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Cluster models are constructed for mononuclear Cr(II) sites of the Cr/SiO2 Phillips catalyst for ethylene polymerization, displaying chromium covalently bound to two oxygen ligands. Based on these models, gradient-corrected density functional theory has been used to compare different routes of initiation and chain propagation with respect to structure, thermodynamical, and kinetical properties. It is shown that, for these sites, propagation mechanisms that involve four-coordinated chromium lead to activation energies that are incompatible with high catalytic activity. In the case of a chromacycloalkane intermediate, the relative rates of β-hydrogen transfer and monomer insertion are in agreement with the observed production of 1-hexene during 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 precursor complex. |
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