Backbone rearrangement during olefin capture as the rate limiting step in molecular olefin polymerization catalysis and its effect on comonomer affinity

Available exptl. data for several metallocenes indicate that the ethene​/propene copolymn. ratio rc can be much more temp. dependent than would be expected if competing insertion transition states (TS) are rate limiting. Detailed exploration of the reaction paths reveals in several cases the existence of a "capture-​like" transition state before the actual insertion, with free energies close to the insertion TS. Movement around these transition states does not just involve monomer and chain, but also clear distortion of the ligand skeleton to allow entry of the monomer. Taking these addnl. TSs into account leads to much improved agreement with expt. for a series of metallocenes and a constrained geometry catalyst system. Depending on catalyst and temp., selectivity is detd. by competing insertion​/insertion, capture​/insertion or capture​/capture. It seems likely that this is a common situation esp. for highly efficient catalysts, complicating (but not preventing) prediction of copolymn. performance.