Thermal degradation of trimethylolpropane/adipic acid hyperbranched poly(ester)s

Thermal degradation of trimethylolpropane/adipic acidhyperbranched poly(ester)s

Tracy Zhang • Bob A. Howell • Patrick B. Smith

Received: 13 September 2013 / Accepted: 9 January 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract Hyperbranched poly(ester)s offer attractive

features for applications in a number of areas, particularly

as platforms for the support of controlled release actives in

the agricultural and biomedical fields. Such materials have

been generated from trimethylolpropane and adipic acid,

and fully characterized using chromatographic, spectro-

scopic, and thermal methods. The thermal stability of these

polymers has been assessed using thermogravimetry and

infrared spectroscopy. The degradation characteristics of

these materials have been compared to those of two linear

adipic acid polymers. The prominent feature of the thermal

degradation of the hyperbranched poly(ester)s is ether

formation while that for the comparable linear poly(ester)s

is ester pyrolysis resulting in chain scission.

Keywords Poly(ester) degradation � Dehydrative

crosslinking � Infrared monitoring � Ether formation � Ester

pyrolysis � Thermogravimetry

Introduction

Hyperbranched polymers are more readily accessible and

much less expensive than their structurally more uniform

counterparts, the dendrimers [1]. Attractive structural fea-

tures of these materials are high branching density, three-

dimensional shape, and most importantly for many

applications, multiple terminal groups [1, 2]. The identity

and number of end groups may be controlled by either the

relative stoichiometry of reactants or by the utilization of

monomers containing functional groups of non-equal

reactivity [3–5]. In either case, the extent of reaction must

be kept below that at which significant crosslinking (gela-

tion) occurs. Among hyperbranched systems, poly(ester)s

are particularly attractive for a number of applications

including controlled release of actives in the agricultural,

personal care, biological, and medical fields. They are also

effective as blend compatibilizers and as structures for

catalyst development. They have been widely used for drug

delivery and tissue engineering [6, 7]. Numerous novel

methods have been developed for the synthesis of these

materials [4, 8–13]. In this case, hyperbranched polymers

derived from trimethylolpropane [2-hydroxymethyl-1,3-

propanediol] and adipic acid [hexanedioic acid] have been

synthesized, and fully characterized using chromatographic,

spectroscopic, and thermal methods. The thermal degrada-

tion of these materials has been examined using thermo-

gravimetry and infrared spectroscopy. The degradation

characteristics of these branched polymers have been

compared to those to two linear poly(ester)s, one derived

from adipic acid and 1,3-propanediol and the other from

adipic acid and 1,10-decanediol.

Experimental

General

Polymerizations were in general carried out in dry glassware

in a nitrogen atmosphere. Infrared spectra were obtained by

ATR using a Thermo Scientific Nicolet 380 FT-IR spectro-

photometer. Absorptions were recorded in wavenumbers

T. Zhang � B. A. Howell (&)

Science of Advanced Materials Program, Department of

Chemistry, Cener for Applications in Polymer Science, Central

Michigan University, Mt. Pleasant, MI 48859, USA

e-mail: [email protected]

