Polymer Journal, Vol. 25, No. II, pp I 161-1168 (1993)

Stereoregularity of Poly(vinyl ether)s with a Narrow Molecular Weight Distribution Obtained by the Living Cationic Polymerization

Sadahito AosHIMA, Yuuichi ITO, and Eiichi KoBAYASHI*

Department of Industrial Chemistry, Faculty of Science and Technology, Science University of Tokyo, Noda, Chiba 278, Japan

(Received March 30, I 993)

ABSTRACT: The stereoregularity ofpoly(isobutyl vinyl ether) with a narrow molecular weight distribution obtained by the living cationic polymerization in toluene at + 40 - 78oC was studied. As an initiating system, 1-(isobutoxy)ethyl acetate1EtAICI2 was used in the presence of methyl acetate or methyl pivalate as an added base, which can stabilize the propagating carbocation by the nucleophilic interaction. The stereostructure was determined as a diad fraction by 13C NMR spectroscopy and compared with that of the non-living polymer obtained in the absence of added bases. The diad fraction and the temperature coefficient of ln(PmiP,) for the living polymerization were equal to those for the non-living polymerization. The L1H.:; -L1H," was determined to be -900 cal mol- 1, indicating that meso-rich poly(isobutyl vinyl ether) was synthesized favorably at low temperature polymerization. A counteranion and an added base in the living cationic polymerization of IBVE did not affect obviously the stereostructure of polymers obtained. The stereo regularity of seven living poly(vinyl ethers) having various side groups was investigated. Benzyl vinyl ether (meso: 89%) and 2-benzoyloxyethyl vinyl ether (meso: 75%) gave meso-rich living polymers in the presence of ethyl acetate as an added base at ooc. On the other hand, the stereostructure of living poly(2-benzoyloxyethyl vinyl ether) obtained in the absence of added bases (meso: 58%) was quite different from that for the living polymerization in the presence of ethyl acetate. The ln(Pml P,) vs. IIT curve had a maximum at about + 20°C, and it decreased with decreasing the polymerization temperature and became constant at the lower temperature below ooc. The facts and the profile about polymerization rates suggested a stabilization of the propagating carbocation by the side ester group of the terminal unit through an intramolecular interaction. KEY WORDS Stereoregularity I Living Cationic Polymerization I Added Base I Stabilization of Carbocation I Side Ester Group I Vinyl Ethers I Benzyl Vinyl Ether I 2-Benzoyloxyethyl Vinyl Ether I Meso-Rich Polymer I Polymerization Mechanism I

Living polymerization has been known to be polymerization, 5 etc. On the other hand, the very useful and versatile for the control of the control of the stereostructure is also an terminal functional groups, the polymer important research, because physical and molecular weight, and its distribution (MWD). chemical properties of polymers (Tg, 6 solubil­ These control has been achieved by various ity, 7 reactivity, 8 etc.) markedly depend on the polymerization systems; for example, living stereostructure of the polymers. Recently, anionic polymerization, 1 living cationic po­ living stereospecific polymerization has been lymerization with HI/12 initiating system, 2 · 3 investigated mainly in the field of anionic and group transfer polymerization,4 immortal coordination polymerizations as follows: iso-

* To whom all correspondence should be addressed.

I 161 S. AosHIMA. Y. ITo, and E. KoBAYASHI tactic9 or syndiotactic living poly(methyl determined. For instance, poly(BnVE) of meso methacrylate), 10 isotactic living poly( vinyl pyr­ 89% obtained at ooc could be used for the idine)s, 11 syndiotactic living poly(o:-olefine)s, 12 preparation of isotactic poly(vinyl alcohol) etc. with a narrow MWD. 16 The meso content of The authors have recently reported on a new living poly(BzOVE) obtained in the absence of polymerization method for vinyl ether mono­ added bases had a maximum value at about mers that gives nearly monodisperse living + 20°C, suggesting an interaction between the polymers with controlled molecular weight, 13 living polymer end carbocation and the side except for a study with the stereostructure. In ester group of the terminal unit. this paper, to examine the stereostructure of living poly( vinyl ether)s, the living polymeriza­ EXPERIMENTAL tion of isobutyl vinyl ether (IBVE) with

