Living Anionic Polymerization of 4-(Î±-Alkylvinyl)styrene Derivatives

Macromolecules 2008, 41, 4235-4244

4235

Living Anionic Polymerization of 4-(R-Alkylvinyl)styrene Derivatives Kenji Sugiyama, Kenji Watanabe, and Akira Hirao* Polymeric and Organic Materials Department, Graduate School of Science and Engineering, H-127, 2-12-1 Ohokayama, Meguro-ku, Tokyo 152-8552, Japan

Mayumi Hayashi Petrochemicals Research Laboratory, Sumitomo Chemical Co., Ltd. 2-1, Kitasode, Sodegaura, Chiba 299-0295, Japan ReceiVed April 9, 2008

ABSTRACT: The anionic polymerizations of four new 4-(R-alkylvinyl)styrene derivatives have been studied under various conditions. These monomers involve 4-(R-ethylvinyl)styrene (2), 4-(R-butylvinyl)styrene (3), 4-(Risopropylvinyl)styrene (4), and 4-(R-tert-butylvinyl)styrene (5). Among them, the R-tert-butyl derivative, 5, was found to undergo selective living anionic polymerization under the conditions in THF at -78 °C as well as in benzene at 25 °C to afford remarkably stable living polymers. The resulting polymers possessed well-controlled molecular weights (Mn ∼ 80 000 g/mol) and narrow molecular weight distributions (Mw/Mn < 1.05) as well as an R-tert-butylvinyl group in all monomer units. On the other hand, the carefully selected conditions are required to achieve the living anionic polymerization of 2, 3, and 4. They could undergo selective living anionic polymerization in THF at -78 °C with use of oligo(R-methylstyryl)potassium as an initiator. In contrast, the living anionic polymerizations of such monomers were not successful under the conditions with sec-BuLi in THF at -78 °C and in benzene at 25 °C. Considering the detailed polymerization results, the stability of chainend carbanions substituted with 4-R-alkylvinyl groups were observed to increase in the following order: living poly(2) ∼ living poly(3) < living poly(4) , living poly(5). This is exactly the same order as those of electrondonating ability and steric bulkiness of the R-substituent, and accordingly, such factors may possibly play an essential role to achieve the selective living polymerization of 4-(R-alkylvinyl)styrene derivatives. The three polymer reactions of poly(4-(R-alkylvinyl)styrene) with m-chloroperoxybenzoic acid, bromine, and sec-BuLi were performed in order to transform the pendant R-alkylvinyl groups into the corresponding epoxide, bromide, and carbanionic species. These reactions were found to proceed cleanly and quantitatively to afford the new functional polymers possessing the above reactive groups in all monomer units and well-defined structures originated from the base polymer.

Introduction

Scheme 1

In general, gelation occurs instantaneously in the anionic polymerization of dual-functionalized monomers represented as divinylbenzenes and R,ω-ethylene glycol dimethacrylates, resulting in the formation of highly cross-linked materials insoluble in all solvents.1–5 Although soluble polymers were obtained in moderate conversions (∼50%) in the anionic polymerization of diisopropenylbenzenes, the occurrence of chain branching followed by cross-linking was not avoidable at the final stage of the polymerization.6–9 Thus, it is difficult to achieve the selective polymerization of such dual-functionalized monomers because of a small difference in reactivity between the same two vinyl groups. Herein, the question has been raised: what happens in the anionic polymerization of an asymmetric dual-functionalized monomer of 4-isopropenylstyrene (1) (hereafter 4-(R-methylvinyl)styrene) possessing both a vinyl group and an R-methylvinyl polymerizable group? The R-methylvinyl group may be less reactive toward anionic species than the vinyl group because of a higher electron density of the β-carbon of an R-methylvinyl group by electron donation of the R-methyl substituent. In addition, the steric hindrance due to the presence of an R-methyl substituent further reduces its reactivity. Accordingly, we speculated that if 1 were treated with an anionic initiator, the more reactive vinyl group would be selectively polymerized and the less reactive R-methylvinyl group would remain unreacted, as illustrated in Scheme 1. * To whom correspondence should be addressed.

We have indeed found for the first time that the selective living anionic polymerization of 1 proceeds as speculated under carefully selected conditions.10,11 The polymers thus obtained were essentially soluble and linear polymers with controllable molecular weights (Mn ) 5000-61 000 g/mol) and narrow molecular weight distributions (Mw/Mn < 1.08). It has been however noticed that the chain-end styryl-type anion was stable only for a short period (∼5 min) and gradually attacked on the pendant R-methylvinyl group to cause chain branching. This attack seems not to be surprising in view of the fact that styryltype anion readily and quantitatively initiates the polymerization of R-methylstyrene under identical conditions. Thus, the success of the selective polymerization of 1 may be resulted from a very delicate balance in reactivity between the chain-end styryltype anion and the R-methylvinyl group. It is therefore desirable to molecularly design the monomer as to clearly differentiate the reactivity between the vinyl group and the R-substituted vinyl groups in order to obtain more stable living anionic polystyrene functionalized with an R-substituted vinyl group.

