Catalyst Site Epimerization during the Kinetic Resolution of Chiral α-Olefins by Polymerization

A new enantiopure C1-symmetric olefin polymerization precatalyst, (1,2-SiMe2)2{η5-C5H2-4-((S)-CHEtCMe3)}{η5-C5H-3,5-(CHMe2)2}ZrCl2, (S)-2, was synthesized, and its use for the kinetic resolution of 3-methyl-substituted racemic α-olefins was investigated. Upon activation with methyl aluminoxane (MAO), selectivity factors for most olefins were greater when (S)-2 was used as the catalyst as compared to its previously reported methylneopentyl analogue, (1,2-SiMe2)2{η5-C5H2-4-((S)-CHMeCCMe3)}{η5-C5H-3,5-(CHMe2)2}ZrCl2, (S)-1. Pentad analysis of polypropylene produced by the two catalysts at various propylene concentrations indicates that (S)-2 undergoes more efficient site epimerization (polymeryl chain swinging prior to subsequent monomer enchainment) at intermediate propylene concentrations compared to (S)-1. At high and low propylene concentrations, however, the two catalysts behave similarly. On the other hand, polymerization of 3,5,5-trimethyl-1-hexene at different olefin concentrations and temperatures illustrated that selectivity differences between the two catalysts are likely not a consequence of inefficient site epimerization for (S)-1.