α‑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; α-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 α-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.