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Highly “Diheterotactic” Polymerization of tert-Butyl Crotonate

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journal contribution
posted on 11.03.1996, 00:00 by Koichi Ute, Takeshi Asada, Koichi Hatada
The polymerization of tert-butyl crotonate with RMgBr or R2Mg (R:  t-C4H9, C6H5) in toluene gave a polymer with high stereoregularity. In particular, (C6H5)2Mg had high initiator efficiency and was useful for the preparation of oligomer suitable for stereostructure analysis. The oligomer of tert-butyl crotonate was transformed into oligo(methyl crotonate) by transesterification, and the oligo(methyl crotonate) was fractionated into the individual homologues from 1mer to 10mer by means of GPC and supercritical fluid chromatography. 1H NMR spectroscopy showed that each oligomer homologue consisted essentially of two diastereomers A and B. The configurational sequences for 6mer-A, 7mer-A, 8mer-A, and 7mer-B were determined by X-ray single crystal analysis to be et(ettt)2, et(ettt)2et, et(ettt)3, and (ettt)3, respectively, where e and t denote erythro and threo diads. Accordingly, this polymerization is highly stereospecific to ettt structure. The ettt structure well explains the nonequivalence in the NMR signals due to each α-CH, β-CH, β-CH3, and t-C4H9O group of this stereoregular poly(tert-butyl crotonate). The polymer chain consisting of regular repetition of an ettt pentad can be regarded as a double heterotactic sequence with respect to the α- and β-positions, and therefore this novel structure of stereoregular polymer should be termed “diheterotactic”.