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)

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 [CH 3 B(C 6 F 5 ) 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 [CH 3 B(C 6 F 5 ) 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 ethylene insertion transition state are all clearly present on the FEPs. This is not always the case for the PEPs, especially for the trans pathway. The calculated FEPs for the chain propagation indicate that the cis pathway is preferred over the trans pathway for both the first and second insertions. For the energetically favorable cis pathway, the ethylene insertion into the Zr−C α bond rather than the ethylene uptake by the metal center is the rate-determining step for both PEP and FEP (with/without solvation effects included). Free dynamic simulations without any constraint on the process after ethylene insertion show that the counteranion can recoordinate to the metal center again in a period of less than 1 ps. In comparison with the estimated time scale of the chain propagation, the counteranion which was displaced by ethylene in the ethylene uptake process has sufficient time to recombine with [Cp 2 ZrR] + again before a new monomer approaches to the metal center. This is in good agreement with very recent experiments. The energetically most favorable pathway for the chain termination corresponds to the cis approach, with a free energy barrier of 18.5 kcal/mol.

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