A density functional and molecular mechanics study of β-hydrogen transfer in homogeneous Ziegler-Natta catalysis

Models of catalytic complexes, of the type Cp₂ZrC₂H₅⁺ and Cp₂ZrC₄H₉⁺ (Cp = η⁵-C₅H₅), for the β-hydrogen transfer processes in homogeneous Ziegler-Natta polymerizations have been studied using density functional methods. We investigated the geometries and the energetics of the processes corresponding to the β-hydrogen transfer either to the metal or to the monomer. A preference for the transfer to the monomer is shown by the noticeably smaller activation barrier. The analysis has been extended through molecular mechanics calculations to models of the catalytic complexes based on the ligands BenzInd and MeBenzInd (BenzInd = (CH₃)₂Si(benz[e]indenyl)₂ and MeBenzInd = (CH₃)₂Si(2-methylbenz[e]indenyl)₂). This analysis, in agreement with experimental results, indicates that the β-hydrogen transfer to the monomer is more difficult in the presence of the MeBenzInd ligand due to the nonbonded interactions of the 2-methyt substituents with the C₂ of both the monomer and the growing chain.