Stereochemistry in Radical Polymerization of Vinyl ... - ACS Publications

Chapter 4

Stereochemistry in Radical Polymerization of Vinyl Esters 1

2,4

3

Yoshio Okamoto , Kazunobu Yamada , and Tamaki Nakano 1

Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Joint Research Center for Precision Polymerization (JRCPP)Japan Chemical Innovation Institute (JCII), Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Takayama-cho 8916-5, Ikoma, Nara 630-0101, Japan

Downloaded by STANFORD UNIV GREEN LIBR on September 20, 2012 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0768.ch004

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3

This chapter describes a new stereoregulation method for free -radical polymerization of vinyl esters using fluoroalcohols as solvents. The stereochemistry of the vinyl ester polymerization was remarkably affected by the fluoroalcohols such as (CF )3COH and (CF )2CHOH. The polymerization of vinyl acetate (VAc) gave a polymer rich in syndiotacticity (up to rr = 50%, r = 72%), while the polymerization of vinyl propionate (VPr), vinyl isobutyrate (ViBu), vinyl pivalate ( VPi), vinyl 2,2-dimethylbutylate (VDMB), and vinyl 2,2-dimethylvalerate (VDMV) gave polymers rich in heterotacticity (up to mr = 61%). A polymer rich in isotacticity (up to mm = 33%) was obtained by the polymerization of vinyl benzoate (VBz). This stereochemistry represents the highest level of content for the radical polymerization of vinyl esters reported so far. The stereochemical effects observed in this study are due to a hydrogen -bonding interaction between the fluoroalcohol molecules and the ester groups of the vinyl ester monomer and the growing polymer. Conversely, the stereochemistry of the polymerization of bulky 2,2bis(trifluoromethyl)propionate (VF6Pi) in fluoroalcohols was similar to that of the bulk polymerization presumably due to the weak hydrogen-bonding interaction between the monomer and the fluoroalcohols. 3

3

Control of free-radical polymerization is drawing much current attention and several living polymerization methods have been developed. Because polymer properties are significantly influenced not only by the molecular weight but also by the main-chain tacticity, stereocontrol of the polymerization is also an important topic in macromoleeular chemistry. However, stereoregulation by free-radical polymerization has been achieved only in limited cases so far (/). 4

Present address: R & D Center, Unitika Ltd., 23 Kozakura, Uji, Kyoto 611-0021, Japan.

© 2000 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

57


58 Vinyl esters afford polymers only by radical polymerization and solvolysis of the obtained polymers leads to polyvinyl alcohol) (PVA). The thermal and mechanical properties of P V A are significantly influenced by a slight change in the main-chain tacticity, and increasing syndiotacticity of P V A improves the properties (2). However, commercially available P V A , which is produced through the polymerization of vinyl acetate (VAc), has an atactic structure. In order to increase the syndiotactic specificity of the vinyl ester polymerization, various bulky monomers including vinyl trifluoroacetate (3,4), vinyl pivalate (VPi) (4,5), vinyl diphenylacetate (6), and vinyl 2,2-bis(trifluoromethyl)propionate (VF6Pi) (7) have been designed. Although the monomer-design method is effective, it lacks versatility and often requires expensive monomers. In the search fora versatile method which can be applied to conventional monomers, we investigated solvent effects on the stereochemistry of the vinyl ester polymerization. The solvent effect on the stereochemistry of V A c polymerization is generally very small. However, the polymerization in phenol has been reported exceptionally to afford a polymer rich in syndiotacticity (rr = 32-33%) (8). This effect has been ascribed to the increased bulkiness of the monomer's side group due to hydrogenbond formation between the solvent and the acetyl group. This work stimulated us to investigate the radical polymerization of vinyl esters in a wide range of protic solvents that would efficiently interact with the ester groups of the monomers and the growing species through the hydrogen-bonding. We were especially interested in fluoroalcohols as solvents because they are more acidic than the corresponding parent alcohols and the monomers containing fluoroalkyi groups are known to give syndiotactic-rich polymers (3,7). The solvents used for the polymerization in this study include C F C H O H , ( C F ) C H O H , and (CF ) COH. The structures of the monomers employed here are shown below. Unambiguous solvent effects on the polymerization stereochemistry were confirmed as discussed hereafter. However, the mechanism of the effects is still not completely clear, and the discussion on this aspect contains speculations because the reaction systems are quite complex. 3

VAc

2

VPr

3

ViBu

2

VPi

3

VDMB

3

VDMV

VF6PÎ

VBz

Polymerization of Vinyl Esters Using Fluoroalcohols As Solvents

Polymerization of V A c Table I shows the results of the polymerization of V A c in various solvents. The V A c polymerization in fluoroalcohols gave polymers with higher syndiotacticity (rr) than that obtained in bulk polymerization or in methanol. The stereochemical effect


59 was larger with a solvent having a smaller pK and higher bulkiness and at lower temperature. Syndiotactic specificity of the V A c polymerization in (CF )3COH at-78 °C reached an rr of 50% (r = 72%) (run 8), which is the highest for the radical polymerization of vinyl esters (9a). This effect may be due to the steric or electrostatic repulsion between the apparently bulky side groups of the monomer and the growing chain-end hydrogen-bound by the fluoroaicohol as will be discussed later. a

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Table I. Radical Polymerization of VAc in Alcoholic Solvents Downloaded by STANFORD UNIV GREEN LIBR on September 20, 2012 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0768.ch004

