Initiation of the Phillips Cr(VI) Catalyst by Alkenes

As part of a continuing effort to better understand the initiation (i.e., the reduction and self-alkylation) of the Phillips commercial Cr(VI)/silica catalyst, this study attempted to isotopically label the initiating group on the starting PE chain. The hexavalent catalyst was reduced by deuterated olefin (C₂D₄ or C₃D₆), then polymerization of C₂H₄ was conducted. This amplified the starting group signal during the subsequent ²H NMR analysis of the polymer. In a second method, the Cr(VI) catalyst was reduced by various olefins, then hydrolyzed by injection of protic liquids. Redox products were then analyzed by GC–MS. These data indicate that the initial chain made on a Cr(VI) site begins with a methyl group. No unsaturation was observed. In addition, oxygen was found in many of the resultant chains, apparently incorporated from the original Cr(VI) as part of the initiation process. It demonstrates that reduction and alkylation are not necessarily separate and independent reactions, but often occur in a concerted process. It further suggests that there is not one initiation mechanism, but many, depending on the reactivity of the individual sites and the monomer used.