T. Zhang � P. B. Smith

Michigan Molecular Institute, Midland, MI 48640, USA

123

J Therm Anal Calorim

DOI 10.1007/s10973-014-3656-z

(cm-1) and absorption intensities were classified in the usual

fashion as very weak (vw), weak (m), medium (m), strong (s),

and very strong (vs) relative to the strongest band in the

spectrum. Nuclear magnetic resonance (NMR) spectra were

obtained using a 5–15 % solution in deuterochloroform using

a Varian INOVA 500 MHz spectrometer. Proton and carbon

chemical shifts are reported in parts-per-million (d) with

respect to tetramethylsilane (TMS) as an internal reference

(d = 0.00). Mass spectra were obtained using electrospray

ionization (ESI/MS) with a Waters Associates LCT Premier

XE instrument interfaced with a Waters Acquity Ultra Per-

formance Liquid Chromatograph or matrix assisted laser

desorption time of flight mass spectrometry (MALDI-TOF

MS) and a Bruker Daltonics Autoflex unit. A MALDI matrix

was determined experimentally but was most usually 2,5-

dihydroxybenzoic acid. Thermal transitions were determined

by differential scanning calorimetry (DSC) using a TA

instruments Q2000 instrument. Samples, contained in stan-

dard aluminum pans, were analyzed at heating rate of 5 or

10 �C min-1. Thermogravimetry was performed using a TA

instruments Q500 instrument. Typically, a heating rate of

5 �C min-1 was used. Samples (4–10 mg) were contained in

a platinum pan. The sample compartment was purged with dry

nitrogen at 50 cm3 min-1 during analysis. TA Universal

Analysis software was used for data analysis. Size exclusion

chromatography (SEC) was performed using a Waters 1525

liquid chromatography equipped with two Agilent PL gel

3 lm MIXED-E columns in series and a Waters 410 refrac-

tive index detector in series with a Wyatt Technologies

DAWN Heleos-II light scattering detector. The solvent was

THF at a flow rate of 1 mL min-1. The sample concentration

was 5 mg mL-1.

Materials

Common solvents and reagents were obtained from Ther-

moFisher Scientific or the Aldrich Chemical Company.

Trimethylolpropane, adipic acid, 1,10-decanediol, and 1,3-

propanediol were from Aldrich.

Synthesis

Hyperbranched poly(ester)

The adipic acid/trimethylolpropane hyperbranched poly(e-

ster) was prepared using two methods.

Solution polymerization

Into a dry 250-mL, three-necked, round-bottomed flask

equipped with a mechanical stirrer and a Soxhlet extractor

filled with 40 g of 4 A molecular sieves and fitted with a

condenser bearing a gas-inlet tube, was placed a solution of

10.02 g (74.5 mmol) of trimethylolpropane, 8.16 g

(55.6 mmol) of adipic acid and 0.48 g (2.78 mmol, 2.5 mol%

based on the number of reactive carboxyl groups present) of

Exo

0.0

–0.1

–0.2Hea

t Flo

w/W

g–1

–0.3–80 –60 –40 –20 0

Temperature/°C

20 40 60 80 100

Fig. 1 DSC curve or a trimethylolpropane/adipic acid hyperbranched

poly(ester)

90

80

70

60

50

40

30

20

10

4000 3000 2000 1000

Tran

smitt

ance

/%

Wavenumber/cm–1

Fig. 2 Infrared spectrum for a trimethylolpropane/adipic acid hyper-

branched poly(ester)

4’m & 4’d

4’t

4m

4dA B

2

1

4m 4’m

4m

4’d 4’d

4d

4’t 4’t

4’t

12

4 4

4

3 A

B A

B

4.5 4.0 3.5 2.0 1.01.5 0.5 ppm2.53.0

O

OCHO

HO

O O

O

O

O

O

O

O

nOCCO

HO

O

O

O

OC

CO

CO

Fig. 3 Proton NMR spectrum of a trimethylolpropane/adipic acid

hyperbranched poly(ester)

T. Zhang et al.

123

p-toluenesulfonic acid in 100 mL of anhydrous dioxane. The

solution was stirred at solvent reflux with the distillate being

cycled through the sieves to remove water formed as the

esterification proceeded. After 20 h, heating was discontin-

ued. The reaction solution was concentrated under reduced

pressure. The concentrated solution was then added dropwise

with vigorous stirring and mixing into 500 mL of water. A

honey-like product separated and the water-solvent liquid

phase was decanted. The polymer was dissolved in chloro-

form and the solution was dried over magnesium sulfate. The

solvent was removed by evaporation at reduced pressure to

provide the polymer as a colorless viscous liquid. The molar

mass of the hyperbranched polyester was about

5,000 g mol-1. The conversion was about 60 %: 1H NMR

(d, CDCl3) 0.75–0.78 (t, CH3), 1.19–1.33 (m, CCH2CH3),

1.54–1.56 (m, OCOCH2CH2), 2.24–2.24 (OCOCH2CH2).

3.33, 3.45 (s, HOCH2 with mono or di ester substituted), 3.55

(s, HOCH2, unreacted TMP), 3.95 (s, COOCH2 with alcohol

mono or di ester substituted), 4.01 (s, COOCH2 with all

three –OH substituted); 13C NMR (d, CDCl3) 7.49–7.56

(CH3), 22.40–22.54 (CCH2CH3), 24.45–24.55 (OCOCH2

CH2), 33.86–34.03 (OCOCH2CH2), 40.82–42.8 (quaternary

carbon atom), 62.22–65.22 (CH2 adjacent to –OH or alcohol

ester substituted), 173.27–174.20 (carbonyl ester group); IR

(ATR, cm-1) 3299 (O–H stretch), 2960, 2865

(C–H saturated), 1733 (C=O ester), 1175 (C–O stretch).