EtA1Cl 2 in the presence of added bases was Materials carried out under various conditions, and the Commercial isobutyl vinyl ether (IBVE), n­ results were compared with those of non-living butyl vinyl ether (NBVE), and neopentyl vinyl polymers. The stereostructure of the living ether (NeoPVE)(Tokyo Kasei) were washed polymers obtained may be concerned with an with an aqueous alkaline solution and then ether group in the vinyl ether monomers, water, and distilled twice over calcium hydride. structures of added bases, counteranions, and 2-Benzoyloxyethyl vinyl ether (BzOVE) was polymerization temperatures, because the prepared by the Bu4 NI-catalyzed substitution living polymer end can be expressed as Scheme reaction of 2-chloroethyl vinyl ether (Nisso 1. Thus, the authors investigated factors Maruzen Chemical) with sodium benzoate at cotrolling the stereostructure of the living a reflux temperature for 12 h, and distilled twice poly(vinyl ether)s in detail. In addition, over calcium hydride15 (yield 51%; bp following seven vinyl ethers (Scheme 2) having 107-1 08°C/2 mmHg) (lit. 17 157°C/3 mmHg). various side groups (alkyl, ether, 14 and ester Benzyl vinyl ether (BnVE) was prepared by the groups 15) were polymerized in the presence of H g( OCOCH 3 ) 2 -catalyzed transetherifica ti on added base or in the absence of added base for between ethyl vinyl ether (Tokyo Kasei) and BzOVE15 to give the living polymers with a benzyl alcohol (Wako Chemical) in the narrow MWD, and the stereostructure was presence of molecular sieves 4A at room temperature for 5 h. The crude product was ® j1 8 distilled under reduced pressure over sodium -- v-y· --ElxAICIJ-x(OCOR') hydride and then over lithium aluminum 6iBu OR 2 hydride16 (yield 38%; bp 69.0°CjlOmmHg) Scheme 1. (lit. 18 69.0°CjlOmmHg). 2-Ethoxyethyl vinyl

CH:z=CH CH2=yH CH:z=YH b-cH2CH(CH3)2 O-cH2C(CH3)J - 0-cHz-@

IBVE NBVE NeoPVE BnVE

EOVE EOEOVE Scheme 2.

1162 Po1ym. J., Vol. 25, No. 11, 1993 Stereoregularity of Living Poly(vinyl ether)s ether (EOVE) and 2-(2-ethoxy)ethoxyethyl and with water to remove the initiator residues, vinyl ether (EOEOVE) were synthesized by the and then neutralized with a dilute aqueous

Bu4 NI-catalyzed substitution reaction of 2- sodium hydroxide (0.5 M). In the cases of chloroethyl vinyl ether with the corresponding EOVE and EOEOVE, the quenched reaction sodium alkoxides at a reflux temperature for mixtures were diluted with dichloromethane, 14 12 h, 19 and distilled twice over calcium and then were treated in a similar manner as hydride14 [each yield bp 126oC for above. The product polymer was recovered EOVE (litY 126°C), 189oC for EOEOVE from the organic layer by evaporation of the (lit. 14 189°C)]. solvent and the remaining monomer under

EtA1Cl 2 and Et2 A1Cl (Nippon Aluminum reduced pressure and vacuum dried overnight. Alkyls) were distilled under reduced pressure. The conversion of the monomer was measured EtuAlClu was prepared by reaction of by gravimetry.

