Variability of Chain Transfer to Monomer Step in Olefin Polymerization

Abstract

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.

Study on the electrodriven action of gelatin hydrogel in silicone oil

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Influence of the ligand structure of hafnocene polymerization catalysts: A theoretical study on chain termination reactions in ethene polymerization

“…Since the steric requirements are much lower for ethene polymerization than for polymerization of higher a-olefins, the latter transition state (2f) is favored in this context, by 13.8 kJ/mol in average for the ''Hfset". There are two exceptions, namely catalysts D11 and H7, favoring the transition state (2e), which is supported by the conclusions of Talarico and Budzelaar for hafnocenes with crowded center [18]. The termination product (2g) has an ethyl chain together with pentene attached to the metal.…”

Tuning the Relative Energies of Propagation and Chain Termination Barriers in Polyolefin Catalysis through Electronic and Steric Effects

“…Selectivity is important in polymerization, and the main side‐reaction is chain termination, which determines the molecular weight (MW) of the resulting polymer . As the MW is such an important property of the product, the factors that affect it have been studied extensively.…”