Stereochemistry of the radical polymerization

of vinyl pentafluorobenzoate

Weihong Liua,*, Yasuhiro Koikeb,c, Yoshi Okamotoa*

aDepartment of Chemistry, Polymer Research Institute, Polytechnic University, Six Metrotech Center, Brooklyn, NY 11201,USAbFaculty of Science and Technology, Keio University, Yokohama 223-8522, Japan

cERATO, Koike Photonics Polymer Project, K2 Town Campus 144-8 Ogura, Saiwai-ku Kawasaki 212-0054, Japan

Received 11 March 2004; received in revised form 29 May 2004; accepted 2 June 2004

Available online 24 June 2004

Abstract

The free-radical polymerization of vinyl pentafluorobenzoate (VPBz) was carried out under various conditions in order to compare its

stereochemistry to that of the vinyl benzoate (VBz) polymerization using similar conditions. Contrary to the stereochemistry of the radical

polymerization of VBz, VPBz favors syndiotactic propagation. The poly(VPBz) obtained in hexafluoroisopropanol (HFIP) with nBu3B–air

at230 8C has a triad syndiotacticity ðrrÞ of 52% which achieved the highest syndiotacticity reported for the radical polymerization of vinyl

esters. The stereochemistry difference for the VPBz polymerization was ascribed to the electron-withdrawing effect of the fluorine on the

aromatic ring. The solvent effect of enhancing the rr specificity in HFIP may be related to the hydrogen-bonding between HFIP and VPBz or

the growing species. It was also found that the glass transition temperatures ðTgÞ of the VPBz polymers apparently increased with an increase

in their diad syndiotacticities ðrÞ : the Tg of poly(VPBz) with r ¼ 72% was 79 8C, which was 25 8C higher than that of poly(VPBz) with

r ¼ 56% obtained in toluene.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Stereochemistry; Radical polymerization; Vinyl ester

1. Introduction

Tacticity control in vinyl polymerization is a major

theme in polymer science since the physical properties of

vinyl polymers are often significantly influenced by the

main-chain stereochemistry. Fully stereocontrolled polym-

erization was only achieved by ionic or coordination

catalysts, which can provide a counterionic species at the

growing ends. Stereocontrol in free-radical polymerization

is especially intriguing because of the absence of the

counterion in this process in spite of the convenience and

industrial importance of radical polymerization [1]. Two

effective methods of stereoregulation in radical polymeriz-

ation were recently reported: one is based on the hydrogen-

bonding interaction of a fluoroalcohol with a monomer and

with the growing species or both [2,3]; another is based on

the coordination between a Lewis acid and a monomer [4].

The polymerizations of various vinyl esters have been

widely investigated because the stereostructure of the vinyl

ester polymers strongly affects the properties of the

poly(vinyl alcohol) derived from the original polymer [5,

6]. The stereochemistry has been found to be affected by the

monomer structure under conventional radical polymeriz-

ation conditions: the polymerization of bulky vinyl esters

such as vinyl pivalate (VPi) [7–9] and vinyl diphenylacetate

[10] are known to give syndiotactic-rich polymers, whereas

that of vinyl acetate (VAc) leads to an almost atactic

polymer. The fluoroalcohol was found to have a remarkable

influence on the stereochemistry in the radical polymeriz-

ation of vinyl esters and the solvent effect also strongly

depends on the monomer structures: the syndiotactic

specificity was enhanced in the polymerization of VAc [2]

and was decreased in the polymerization of VPi [2], vinyl

isobutylate (ViBu) [11] and vinyl benzoate (VBz) [11]

(Scheme 1).

Imai et al. [12] reported that the polymerization of VBz

affords slightly isotactic-rich polymers under conventional

radical polymerization conditions. Recently, Yamada et al.

0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymer.2004.06.003

Polymer 45 (2004) 5491–5495

www.elsevier.com/locate/polymer

* Corresponding authors. Tel.: þ1-718-260-3965; fax: þ1-718-260-

3125.

E-mail addresses: whliu888@hotmail.com (W. Liu), wliu@poly.edu

(W. Liu), yokamoto@poly.edu (Y. Okamoto).

[11] have studied the radical polymerization of vinyl esters

using fluoroalcohols as solvents. They have also found that

the polymerization of VBz in hexafluoroisopropanol (HFIP)

yielded a polymer with a triad isotacticity ðmmÞ of 33%.

