The polymerization of tert-butyl crotonate with RMgBr or R(2)Mg (R : t-C4H9, C6H5) in toluene gave a polymer with high stereoregularity. In particular, (C6H5)(2)Mg 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. H-1 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)(2)et, et(ettt)(3), and (ettt)(3), respectively, where e and t denote erythro and three diads. Accordingly, this polymerization is highly stereospecific to ettt structure. The ettt structure well explains the nonequivalence in the NMR signals due to each alpha-CH, beta-CH, beta-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 alpha- and beta-positions, and therefore this novel structure of stereoregular polymer should be termed "diheterotactic".