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” (BHTA) path resembles a metal hydride−bis(olefin) complex, whereas the alternative BHTB 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” BHTA path. Increasing the amount of space around the metal (e.g., in lanthanocenes) changes BHTA into a two-step path (BHTC), 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” BHTB 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 BHTA or β-hydrogen elimination will be the dominant chain-transfer pathway, whereas BHTB may be relevant for some Hf complexes of intermediate crowding. The BHTB 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.