Tacticity control in the radical polymerization of 2,2,2-trifluoroethylmethacrylate with fluoroalcohol

Weihong Liua, Kai Tanga, Yinzhong Guoa, Yasuhiro Koikeb,c, Yoshi Okamotoa,*

aPolymer 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 6 February 2003; received in revised form 24 March 2003; accepted 26 March 2003

Abstract

The free-radical polymerization of 2,2,2-trifluoroethyl methacrylate (TEMA) was carried out in fluoroalcohols to achieve stereoregulation.

The polymerization reactivity at low temperature and syndiotactic specificity were enhanced by the use of fluoroalcohol as a solvent. The

polymer having triad syndiotacticity (rr) of 70% was obtained in perfluoro-t-butyl alcohol. It was noted that the stereochemistry was nearly

independent of reaction temperature. The stereoeffect of fluoroalcohols seemed to be due to the hydrogen-bonding interaction between the

alcohol and the monomer or growing species. The hydrogen-bonding formation was determined by FTIR. The copolymerization of TEMA

with methyl methacrylate (MMA) in hexafluoroisopropanol afforded a copolymer with syndiotactic specificity. By this method, a cladding

material for an optical fiber based on poly(methyl methacrylate) (PMMA) with high mechanical strength and low refractive index could be

obtained.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Tacticity; Radical polymerization; Fluoroalcohol; 2,2,2-Trifluoroethyl methacrylate; Copolymerization; Optical fiber

1. Introduction

The tacticity control in vinyl polymerization is a major

theme in polymer science because the physical properties of

vinyl polymers are often significantly influenced by the main-

chain stereochemistry. Fully stereocontrolled polymerization

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 a counterion in this

process in spite of the convenience and industrial importance

of radical polymerization [1]. Recently, two effective meth-

ods of stereoregulation in radical polymerization were

reported: one is based on the hydrogen-bonding interaction

of a fluoroalcohol with a monomer, and also with growing

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

between a Lewis acid and a monomer [4]. However, the

stereoeffect strongly depends on the monomer structures. For

instance, fluoroalcohols enhanced the syndiotactic specificity

of the polymerization of methyl methacrylate (MMA), but

decreased the specificity in the polymerization of t-butyl

methacrylate, and little affected the stereochemistry of the

polymerization of isopropyl methacrylate [5].

Recently, fluorinated acrylic polymers have acquired an

increasing interest in various applications such as optical

materials [6], photoresists [7] and high performance coat-

ings [8]. In particular, copolymers containing 2,2,2-trifluoro-

ethyl methacrylate (TEMA) units have been used as

cladding materials in optical fibers [9]. TEMA has been

polymerized by various techniques [10–12]. However, tac-

ticity control in the free-radical polymerization of TEMA

has not been investigated. In order to improve the mechan-

ical properties of the cladding material for an optical fiber

based on poly(methyl methacrylate) (PMMA), we have

examined the stereochemistry of the free-radical polymer-

ization of TEMA and the copolymerization of TEMA with

MMA in fluoroalcohol.

2. Experimental

2.1. Materials

TEMA and MMA were obtained from Aldrich and

distilled before use. Toluene (Aldrich; purity >99%),

Journal of Fluorine Chemistry 123 (2003) 147–151

* Corresponding author. Fax: þ1-718-260-3508.

E-mail address: yokamoto@poly.edu (Y. Okamoto).

0022-1139/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0022-1139(03)00114-3

(CF3)2CHOH (HFIP) (Aldrich; purity >99%), (CF3)3COH

(PFTB) (Aldrich; purity >99%) were used as received. 2,20-Azobisisobutyronitrile (AIBN) was purified by recrystalli-

zation from methanol. Tri-n-butylborane (nBu3B) was

obtained as an ether solution (1.0 M) (Aldrich) and used

after removal of the solvent.

2.2. Polymerization

2.2.1. Homopolymerization

Polymerization was carried out in a glass tube under an

argon atmosphere. Using an nBu3B–oxygen as an initiator

[13], a typical polymerization was performed as follows:

nBu3B was added with a hypodermic syringe to a monomer

solution kept at �30 8C. The polymerization reaction was

initiated by introducing a small amount of air. The poly-

merization reaction was terminated by adding a small

amount of a methanol solution of 2,6-di-t-butyl-p-cresol.

The polymerizations with AIBN were carried out using

standard methods. The polymer obtained was precipitated

into a methanol–water mixture (4/1, v/v). The polymer was

collected by centrifuging and dried under vacuum.

2.2.2. Copolymerization of TEMA with MMA

in fluoroalcohol

TEMA (4.0 mmol), MMA (16.0 mmol), AIBN (0.2

mmol) and HFIP (8 ml) were charged in a glass tube, which

was then degassed and refilled with argon in three vacuum

freeze–thaw cycles. The tube was sealed and heated at 50 8Cfor 24 h. After the tube was cooled to room temperature, it

was opened, and then the fluoroalcohol was recovered by

distillation with a trap cooled to �78 8C under reduced

pressure. The copolymer obtained was dissolved in chloro-

form and precipitated into a large amount of methanol,

isolated by centrifuging and dried under vacuum.

