Stereoregular Oligomers of Methyl Methacrylate: X-Ray Crystal Structure Analysis and 1H NMR Spectrum of the Trimer

Abstract

A trimer of methyl methacrylate was isolated by HPLC from the oligomer mixture prepared in toluene with t-BuMgBr, and the crystal and molecular structure was determined by single crystal X-ray analysis. A monoclinic crystal of the trimer grown from heptane belonged to the space group P21/n having cell constants a=10.047, b=24.743, c=9.026 Å and β=109.02°. The final R value was 0.050. The configuration of the trimer was proved to be meso-meso. This trimer had been differently assigned to meso-racemo isomer in our previous report [K. Hatada et al., Polym. J., 19, 1325 (1987)]. The conformation in the crystal is ttg+tg+ (as (R,S,R)-isomer) along the main chain skeletal sequence t-Bu-CCCCCC-Hω. However, the more extended state ttttg+ was found to be predominant in solution, from the considerations of the chemical shifts of the methylene protons, 4J-correlation peaks observed in 1H COSY spectrum, and 3J vicinal coupling constants between the methine and methylene protons in the ω-end unit. The chemical shift between the nonequivalent methylene protons in the ω-end unit is larger for racemo-sequence than for meso-sequence, while the opposite is true for in-chain monomeric units.