Density functional study of ethylene polymerization on zirconocene catalysts

Abstract

Polymerization of ethylene on zirconocene catalysts was studied with the density functional theory. The approach proposed previously for the model system Cp₂ZrEt⁺+C₂H₄ was extended to substituted zirconocene catalysts (RCp)₂ZrEt⁺, which allowed us to compare the results of calculations with the experimentally found rate constants of chain propagation. The ethyl fragment was demonstrated to be an adequate model for a growing polymer chain. The order of decrease in the activation energy of chain propagation calculated for compounds with R=H, Me, Prⁿ, and Buⁿ is in qualitative agreement with the experimental data. However, the quantitative description gave underestimated theoretical rate constants of chain propagation compared to the experimental values. In the case of polyalkyl-substituted zirconocenes (R=1.2-Me₂ or Me₄), no correlation between the calculated and experimental characteristics was observed. The results were explained using the simplest model reaction of the replacement of the MeMAO⁻ counterion by an ethylene molecule accompanied by displacement of the former to the outer coordination sphere of the Zr atom. This step was demonstrated to control the kinetics of the process. It was concluded that the isolated-cation model used in early investigations is not adequate and calls for modification with regard to the effect of the counterion.