a-Agostic Interactions and Growing Chain Orientation for Olefin Polymerization Catalysts

Growing chain orientational preference is one of the key factors determining the stereoselectivity of olefin polymerization and related reactions such as carboalumination; alpha-agostic interactions are an important component of this preference. Using insertion of ethene in the M-CH3 bond as a model, the intrinsic orientational preference was evaluated for a number of important catalyst types through determination of the energy profile for methyl group rotation at the insertion TS. Ti and Zr metallocenes show a pronounced preference (Ti, 10 kcal/mol; Zr, 6 kcal/mol) for a classical a-agostic arrangement with a single short M···CH contact and an elongated C-H bond; on CH3 rotation the agostic elongation mostly disappears. In contrast, for all non-metallocene systems studied the orientational preference is much smaller or even opposite that of metallocenes. Moreover, on CH3 rotation the agostic C-H bond elongation gets spread out over two C-H bonds rather than disappearing. These results point to greater chain orientation flexibility for non-metallocene catalysts.