Melt polymerization


fitted with a magnetic stirring bar and a condenser bearing

a gas-inlet tube, was placed 5.02 g (37.4 mmol) of trime-

thylolpropane, 4.08 g (27.7 mmol) of adipic acid, 0.24 g

(1.39 mmol, 2.5 mol% of the reactive carboxyl groups

present) of p-toluenesulfonic acid and 5.0 g of anhydrous

magnesium sulfate to act as a water scavenger. The flask

was mounted in an oil bath maintained at 140 �C and the

mixture was stirred. Periodically aliquots of the mixture

were removed for analysis by GPC. Typically a molar mass

of 7,000–8,000 g mol-1 was achieved within 3 h. The

crude product was dissolved in chloroform. The drying

agent was removed by filtration and the polymer was

purified as described above. The conversion was about

70 %. The characterization data was the same as that for

the material obtained from solution polymerization.

12

4 4

43 A

B A

BB

1

2

A

4

3

C=O

180 160 140 120 100 80 60 40 20 ppm

O

O

O

O

O

n

O

Fig. 4 Carbon-13 NMR spectrum of a trimethylolpropane/adipic acid

hyperbranched poly(ester)

O

O

O

O

O

O

O

O

OO

OO

O

O

O

O

O

O

O

OO

OO

O

O

O

O

O

O

O

O

O

O

O

O

O

OH

OH

OH

OH

OHOH

OH

OH

OH

OH

OH

OH

HO OH

HO

HO

O

O

OH

Excess TMP Adipic acid (AA)

Scheme 1 Synthesis of trimethylolpropane/adipic acid hyperbranched poly(ester)

Thermal degradation of trimethylolpropane/adipic acid hyperbranched poly(ester)s

123

Linear poly(ester)s

Two linear adipic acid polymers, poly(decylene glycol

adipate) and poly(propylene glycol adipate), were prepared

using procedures analogous to those used for the prepara-

tion of the hyperbranched poly(ester)s.

Poly(decylene glycol adipate)


equipped with a mechanical stirrer and Soxhlet extractor filled

with 40 g of 4 A molecular sieves and fitted with a condenser

Tg = 42 °C

Exo

–1

–2

–3

–4–100 –50 0 50 100 150

1

0

Hea

t flo

w/W

g–1

Temperature/°C

Fig. 5 DSC curve for poly(decaylene adipate)

90

100

80

70

60

50

40

30

20

10

4000 3000 2000 1000

Tran

smitt

ance

/%

Wavenumber/cm–1

Fig. 6 Infrared spectrum of poly(decylene adipate)

1 AB & 2

3 – 5

A B

O

C CH2 22 2 2CH CH CH 2CH 2CH 2CH 2CH 2CH 2CH 2CH 2CH 2CHC CH

O

- - - - - - - - - - - - - - - -OO

1 2 3 4 5

8 7 6 5 4 3 2 1 ppm9

n

Fig. 7 Proton NMR spectrum of poly(decylene adipate)

C=O

1A

2, 4 & 5

3B

A B 1 2 3 4 5

O O

C CH2 22 2 2CH CH CH C CH- - - - - -O 2 2CH 2CH 2CH 2CH 2CH 2CH 2CH 2CH 2CH- - - - - - - - - -On

180 160 140 120 100 80 60 40 20 ppm

Fig. 8 Carbon-13 NMR spectrum of poly(decylene adipate)

Tg = –55 °C

Exo

Hea

t flo

w/W

g–1

Temperature/°C

–0.2

0.2

0.0

–0.4

–0.6

–0.8

–1.0

–1.2–100 –50 0 50 100

Fig. 9 DSC curve for poly(propylene adipate)

90

100

80

70

60

50

40

30

20

10

4000 3000 2000 1000

Tran

smitt

ance

/%

Wavenumber/cm–1

Fig. 10 Infrared spectrum of poly(propylene adipate)