EtA1Cl 2 with an equimolar amount of Et2A1Cl at room temperature for 8 h, and the crude Measurement product was fractionally distilled at 95°C/l4 The MWDs of the polymers were measured mmHg. 14 As a solvent or an added base, using size exclusion chromatography (SEC) in toluene, hexane, ethyl acetate, methyl acetate, chloroform at 40oC with a CCPD instrument methyl pivalate, isopropyl acetate, methyl (TOSOH) equipped with three polystyrene gel chloroacetate, and tetrahydrofuran (THF) columns (TSK gel G-2000, 3000, and 4000; (Tokyo Kasei) were distilled twice over calcium 8.0mm (i.d.) x 300mm each) and a refractive hydride or metallic sodium for THF just before index detector. The number-average molecular use. 1-(Isobutoxy)ethyl acetate (1), 1-(isobu­ weight (Mn) and M wMn were calculated for toxy)ethyl pivalate, and 1-(isobutoxy)ethyl SEC curves on the basis of the polystyrene t-butylacetate as a cationogen were prepared calibration. from IBVE and the corresponding carboxylic 13C NMR spectra of the polymer samples acids, 13 and distilled over calcium hydride except for poly(Bn VE) were recorded at 2rc under reduced pressure. 1-(Isobutoxy)ethyl in CDCI 3 10 wt%) using a JEOL FX90Q trifluoroacetate was synthesized in situ from spectrometer pulsed Fourier transform system IBVE and trifluoroacetic acid in 1 : 1 molar operating at 22.49 MHz. Instrument conditions ratio in hexane, and the quantitative formation were n/4 pulse of 11 ps, 2.0 s repetition time, was confirmed by 1 H NMR spectroscopy. 13•20 and 4500Hz sweep width. The number of accumulation was over 104 . The ratios of Polymerization meso(m) to racemo(r) in the diad were The polymerization was carried out at determined from methylene area of the main + 70 - 78°C under a dry nitrogen atmo­ chain in the 13C NMR spectra. 21 The diad sphere in a glass tube baked at 250oC equipped fraction of poly(Bn VE) was calculated from with a three-way stopcock. 13 The reaction was the triad fraction determined from the signals initiated by addition of EtA1Cl 2, Et2A1Cl, or of methylene proton in the side chain observed EtuAlClu solution in hexane into a mixture at 90 MHz 1 H NMR spectroscopy at 27°C in of monomer, added base, and cationogen in · CDC1 3 . 18 toluene or hexane at the polymerization tem­ perature by a medical syringe. After a prop­ RESULTS AND DISCUSSION er interval, the polymerization was quench­ ed with 0.3 wt% ammoniac methanol. The Stereoregularity of the Living and Non-Living quenched reaction mixture was sequentially Poly(IBVE) washed with dilute hydrochloric acid (0.6 M) The stereoregularity of the living poly(IBVE)

Polym. J., VoL 25, No. II, 1993 1163 S. AosHIMA, Y. ITo, and E. KOBAYASHI

meso/racemo Temp.(°C) 0 -20 -40 -60 -80 3 66/34 Mn=19x1o Mw/Mn=1.04 Yield=96% (A) Mn( cai.)=19X103

66/34 Mn=25 x1o3 Mw/Mn=1.90 (B) Yield=93%

....,..--...,5-x1.._o_42x__,_1..,.o4_s_x1._o_3,..--- MW(PSt) 20 25 30 35 EV(ml)

Figure 1. MWDs and tacticity of poly(IBVE) obtained Figure 2. Relationships between polymerization tem­ with 1/EtAICI2 in the presence of (A) or in the absence of perature and tacticity of poly(IBVE) obtained with 1/ (B) methyl acetate in toluene at +40oC: [IBVE] 0 =0.76 M, EtAlCl2 in the presence of (0) or in the absence of (D) [1]0 = [EtAICI2 ] 0 =4mM, [methyl acetate]= 1.0 M. methyl pivalate in toluene at +40- -78°C, yield >90%:

