Molecular-level insight into Cr/silica Phillips-Type catalysts: Polymerization-active dinuclear chromium sites

Prospects of ethylene polymerization over dinuclear Cr(II) sites of the reduced Cr/SiO2-based Phillips catalyst have been investigated by means of cluster models and gradient-corrected density functional theory. The starting structure for polymerization is considered to arise through reactions between ethylene and the dichromium site. A feasible route is found to a structure with a butano moiety suspended between the two chromium centers. The bridging alkano ligand may grow by facile insertion of ethylene into either of the chromium-carbon bonds. The relative rates of monomer insertion and termination through beta-hydrogen transfer, as a function of the length of the bridging organic ligand, agrees with the production of 1-hexene observed during early stages of polymerization. (C) 2002 Elsevier Sclence (USA).