Theoretical models of ethylene polymerization over a mononuclear chromium(II)/silica site

Cluster models are constructed for mononuclear Cr(II) sites of the Cr/SiO2 Phillips catalyst for ethylene polymerization, displaying chromium covalent ly bound to two oxygen ligands. Based on these models, gradient-corrected d ensity functional theory has been used to compare different routes of initi ation and chain propagation with respect to structure, thermodynamical, and kinetical properties. It is shown that, for these sites, propagation mecha nisms that involve four-coordinated chromium lead to activation energies th at are incompatible with high catalytic activity. In the case of a chromacy cloalkane intermediate, the relative rates of beta-hydrogen transfer and mo nomer insertion are in agreement with the observed production of 1-hexene d uring early stages of polymerization. However, the anchoring site needs to be fairly strained before the activation energies drop significantly below 100 kJ/mol. On the other hand, a monoalkylchromium site supports insertion of ethylene into the Cr-alkyl bond according to the Cossee mechanism, with an activation energy of 56 kJ/mol relative to the ethylene-chromium precurs or complex. (C) 2000 Academic Press.