10.1021/ma8007866 CCC: $40.75  2008 American Chemical Society Published on Web 05/30/2008

4236 Sugiyama et al.

Macromolecules, Vol. 41, No. 12, 2008 Scheme 2

The present study deals with the anionic polymerization behaviors of the following four new asymmetric dual-functionalized 4-(R-alkylvinyl)styrene derivatives, as illustrated in Scheme 2: 4-(R-ethylvinyl)styrene (2), 4-(R-butylvinyl)styrene (3), 4-(R-isopropylvinyl)styrene (4), and 4-(R-tert-butylvinyl)styrene (5). Since the R-alkyl substituents of ethyl, butyl, isopropyl, and tert-butyl groups are more electron rich and sterically bulkier than a methyl group, the reactivity difference between the vinyl and either of the R-alkylvinyl groups increases, and for this reason, the substitution of such groups would be expected to suppress the chain-end anion attack to achieve the selective living anionic polymerization. Experimental Section Materials. All the reagents (>98% purities) were purchased from Tokyo Kasei Kogyo Co. Ltd. or Aldrich Japan and used as received unless otherwise stated. 4-Chlorostyrene (>99%, Hokko Chemical Industry Co., Ltd.) was distilled over CaH2 under reduced pressure. Propionic, valeric, isobutylic, and pivalic anhydrides were distilled over CaH2 under reduced pressures. Tetrahydrofuran (THF) was refluxed over Na wire, distilled over LiAlH4 under nitrogen, and then distilled from its sodium naphthalenide solution under highvacuum conditions (10-6 Torr). Benzene was washed with concentrated H2SO4 and H2O, dried over MgSO4, distilled over CaH2 under nitrogen, and finally distilled from its 1,1-diphenylhexyllithium solution under high-vacuum conditions. R-Methylstyrene was washed with 10% NaOH aqueous solution, dried over MgSO4, distilled over CaH2, and finally distilled over dibutylmagnesium (ca. 5 mol %) on the high-vacuum line into ampules equipped with break-seals that were prewashed with potassium naphthalenide in THF. sec-Butyllithium (sec-BuLi) was used as received and diluted appropriately with heptane under high-vacuum conditions. Potassium tert-butoxide (tert-BuOK) was prepared by the colorimetric titration of tert-butanol in THF with potassium naphthalenide in THF at -78 °C to a colorless end point under high-vacuum conditions. Oligo(R-methylstyryl)lithium was prepared by mixing sec-BuLi to a 3-fold excess of R-methylstyrene in THF first at 25 °C for 30 s followed by reacting at -78 °C for 15 min. Oligo(Rmethylstyryl)potassium was prepared by treatment of oligo(Rmethylstyryl)lithium with a 5-fold excess of tert-BuOK to replace the countercation of Li+ by K+. Measurements. Both 1H and 13C NMR spectra were measured on a Bruker DPX300 in CDCl3. Chemical shifts were recorded in ppm downfield relative to tetramethylsilane (δ 0) and CDCl3 (δ 77.1) for 1H and 13C NMR, respectively. Size exclusion chromatography (SEC) was performed on an Asahi Techneion AT-2002 equipped with a Viscotek TDA model 302 triple detector array using THF as a carrier solvent at a flow rate of 1.0 mL/min at 30 °C. The molecular weight (Mn) and molecular weight distribution (Mw/ Mn) were determined by a right angle laser light scattering (RALLS) and a refractive index (RI) detectors with a calibration curve made by standard polystyrene samples. 4-(r-Ethylvinyl)styrene (2). Under nitrogen, 4-vinylphenylmagnesium chloride (Grignard reagent) was prepared by slowly adding 4-chlorostyrene (25.6 g, 184 mmol) in THF (65 mL) to Mg (5.82 g, 240 mmol) in THF (115 mL) under reflux over a period of 1 h followed by stirring for an additional 2 h. The Grignard reagent thus prepared was reacted with propionic anhydride (35.9 g 276 mmol) in THF (50 mL) at 0 °C, and the reaction mixture was allowed to react at 25 °C for 24 h. Then, the reaction mixture was neutralized with 2 N HCl(aq), extracted with ether, washed with

