Theoretical Elucidation of the Effect of Counteranions on the Olefin Polymerization Activity of (Pyridylamido)Hf(IV) Catalyst by QM and REMD Studies: MeB(C6F5)3– versus B(C6F5)4–

Abstract

1-Octene insertion reaction by the cationic active species of (pyridylamido)Hf(IV) catalyst (HfCat+) was investigated, focusing on the effect of the counteranions (CAs), MeB(C6F5)3– and B(C6F5)4– on catalyst activity. Quantum mechanical (QM) calculation showed a remarkable difference in TS structures that HfCat+–MeB(C6F5)3– complex forms an “inner-sphere” ion pair (ISIP), while HfCat+–B(C6F5)4– forms an “outer-sphere” ion pair (OSIP). However, the activation free energies of 1-octene insertion by both complexes are almost identical to each other. Meanwhile, replica exchange molecular dynamics (REMD) calculation revealed that the time duration of the HfCat+ capturing 1-octene monomers throughout the REMD trajectories of HfCat+–B(C6F5)4– complex was ∼2.5 times that of HfCat+–MeB(C6F5)3–. This is because B(C6F5)4– is more likely to dissociate from HfCat+ to form an OSIP than MeB(C6F5)3– and thus allows monomers to approach HfCat+ more easily. Using these microscopic data, we have numerically evaluated the 1-octene polymerization reaction rate constants and succeeded in qualitatively reproducing the experimentally observed tendency of the polymerization reaction with B(C6F5)4– faster than with MeB(C6F5)3–. Finally, it is theoretically elucidated that the monomer capture process before the monomer insertion process determines the overall polymerization reaction rates in this catalytic system.