[IBVE]0 =0.76M, [1]0 /[EtAICI2] 0 =4mM/20mM, [me­ was determined by 13C NMR spectroscopy and thyl pivalate] = 1.0 M. compared with that of the non-living one. Living cationic polymerization of IBVE was achieved with 1/EtA1Cl2 in the presence of ln(Pm/P,)=(LlS!' -LlS/")/R methyl acetate in toluene at + 40°C, to give -(LlH!' -LlH/")/RT (1) the polymer with a narrow MWD (Mw/M.= 1.04, at 96% conv.) as shown in Figure 1. where P m and P, are the parameter of the meso Non-living polymer with a broad MWD and racemo placements, respectively. The (Mw/M.= 1.90, at 93% conv.) was obtained in values of ln(Pml P,) of the polymers obtained the absence of the added base. The stereo­ in the living and non-living systems were almost regularity of the living polymer was found to equal at each temperature, and increased with be meso-rich (meso/racemo = 66/34), and the a decrease in the polymerization temperature. diad fraction was equal to that of the non-living The increase was a general tendency for the polymer. cationic polymerization of vinyl ethers in To obtain further information on the non-polar solvents. The value of LlH!' -LlH/" stereoregularity of each polymer, the de­ and that of L1 s,; - L1 S,"' at + 40°C were found pendence on the polymerization tempera­ to be -900calmol- 1 and -lOOOcalmol- 1 , ture was investigated. The polymerization of respectively. These results indicate that the IBVE was carried out at + 40 - 78oC with living and the non-living polymerizations of 1/EtAlC1 2 as an initiating system in toluene in IBVE exhibit not only the same stereo­ the presence of methyl pivalate. Methyl regularity but also the same temperature pivalate used as an added base is suitable for dependence of the tacticity. That is, although a faster living polymerization of IBVE than the counteranion was generally known to methyl acetate, 22 especially at low tempera­ influence such stereoregulation, the basic tures. As shown in Figure 2, the diad fraction additive used here does not affect the was plotted against 1/T by the following stereoregulation but induces mainly living

1164 Polym. J., Vol. 25, No. II, 1993 Stereoregularity of Living Poly( vinyl ether)s nature of the growing polymer carbocation by as shown in Table I, indicating that the effect suppressing unfavorable chain transfer and of the steric hindrance (R1 and R2 in Scheme termination reactions. 1) and nucleophilicity of added bases was unexpectedly small. Effect of Added Base and Counteranion on the The effect of the counteranions was also in­ Stereostructure of the Living Poly(IBVE) vestigated by using the combination of four ini­ Since the living polymer end carbocation was tiators containing various aliphatic acid resi­ expressed as Scheme I, the stereostructure of dues (-OCOCH3, -OCOC(CH3)3, -OCOCH2- the penultimate unit is thought to be controlled C(CH3h, and -OCOCF3) and three alkyl­ by relative arrangement of the living polymer aluminum halides (EtA1Cl 2 , Et1.5AlClL 5 , and end, added base, counteranion, and coming Et2AIC1). It can be seen from Table II that IBVE. To elucidate the effect of the structure there is no remarkable change in meso %, and of added bases, the authors herein employed various types of acid residues or alkylaluminum methyl acetate, methyl pivalate, isopropyl halides had little influence on the stereo­ acetate, and methyl chloroacetate as a basic regularity of the polymers. Further inspection additive in the polymerization of IBVE with of Table II shows that the initiator [CH3CH­ 1/EtAlC12 in toluene at ooc. In any cases, the (OiBu)OCOC(CH3h] having a more bulky meso % unchanged in the range of 62 66% acid residue near the carbonyl group than 1 gives somewhat meso-rich polymer (meso: 73%), while the initiator having an electron­ Table I. Effect of added base on the stereoregularity of living poly(IBVE)• withdrawing group [CH3CH(OiBu)OCOCF 3] gives somewhat less meso polymer (meso: 65%) Added base M. X 10- 3 Mw!M. mesojracemo than that obtained by initiator 1.