2.3. Measurements

The 1H NMR spectra were obtained with a Bruker ACF

300 spectrometer with chloroform-d as a solvent at 50 8C.

FTIR spectra were obtained with a Perkin-Elmer FTIR-1600

spectrometer. Size exclusion chromatographic (SEC) ana-

lysis 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. Chloroform was

used as an eluting solvent with a flow rate of 1.0 ml/min

at 30 8C. The molecular weight of the polymers was cali-

brated with a PMMA standard. The differential scanning

calorimetry (DSC) measurement was performed on a DSC

2920 module in conjunction with the TA Instrument 5100

system at a heating rate of 10 8C/min under a nitrogen

atmosphere. The midpoint of the heat capacity transition

was taken as the glass transition temperature (Tg). The

refractive index of the film was obtained with a Metricon

Model 2010 prism coupler.

3. Results and discussion

3.1. Homopolymerization of TEMA

The polymerization of TEMAwas carried out in HFIP and

PFTB to examine the effects of the fluoroalcohols on the

reaction stereochemistry. The polymerization in toluene was

also performed as a control experiment. The results obtained

are summarized in Table 1. It was noted that fluoroalcohols

enhanced the polymerization reactivity at low temperature.

Under the same conditions with a nBu3B–oxygen initiator at

�30 8C, the use of fluoroalcohols resulted in higher polymer

yields compared with that for the polymerization in toluene.

Also the polymerization in PFTB gave a high molecular

weight polymer, whereas that in toluene only afforded an

oligomer. Similar effects were also observed in the poly-

merization of MMA and ethyl methacrylate [3,5].

The fluoroalcohols used also enhanced the triad syndio-

tactic content (rr). The triad tacticity of the poly(TEMA)

obtained can be determined by 1H NMR analysis. Fig. 1

shows the 1H NMR spectra in the a-methyl region of the

poly(TEMA) obtained in toluene at 60 8C and in PFTB at

�30 8C. The triad concentration for syndiotactic (rr), hetero-

tactic (mr) and isotactic (mm) sequences can be calculated as

Table 1

Radical polymerization of TEMAa

Run Solvent Initiator Temperature (8C) Time (h) Yieldb (%) Mnc (�104) Mw/Mn

c Triad tacticityd (mm/mr/rr)

1 Toluene AIBN 60 24 95 3.84 1.27 5/39/56

2 HFIP AIBN 50 24 78 3.79 1.64 4/32/64

3 PFTB AIBN 40 24 36 8.86 1.99 3/28/69

4 Toluene nBu3B/air �30 48 12 0.54 2.02 NAe

5 HFIP nBu3B/air �30 48 17 1.03 6.41 3/29/68

6 PFTB nBu3B/air �30 48 32 5.74 1.66 2/28/70

a Conditions: [TEMA]0 ¼ 2.0 mol/l; [AIBN]0 ¼ 0.02 mol/l; [nBu3B]0 ¼ 0.1 mol/l.b Methanol-insoluble part (runs 1–3), methanol–water (4/1, v/v)-insoluble part.c Determined by SEC with PMMA standard.d Determined by 1H NMR.e Tacticity is not available since this product was oligomeric.

148 W. Liu et al. / Journal of Fluorine Chemistry 123 (2003) 147–151

indicated in Fig. 1. The triad concentration for the polymer

obtained in toluene (rr/mr/mm ¼ 56/39/5) is exactly the same

as that for the poly(TEMA) obtained by free-radical poly-

merization at 65 8C in dioxane determined by 13C NMR

analysis reported by Passaglia et al. [12]. The polymer

obtained in PFTB at �30 8C has 14% higher syndiotactic

content than that in toluene at 60 8C. The tacticity of the

polymer obtained in PFTB is nearly the same as that obtained

in HFIP. Unlike the stereochemistry in the polymerization of

ethyl methacrylate which strongly depended on temperature

in PFTB [3], the stereochemistry in the polymerization of

TEMA was nearly independent of reaction temperature

reported here in PFTB. This may be due to a little bulkiness

and electron withdrawing of the trifluoroethyl group. Those

may make the hydrogen bonding between monomer and the

fluoroalcohol weaker in TEMA than in ethyl methacrylate.

No obvious temperature dependence of tacticity was also

observed in the polymerization of t-butyl methacrylate in

PFTB [3].

3.2. Copolymerization of TEMA with MMA

The copolymerization of TEMA with MMA was per-

formed in HFIP at 50 8C using AIBN as an initiator. The

Fig. 1. 1H NMR spectra of poly(TEMA) (a-CH3 region): (a) run 1 in Table 1; (b) run 6 in Table 1.

Fig. 2. 1H NMR spectrum of poly(MMA-co-TEMA).

