Variability of Chain Transfer to Monomer Step in Olefin Polymerization

Computational studies of a variety of polymerization catalyst models have revealed an unexpected fluidity in chain termination mechanisms. For many early transition metal olefin polymerization catalysts two distinct transition states exist for β-hydrogen transfer to monomer, which differ mainly in the M−H distance and CMC angle. The transition state for the “classical” (BHT A ) path resembles a metal hydride−bis(olefin) complex, whereas the alternative BHT B path involves direct transfer of an alkyl β-hydrogen to a coordinated olefin without any metal−hydride interaction. The two transition states are separated by a second-order saddle point that is just a few kcal/mol above the highest of the two transition states, indicating a flat potential-energy surface between the two paths. Of the group IV metals, Zr (in contrast to Ti and Hf) appears to have an intrinsic preference for the “classical” BHT A path. Increasing the amount of space around the metal (e.g., in lanthanocenes) changes BHT A into a two-step path (BHT C ), showing two β-hydride elimination transition states around a hydride−bis(olefin) complex local minimum. Decreasing the amount of space by using sterically demanding ligands results in a shift toward the “new” BHT B path. However, β-hydrogen elimination becomes more favorable at the same time, and our results suggest that for most early transition metal catalysts (typically 14- e metal alkyls) either BHT A or β-hydrogen elimination will be the dominant chain-transfer pathway, whereas BHT B may be relevant for some Hf complexes of intermediate crowding. The BHT B path is expected to be more important for systems that are less unsaturated (16- e transition metal alkyls; 6- e main-group metal alkyls) and also for “hetero-olefin” derivatives (alkoxides, amides), where β-hydrogen elimination is strongly endothermic.