NaHCO3(aq), and dried over MgSO4. Removal of the solvent under reduced pressure followed by flash column chromatography (hexanes/ethyl acetate ) 20/1, v/v) yielded ethyl 4-vinylphenyl ketone (16.4 g, 103 mmol) as a colorless liquid in 56% yield. Then, ethyl 4-vinylphenyl ketone (16.4 g, 103 mmol) thus synthesized was reacted with methyltriphenylphosphonium bromide (43.6 g, 122 mmol) and potassium tert-butoxide (15.0 g, 134 mmol) in THF (120 mL) at 0 °C under nitrogen. The reaction mixture was stirred at 25 °C for 3 h and quenched with a small amount of water. The organic layer was extracted with ether and dried over MgSO4. It was concentrated and poured into hexane to precipitate triphenylphosphine oxide. After removal of solvents, flash column chromatography (hexanes) followed by fractional distillation gave 2 as a colorless liquid (6.76 g, 42.7 mmol); bp 45.0-47.0 °C/1 mmHg. 300 MHz 1H NMR (CDCl3): δ ) 7.38 (s, 4H, Ar), 6.72 (dd, 1H, J ) 18 and 11 Hz, CH2dCH), 5.75 and 5.24 (2d, 2H, J ) 18 Hz, CH2dCH), 5.30 and 5.06 (d, 2H, CH2dCCH2), 2.52 (m, 2H, CH2CH3), 1.11 (t, 3H, J ) 7 Hz, CH2CH3). 75 MHz 13C NMR (CDCl3): δ ) 149.6 (CH2dCCH2), 140.9 (ArC1), 136.6 (CH2dCH), 136.5 (ArC4), 126.5 (ArC3), 126.2 (ArC2), 113.6 (CH2dCH), 111.0 (CH2dCCH2), 28.0 (CH2CH3), 13.0 (CH2CH3). Anal. Calcd for C12H14: C, 91.08; H, 8.92. Found: C, 91.20; H, 8.98. 4-(r-Butylvinyl)styrene (3). The title compound 3 was synthesized in 62% yield by a similar procedure mentioned above, except for use of valeric anhydride. After usual work-up, flash column chromatography followed by fractional distillation gave pure 3; bp 60.0 - 62.0 °C/0.5 mmHg. 300 MHz 1H NMR (CDCl3): δ ) 7.42 (s, 4H, Ar), 6.76 (dd, 1H, J ) 18 and 11 Hz, CH2dCH), 5.79 and 5.28 (2d, 2H, J ) 18 Hz, CH2dCH), 5.33 and 5.10 (2s, 2H, CH2)CCH2), 2.55 (t, 2H, CH2(CH2)2CH3), 1.52-1.36 (m, 4H, CH2(CH2)2CH3), 0.95 (t, 3H, J ) 7 Hz, CH3). 75 MHz 13C NMR (CDCl3): δ ) 148.3 (CH2dCCH2), 140.9 (ArC1), 136.6 (CH2dCH), 136.6 (ArC4), 126.3 (ArC3), 126.2 (ArC2), 113.6 (CH2dCH), 112.0 (CH2dCCH2), 35.0 (CH2CH2CH2CH3), 30.6 (CH2CH2CH2CH3), 22.5 (CH2CH2CH2CH3), 14.0 (CH3). Anal. Calcd for C14H18: C, 90.26; H, 9.74. Found: C, 90.00; H, 9.70. 4-(r-Isopropylvinyl)styrene (4). The title compound 4 was synthesized in 56% yield by a similar procedure mentioned above, except for use of isobutyric anhydride. After usual work-up, flash column chromatography followed by fractional distillation gave pure 4; bp 63.0-65.0 °C/1.5 mmHg. 300 MHz 1H NMR (CDCl3): δ ) 7.39-7.31 (m, 4H, Ar), 6.72 (dd, 1H, J ) 18 and 11 Hz, CH2dCH), 5.75 and 5.24 (2d, 2H, J ) 18 Hz, CH2dCH), 5.17 and 5.04 (2s, 2H, CH2dCCH), 2.78-2.91 (m, 1H, CH(CH3)2), 1.11 (d, 6H, J ) 7 Hz, CH3). 75 MHz 13C NMR (CDCl3): δ ) 155.3 (CH2dCCH), 142.3 (ArC1), 136.6 (CH2dCH), 136.5 (ArC4), 126.8 (ArC3), 126.1 (ArC2), 113.6 (CH2dCH), 110.0 (CH2dCCH), 32.1 (CH(CH3)2), 22.1 (CH3). Anal. Calcd for C13H16: C, 90.64; H, 9.36. Found: C, 90.81; H, 9.31. 4-(r-tert-Butylvinyl)styrene (5). The title compound 5 was synthesized in 56% yield by a similar procedure mentioned above, except for use of pivalic anhydride. After usual work-up, flash column chromatography followed by fractional distillation gave pure 4; bp 60.0-62.0 °C/0.5 mmHg. 300 MHz 1H NMR (CDCl3): δ ) 7.36 and 7.14 (2d, 4H, J ) 8 Hz, Ar), 6.75 (dd, 1H, 1J ) 18 Hz, 2J ) 11 Hz, CH dCH), 5.77 and 5.25 (2d, 2H, 1J ) 18 Hz, 2 CH2dCH), 5.20 and 4.80 (2d, 2H, 1J ) 1 Hz, CH2dCC), 1.15 (s, 9H, CH3). 75 MHz 13C NMR (CDCl3): δ ) 159.7 (CH2dCC), 143.3 (ArC1), 136.7 (CH2dCH), 135.7 (ArC4), 129.3 (ArC3), 125.3 (ArC2), 113.4 (CH2dCH), 111.6 (CH2dCC), 36.2 (C(CH3)3), 29.8 (CH3). Anal. Calcd for C14H18: C, 90.26; H, 9.74. Found: C, 90.06; H, 9.64. These monomers, 2-5, were finally distilled over dibutylmagnesium (ca. 5 mol %) under high-vacuum conditions (10-6 Torr). They were diluted with THF (ca. 0.5 M) or benzene (ca. 1.0 M). Anionic Polymerization. All operations were carried out under high-vacuum conditions (10-6 Torr) in the all-glass reactors equipped with break-seals that was prewashed with the initiator solution. Two polymerization procedures have been adapted to examine the anionic polymerization behaviors of 2-5. The first polymerization is conducted in THF at -78 °C with either sec-