T. Zhang et al.

123

bearing a gas-inlet tube, was placed a solution of 14.02 g

(95.9 mmol) of adipic acid and 16.71 g (95.8 mmol) of 1,10-

decanediol and 0.825 g (4.79 mmol) of p-toluenesulfonic

acid in 120 mL of anhydrous dioxane. The solution was

stirred at solvent reflux with the distillate being cycled

through the sieves to remove water formed as the esterifica-

tion proceeded. After 20 h, heating was discontinued and the

solvent was removed by evaporation at reduced pressure to

provide the polymer as a light yellow solid. The molar mass of

this linear polyester was about 30,000 g mol-1. The conversion

was 85 %: 1H NMR (d, CDCl3) 1.27–1.34 (COOCH2CH2CH2

CH2CH2),1.52–1.75(OCOCH2CH2 ? COOCH2CH2),2.21–2.27

(OCOCH2CH2), 3.95–4.06 (COOCH2); 13C NMR (d,

CDCl3) 24.19 (OCOCH2CH2), 25.67 (COOCH2CH2CH2),

28.38–28.99 (COOCH2CH2CH2CH2CH2), 33.71(OCOCH2

CH2), 173.26 (carbonyl ester group); IR (ATR, cm-1) 2919,

2852 (C–H saturated), 1729 (C=O ester), 1168 (C–O

stretch).

Poly(propylene glycol adipate)

Into a dry 100-mL, three-necked, round-bottomed flask fitted

with a magnetic stirring bar and a condenser bearing a

gas-inlet tube was placed 5.02 g (34.3 mmol) of adipic acid,

2.61 g (34.3 mmol) of 1,3-propanediol, 0.29 g (1.71 mmol,

2.5 mol% with respect to active carboxyl groups present) of

p-toluenesulfonic acid as catalyst and 5.0 g of anhydrous

magnesium sulfate as a water scavenging agent. The flask

was mounted in an oil bath and the mixture was maintained at

140 �C with stirring for 12 h. The residual material was

dissolved in chloroform and the solution was dried over

anhydrous magnesium sulfate. The solvent was removed by

evaporation at reduced pressure to afford the polymer as a

white waxy solid: 1H NMR (d, CDCl3) 1.45 (OCOCH2CH2),

1.76 (COOCH2CH2), 2.13 (OCOCH2CH2), 3.94

(COOCH2); 13C NMR (d, CDCl3) 23.51 (OCOCH2CH2),

27.05–27.38 (COOCH2CH2), 30.81–32.90 (OCOCH2CH2),

60.1(COOCH2), 173.27–174.20 (carbonyl ester group); IR

(ATR, cm-1) 3299 (O–H stretch), 2960, 2865 (C–H satu-

rated), 1733 (C=O ester), 1175 (C–O stretch).

Results and discussion

Hyperbranched poly(ester)s find application in a number of

areas and are of increasing interest. However, little is

known about the thermal stability of these materials. A

hyperbranched poly(ester) derived from trimethylolpropane

and adipic acid has been prepared under conditions that

prohibit gelation and assure that hydroxyl chain ends are

present [4, 14–17] The polymer can readily be prepared by

either solution or melt polymerization and has been fully

characterized by chromatographic, spectroscopic and thermal

methods. The molar mass of the polymer determined by SEC

using light scattering detection was found to be Mn = 3,300,

Mw = 8,600 with a dispersity of 2.6. The polymer displays a

glass transition temperature (Tg) of -29 �C (Fig. 1).

The infrared spectrum of the polymer contains charac-

teristic absorptions for a hydroxyl group (3,458 cm-1),

A B 21

1

2

A B

2CH 2CH C-O 2CH 2CH 2CH--2CH

OO

C- 2CH- - - - - -On

8 7 6 5 4 3 2 1 ppm9

Fig. 11 Proton NMR spectrum of poly(propylene adipate)

A B 1 2

1

2

A B

C=O

2CH 2CH 2CH 2CH 2CH--2CHC-

O O

2CH- -C-O- - - -On

180 160 140 120 100 80 60 40 20 ppm

Fig. 12 Carbon-13 NMR spectrum of poly(propylene adipate)

–0.5

0.0

0.5

1.0

343.80 °C

392.85 °C 1.5

2.0120

100

80

60

40

20

00 100 200 300 400 500

Temperature/°C

Mas

s/%

Der

iv. m

ass

chan

ge/%

°C

–1

Fig. 13 Mass loss as a function of temperature for a trimethylolpro-

pane/adipic acid hyperbranched poly(ester)


123

aliphatic C–H (2,964, 2,937, 2,883 cm-1), ester carbonyl

(1,735 cm-1), and ester/alcohol C–O (1,177 cm-1) as

shown in Fig. 2.

