Computational Support for Phillips Catalyst Initiation via Cr-C Bond Homolysis in a Chromacyclopentane Site

Using density functional theory, we examine a possible homolysis initiation mechanism for the Phillips catalyst, starting from Cr-II sites exposed to ethylene. Spin-crossing in an abundant quintet bis(ethylene) Cr-II site leads to cycloaddition to form a chromacyclopentane site. One Cr-C bond then homolyzes to generate a tethered n-butyl radical: [Cr(CH2)(3)CH2 center dot]. If the radical attaches to a nearby inorganic Cr site, it yields two alkylCr(III) sites capable of Cossee-Arlman polymerization. The overall computed barrier for this initiation process is 132 kJ/mol, which is comparable to the 120 kJ/mol value that we estimated from reported initiation times in industrial reactors. Poisson statistics suggest that this mechanism could activate similar to 35% of Cr sites on a commercial catalyst with a loading of 0.4 Cr/nm(2). Pairwise Cr grafting, amplification by complementary initiation reactions, or the creation of dangling bonds that form as the silica support fractures, might, explain the apparent increase in per-site activity at lower Cr loadings.