CH3 COOCH3 19 1.04 66/34 (CH3),CCOOCH3 20 1.07 65/35 Stereoregularity of Various Living Poly( vinyl CH3 COOCH(CH3) 2 22 1.08 62/38 ethers) CICH2COOCH 3 19 1.20 64/36 The authors have recently succeeded in the living cationic polymerization of various vinyl • Polymerization conditions: [IBVE] =0.76 M, [1] = 0 0 ethers, for example, alkyl vinyl ethers of a [EtAlC12] 0 =4mM, [added base]=l.OM, in toluene at +40oC; yield, >90%; M. (cal.)=l9x 10 3 variety of the steric crowding (IBVE, NBVE, 24

Table II. Effect of counteranion on the stereoregularity of living poly(IBVE)•

R' ofOCOR' /EtxAICI,_x M. X 10- 3 Mw!M. mesojracemo group•

CH3- I EtAIC12 20 1.20 69/31 (CH3hC- /EtAICI2 19 1.18 73/27 (CH3hCCH2- I EtAIC12 19 1.19 66/34 CF3- /EtAICI2 19 1.19 65/35

CH3- /EtAlCI2 20 1.20 69/31 CH3- I Et1.5AIC11.5 20 1.06 70(30 CH3-- /Et2AIC1 23 1.08 70/30

Polymerization conditions: [IBVE] 0 =0.76M, [CH3CH(OiBu)OCOR']0 =[EtxAICI3_xJo=4mM, [ClCH2 - COOCH3] =!.OM, in toluene at -2ooc; yield, >90%; M. (cal.)= 19 x 10 3 • • Fragment of cationogen CH3-CH-0-CO-R'. I OiBu

Polym. J., Vol. 25, No. II, 1993 1165 S. AoSHIMA, Y. ITO, and E. KOBAYASHI

Table IIII. Tacticity of living poly( vinyl ethers)

Monomer Added base M.x!0- 3 Mw/M. mesojracemo

IBVE" CH3COOC2H 5 19 1.05 70/30 NBVEb CH3COOCH3 20 1.04 70/30 NeoPVEb CH3COOCH3 18 1.03 66/34 BnVE" CH3COOC2H 5 20 1.06 89/11 EOVE"·c THF (0.2M) 20 1.10 62/38 EOEOVE"·c THF (0.2M) 16 1.10 61/39 BzOVE' CH3COOC2H 5 15 1.10 75/25 None 15 1.09 58/42

Polymerization conditions: [monomer] 0 = 10 vol %, [1 ] 0 /[EtA1Cl2] 0 = 4 mM/20 mM, [added base] = 1.0 M, at ooc; yield >90%. ' In toluene or b in hexane. c [1] 0 /[Et!.SA1Cl!.S]o=4mM/20mM.

NeoPVE,24 and BnVE 16), vinyl ethers having Temp.( 0 C) oxyethylene groups14 (EOVE and EOEOVE) 6040 20 0 -20 and a vinyl ether having an ester group15 (BzOVE). Table III summarized the stereo­ regularity of the living polymers obtained in 0.8 the presence of added bases at oac. Since these monomers have various reactivity, the most 0.6 suitable initiating system and added base were used, respectively. (It was already confirmed in the previous section that such minor differences fo.4 of conditions hardly affected the stereostrcutre 0.2 of the polymer obtained.) The meso diad frac­ tions of polymers having the aliphatic alkyl 0.0 group were about 70%, but that of the poly­ mer. having benzyl group [poly(BnVE)] was 89%. The meso regulation of the polymers having benzoyloxy ester group [poly(BzOVE)] Figure 3. Relationships between polymerization tem­ and ether groups [poly(EOVE) and poly­ perature and tacticity of poly(IBVE)(bloken line taken from Figure 2), poly(BnVE)(l:.), poly(BzOVE)(D) ob­ (EOEOVE)] were 75% and 61-62%, re­ tained with 1jEtA1Cl2 in the presence of ethyl acetate in spectively. toluene at -20°C, yield >90%: [monomer]0 = 10 The effect of the monomer structure and the vol%, [1] 0 /[EtA!Cl2 ] 0 =4mM/20mM, [ethyl acetate]= polymerization temperature on the tacticity 1.0 M; poly(EOEOVE)( (>)obtained with 1/Et!.SAlCl!.S in the presence of THF in toluene at -20°C, yield of poly(IBVE), poly(BnVE), poly(EOEOVE), >90%: [EOEOVE]0 = !Ovol%, [1] 0 /[Et!.SA1Cl1.5] 0 =4 and poly(BzOVE) was investigated at + 60 mMj20mM, [THF]=0.2M. - 20oc in detail. Poly(BnVE) had a slightly large temperature coefficient in comparison diad, that is, the living cationic polymeriza­ with poly(IBVE), poly(BzOVE), and poly­ tion of all vinyl ethers employed here gives (EOEOVE) (Figure 3). The LJH,; -LJH," and favorably meso-rich polymers at a low LJS,; -LJS," for the diad fraction detennined polymerization temperature. The values of by eq I are listed in Table IV. The LJH:; for LJH:;- LJH," show that Bn VE is the most fa­ meso diad was smaller than LJH," for racemo vorable monomer to synthesize the meso-rich

