A DENSITY-FUNCTIONAL STUDY ON OLEFIN INSERTION AND HYDROGEN-TRANSFER IN THE REACTION BETWEEN CL2TI+-ETHYL AND ETHYLENE - POSSIBLE IMPLICATIONS FOR THE STEREOCHEMISTRY AND CHAIN TERMINATION IN OLEFIN POLYMERIZATION

Calculations based on density functional theory have been carried out on the reaction between Cl2Ti+-ethyl (1) and ethylene. In this study 1 was taken as a model for the cationic metallocenes of group-4 elements, which have been developed by Kaminsky and Brintzinger as efficient catalysts for the polymerization of olefins. The ground state structure of 1 has a beta-agostic conformation in which a single C-beta-H bond is directed towards the metal center. It was assumed that this conformation also serves as a model for the resting state of the growing chain attached to the cationic group-4 metallocenes between insertions. Two paths were considered for the reaction between 1 and ethylene. The first (2) has ethylene approaching the agostic C-beta-H bond, whereas ethylene in the second approach (3) attacks the Ti-C-alpha link from the side opposite to the C-beta-H bond. The front-side attack (2) results in a transfer of hydrogen from the beta-carbon of ethyl to ethylene and represents a chain-terminating step with an activation energy of 5.3 kcal/mol. It was not possible to locate a path leading to olefin insertion from the front-side attack (2). The back-side attack (3) resulted readily in insertion with an activation energy of 3.9 kcal/mol. The study made use of full transition state optimization as well as a tracing of the reaction paths by the intrinsic reaction coordinate (IRC) method of Fukui. Previous investigations have all assumed that olefin insertion takes place via a front-side approach (2) based on the known stereochemistry of alpha-olefin polymerization. The present study suggests that insertion takes place by a back-side approach (3), and this suggestion is discussed in connection with the known stereochemistry of olefin polymerization.