The proton and carbon-13 NMR spectra for the polymer

are shown in Figs. 3 and 4. Notable peaks may be observed

at d 3.33–4.01 in the proton spectrum and in the carbon-13

spectrum at d 62.2–65.2 and ester carbonyl (absorption at d173.3–174.2) (Scheme 1).

For purposes of comparison, two linear adipic acid

poly(ester)s were also prepared. The first of these, from

1,10-decanediol, of number average molar mass 30,000

and dispersity of 1.68, displays a glass transition temper-

ature of 42 �C and a melting temperature of 72 �C (Fig. 5).

The infrared spectrum of this material (Fig. 6) contains

ester carbonyl absorption at 1,730 cm-1 and ester C–O

absorption at 1,176 cm-1.

Both the proton and the carbon-13 NMR spectra are

consistent with the linear structure. The proton spectrum

contains resonance for the methylene groups of the adi-

pate units as well as those from the decylene unit

(Fig. 7). The carbon-13 spectrum contains absorption

from the carbonyl ester functional group at d 173.3

(Fig. 8).

A second linear adipic acid poly(ester) was prepared

using 1,3-propanediol as the difunctional alcohol. This

polymer with a number average molar mass 7,000 and

dispersity of 1.37, displays a glass transition at -55 �C,

and a melting temperature at 40 �C (Fig. 9).

Characteristic infrared absorptions are present at 2,926

and 2,865 cm-1 (aliphatic C–H), 1,733 cm-1 (ester car-

bonyl), and 1,175 cm-1 (ester C–O) (Fig. 10).

The proton NMR spectrum of this material is shown in

Fig. 11 and contains characteristic absorption for the

methylene groups of both adipate and propylene units. The

corresponding carbon-13 spectrum in Fig. 12 contains

carbonyl ester absorption at d 172.0–172.3.

The thermal stability/degradation of all three polymers

was examined using thermogravimetry and infrared spec-

troscopy. The mass loss curve for degradation of the hy-

perbranched polymer is displayed in Fig. 13. As it can be

seen, the extrapolated onset temperature for degradation is

345 �C and the temperature of maximum degradation rate

is 392 �C. Plots of mass loss as a function of time are

contained in Fig. 14. At 150 �C, *5 % of the initial

sample mass is lost within 5 h. At 250 �C, the initial mass

loss occurs much more rapidly with almost 30 % of the

initial sample mass lost within 5 h. The polymer is then

relatively stable at this temperature—further mass loss

occurs very slowly—the mass loss at 20 h is 28 %. This

Starting material

1 h2 h3 h4 h5 h6 h12 h

90

80

70

60

50

40

30

20

10

Tra

nsm

ittan

ce/%

4000 3000 2000 1000

Wavenumber/cm–1

Fig. 15 Thermal degradation of a trimethylolpropane/adipic acid

hyperbranched poly(ester) at 180 �C in nitrogen at 1, 2, 3, 4, 5, 6, and

12 h

Starting material

6 hours

1 hour

90

100

80

70

60

50

40

30

20

10

4000 3000 2000 1000

Tran

smitt

ance

/%

Wavenumber/cm–1


hyperbranched poly(ester) at 300 �C in N2 at 1 and 6 h

150 °C 250 °C

100

98

96

94

92

900 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200

Mas

s/%

100

90

80

70

60

Mas

s/%

Time/minTime/min

Fig. 14 Thermal stability of

trimethylolpropane/adipic acid

hyperbranched poly(ester) at

150 and 250 �C

T. Zhang et al.

123

rapid initial mass loss probably reflects dehydrative ether

formation leading crosslinking to generate a structure that

is relatively stable at 250 �C.