This was the highest isotacticity observed for a free-radical

polymerization of vinyl esters.

In the course of our current investigation on fluorine-

containing polymers, we have recently prepared vinyl

pentafluorobenzoate (VPBz) (Scheme 1) and investigated

its homo- and copolymerizations [13]. VPBz readily

polymerized in bulk with a free-radical initiator. Therefore,

in the present study we will discuss in detail the

stereochemistry of the VPBz polymerization under various

conditions. The results obtained were compared with those

of VBz polymerizations. The correlation between the glass

transition temperature ðTgÞ and tacticity of the resulting

polymers was also examined.

2. Experimental

2.1. Materials

Toluene (Aldrich; purity . 99%), methanol (MeOH)

(Aldrich; purity . 99%) (CF3)2CHOH (HFIP) (Aldrich;

purity . 99%), mercury (II) oxide (Aldrich, purity 98%),

calcium carbide (Aldrich), and pentafluorobenzoic acid

(Fluorochem USA, purity . 99%) were used as received.

VAc (Aldrich) was distilled before use. 2,20-Azobisisobutyr-

onitrile (AIBN) was purified by recrystallization from metha-

nol. Tri-n-butylborane (nBu3B) was obtained as an ether

solution(1.0 M)(Aldrich)andusedafter removalofthesolvent.

2.2. Synthesis of VPBz

The monomer has been synthesized via a vinyl

interchange reaction used in the preparation of diaroyl-

methane [14]. Here, we reported an alternative method. A

typical synthetic procedure is as follows: a flask was

equipped with a magnetic stir, a thermometer, a gas inlet

tube and a reflux condenser. The upper end of the condenser

was attached to a small gas-washing bottle. The flask was

charged with 424 g (2.0 mol) of pentafluorobenzoic acid,

0.2 g of hydroquinone, and 20 g of mercuric oxide.

Acetylene was produced slowly by dropping water onto

calcium carbide. It was passed through a spiral trap cooled

in dry ice-acetone bath, an empty wash bottle, a sulfuric acid

wash bottle, a soda-lime tower, and then into the reaction

flask through the gas inlet tube. The flask was heated in oil

bath until the acid just melted (about 120 8C) and then the

reaction mixture was cooled to 80 8C and kept for 3 h. The

mercury salt was removed by centrifugation and the

resulting liquid product was purified by distillation under

reduced pressure. Yield 63% (300 g). Bp: 50 8C/1.0 mm Hg.1H NMR (CDCl3) d (ppm): 4.80 (q, 1H), 5.10(q, 1H),

7.40(q, 1H). 19F NMR (CDCl3) d (ppm): 2137.60 (s, 2F);

2147.60 (s, 1F); 2160.80 (s, 2F).

2.3. Polymerization

Using nBu3B–oxygen as an initiator, a typical polym-

erization was performed as follows: the monomer and

solvent were charged in a glass tube equipped with a three-

way stopcock, which was then degassed and refilled with

argon in three vacuum freeze–thaw cycles. nBu3B was then

added via a syringe under an argon atmosphere to the

monomer solution at 230 8C. The polymerization reaction

was initiated by introducing a small amount of air. After a

specified time, the polymerization reaction was terminated

by adding a small amount of a methanol solution of 2,6-di-

tert-butyl-p-cresol. The polymerizations with AIBN were

carried out using standard methods in a glass tube under an

argon atmosphere. The polymer obtained was precipitated

into methanol, collected by centrifuging and dried under

vacuum at 50 8C.

2.4. Methanolysis of the polymers

The methanolyses of poly(VPBz)s were performed in a

mixed solvent system containing dimethylsulfoxide

(DMSO) and MeOH (9/1, v/v) at 60 8C using methanolic

KOH as the catalyst. The poly(vinyl alcohol) (PVA)

obtained was purified by precipitation into methanol and

dried under vacuum.

2.5. Measurements

The 1H and 19F NMR spectra were obtained with a

Bruker ACF 300 spectrometer. Size exclusion chromato-

graphic (SEC) analysis was accomplished on a system with

a Waters 510 pump in line with TSK gel HMXL and H5000

columns, and with dual detectors: a Waters 440UV

absorbance detector and a Waters R401 differential

refractometer. Tetrahydrofuran (THF) was used as an

eluting solvent with a flow rate of 1.0 ml min21 at 30 8C.

