Chromium Allyls on Silica: A Study of Structure and Reactivity

The reactions of Cr(2-Me-allyl) 3 with the surface of silica dehydrated at temperatures in the range 200−800 °C have been studied by quantitative gas chromatography measurements and diffuse reflectance infrared spectroscopy and as catalysts for polymerization of ethylene. While relatively high molecular weight polyethylene with a negligible amount of side chain branches was produced with Cr(2-Me-allyl) 3 on silica dehydrated at 200 °C, the catalyst based on silica dehydrated at 800 °C produced polymer with a high concentration of short-chain branches as well as a broad distribution of oligomers. During preparation, Cr(2-Me-allyl) 3 reacts primarily with hydroxyl groups with release of the allyl ligands as isobutene to form supported chromium complexes; the reaction stoichiometry depends on the density of surface hydroxyl groups. Diffuse reflectance infrared spectroscopy shows the presence of π-bonded allyl ligands only on samples prepared with highly dehydrated silica. All catalysts give bands typical for vinyl groups, indicating the presence of σ-bonded allyl ligands. CO adsorption is more extensive on Cr(2-Me-allyl) 3 reacted on highly dehydrated silica compared with samples prepared on silica dehydrated at lower temperatures, reflecting different levels of coordinative unsaturation of chromium for the catalysts. The diffuse reflectance infrared spectra recorded in the presence of ethylene at ambient temperature reflect the results from the slurry polymerization experiments performed under realistic conditions.