W. Liu et al. / Journal of Fluorine Chemistry 123 (2003) 147–151 149

copolymerization afforded a random copolymer of TEMA

and MMA, and the yield reached to 95%. Based on the SEC

analysis, the number-average molecular weight (Mn) and the

polydispersity (Mw/Mn) of the copolymer were found to be

5:83 � 104 and 2.09, respectively. The copolymer composi-

tion was determined by 1H NMR as shown in Fig. 2. By

comparing the peak area of the side groups of monomeric

units, we determined that the copolymer contains 20 mol%

of TEMA units. Based on the 1H NMR analysis, the

copolymer has mainly syndiotactic structure (rr/mr/mm ¼70/27/3). This means that the fluoroalcohol is also effective

in enhancing syndiotactic specificity in the copolymeriza-

tion.

3.3. Mechanism study

The hydrogen-bonding interaction of the alcohol with the

monomer and the growing species may be related to both

effects mentioned above. The presence of hydrogen bonding

between methacrylate and fluoroalcohol has been proved by

an NMR study in deuterated chloroform [5]. The hydrogen

bonding between TEMA and PFTB was further observed in

situ by FTIR spectroscopy. The pure monomer containing a

carbonyl group yields a carbonyl stretching mode centered

at about 1738 cm�1, while the solvents show no absorption

in the carbonyl vibration region from 1660 to 1800 cm�1.

Fig. 3 shows the FTIR spectra of pure TEMA, TEMA in

toluene (½M� ¼ 2:0 mol/l) and TEMA in PFTB (½M� ¼ 2:0mol/l). Compared with the carbonyl vibration of the pure

monomer, the monomer’s carbonyl absorption band in

toluene was centered at the same position, but it became

much narrower. The monomer’s carbonyl absorption band in

PFTB was shifted to 1718 cm�1 and became much broader.

This may be attributed to the strong hydrogen-bonding

interaction between the monomer and the fluoroalcohol.

In addition, the vinyl bond absorption was also affected

by the hydrogen-bonding. The monomer’s vinyl bond

absorption band was centered at 1639 cm�1. It became much

broader in PFTB.

The mechanism of the proposed hydrogen-bonding-

mediated polymerization reaction is already reported for

other methacrylates [5]. Due to the hydrogen-bonding inter-

action between monomers or growing species and fluoroal-

cohols, the side groups of the monomers and growing

species become apparently bulkier than those without the

hydrogen bonding. This may result in larger steric effects in

the propagation. Hence, the rr specificity is enhanced by the

fluoroalcohol. However, in the case of methacrylate with a

bulky side group such as t-butyl methacrylate, the rr speci-

ficity is decreased by the fluoroalcohol [3,5]. Therefore, the

stereoeffect enhancing the rr specificity with fluoroalcohol is

limited to those methacrylates with little bulky side groups.

The effect of fluoroalcohols on the molecular weight of

the product may be mainly due to the following reason: the

apparently bulkier growing species bound with the alcohols

suppresses bimolecular termination.

3.4. Physical properties

Tg is an important physical property to evaluate an

amorphous polymer. Based on DSC determination, the

poly(TEMA) obtained in PFTB at �30 8C (run 6 in

Table 1) exhibits a Tg at 78 8C, whereas that obtained in

toluene at 60 8C (run 1 in Table 1) exhibits a Tg at 74 8C. The

two polymers have 8.5% of difference in diad syndiotactic

(r) content. The Tg difference of about 4 8C is due to the

different r contents. This was similar to PMMA whose Tg

was increased about 5 8C with an increase of r content from

78 to 90% [14]. Thus, the Tg of poly(TEMA) could be

improved with an increase of syndiotacticity.

The copolymer of MMA and TEMA (MMA/TEMA ¼80/20, mol/mol) was determined to have a Tg of 114 8C by

DSC. The higher Tg was attributed to the syndiotactic

structure of the copolymer. More importantly, the copolymer

exhibited a lower refractive index than PMMA. The refrac-

tive index of the film of PMMA at the wavelength of

632.8 nm was 1.4904, whereas that of the copolymer was

1.4599. The lower refractive index of the copolymer was

related to the presence of fluorinated units. To design

cladding materials for plastic optical fibers, both high

mechanical strength and low refractive index are required

[9]. Hence, by the copolymerization in fluoroalcohol, we

may obtain a good cladding material for an optical fiber

based on PMMA.

4. Conclusions

The fluoroalcohols enhanced both polymerization reac-

tivity at low temperature and syndiotactic specificity in the

radical polymerization of 2,2,2-trifluoroethyl methacrylate.

The hydrogen bonding of fluoroalcohols with monomers

and growing radicals may be responsible for those effects,

which were determined by FTIR. The stereoeffect was also

observed in the copolymerization of 2,2,2-trifluoroethylFig. 3. FTIR absorption of carbonyl and vinyl groups of: (a) TEMA; (b)

TEMA in toluene; (c) TEMA in PFTB.

150 W. Liu et al. / Journal of Fluorine Chemistry 123 (2003) 147–151

methacrylate with methyl methacrylate. The resulting copo-

lymer with triad tacticity of rr/mr/mm ¼ 70/27/3 is likely to

provide a good cladding material for PMMA optical fibers.

Acknowledgements

This work was supported in part by the Japan Science and

Technology Corporation through the Grant for ERATO-

Photonic Polymer.

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