The Union Carbide(UC) catalyst, one of the first ethylene polymerization (EP) catalysts based on supported organometallic compounds, is based on chromocene dispersed on silica; it is distinct from the corresponding Phillips catalysts, based on supported chromium oxide, because it is responsive to H2 enabling tuning the resulting polymer. Yet, despite 50 years of research, the structure of its active sites has also been controversially discussed and remains elusive. Based on a combination of IR, EPR spectroscopies, labeling experiments, and modeling at the DFT level, we identified monomeric surface-supported Cr(III) hydrides, (≡SiO)Cr(Cp)-H, as the active sites of the UC catalyst. These active sites, with distinct EPR signatures, are exclusively formed at high Cr loadings because their formation requires a combination of congruent factors, namely the presence of grafted as well as adsorbed chromocene together with residual surface OH groups, and involves a C-H bond activation step of the Cp ring. These Cr hydrides readily initiate polymerization, yielding Cr(III) alkyl species that insert ethylene monomers through a Cossee Arlman-type mechanism, as evidenced by labeling and spectroscopic studies. This information inspired the design of a well-defined analog, that was prepared and characterized using SOMC by grafting CpCr(CH(SiMe3)2)2 on partially dehydroxylated silica. The resulting surface Cr sites show similar EPR signatures and produce similar polyethylene as the UC catalyst, further supporting the presence of similar active sites and opening the way to generating new classes of catalysts.
Methods and additional data.