Methyl side chain formation on the CrCp2/SiO2 catalyst during polymerisation of ethylene: Spectroscopic analyses and theoretical modelling

Polymerisations at different ethylene pressures and temperatures in the pre sence of deuterium as a chain transfer agent have been performed with a CrC p2/SiO2 catalyst. The polymers formed have been analysed by H-1, H-2, and C -13 NMR spectroscopy. The results from the NMR analyses, DRIFTS analysis of the catalyst in the presence of ethylene, and theoretical modelling (DFT) have been used to investigate the mechanism for methyl branch formation. In direct evidence for the presence of an isomerised chromium site has been fo und, where the chromium is bonded to a secondary carbon atom within the pol ymer chain. The DFT calculations indicate an energy barrier of 17 kcal/mol for the isomerisation, where a hydrogen atom is transferred from the alpha- carbon to the beta-carbon through an olefin-chromium-hydrid intermediate. P ropagation from the isomerised site gives a methyl branch. Ethyl branches a re also observed, which may be explained by repeated isomerisation. The ene rgy barrier for propagation from the isomerised site is significantly highe r than for the regular site where chromium is bonded to the chain end, with barriers of 12 and 8 kcal/mol, respectively. The results support a model i n which the observed strong pressure dependence of the activity on the part ial pressure of ethylene is due to a higher rate of isomerisation than prop agation from this site at low ethylene pressures. (C) 2000 Academic Press.