The molecular weight of the polymers was calibrated with

polystyrene (PS) standards. Differential scanning calorime-

try (DSC) measurement was performed on a DSC 2920

module in conjunction with a TA Instrument 5100 system

with a heating rate of 10 8C min21 under a nitrogen

atmosphere. The midpoint of the heat capacity transition

was taken as the glass transition temperature.

Scheme 1. Monomer structure.

W. Liu et al. / Polymer 45 (2004) 5491–54955492

3. Results and discussion

3.1. Polymerization of VPBz

The bulk polymerization of VPBz at 60 8C using AIBN

as an initiator for 48 h afforded a clear resin with a

quantitative yield. The solid was not soluble in any organic

solvents, suggesting that cross-linking may have occurred at

high conversion. The bulk polymerization of VBz was also

reported to produce a hard, brittle, and cross-linked clear

resin. A detailed mechanism has been proposed to account

for the cross-linking based on free radical addition to the

benzoate nucleus [15]. In the case of the VPBz polymeriz-

ation, the chain-transfers to the monomer along with

transfer to the macromolecular main-chain may be also

responsible for the cross-linking. The chain-transfer to the

macromolecular main-chain has also been demonstrated for

the VAc polymerization [16]. In order to obtain a soluble

polymer, the bulk polymerization was terminated when the

monomer conversion was less than 60%. As a result of

chain-transfer, the molecular weight of the polymer

obtained in the bulk polymerization was much higher than

that obtained in the solution polymerization. The poly-

dispersity was also broader in the bulk polymerization than

that obtained in the solution polymerization.

The solution polymerizations were carried out in toluene,

methanol, and HFIP using AIBN and nBu3B–oxygen as

initiators. It was noted that the resulting polymer precipi-

tated out during polymerization in methanol and the

reaction mixture gelled during polymerization in HFIP.

The polymers obtained were soluble in common organic

solvents such as acetone, THF and chloroform. It seems that

chain-transfer to the macromolecular main-chain was

minimized by the presence of the solvents since the

polydispersity were less than 2.0 in all the solution

polymerization cases. Moreover, under the same condition

with an nBu3B–oxygen initiator at 230 8C in HFIP, a

higher polymer yield was obtained compared to the

polymerization in toluene, suggesting that the polymeriz-

ation was enhanced by the hydrogen-bonding between the

solvent and the monomer or the growing species. Similar

effects were also observed in the polymerization of MMA,

ethyl methacrylate [3] and trifluoroethyl methacrylate [17].

3.2. Stereochemistry characteristic

The tacticity for the vinyl ester polymer can be

determined based on –OH signals of the corresponding

PVA derived from the original polymer in 1H NMR

spectrum taken in dimethylsulfoxide-d6 [18]. A typical

assignment is indicated in Fig. 1. The triad tacticities of the

polymers are listed in Table 1. On the other hand, the 19F

NMR spectra of the VPBz polymers were recorded in

CDCl3 and acetone-d6. The signals centered at 2161.0,

2148.0 and2139.0 ppm were assigned to the meta-, para-

and ortho-position fluorines on the pentafluorophenyl

moiety, respectively. Comparing with the 19F NMR spectra

of two samples with different tacticities, we found that the

fluorines on the aromatic ring are also sensitive to the

stereochemistry in the 19F NMR spectrum in CDCl3. The

signal at2139.1 ppm in the ortho-fluorine region decreased

with a decrease in isotacticity (Fig. 2). The ratio of the peak

at 2139.1 ppm was calculated to be 19% for the sample

obtained at 50 8C in toluene (run 2 in Table 1, Fig. 2(A)) and

8% for the sample obtained at 230 8C in HFIP (run 10 in

Table 1, Fig. 2(B)), respectively, which is nearly the same as

the triad isotactic content listed in Table 1. In addition, the

signals centered at about 2138.5 ppm increased with an

increase in syndiotacticity and the signals centered at about

2138.8 ppm decreased with a decrease in heterotacticity.

Thus, the 19F NMR spectra may be interpreted in terms of

triad stereosequences as shown in Fig. 2.

