Diastereoselectivity switching in α,ω-dienes cyclopolymerization via rational monomer modification

Cyclic polyolefins (COPs), valued for their distinctive properties, are primarily synthesized through the cyclopolymerization of non-conjugated α,ω-diolefins catalyzed by transition metals (TMs). The microstructure of COPs, particularly the configuration of the cyclic units (cis or trans) and the degree of cyclization, can be finely tuned by selecting appropriate catalytic systems. Numerous studies have shown that most TM-based catalysts favor the formation of trans-enriched polymers from 1,5-hexadiene (1,5-HD), with pyridyl-amido complexes being a notable exception, as they preferentially yield the cis counterpart. In this study, combining experimental and theoretical approaches, we demonstrate that introducing a methyl substituent into the monomer (2-methyl-1,5-hexadiene, MHD) enhances trans-selectivity in both C2 and Cs-metallocene catalysts, and notably reverses the diastereoselectivity of the pyridyl-amido hafnium complex. While the cyclization ratio remains unaffected by the methyl group, a significant impact is observed on the resulting molecular weights, with a marked decrease in molecular weight capability, particularly for the C2-metallocene system.