Macromolecules, Vol. 41, No. 12, 2008

Polymerization of 4-(R-Alkylvinyl)styrene Derivatives 4237

Figure 1. 1H NMR spectrum of poly(2). Table 1. Anionic Polymerization of 4-(r-Alkylvinyl)styrenes (2-5) with Oligo(r-methylstyryl)potassium in THF at -78 °Ca -3 monomer type, initiator time Mn × 10 mmol (mmol) (h) calcb obsc

Mw/Mnd

coupling product (%)e

2, 6.07 0.0855 1 11.9 11.6 1.08 2, 13.4 0.108 1 18.9 21.6 1.08 2, 18.5 0.0929 1 32.2 37.5 1.07 3, 4.99 0.0851 1 11.4 9.61 1.07 3, 5.86 0.0555 1 20.3 18.1 1.08 3, 4.90 0.0785 24 12.2 10.6 1.04 12 4, 4.98 0.0839 1 10.6 9.15 1.07 4, 12.2 0.0737 1 29.3 29.3 1.12 4, 4.08 0.0707 24 10.5 10.6 1.03 5 5, 4.38 0.0830 48 10.4 8.50 1.06 a Yields of polymers were always quantitative. b Mn(calc) ) MW of oligo(R-methylstyryl) group + [M]/[I] × [MW of monomer]. c Determined by RALLS. d By SEC. e Determined by the SEC area ratio.

BuLi or oligo(R-methylstyryl)potassium. A monomer in THF (ca. 0.5 M) precooled at -78 °C was added to the initiator in THF (0.025-0.050 M) with vigorous shaking at -78 °C. The polymerization mixture was allowed to stand for an appropriate time at -78 °C and then terminated with a small amount of degassed methanol at -78 °C to avoid the unwanted oxidation followed by coupling reaction between intermediate polymers. The second polymerization is carried out in benzene at 25 °C with sec-BuLi as an initiator. A monomer in benzene (ca. 1.0 M) was mixed with the initiator solution (0.025-0.050 M) at 25 °C, and the polymerization mixture was allowed to stand for 45 min at 25 °C. The same termination as that mentioned above was conducted. The postpolymerization was carried out as follows: Similar to the polymerization mentioned above, the first polymerization of either 4-(R-alkylvinyl)styrene derivative was carried out with oligo(R-methylstyryl)potassium in THF at -78 °C for 30 min, and a small amount of the polymerization solution was taken to determine the molecular weight and molecular weight distribution of the first polymer. Then, the second monomer in THF was added to the polymerization solution. The polymerization mixture was allowed to stand at -78 °C for 30 min and quenched with a small amount of degassed methanol at -78 °C. The polymers were precipitated by pouring the reaction mixture into a large amount of methanol, purified by reprecipitation from THF solution to

Figure 2. SEC curves of poly(3) obtained under the conditions with oligo(R-methylstyryl)potassium in THF at -78 °C after 1 h (a) and 24 h (b).

methanol twice, and finally freeze-dried from their absolute benzene solutions. Epoxidation of Poly(5). The following careful operation is needed for this epoxidation reaction. If a large excess of m-chloroperoxybenzoic acid to alkene was used, undesirable coupling products (∼10%) were often formed. Therefore, the reaction was repeated three times with sequential use of 0.8, 0.4, followed by 0.1 equiv of m-chloroperoxybenzoic acid. Under nitrogen, m-chloroperoxybenzoic acid (2.56 mmol) in dichloromethane (10 mL) was slowly added to poly(5) (0.555 g, Mn ) 33 700, 2.98 mmol for CdC bond)



Figure 3. SEC curves of polymers obtained under the conditions with oligo(R-methylstyryl)potassium in THF at -78 °C for 1 h. Table 3. Anionic Polymerization of 4-(r-Alkylvinyl)styrenes

(2-5) with sec-BuLi in THF at -78 °Ca

monomer type, sec-BuLi time mmol (mmol) (min)

Mn × 10-3 calc

obsb

coupling Mw/Mnc product (%)d

2, 8.19 0.140 5 9.31 10.4 1.09 13 3, 5.94 0.121 5 9.20 8.69 1.07 9 4, 6.68 0.109 5 10.6 10.5 1.05 4, 5.99 0.106 1h 9.78 8.19 1.24 13 5, 9.23 0.0570 1 h 30.2 33.7 1.02 5, 14.5 0.0367 1 h 74.0 83.1 1.04 5, 6.49 0.123 24 h 9.89 9.24 1.03 a Yields of polymers were always quantitative. b Determined by RALLS. c By SEC. d Determined by the SEC area ratio.

