Influence of the Counterion MeB(C6F5)3- and Solvent Effects on Ethylene Polymerization Catalyzed by [(CpSiMe2NR)TiMe]+: A Combined Density Functional Theory and Molecular Mechanism Study
journal contributionposted on 05.01.2004 by Zhitao Xu, Kumar Vanka, Tom Ziegler
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A combined density functional theory and molecular mechanism (QM/MM) method has been used to study the first (R = Me) and the second (R = propyl) insertion of the ethylene monomer into the Ti−R bond of (CpSiMe2NtBu)(R)Ti-μ-MeB(C6F5)3. The present extensive theoretical investigation provides a comprehensive analysis of the chain propagation process for ethylene polymerization catalyzed by CGC in the presence of the counterion MeB(C6F5)3-. Both the counterion and solvent effects play important roles in the first and second ethylene propagations and therefore cannot be ignored in the mechanistic study. The influence of the counterion and the solvent not only changes the reaction barriers but also alters the rate-determining step for the chain propagation, from insertion to uptake. The cis approach is preferred over the trans approach for the second ethylene propagation in the gas phase as well as in solution. The total reaction barrier for the first ethylene propagation has been calculated to be slightly higher than that for the second ethylene propagation, in general agreement with experimental findings. The total barriers in solution are 11.7 and 8.5 kcal/mol for the first and second ethylene propagations, respectively. These barriers are comparable to the activation energy (13.3 kcal/mol) found experimentally for the same catalyst (CGC).