane-1,5-dione when

physically mixed with commercial grade PLLA, or used as a copolymer, films of

similar formulations could be aged under outdoor natural conditions to confirm the

photodegradation mechanisms found using accelerated laboratory studies. In addition,

monitoring the mechanical properties during the ageing study would provide a

quantitative assessment of property change until embrittlement to guide their potential

use as degradable packaging films.

The biodegradation rate of poly(L-lactide) is impacted by the temperature, the relative

humidity and the type of microorganisms present in the environment. However, the

influence of OPD incorporation is unknown. Comparative studies could be performed

between PLLA and PLLA - OPD copolymers and blends placed in compost (where

the conditions are controlled) and in natural environments so that the influence of OPD

on the biodegradation of PLLA can be assessed.

Further work regarding the photodegradation of poly(L-lactide-co-OPD) copolymers

could focus on varying the concentration of incorporated OPD and artificially ageing

the copolymers to determine what the effect of a wider concentration range of OPD is

on crosslinking and chain scission mechanisms of degradation.

Appendices 225

Appendices

CHAPTER 2

1H NMR spectrum of OPD

DSC thermograms of OPD

226 Appendices

DSC thermograms of the transparent and opaque sections of the PLLA-OPD 10

wt% film

ATR-FTIR average spectra of PLLA –OPD 4 wt% film before and after one

and ten irradiation days (average of 9 spectra after baseline correction and


Appendices 227

ATR-FTIR average spectra of PLLA –OPD 6 wt% (top) and PLLA - OPD 8

wt% (bottom) films before and after one and ten irradiation days (average of 9

spectra after baseline correction and normalization with the -CH3 bending band

at 1455 cm-1).

228 Appendices

UV-visible spectra from the PLLA – OPD 2 wt% (top) and PLLA – OPD 4 wt%

(bottom) films as a function of irradiation time in the QUV.

Appendices 229

UV-visible spectra from the PLLA – OPD 6 wt% (top) and 8 wt% OPD

(bottom) films as a function of irradiation time in the QUV.

230 Appendices

CHAPTER 3

DSC thermograms of polyOPD

DSC thermograms from the cooling cycle of purified extrudate of PLLA – tin

(II) octanoate formulation without OPD

Appendices 231

DSC thermograms from the cooling cycle of purified extrudates of PLLA - OPD

(top: 5 wt% OPD; bottom: 10 wt% OPD).

232 Appendices

DSC thermograms from the cooling cycle of purified extrudates of PLLA –

OPD 15 wt%

Appendices 233

DSC thermograms from the cooling cycle of double-purified extrudates after 20

(top) and 40 minutes (bottom) residence times.

234 Appendices

DSC thermograms from the cooling cycle of double-purified extrudates after 60

(top) and 80 minutes (bottom) residence times.

Appendices 235

CHAPTER 4

DSC thermograms of compound 5 (top) and compound 6 (bottom).

236 Appendices

DSC thermograms of red crystals.

GPC traces of the homopolymerisation of L-lactide using DBU and benzyl

alcohol measured in chloroform (the traces were baseline-corrected and

normalized).

Appendices 237

CHAPTER 5

DSC thermogram of TOSUO

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