Figure 4. SEC curves of poly(3) (solid line) and base polymer (dashed line). Table 2. Postpolymerization of 4-(r-Alkylvinyl)styrenes (2-4)

with Oligo(r-methylstyryl)potassium in THF at -78 °Ca

stage

time (min)

monomer type, mmol

initiator (mmol)

Mn × 10-3 calcb

obsc

Mw/Mnd

1st 30 2, 4.96 0.124 6.76 7.34 1.09 2nd 30 2, 6.30 0.0889 18.5 19.2 1.11 1st 30 3, 4.43 0.122 7.15 7.08 1.08 2nd 30 3, 5.76 0.0866 19.5 21.1 1.07 1st 30 4, 5.33 0.119 8.07 8.07 1.07 2nd 30 4, 6.00 0.0794 21.2 20.3 1.05 1st 30 5, 3.89 0.120 6.42 6.22 1.06 2nd 30 5, 6.41 0.0852 20.2 19.5 1.04 a Yields of polymers were always quantitative. b Mn(calc) ) MW of oligo(R-methylstyryl) group + [M]/[I] × [MW of monomer]. c Determined by RALLS. d By SEC.

dissolved in dry dichloromethane (70 mL) at 0 °C, and the reaction mixture was allowed to react at 25 °C for an additional 3 h. The reaction was terminated with 10% K2CO3(aq). After removal of the solvents, the residual polymer was dissolved in THF, and the resulting THF solution was poured into methanol to precipitate the polymer. The polymer was recovered by filtration and freeze-dried from its dry benzene solution. The reaction of poly(5) with

m-chloroperoxybenzoic acid followed by isolation of the polymer was repeated two more times. The objective polymer was purified by reprecipitation from THF to methanol and freeze-dried from its dry benzene solution. The polymer yield was 80%. 300 MHz 1H NMR (CDCl3): δ ) 6.00-7.30 (broad, 4H, Ar), 2.53 and 3.03 (broad, 2H, CH2O), 1.20-2.40 (broad, 3H, main chain), 0.8-1.20 (broad, 9H, C(CH3)3). Bromination of Poly(5). Under nitrogen, bromine (0.360 g, 2.25 mmol) in dichloromethane (10 mL) was carefully added to poly(5) (0.400 g, Mn ) 9240, 2.14 mmol for CdC bond) dissolved in dry dichloromethane (10 mL) at 0 °C, and the reaction mixture was allowed to react for an additional 1 h at 25 °C. Then, a small amount of sodium hydrogen sulfite was added to quench the reaction. After removal of the solvents, the residual polymer was dissolved in THF, and the resulting THF solution was poured into a large amount of methanol to precipitate the polymer. The polymer was further purified by reprecipitation from THF to methanol two more times and freeze-dried from its absolute benzene solution. The objective polymer was obtained in 93% yield. 300 MHz 1H NMR (CDCl3): δ ) 6.20-7.60 (broad, 4H, Ar), 4.31 and 4.70 (broad, 2H, CH2Br), 1.30-2.50 (broad, 3H, main chain), 1.04-1.30 (broad, 9H, C(CH3)3). Similarly, bromination of poly(4) was carried out under the same conditions, and the quantitatively brominated polymer was obtained in 97% yield. 300 MHz 1H NMR (CDCl3): δ ) 6.00-7.60 (broad, 4H, Ar), 4.01 and 4.20 (broad, 2H, CH2Br), 2.53 (broad, 1H, CH(CH3)2), 0.80-2.30 (broad, 9H, CH2CH and CH(CH3)2).


Polymerization of 4-(R-Alkylvinyl)styrene Derivatives 4239

synthesized, four new styrene derivativess4-(R-ethylvinyl)styrene (2), 4-(R-butylvinyl)styrene (3), 4-(R-isopropylvinyl)styrene (4), and 4-(R-tert-butylvinyl)styrene (5)swere synthesized, and their anionic polymerization behaviors were studied under various conditions.

Figure 5. SEC curves of poly(4) obtained after 5 min (a) and 1 h (b) under the conditions with sec-BuLi in THF at -78 °C.

Lithiation of Poly(4). Lithiation of poly(4) was carried out with sec-BuLi in tert-butylbenzene at 25 °C for 168 h followed by quenching with Me3SiCl under high-vacuum conditions. Poly(4) (0.150 g, Mn ) 8980, 0.871 mmol for CdC bond) dissolved in tert-butylbenzene (10 mL) was mixed with sec-BuLi (1.90 mmol) in heptane (2.0 mL) at 0 °C, and the reaction mixture was allowed to react at 25 °C for 168 h. Then, the reaction mixture was diluted with THF (20 mL) at -78 °C followed by addition of Me3SiCl (2.81 mmol) in THF (1.0 mL), and the reaction mixture was allowed to react at -78 °C for an additional 1 h. After removal of solvents, the residual polymer was dissolved in THF, and the resulting THF solution was poured into a large amount of methanol to precipitate the polymer. The polymer was purified by reprecipitation from THF to methanol two more times and freeze-dried from its absolute benzene solution. The objective polymer was obtained in 91% yield. 300 MHz 1H NMR (CDCl3): δ ) 5.50-7.30 (broad, 4H, Ar), 0.50-3.00 (broad, 21H, aliphatic protons), -0.11 (broad, 9H, Si(CH3)3).

