13C NMR Analysis of Uniform Heterotactic Oligo(allyl methacrylate) Aiming at Understanding Even—Odd Alternation in Distribution of Degree of Polymerization

Abstract

The heterotactic oligo(allyl methacrylate)s prepared with t-butyllithium/bis(2, 6-di-t-butylphenoxy)methylaluminum in toluene at −95°C exhibit even—odd alternation in distribution of degree of polymerization (DP) except for 8 mer; oligomers with odd-number DP preferentially formed over those with even-number DP. Uniform oligomers with DPs from 5 to 11 were isolated by fractionating the oligomer with the aid of supercritical fluid chromatography (SFC), and their stereochemical structures were investigated by13C NMR spectroscopy. The NMR analysis for initiating chain-ends of the uniform oligomers revealed evenmdash;odd alternation in distribution of stereoregularity near the initiating chain-end. On the other hand, terminating chain-end analysis showed that the living anions predominantly stayed in r-ended antion state regardless of the DP s of living anions. The fraction of the odd—number DP oligomers (55%), calculated from the peak intensities in the SFC trace, was found to be much lower than that estimated from chain-endanalysis of the original unfractionated oligomer on the assumption that propagation reaction proceeded in a completely heterotactic manner (76%). Structural analysis in the main chain of the each uniform oligomer demonstrated that even—number DP oligomers contained stereochemical defects such as mm sequence, while odd—number DP oligomers consist of almost perfect heterotactic sequence. These results indicate that high-level stereoregulation in the propagation process is also an important factor for the observation of evident even-odd alternation in DP distribution.