Die Mukromolekulare Chemie 176, 11 87-1 190 ( 1 975)

Department of Synthetic Chemistry, Faculty of Engineering University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

Short Communication

Asymmetric-selective Copolymerization of (R)- and (S)-tert-Butylethylene Oxides

Initiated by Potassium tert-Butoxide

Akira Sato, Tsuneo Hirano, and Teiji Tsuruta

(Date of receipt: January 14, 1975)

As part of our study on the optical rotatory phenomena and on the conforma- tions of polyethers’. ’), we synthesized (R)-3,3-dimethyl-l,2-epoxybutane [(R)- rert-butylethylene oxide] ((R)-rert-BuEO) and carried out its polymerization with catalysts such as amorphous Zn(OCH,),, Zn(C,H,),-H,O (1 :0,8), and potassium rert-butoxide (tert-BuOK). In the rert-BuOK-initiated bulk copoly- merization of (R)- and (S)-tert-BuEOs, we noticed an asymmetric-selective poly- merization occurred when the starting (R)/(S) monomer mole ratio was different from one. This result is quite different from those expected from the “iso,syn” regulation mechanism proposed by Price et a1.3-5) for tert-BuOK-catalyzed bulk polymerization of “racemic” tert-BuEO, and from KOH-initiated non- stereoselective6* ’) polymerization of epoxypropane (propylene oxide). Our observation of asymmetric selection due to the chiral structure of the growing chain end moiety may be the first example of a base-catalyzed polymerization of an epoxyalkane.

Optically active (R)-3,3-dimethyl- 1,2-butanediol, prepared from 1 -hydroxy- 3,3-dimethyl-2-butanone by yeast fermination”, was converted via tosylate to (R)-tert-BuEO; [.]is -20,4 (neat), - 18,l (c= 1,83gdl-’, benzene), and - 21,5 (c= 2,15 g dl ~ ’, cyclohexane). Racemic tert-BuEO was prepared from pinacolone by bromination, reduction with NaBH4 and ring closure with KOH3s9’. These epoxy compounds were purified by reflux over KOH pellets and CaH2. Potassium rerr-butoxide was prepared from tert-butyl alcohol and potassium metal and was purified by sublimation.

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A. Sato, T. Hirano, and T. Tsuruta

0 - ‘ I I

The copolyrnerization of (R)- and (S)-tert-BuEOs by tert-BuOK was carried out in bulk as follows. A given amount of (R)- and (S)-tevt-BuEOs was added by trap-to-trap distillation to tert-BuOK and the mixture was transferred into three or b u r ampoules under dry nitrogen atmosphere. These ampoules were sealed and kept at 60°C. At a certain time interval, one of these ampoules was opened and the unreacted monomer was recovered from the reaction mixture by trap-to-trap method. The remaining mixture was diluted with benzene, washed successively with 1 M HCl, H 2 0 , 2% NaHCO,, and H 2 0 , and then freeze-dried to obtain the polymer. The [.I;’ and [MI$& of the 2% solution of the monomer or the recovered monomer in cyclohexane were measured on Perkin-Elmer 241 autopolarimeter. The molecular weight of the polymer was determined by vapor pressure osmometer, Mechrolab Osmometer model 301A.

Two significant features of the results obtained in the tert-BuOK-initiated bulk copolymerization of (R)- and (S)-tert-BuEOs are shown in Figs. 1 and 2.

- 5000 m 3 - 2 4000 m

z rn .-

3 0

3000 P

2000

1000

Fig. 1. Molecular weight of the polymer obtained at various conversions in the rerr-BuOK-initiated bulk copolymerization of ( R ) and@)-rerr-BuEOsat 60°C; mole ratio of rerr-BuOK to monomer 0,019

As!~mmetric-se/ec.rive Copolymrrization

e f

Fig. antipode, 2. The I O ( R ) / [ ( R ) + ( S ) ] , amount of (R)-

at 60°C; mole ratio of rut- 0 52 O\;*

5 : m<\ in the unreacted monomer recovered from the reaction 0

system of rut-BuOK-initi- v)

ated bulk copolymerization of (R)- and (S)-rut-BuEOs

BuOK to monomer 0,015 (0, A) and 0,019 ( 0 )