The probability of the syndiotactic propagation ðPSÞ (i.e.

r content) can be calculated based on the triad tacticity listed

in Table 1. The value of PS for the bulk polymerization of

VPBz at 60 8C was calculated to be 0.56. This value is

higher than the one reported for the VBz bulk polymeriz-

ation ðPS ¼ 0:49Þ at 60 8C [12]. The stereochemistry of

the VBz polymerization ðPS , 0:5Þ was attributed to the

p-stacking effect of the monomer’s aromatic ring [11,19].

The favored syndiotactic-specific propagation for the VPBz

polymerization may be due to the electron-withdrawing

effect of the fluorine on the aromatic ring. It has been

Fig. 1. 1H NMR spectra of PVA (hydroxyl and methine regions) derived

from run 10 in Table 1.

Fig. 2. 19F NMR spectra of poly(VPBz)s (ortho-fluorine region) in CDCl3:

run 2 in Table 1 (A) and run 10 in Table 1 (B).

W. Liu et al. / Polymer 45 (2004) 5491–5495 5493

reported that both steric and electrostatic effects of the side

chain are responsible for the syndiotactic-specific propa-

gation in the polymerization of fluorinated vinyl esters [20].

Solvent effects were also studied using toluene, MeOH

and HFIP as the solvents. The triad tacticities of the

obtained poly(VPBz)s are listed in Table 1. Toluene has

little effect on the stereochemistry compared with the bulk

polymerization. However, the solvent effect was observed

in the VPBz polymerization in alcoholic solvents resulting

in an increase in the rr specificity compared to the bulk

polymerization. The effect became more pronounced in

HFIP. This is interesting because vinyl esters with bulky

groups such as VPr, VPi, ViBu, vinyl 2,2-dimethylbutylate

(VDMB) and vinyl 2,2-dimethylvalerate (VDMV) usually

produce polymers with higher mr specificity in fluoro-

alcohol solvents [11]. On the other hand, as reported in the

polymerization of VBz, the mm specificity was enhanced in

fluoroalcohol solvents, which was ascribed to the p-

stacking effect and hydrogen-bonding effect that coexist.

However, the p-stacking effect may be dominant over the

hydrogen-bonding effect [11]. The difference in the solvent

effect between VBz and VPBz polymerizations may be also

cooperated with the electron-withdrawing effect of the

fluorine on the aromatic ring. The solvent effect of

enhancing the rr specificity in HFIP may be related to the

hydrogen-bonding between HFIP and VPBz or the growing

species.

The stereochemistry in the polymerization of VPBz

was found to be dependent on the reaction temperature

reported here in HFIP. Based on the Fordham plot of

the tacticity data for runs 4, 7 and10, the differences of

activation enthalpy and that of activation entropy

between isotactic and syndiotactic propagations were

determined to be DH–i 2 DH–

S ¼ 884 cal mol21 and

DS–i 2 DS–S ¼ 1:8 cal deg21 mol21, respectively, by

using Eq. (1) [21]:

lnðPi=PSÞ ¼ ðDS–i 2 DS–S Þ=R2 ðDH–i 2 DH–

S Þ=RT ð1Þ

where Pi and PS are the mole fractions of isotactic and

syndiotactic diads, respectively, R is the gas constant

(1.987 cal mol21 K21), and T is the polymerization

temperature in K. The positive value of DH–i 2 DH–

S

in the solution polymerization using HFIP as solvent

indicates that syndiotactic propagation is favored by

enthalpy.

In the polymerization of VPBz in MeOH, the differences

in activation enthalpy and that of activation entropy

between isotactic and syndiotactic propagations were

determined to be DH–i 2 DH–

S ¼ 218 cal mol21 and

DS–i 2 DS–S ¼ 0:10 cal deg21 mol21, respectively,

suggesting that the syndiotactic propagation in MeOH is

also favored by the enthalpy. However, the value of

enthalpy is much lower than that in HFIP, indicating that

the temperature dependence on the stereochemistry is less

obvious in MeOH.

As a result of the temperature and solvent effects

discussed above, the polymerization of VPBz in HFIP at

230 8C gave a polymer with rr ¼ 51:7% (run 10 in Table 1)

as evident from the 1H NMR spectrum of the corresponding

PVA derived from the poly(VPBz) (Fig. 1). This achieved

the highest syndiotacticity reported for the free-radical

polymerization of vinyl esters.