To further document the changes occurring as degra-

dation proceeds, the process in both nitrogen and air, was

monitored using infrared spectroscopy. Films of the poly-

mer were prepared on glass slides and placed in an oven at

a fixed temperature. Slides were removed as a function of

time and the infrared spectrum of the film recorded using

the attenuated total reflectance (ATR) technique. If soluble,

the samples were rinsed from the slide using deuterochlo-

roform and the proton NMR spectrum recorded. At high

degrees of degradation, the residue was highly crosslinked

and insoluble in deuterochloroform. Results from degra-

dation of the hyperbranched polymer in nitrogen at 180 �C

are presented in Fig. 15. The rapid conversion of alcohol

chain ends (hydroxyl absorption at 3,441 cm-1) to ether

functionality (increase in intensity of the band at

6 5 4 3 ppm

Fig. 17 Proton NMR spectra for samples of a trimethylolpropane/

adipic acid hyperbranched polymer undergoing degradation at 180 �C

in nitrogen at 10, 20, 30, and 60 min

1 hour

Starting material

10 mins

20 mins

30 mins

50 mins

40 mins

3 hours

6 hours

90

8070

60

5040

30

2010

0

Tra

nsm

ittan

ce/%

4000 3000 2000 1000

Wavenumber/cm–1


hyperbranched poly(ester) at 300 �C in air at 10, 20, 30, 40, 50 min,

and 1, 3, and 12 h

–0.5

0.0

0.5

1.0

1.5

2.0

2.5

0 200 400

367.35 °C

404.83 °C

800600

100

80

60

40

20

0

Mas

s/%

Temperature/°C

Der

iv. m

ass

chan

ge/%

°C

–1

Fig. 19 Thermal degradation of poly(decylene adipate)

O C

O

C

O

O

OH

OH

O C

O

C

O

O

OH

O

HO

O

C

O

C

O

O H2O

Scheme 2 Dehydrative crosslinking for trimethylolpropane/adipic acid hyperbranched poly(ester)s


123

150 °C 250 °C

100100.0

99.5

99.0

98.5

98.0

98

96

94

920 200 400 600 800 1000 12000 200 400 600 800 1000 1200

Mas

s/%

Mas

s/%

Time/min Time/min

Fig. 20 Thermal stability of

poly(decylene adipate) at 150

and 250 �C

Starting material1 hour2 hours

3 hours4 hours5 hours6 hours

12 hours

90

80

70

60

50

40

30

20

10

Tra

nsm

ittan

ce/%

4000 3000 2000 1000

Wavenumber/cm–1

Fig. 21 Structural changes which accompany the thermal degrada-

tion of poly(decylene adipate) at 180 �C in nitrogen

Starting material

1 hour

2 hours3 hours

4 hours5 hours6 hours

12 hours

90

80

70

60

50

40

30

20

10

Tra

nsm

ittan

ce/%

4000 3000 2000 1000

Wavenumber/cm–1


tion of poly(decylene adipate) at 180 �C in air at 1, 2, 3, 4, 5, 6, and

12 h

Starting material

10 mins

20 mins30 mins

40 mins

50 mins

1 hour

3 hours

12hours

90100110

8070605040302010

Tra

nsm

ittan

ce/%

4000 3000 2000 1000

Wavenumber/cm–1


tion of poly(decylene adipate) at 300 �C in air at 10, 20, 30, 40,

50 min, and 1, 3, and 12 h

Starting material

1 hour

2 hours

12 hours

90100

80706050403020100

–10–20–30

4000 3000 2000 1000Tr

ansm

ittan

ce/%

Wavenumber/cm–1

Fig. 24 Thermal stability of poly(propylene adipate) at 180 �C in

nitrogen at 1, 2, and 12 h

Starting material

10 mins

20 mins30 mins40 mins50 mins1 hour3 hours

12 hours

9080706050403020100

–10–20–30

4000 3000 2000 1000

Tran

smitt

ance

/%

Wavenumber/cm–1

Fig. 25 Thermal degradation of poly(propylene adipate) at 300 �C in

air at 10, 30, 40, 50 min, and 1, 3, and 12 h

5 4 3 2 1 ppm

Fig. 26 Proton NMR spectra depicting thermal degradation of

poly(propylene adipate) at 300 �C in air at 30, 40 min, and 1, 12 h

T. Zhang et al.

123

1,208 cm-1) is readily evident. Approximately, 90 % of

the hydroxyl absorption is removed after only 60 min

under these conditions; none remains after 12 h. The same

transformation at 300 �C is depicted in Fig. 16. From this

set of spectra the conversion of alcohol to ether is very

clearly apparent and is complete within 1 h. This process is

depicted in Scheme 2. Intermolecular crosslinking which

accounts for the ultimate insolubility of the material is

illustrated. Intramolecular crosslinking, of course, to form

cyclic structures also occurs. The proton NMR spectra for

the films degraded at 180 �C are collected in Fig. 17. The

films crosslinked after 30 min. Ether formation is verified

by the growth in the intensity of the absorption at d 3.4.

