Combined Car−Parrinello QM/MM Dynamic Study on the Propagation and Termination Steps of Ethylene Polymerization Catalyzed by [Cp2ZrR(μ-Me)B(C6F5)3] (R = Me, Pr)

Abstract

Combined Car−Parrinello and QM/MM dynamic simulations have been carried out in order to investigate the polymer chain propagation and termination processes in ethylene polymerization catalyzed by zirconocene with the counteranion [CH3B(C6F5)3]- included. A complete account is given not only of the potential energy profile (PEP) but also of the free energy profile (FEP) for the first and second steps of ethylene insertion, as well as of the chain termination. Solvation effects were included on the basis of a continuum model. Special attention was given to the counteranion mobility in the catalytic process and the possible differences between PEP and FEP. Two different modes of attack by ethylene, cis and trans to the counteranion [CH3B(C6F5)3]-, were considered. In general, the FEPs differ from the PEPs in the barrier height and the shape. The FEP gives a higher barrier over PEP due to entropic contributions. The transition state of ethylene uptake by the metal center, the ethylene π-complex, and the ethyl...