\ *\ ‘1; 60 -

I I

Fig. 2 shows that the optical rotation of the recovered monomer from the reaction mixture decreased rapidly as the polymerization proceeded. No racemization of the monomer takes place during polymerization because the unreacted monomer, recovered from the reaction system of tert-BuOK-ini- tiated homopolymerization of(R)-tert-BuEO at about 77% conversion, showed within experimental error the same specific rotation, [a]:g5- 89,3 (c= 1,67g dl- ’, cyclohexane), as that of the starting (R)-monomer, [a] :& - 88,5 (c= 2,15 g dl- ’, cyclohexane). In addition, the polymer isolated from the (R)- and (S)-copolymerization system showed an optical activity corres- ponding to the amounts of the monomer antipodes incorporated into the polymer. Therefore, the decrease in the specific rotation of the recovered monomer should be ascribed to the asymmetric-selective polymerization. Obviously, the enantiomorphic catalyst site control mechanism for stereoselec- tive polymerization6, 7 , does not hold for the present copolymerization, since the recovered monomer showed a different specific rotation from that of the starting monomer.

If the “iso,syn” regulation on the K + counter ion3-5) is strong enough even in polymerizations such as shown in Fig. 2, i.e. when the amounts of the starting (R)- and (S)-monomers are unequal, an increase in the optical rotation of the unreacted monomer can be expected as the polymerization proceeds. Our results in Fig. 2 do not meet this expectation, indicating that

1189

A. Sato, T. Hirano, and T. Tsuruta

the “iso,syn” inechanism does not operate when either of the antipodes is in excess in the polymerization system.

Thus, a reasonable interpretation of the results in Fig. 2 is a growing chain control mechanism: when (R)-monomers are in excess in the polymeriza- tion system, a growing chain end moiety consisting of a predominant amount of (R)-monomer residue is likely to take a kind of chiral structure and in- corporates an (R)-monomer preferentially.

According to our conformational analysis of poly[(R)-oxy( 1 -tert-butylethyl- ene)] (poly(R)-tert-BuEO]) in solution, more than 75% of the polymer seg- ments take a helical structure locally and statistically due to the steric hindrance of the bulky tert-butyl side groups2’, in harmony with the (9/4) helical structure reported for thecrystalline state”’. In the light of these results on the conforma- tion of poly(tevt-BuEO) molecules, the helical structure of the growing chain end moiety should play an important role in the chiral structure responsible for the stereoselection in the propagation process.

Detailed studies are in progress and will be reported elsewhere.

Optical rotation and conformation of poly((S)-isopropylethylene oxide) in compari- son with those of poly((S)-propylene oxide): K. Tsuji, T. Hirano, T. Tsuruta, Makromol. Chem., in press

* ) Conformational analysis of poly(a1kylrne oxide)s: K. Tsuji, T. Hirano, T. Tsuruta, A. Abe, 21st Annual Meeting of Soc. Polymer Sci., Japan 1972, preprint p. 81

3, C. C. Price, M. K. Akkapeddi, B. T. DeBona, B. C. Furie, J. Amer. Chem. SOC. 94, 3964 (1972)

4, C. C. Price, M. K. Akkapeddi, J. Amer. Chem. SOC. 94, 3972 (1972) 5 , C. C. Price, Accounts Chem. Res. 7, 294 (1974) 6 , S. Inoue, N. Yoshida, Y. Yokota, T. Tsuruta, Kogyo Kagaku Zasshi (J. Chem.

Soc. Japan, Ind. Chem. Sect.) 68, 220 (1965) ’) T. Tsuruta, J. Polymer Sci. Part D, 6, 179 (1972) * ) (a) P. A. Levene, A. Walti, J. Biol. Chem. 68, 415 (1926): (b) J. P. Guettk, N. Spassky,

9, C. C. Price, €4. Fukutani, J. Polymer Sci. Part A-I, 6, 2653 (1968) l o ) H. Sakakihara, Y. Takahashi, H. Tadokoro, N. Oguni, H. Tani, Macromolecules

Bull. Soc. Chim. Fr. 11, 4217 (1972)

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