Some elimination to form olefinic chain ends (peaks at d*5) is also observed.

Although, degradation in air retains the overall features

of the degradation occurring in nitrogen, it is somewhat

more complex (Fig. 18).

The absorption due to hydroxyl groups rapidly dimin-

ishes in intensity while that at 1,223 cm-1 increases. The

growth of bands at 3,050 and 1,605 cm-1 as the degrada-

tion proceeds suggests that olefin formation is more

prominent in air than in nitrogen.

The linear poly(ester)s are subject to a different mode of

degradation as might be expected [18–23]. A prominent

reaction for the thermal degradation of linear poly(ester)s is

a classical ester pyrolysis to cleave the polymer mainchain

and form polymer with carboxyl and olefinic chain ends.

This generally occurs randomly along the polymer main-

chain. Secondary breakdown of end-groups, particularly

carboxyl end-groups, may lead to the formation of cyclic

structures (ketones, anhydrides) and small volatile pro-

ducts. Olefinic end groups lead to the formation of alde-

hydes, dienes, and cyclic ethers. These processes often

occur at appreciably higher temperatures than does the

degradation of the hyperbranched polymer noted above.

Radical processes may also occur.

The relative stability of linear poly(ester)s compared to

that of their hyperbranched counterparts is reflected in the

thermal degradation behavior of two polymers derived

from adipic acid. A curve for the degradation of

poly(decylene adipate) is displayed in Fig. 19.

The decomposition of this material occurs at somewhat

higher temperature (extrapolated onset temperature for

degradation of 366 �C) and lacks the heterogeneity of the

decomposition of the hyperbranched polymer. This relative

stability is also reflected in mass loss at constant temperature

depicted in Fig. 20. At 150 �C, after an initial small mass

loss, the polymer is stable for 20 h. Even at higher temper-

ature, 250 �C, the mass loss for the polymer is only 7 % of

the initial mass at 20 h. This is in contrast to a 30 % mass loss

for the hyperbranched polymer under the same conditions.

The structural changes occurring upon degradation of

the polymer at 180 �C in nitrogen are depicted in Fig. 21.

This figure contains the infrared spectra for samples held at

180 �C for 1, 2, 3, 4, 5, 6, and 12 h.

The most obvious change is the initial growth of bands

at 3,066, 3,176 cm-1 reflecting the formation of structures

containing alkene units. These bands initially increase and

then decrease in intensity as a function of time at 180 �C.

This probably reflects initial formation of unsaturated

moieties which undergo cycloaddition/crosslinking with

extended time at high temperature. Similar structural

changes occur, but much more rapidly in air (Fig. 22).

These changes are even more rapid at 300 �C (unsatu-

ration is generated and then consumed within 0.5 h) as

shown in Fig. 23. The degradation at 300 �C is both more

complex and more complete at extended times at 300 �C.

At 3 h, the sample has undergone a major transformation

and at 12 h is, although retaining some identifiable struc-

tural features, largely insoluble char.

The linear polymer obtained from a small diol,

poly(propylene adipate), displays a thermal stability greater

than that for poly(decylene adipate). At 180 �C in nitrogen,

it undergoes no significant degradation up to 12 h

(Fig. 24). The polymer does undergo degradation at 300 �C

in air (Fig. 25).

The greatest structural changes (1,160 cm-1) probably

reflect ester cleavage or cyclization reactions. Significant

structural changes are also evident in the proton NMR

spectra presented in Fig. 26.

Conclusions

A hyperbranched poly(ester) has been prepared from

trimethylolpropane and adipic acid under conditions which

avoid gelation and assure the presence of hydroxyl end

groups. The polymer was fully characterized using chro-

matographic, spectroscopic, and thermal methods. The

thermal degradation of this material was evaluated using

thermogravimetry and infrared spectroscopy. In both

nitrogen and air, the polymer undergoes dehydrative ether

formation to form a crosslinked structure. The thermal

behavior of this material was compared with that of two

analogous linear polymers, poly(decylene adipate) and

poly(propylene adipate). In general, the linear polymers

display significantly greater thermal stability than does the

hyperbranched poly(ester). In addition, the degradation of

the linear polymer is more complex with ester cleavage,

alkene formation, and cyclization reactions being promi-

nent. For the linear poly(ester)s, that derived from the

smaller diol [poly(propylene adipate)] is the more ther-

mally stable.


123

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