Stereoregular Oligomers of Methyl Methacrylate III. 1H and 13C NMR Spectra of the Pure-Isotactic and Pure-Syndiotactic Oligomers from Dimers to Octamers

Abstract

Pure-isotactic (it-) and pure-syndiotactic (st-) MMA oligomers from dimers to octamers were isolated from stereoregular living polymerization systems of MMA in toluene at −78°C with t-BuMgBr and t-BuLi/Et3Al, respectively, and their 1H and 13C NMR spectra were investigated in detail. The peak assignments for all the 1H and 13C signals except for methoxy signals were unambiguously made by 1H COSY and long-range 13C-1H COSY (COLOC). NMR chemical shifts of protons and carbons in the monomeric units of the third and farther positions from the α- and ω-ends were nearly the same as those in the corresponding stereosequences of high molecular weight PMMA. Both the it- and st-oligomers were found to take the extended, all trans conformation in solution, from the observed patterns of the 4JHH long-range correlation peaks in the 1H COSY spectra. The 13C-T1 values of the it-hexamer were larger than those of the st-hexamer for any carbon in any monomeric unit, indicating differences in segmental mobility.