Results and Discussion The 4-(R-alkylvinyl)styrene derivatives herein studied were synthesized in moderate yields by the reaction of the Grignard reagent derived from 4-vinylphenyl chloride with the corresponding acid anhydrides followed by treatment with the Wittig reagent. In addition to 4-(R-methylvinyl)styrene (1) previously

Anionic Polymerizations of 2-5 with Oligo(r-methylstyryl)potassium in THF at -78 °C. As previously reported, the anionic polymerization of 1 with either sec-BuLi or oligo(R-methylstyryl)lithium was very rapid and complete within 5 min in THF at -78 °C.10 Surprisingly, no gelation occurred under such conditions, and soluble polymers were quantitatively obtained. However, the resulting polymers exhibited multimodal distributions ranging from 104 to 105 g/mol in molecular weight along with sharp peaks corresponding to the targeted Mn values (∼104 g/mol). Thus, it may be considered that the vinyl group of 1 is first polymerized selectively, followed by attack of the chainend styryl-type anion on the pendant R-methylvinyl group. In contrast to these results, the anionic polymerization of 1 with the initiator prepared from oligo(R-methylstyryl)lithium and a 5-fold excess of tert-BuOK was found to proceed in a living manner, affording the soluble linear polymers with wellcontrolled Mn values (9200-61 000 g/mol), in agreement with those calculated from [monomer] to [initiator] feed ratios and narrow molecular weight distributions (Mw/Mn < 1.05). Since Lochmann and co-workers previously reported that the Li+ countercation of RLi can be exchanged to K+ by adding a large excess of tert-BuOK to shift the equilibrium, oligo(R-methylstyryl)lithium is considered to convert to oligo(R-methylstyryl)potassium for the same reason.12,13 This is strongly supported by the fact that the similar satisfactory results were obtained by the anionic polymerization of 1 with the initiator bearing K+ such as potassium naphthalenide and cumylpotassium.10,11 The satisfactory results can be explained as follows: The Li+ of chain-end anion is strongly coordinated by a few THF molecules to dissociate the ion pairs, resulting in the production of a highly reactive solvent-separated ion pair and/ or free ion, while the coordination of THF toward the K+ is less favorable because of its bigger size. Accordingly, the chainend anion with Li+ is more reactive than that with K+ in THF, thus impeding the anion attack on the pendant R-methylvinyl group.14 Unfortunately, however, the anion attack gradually occurred to broaden the molecular weight distributions by allowing the polymerization mixture to stand for 1 h. Accordingly, the chain-end styryl-type anion was stable and coexisted with the R-methylvinyl group only for a short period (∼5 min) even under the conditions in THF at -78 °C. Next, the anionic polymerization of the R-ethyl derivative, 2, was performed with oligo(R-methylstyryl)potassium under the same conditions. The polymerization mixture was allowed to stand for an additional 0.5 h, although the polymerization concluded within 5 min. The polymerization mixture showed a characteristic red in color that remained visually unchanged after 1 h. The polymer yield was quantitative. The 1H NMR spectrum of the resulting polymer shown in Figure 1 revealed that the vinyl protons at 6.72, 5.75, and 5.24 ppm disappeared completely, and on the other hand, the signals corresponding to R-ethylvinylene protons at 4.99 and 5.23 ppm remained with keeping the expected intensity ratio of R-ethylviylene protons (2H) to aromatic protons (4H). The polymer exhibited a sharp monomodal SEC peak (Mw/Mn ) 1.08). The molecular weight (Mn ) 11 600 g/mol) absolutely determined by RALLS agreed well with the calculated value (Mn ) 11 900 g/mol). Similarly, polymerswithcontrollablemolecularweights(Mn )21 600-37 500 g/mol) and narrow molecular weight distributions (Mw/Mn < 1.08) could be obtained by the same polymerization system, as listed in Table 1.



Figure 6. SEC curves of poly(4-(R-alkylvinyl)styrene)s obtained with sec-BuLi in benzene at 25 °C for 45 min.

Figure 7. 1H NMR spectrum of poly(5) obtained under the conditions with sec-BuLi in benzene at 25 °C. Table 4. Anionic Polymerization of 4-(r-Alkylvinyl)styrenes

(2-5) in Benzene at 25 °C for 45 min with sec-BuLia

monomer type, mmol

sec-BuLi (mmol)

Mn × 10-3 calc

obsb

Mw/Mnc

coupling product (%)d

2, 5.85 0.0899 10.6 36.9 multimodal 3, 5.41 0.116 8.09 15.2 multimodal 4, 6.49 0.120 9.38 10.2 42 5, 5.02 0.0919 10.2 9.62 1.04 a Yields of polymers were always quantitative. b Determined by RALLS. c By SEC. d Determined by the SEC area ratio.

All of the analytical results clearly indicate that the vinyl group is selectively polymerized, and on the other hand, the R-ethylvinyl group remains completely unreacted and coexists

with the chain-end styryl-type anion generated from 2. Thus, the replacement of a methyl by an ethyl group at the R-position has been very effective to suppress the anion attack leading to chain branching. The anionic polymerizations of other three monomers, 3-5, were also conducted under the same conditions. Yields of polymers were quantitative in all cases. As shown in Table 1, all resulting polymers possess well-controlled molecular weights and narrow molecular weight distributions. High molecular weight shoulders were not formed. Thus, these results confirm the selective living anionic polymerization of each of 3-5. By extending the polymerization time of 24 h, small shoulders corresponding to twice the molecular weights of the main peaks