1166 Polym. J., Vol. 25, No. 11. 1993 Stereoregularity of Living Poly( vinyl ether)s

Table IV. !JH:; -!JH," and !Js:: -!JS," values for diad Temp.( 0 C) fraction o.s r---s...,.o_4r-0_2,....o-..o_-.....,2_o _ ___,

!JH"-!JH"! !JS::-!JS,"! Monomer (Added base) m ' cal mol· 1 cal mol· 1 at + 40oc -c.... 0.4 IBVE (CH3 COOC2H 5) -900 -1000 I I Bn VE (CH3COOC2 H 5) -1700 -700 I EOEOVE' (THF) -800 -600 J I 0.2 I BzOVE (CH3 COOC2 H 5 ) -1000 -400

Polymerization conditions: [monomer] 0 = I 0 vol %, [1 ] 0 / ··--- [EtAICI2 ] 0 =4mM/20mM, [added base]= !.OM, in tolu­ ene at -20oC; yield >90%.

a [1] 0/[Et15AlC1 15] 0 =4mM/20mM, [THF]=0.2M. Figure 4. Relationships between polymerization tem­ perature and tacticity of poly(BzOVE) obtained with 1/

EtAlCI2 in the presence (0) or in the absence of ethyl polymer. The side group of the growing Bn VE acetate (e) in toluene at + -20°C, yield >90%: unit may prefer the favorable conformation of [BzOVE]0 =0.56 M, [1]0 /[EtA1Cl2 ] 0 =4mM/20mM, the intermediate carbocation to induce one side [ethyl acetate]= 1.0 M. attack of the monomer. On the other hand, the difference between and LJS," was a little Intermolecular Intramolecular smaller. Stereostructure of Poly(BzO VE) and the 88 Polymerization Mechanism ..... bi3 Recently, the living cationic polymerization =0 of BzOVE with 1/EtA1Cl2 in the absence of added bases was also succeeded. 25 Further­ 0 more, four p-substituted BzOVE derivatives

(p-X-C 6 H 4 COOC 2 H 4 0CH = CH 2 : X= Scheme 3. CH30, CH3 , H, Cl) were polymerized without added bases. 15 In any cases, living polymers (BzOVE) obtained at + 70 - 20oc in the with a narrow MWD were obtained, whereas absence of added bases was determined as the polymerization rates depended on its shown in Figure 4. In the range of + 70 to p-substituted groups as follows: CH30 < CH3 about + 20°C, the temperature coefficient was < H < Cl. On the basis of the results, it was almost identical with that in the presence of suggested that the propagating carbocations ethyl acetate. On the other hand, at the were stabilized not only by the externally added temperature below + 20°C, ln(Pm/ P,) de­ bases but also by the pendant ester group of creased steeply and then gradually became the terminal unit. However, the detailed constant. These characteristic features suggest stabilization mechanism was not discussed yet. the change of interaction mode of the The stereoregularity of polymers is known carbocation and the pendant ester group of the to be correlated to the polymerization terminal unit at the critical temperature mechanism, and gives the significant informa­ ( + 20°C). That is, at lower temperatures, the tion regarding the nature of propagating growing carbocation was thought to be species (the degree of dissociation, the stabilized through the intramolecular interac­ coordination state, etc.). 26 To obtain such tion with the pendant ester group of the information, the stereoregularity of poly- terminal unit as shown in Scheme 3. On that

