Monomer Discrimination in the Enantioselective Living Coordinative Chain Transfer Polymerization of α,ω-Nonconjugated Dienes vs α-Olefins

Abstract

The stereoselective living coordinative polymerization (LCP) of α-olefins and α,ω-nonconjugated dienes can be achieved using initiators derived from the homochiral, configurationally stable, monocyclopentadienyl, 7- and 9-membered-ring cycloamidinate group 4 metal complexes, 1–3, respectively, as preinitiators in combination with borate B1 as activating co-initiator. However, yield, molar mass (M_n), and degree of stereoregularity in these LCPs were found to be highly dependent upon initial monomer concentration due to decreased magnitudes for monomer complexation and site isomerization, K_M, and K_SI, respectively. The enantioselective living coordinative chain transfer polymerization (LCCTP) of 1,5-hexadiene using the initiator derived from 1/B1 and excess equivalents of ZnEt₂ as chain transfer agent (CTA) provides isotactic cis/trans poly(methylene-1,3-cyclopentane) (PMCP) in high yield. In contrast, under identical conditions, all attempts to carry out the enantioselective LCCTP of 1-hexene failed. It is proposed that this unprecedented discrimination of polymerizability between the two monomer types for LCCTP is due to stark differences in conformational freedom of the growing polyolefin chains. More specifically, in the case of the rigid-rod nature of growing PMCP, reversible chain transfer is not sterically impeded and can occur as required, whereas the unrestricted conformational flexibility of side chains in poly(1-hexene) creates a steric barrier that prohibits chain transfer to occur after just a few monomer insertions. Collectively, these results provide insights for designs of the next generation of enantiopure and configurationally stable cycloamidinate preinitiators that can potentially lead to the successful enantioselective LCCTP of both α,ω-nonconjugated dienes and α-olefins with high stereoregularity.