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 β-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.