Initiation of the\nPhillips Cr(VI) Catalyst by Alkenes

As part of a continuing effort to better understand the\ninitiation\n(i.e., the reduction and self-alkylation) of the Phillips commercial\nCr(VI)/silica catalyst, this study attempted to isotopically label\nthe initiating group on the starting PE chain. The hexavalent catalyst\nwas reduced by deuterated olefin (C<sub>2</sub>D<sub>4</sub> or C<sub>3</sub>D<sub>6</sub>), then polymerization of C<sub>2</sub>H<sub>4</sub> was conducted. This amplified the starting group signal during\nthe subsequent <sup>2</sup>H NMR analysis of the polymer. In a second\nmethod, the Cr(VI) catalyst was reduced by various olefins, then hydrolyzed\nby injection of protic liquids. Redox products were then analyzed\nby GC–MS. These data indicate that the initial chain made on\na Cr(VI) site begins with a methyl group. No unsaturation was observed.\nIn addition, oxygen was found in many of the resultant chains, apparently\nincorporated from the original Cr(VI) as part of the initiation process.\nIt demonstrates that reduction and alkylation are not necessarily\nseparate and independent reactions, but often occur in a concerted\nprocess. It further suggests that there is not one initiation mechanism,\nbut many, depending on the reactivity of the individual sites and\nthe monomer used.