Chain-End Stereostructure of Heterotactic Poly(ethyl methacrylate) Prepared with t-Butyllithium/Bis(2,6-di-t-butylphenoxy)methylaluminum as Studied by 13C NMR Spectroscopy

Abstract

Heterotactic poly(ethyl methacrylate)s [poly(EMA)s] were prepared with t-butyllithium/bis(2,6-di-t-butyl-phenoxy)methylaluminum in toluene at −78°C and −95°C. The end-group signals in 13C NMR spectra of the poly(EMA) were assigned in terms of triad configurations near the initiating and terminating chain-ends. The analysis for the initiating chain-end revealed that (1) the dimer anion preferentially undergoes racemo addition to form a trimer anion with racemo (r) diad (t-Bu-M-M−→t-Bu-r-M−), (2) the trimer anion with r diad favors meso (m) addition (t-Bu-r-M−→t-Bu-r-m-M−), and (3) the m-selectivity is much enhanced by lowering the polymerization temperature. The analysis of the terminating chain-end showed that (1) living polymer having r diad at the chain end (〰rM−) exists more than that with m diad (〰rM−), suggesting the higher stability of the r-ended anion, and (2) the living anion favors r-addition in their termination reaction with alcohol, indicating the inherently r-specific nature of the propagating species. Based on these structural analyses the mechanism of stereospecificity in the heterotactic polymerization is discussed.