Polym. J., Vol. 25, No. II, 1993 1167 S. AOSHIMA, Y. ITO, and E. KOBAYASHI occasion, the side ester group interacted with (1990). the carbocation did not interfere the monomer 9. K. Hatada, K. Ute, K. Tanaka, Y. Okamoto, and insertion from each side to give non­ T. Kitayama, Polym. J., 18, 1037 (1986). 10. T. Kitayama, T. Shinozaki, T. Sakamoto, M. stereoregular polymers. In contrast, at higher Yamamoto, and K. Hatada, Makromo/. Chern., temperatures the carbocation may be stabilized Suppl., 15, 167 (1989). by side ester groups of poly(BzOVE) and/or 11. A. Soum and M. Fontanille, Makromo/. Chern., 181, BzOVE monomer, since the intramolecular 799 (1980). 12. Y. Doi, S. Ueki, and T. Keii, Macromolecules, 12, interaction between the carbocation and the 814 (1979). pendant ester group of the terminal unit 13. S. Aoshima and T. Higashimura, Macromolecules, becomes difficult at such higher temperatures. 22, 1009 (1989). Consequently, the temperature dependence of 14. S. Aoshima, H. Oda, and E. Kobayashi, J. Polym. Sci., A, 30, 2407 (1992). the steric structure of poly(BzOVE) (Figure 4) 15. S. Aoshima, H. Kamae, and E. Kobayashi, Polym. may be accounted for by assuming that, in the Prepr. Jpn., 40, 243 (1991). absence of an added base and at temperatures 16. S. Aoshima, S. lwasa, and E. Kobayashi, Polym. J., below + 20°C, the growing end is in an to be published. 17. 0. N. Mikhant'eva, V. S. Romanova, and V. B. intramolecular interaction with the terminal Mikhant'ev, Zh. Prink/. Khim. (Lenigrad), 58, 1112 pendant ester group. (1985); Chern. Abstr., 103, 178662a (1985). 18. H. Yuki, K .. Hatada, K. Ota, I. Kinoshita, S. Murahashi, K. Ono, andY. Ito, J. Polym. Sci., A-1, REFERENCES 7, 1517 (1969). 19. H. J. Schneider, U.S. Patent 3,062,892 (1962); Chern. 1. M. Szwarc, Nature (London), 178, 1168 (1956). Abstr., 58, 10082 (1963). 2. M. Miyamoto, M. Sawamoto, and T. Higashimura, 20. S. Aoshima, Y. Ito, and E. Kobayashi, Polym. Prepr. Macromolecules, 17, 265 (1984). Jpn., 40, 1713 (1991). 3. T. Higashimura, S. Aoshima, and M. Sawamoto, 21. K. Hatada, T. Kitayama, N. Matsuo, and H. Yuki, Makromo/. Chern., Macromol. Symp., 3, 99 (1986). Polym. J., 15, 719 (1983). 4. D. Y. Sogah, W. R. Hertler, 0. W. Webster, and G. 22. S. Aoshima, K. Shachi, E. Kobayashi, and T. M. Cohen, Macromolecules, 20, 1473 (1987). Higashimura, Makromol. Chern., 192, 1749 (1991). 5. S. Asano, T. Aida, and S. Inoue, J. Chern. Soc., Chern. 23. F. A. Bovey, J. Polym. Sci., 46, 59 (1960). Commun., 1148 (1985). 24. S. Aoshima, K. Shachi, and E. Kobayashi, Polym. 6. T. Kitayama, K. Ute, and K. Hatada, Br. Polym. J., J., to be published. 23, 5 (1990). 25. S. Aoshima and T. Higashimura, Polym. Bull., 15, 7. K. Hatada, T. Kitayama, Y. Okamoto, K. Ohta, Y. 417 (1986). Umemura, and H. Yuki, Makromol. Chern., 179,485 26. C. E. H. Bawn and A. Ledwith, Quart. Rev., 16, 361 (1978). (1962). 8. B. Tarnai and N. Goudargian, Polym. Bull., 23, 295

1168 Po1ym. J., Vol. 25, No. 11, 1993