Polymerization of 4-(R-Alkylvinyl)styrene Derivatives 4241 Scheme 3

appeared in the polymers by the polymerizations of 3 and 4. Comparing two peak areas (main peaks and shoulders), it was determined that the degrees of the chain- branching were 12% and 5%, respectively. Figure 2 represents SEC profiles of the poly(3) obtained after 1 and 24 h as a typical example. As was readily expected, the living anionic polymer of 5 was very stable and maintained a narrow molecular weight distribution even after 48 h. Thus, the replacement of a methyl group by a tertbutyl group was found most effective. This is possibly because the tert-butyl group possesses the most electron-donating ability and the highest steric bulkiness among the R-substituents herein studied. Some representative SEC profiles of the polymers are shown in Figure 3. In order to clarify the persistency of the living anionic polymer derived from either 2, 3, or 4, the postpolymerization of each monomer was carried out in THF at -78 °C with oligo(R-methylstyryl)potassium, similar to those for its homopolymerization. A two-step sequential monomer addition was employed. The first polymerization was conducted for 30 min, and then the second monomer was sequentially added in situ to continue the polymerization. As a typical example, SEC profiles of poly(3) obtained at the first and second polymeri-

Figure 8. SEC curves of the resulting polymer before (dashed line) and after (solid line) epoxidation of poly(5).

zation steps are shown in Figure 4. As can be seen, the molecular weight of the second polymer increases as designed, while the molecular weight distributions remains narrow and no polymer obtained by the first polymerization remains at all. These results are summarized in Table 2. Thus, the living character of the polymerization of 3 has been successfully demonstrated. Similarly, the satisfactory results were obtained by the postpolymerization of 2 or 4 (also see Table 2). Anionic Polymerizations of 2-5 with sec-BuLi in THF at -78 °C. The anionic polymerizations of 2-5 with sec-BuLi in THF at -78 °C were investigated in order to examine the effect of countercation on the polymerization. At first, the polymerizations were stopped after 5 min to compare the previous polymerization result of 1 where chain branching occurred in 56 mol % of active chain end. The 1H NMR spectra of the resulting polymers all showed that the vinyl groups completely disappeared, and on the other hand, the R-alkylvinyl functions remained. Upon analysis of the polymers by SEC, it was found that there was a remarkable decrease in the degree of chain branching by replacing a methyl by an ethyl or a butyl at the R-position. For example, the SEC profile of poly(2) exhibited a sharp monomodal main peak along with a small shoulder with a molecular weight double that of the main peak. The amount of dimer was only 13% and found to be 9% in the case of poly(3) obtained under the same conditions. As listed in Table 3, the Mn values of the polymers (main peaks) agree with those calculated. The living anionic polymerization behavior was improved by introducing isopropyl group into the R-position. The high molecular weight shoulder was almost negligible in poly(4). However, the chain-end anion attack gradually occurred with time, and a high molecular weight shoulder was formed in 13% after 1 h, as shown in Figure 5. A remarkably stable living anionic polymer was obtained when the R-tert-butyl derivative 5 was polymerized under the above conditions. No shoulder was formed after 1 h and even 24 h. The polymers with controllable Mn values up to at least 80 000 g/mol and narrow molecular weight distributions (Mw/ Mn < 1.04) could be obtained (see also Table 3). Once again, the results have clearly demonstrated that both the steric



Figure 9. 1H NMR spectrum of the resulting polymer after epoxidation of poly(5). Table 5. Epoxidation, Bromination, and Lithiation of Poly(4) and

Poly(5) starting polymer reaction epoxidation epoxidation bromination bromination lithiation (silylation) a Determined

resulting polymer

type

Mn × 10-3

Mn,calc × 10-3

Mn,obs × 10-3a

Mw/Mnb

poly(5) poly(5) poly(4) poly(5) poly(4)

9.24 33.7 9.15 9.24 8.98

10.0 36.6 17.2 17.1 15.8

11.0 38.5 19.3 16.4 14.8

1.03 1.03 1.07 1.03 1.09

by RALLS.

b

By SEC.

hindrance and electron-donating ability of the tert-butyl group play an important role to suppress the chain-end anion attack on the pendant R-tert-butylvinyl group. Anionic Polymerizations of 1-5 in Benzene at 25 °C. The living anionic polymerization of styrene can be achieved under the conditions in THF at -78 °C as well as in hydrocarbon solvents such as benzene and cyclohexane at room temperature or higher temperatures. The latter conditions are essential for the preparation of industrially important thermoplastic elastomers of polystyrene-block-polyisoprene (or poly(1,3-butadiene))-block-polystyrene, since high cis-1,4-structures of the middle blocks are required.15 Thus, the anionic polymerization behaviors of 1-5 were examined with sec-BuLi in benzene at 25 °C for 45 min. No gelation occurred, and soluble polymers were quantitatively obtained in all cases under such conditions. However, the poly(1), poly(2), and poly(3) possessed multimodal distributions ranging from 104 to 105 (target Mn values ∼104). In the polymerization of 4, a high molecular weight fraction including dimer and trimer was formed to a certain extent (∼15%) under the same conditions. On the other hand, the poly(5) was found to possess a narrow monomodal SEC distribution, and no high molecular weight fraction was formed as shown in Figure 6. Furthermore, the molecular weight distribution was maintained even after 17 h. Figure 7 shows the 1H NMR spectrum of the resulting polymer. One can observe that the vinyl group is no longer present, and the R-tert-butylvinyl group remains completely unreacted