3.3. Correlation between glass transition temperature and

tacticity

Glass transition temperature is an important physical

property to evaluate an amorphous polymer. In general, the

Tg of an amorphous polymer can be influenced by its

tacticity and molecular weight. To investigate the effect of

Table 1

Radical polymerization of VPBz

Run Solvent Initiator Temperature (8C) Yielda (%) Mnb ( £ 1024) Mw=Mn

b Tacticityc Tg (8C)

mm=mr=rr rd

1 None AIBN 60 26 10.27 4.60 19.5/49.0/31.5 56.0 50

2 Toluene AIBN 50 20 1.56 1.64 19.5/48.5/32.0 56.2 54

3 MeOH AIBN 50 30 1.78 1.60 17.7/48.5/33.8 58.0 61

4 HFIP AIBN 50 60 1.84 1.63 17.7/48.5/33.8 58.0 54

5 Toluene nBu3B/air 0 12 1.45 1.64 14.5/46.5/37.0 60.2 67

6 MeOH nBu3B/air 0 58 1.58 1.63 15.2/48.2/36.6 60.7 69

7 HFIP nBu3B/air 0 58 1.61 1.70 12.0/45.3/42.8 65.4 74

8 Toluene nBu3B/air 230 Trace – – – – –

9 MeOH nBu3B/air 230 15 1.09 1.75 14.3/49.6/36.7 61.2 65

10 HFIP nBu3B/air 230 29 1.52 1.58 7.0/41.3/51.7 72.4 79

Conditions: run 1, AIBN: 0.18 mol% to vinyl group, polymerization time 15 h; runs 2–10, [VPBz]0 ¼ 2.0 mol L21, [AIBN]0 ¼ 0.02 mol L21,

[nBu3B]0 ¼ 0.1 mol L21, polymerization time: 48 h.a Methanol—insoluble part.b Determined by SEC with PS standard.c Determined by 1H NMR of PVA in DMSO-d6.d r ¼ rr þ mr=2:

W. Liu et al. / Polymer 45 (2004) 5491–54955494

tacticity on the Tg of the VPBz polymer, DSCmeasurements

were performed for all the poly(VPBz) samples. The

polymers with different tacticities exhibited different DSC

curves as illustrated for the two polymers (runs 2 and 10 in

Table 1) in Fig. 3. The polymers exhibited a clear thermal

transition in the DSC plot. The poly(VPBz)s obtained with a

tacticity range of r ¼ 56–72% and with the Mn range of

1.1–1.8 £ 104 permit us to plot the Tg vs. tacticity (r

content). As seen from the plot (Fig. 4), the Tg apparently

increased with an increase in r content in the range of r ¼

56–72%: Similar phenomena about Tg increase with an

increase in r-content in the range of r . 50% have been

reported for PMMA [22] and poly(t-butyl acrylate) [23].

4. Conclusions

The stereochemistry of the bulk polymerization of VPBz

was quite different from that of VBz: the syndiotactic

propagation was favored in the VPBz polymerization. Both

solvent and temperature effects in the solution polymeriz-

ation of VPBz led to an increase in syndiotacticity, which is

also different from the result of VBz polymerization. The

different stereochemical characteristics for the VPBz

polymerization were ascribed to the electron-withdrawing

effect of the fluorine on the aromatic ring. As a result, the

polymerization of VPBz in HFIP at 230 8C produced a

polymer with rr ¼ 52%; which achieved the highest

syndiotacticity reported for the radical polymerization of

vinyl esters. The glass transition temperature of poly(VPBz)

was found to be influenced by its tacticity: a higher r content

led to a higher glass transition temperature.

Acknowledgements

This work was supported in part by the Japan Science

and Technology Corporation through a Grant for ERATO-

Photonic Polymer.

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Fig. 4. Correlation between glass transition temperature and tacticity.

Fig. 3. Typical DSC curves for poly(VPBz)s: run 2 in Table 1 (A) and run

10 in Table 1 (B). (Second heating, heating rate: 10 8C min21, nitrogen

flow: 40 ml min21).

W. Liu et al. / Polymer 45 (2004) 5491–5495 5495