Run

Solvent of Solvent

1 2

—

None (CH ) COH

19 16 CH OH CF CH OH 12.4 (CF )2C(CH )OH 9.6 (CF ) CHOH 9.3 (CFJ) COH 5.2 (CF ) COH 5.2 3

3 4

3

3

3

5 6 7 8

2

3

3

3

2

3

3

3

Temp. Time h °C 1 24 24

Ac

Yield M xlO b

n

20 20 20 20 20

24 24

71 73 58 62 64

20 20 -78

24 24 168

81 94 50

5.5 6.4 1.1 1.3 1.9 1.7 6.2 8.3

c

MJM

n

%

d

Tacticity /9b r° mm mr rr

1.9 1.6 1.9 1.9 2.7

22.6 20.9 22.2 19.8 17.9

28.5 29.5 28.6 30.3 32.1

52.9 54.3 53.2 55.2 57.1

2.0 1.8 1.5

18.6 49.1 32.3 13.0 49.4 37.6

57.7 62.3 72.2

5.4

48.9 49.5 49.2 49.9 50.0

44.9 49.8

a

[VAc] = 2.2 M (20 vol%). Polymerizations at 20°C and -78°C were initiatied with AIBN (0.15 M) under UV light irradiation and with («Bu) B (0.20 M) in the presence of a small amount of air, respectively. Et 0-insoluble part. Determined by GPC of original polymers using standard polystyrenes in THF. Determined by H or C NMR of PVA in DMSO-i/ . Calculated on the basis of triad tacticity (r = rr + mr/2). SOURCE: Adapted from reference 9b. t)

3

2

L d

!

, 3

6

u

4

The differences in activation enthalpy (AH*) and activation entropy (Δ5 ) between isotactic and syndiotactic-specific propagation for the V A c polymerization are summarized in Table II along with the data for the bulk polymerization ofVF6Pi. The data were obtained from the results of polymerization at different temperatures. The large positive values of AH* - AH * and the small positive values of AS* - AS * in the V A c polymerization in the fluoroalcohols indicate that the syndiotactic-specific propagation was favored by enthalpy and disfavored by entropy. These results are similar to those for the VF6PÎ polymerization. This suggests that the stereochemical mechanism of the V A c polymerization in the bulky fluoroalcohols may be similar to that for the VF6Pi polymerization. Bulkiness of V A c and the growing chain hydrogen-bound by the bulky solvents may have similar steric effects to that of the bulky VF6PÎ monomer and its growing radical. S

S

Polymerization of Monomers Bulkier than V A c Polymerizations of the monomers bulkier than V A c including VPr, V i B u , V P i , V D M B , and V D M V were also examined using the fluoroalcohols as solvents. In contrast to the syndiotacticity-enhancing effect of the fluoroalcohols in the V A c polymerization, the solvents appeared to be effective for the polymerization of these monomers in increasing heterotactic triad content (mr) and at the same time meso


60 diad content (m) o f the polymers ( T a b l e III). The polymerization o f V P i in (CF )3COH at - 4 0 °C gave an mr o f 6 1 % (run 11), which is thehighest reported for the radical homopolymerization o f v i n y l monomers (9b). T h e mechanism o f the heterotactic-specific polymerization w i l l be discussed later. 3

Table II. Activation Parameters for the Polymerization of VAc and VF6Pi


Monomer VAc

Solvent

[Solvent] vol%

0

None PhOH 3

3

VF6Pi a

2

-0.3 ± 0 . 2

-10±40

a

10 80 80 80 80 0

Reference

AS*-AS*

cal/degTnol

cal/mol

0

CH OH CF CH GH (CF^HOH (CFÔH None

AH*-AH*

9b

-0.35 0.0 ± 0.3 0.3 ± 0.3 1.1 ±0.3 0.9 ±0.2 0.2

28 80 ± 7 0 200 ± 7 0 520 ± 7 0 550 ± 5 0 460

8 9b 9b 9b 9b 7

wt %.

Table III. Radical Polymerization of Bulkier Monomers than VAc* b

Temp. Time Conv. M xl0 °C ~TT %

Run Monomer Solvent 1 2 3 4

VPr

n

5 6 7

ViBu VPi VPi VDMB VDMV VF6PÎ

None None None None None None None

8 9 10 11 12 13 14

VPr

(CFÔH

ViBu VPi VPi VDMB VDMV VF6PÎ

(CFÔH (CFÔH. (CFÔH (CF^COH (CF ) COH (CFÔH 3

20 20 20 -40 20 20 20

3

2 4 1 24 4 4 9 24

20 20 20

24 24

-40 20 20 20

24 24 24 24

31 58 65 52 c

76 45 72

c

83 70 73 84 63 53 25

c

c

d

4

c

w

4.3 1.7 7.1 15.3 8.3 21.7 4.0

1.6 3.1 1.4

4.7

1.8

2.9 2.4 4.2 1.9 1.9 1.2

1.7 2.0

1.9 2.1 1.6 2.1

1.8 3.0 1.6 1.9

Tacticity mm mr rr 19.5 49.9 30.5 16.7 13.7 11.1 14.5 12.9 Π.2

49.8 49.2 47.9 48.7 48.4 44.5

33.6 37.1 41.0 36.8 38.8 44.4

14.6 53.4 32.1 16.9 57.0 26.1 20.7 21.3 21.0 20.3 12.9

57.5 61.0 57.7 56.0 45.7

a

21.8 17.7 21.4 23.7 41.4

Polymerizations at 20 °C and -40 °C were initiated with Ai Β Ν (0.15 M) under UV light irradiation and with («Bu) B (0.20 M) in the presence of a small amount of air, respectively. Determined by H NMR of reaction mixture in acetone-t/. Determined by GPC of original polymers using standard polystyrenes in THF. Determined by Ή o r X NMR of PVA in DMSO-

Stereochemistry in Radical Polymerization of Vinyl ... - ACS Publications

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