resulting from the relative signal intensities of R-tert-butylvinyl protons (4.7 and 5.1 ppm, 2H) to aromatic protons (6.3-6.9 ppm, 4H). The molecular weight determined by RALLS agreed well with the calculated value from the feed ratio, as listed in Table 4. Thus, a stable living polymer could be obtained only by the polymerization of 5 under the conditions with sec-BuLi in benzene at 25 °C. In conclusion, new living anionic polystyrenes having an R-tert-butylvinyl group in all monomer units have been obtained by the anionic polymerization of 5 under the conditions with either oligo(R-methylstyryl)potassium or sec-BuLi in THF at -78 °C and with sec-BuLi in benzene at 25 °C. On the other hand, proper choice of conditions is required to achieve the living polymerizations of 2-4. For example, either 2, 3, or 4 was observed to undergo anionic polymerization in THF at -78 °C with use of oligo(R-methylstyryl)potassium as an initiator to give stable living anionic polymers for at least 1 h or more, while the polymers accompanied more or less by chain branching were obtained with sec-BuLi in THF at -78 °C and in benzene at 25 °C. Considering the detailed results of the anionic polymerizations of 1-5, the stability of chain-end carbanions substituted with 4-R-alkylvinyl groups was increased in the following order: living poly(1) < living poly(2) ∼ living poly(3) < living poly(4) , living poly(5). This order exactly correlates with those of steric bulkiness and electron-donating ability of the R-alkyl group, and therefore the two factors play a key role to suppress the chain-end anion attack on the pendant R-alkylvinyl group. Polymer Reaction: Alternative Route to New Functional Polymers. The polymers herein synthesized have reactive pendant R-alkylvinyl groups that can undergo many kinds of reactions to transform into a variety of functional groups. If the reaction proceeds quantitatively, the resulting polymers have functional groups in all monomer units in addition to precisely controllable molecular weights and narrow molecular weight distributions characteristic of the base polymer. Thus, the combination of living anionic polymerization and transformation of R-alkylvinyl group into other functionalities via the polymer reactions will provide an alternative synthetic route to other welldefined functional polymers. In this section, we have introduced the three polymer reactions using poly(4-(R-alkylvinyl)styrene),


as shown in Scheme 3. They involve epoxidation, bromination, and lithiation reactions. The first example is the epoxidation of poly(5) with m-chloroperoxybenzoic acid. The careful reaction condition of choice is required in this reaction. For example, the direct use of a large excess of m-chloroperoxybenzoic acid to the CdC bond of poly(5) always caused the unwanted coupling reaction among the intermediate polymers, although the conversion to epoxide was quantitative. For this reason, 0.8 equiv of m-chloroperoxybenzoic acid was first used, and the polymer was recovered after the reaction. Then, 0.4 equiv followed by 0.1 equiv was used in the second and third reaction steps. The SEC profile of the resulting polymer thus obtained shown in Figure 8 exhibits a narrow monomodal distribution without any shoulder and tailing. In the 1H NMR spectrum of the polymer shown in Figure 9, one can observe that the signals at 4.75 and 5.15 ppm corresponding to vinylene protons of the R-tert-butylvinyl group are no longer present, while new signals at 2.53 and 3.03 ppm assignable for the epoxy ring protons appear at the expected intensity, indicating the quantitative epoxidation. By FT-IR measurement, the absorption band of the CdC bond stretching vibration (1628 cm-1) completely disappeared, and a new band at 1260 cm-1 attributed to the epoxy stretching vibration appeared after the epoxidation reaction. The molecular weight of 38 500 g/mol determined by RALLS is consistent with that calculated (Mn ) 36 600 g/mol) assuming the quantitative reaction, and importantly, the narrow molecular weight distribution was maintained as such (Mw/Mn ) 1.03). All of the analytical results clearly confirm that the epoxidation reaction proceeds cleanly and quantitatively to afford a new functional polystyrene with reactive epoxy group in all monomer units with completely maintaining well-defined chain structures of the starting poly(5) obtained by living anionic polymerization. The results are summarized in Table 5. Similarly, the bromination of both poly(4) and poly(5) with a 1.05-fold excess of bromine proceeded quantitatively without any other side reaction, as illustrated in Scheme 3. The third example is the lithiation reaction of poly(4) with sec-BuLi carried out under the conditions in tert-butylbenzene at 40 °C for 168 h. The intermediate lithiated polymer was quenched with Me3SiCl, and the resulting polymer was characterized. The 1H NMR of the resulting polymer showed that the signals of vinylene protons of the R-isopropylvinyl group disappeared completely, while a new peak at -0.11 ppm characteristic of the silylmethyl protons appeared at the expected intensity. The FT-IR spectrum showed the appearance of a new absorption band at 1249 cm-1, corresponding to the Si-CH3 bond and complete disappearance of the absorption band at 1628 cm-1 corresponding to the CdC bond. The molecular weight determined by RALLS agreed with that calculated, and the narrowness of molecular weight distribution remained along with a small amount of shoulder (

Living Anionic Polymerization of 4-(Î±-Alkylvinyl)styrene Derivatives

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