Reexamining the evidence for proton transfers in ethylene polymerization.

Delley et al. (1) propose a new proton transfer mechanism to explain the mysterious initiation mechanism (2) of the Phillips polymerization catalyst. The authors’ mechanism invokes dormant (D) sites and active Si(OH)CrR sites, as summarized in Fig. 1. A combination of transition state theory, Delley et al.’s (1) density functional theory (DFT) results, and standard microkinetic modeling techniques (3) for the mechanism of Fig. 1 yield several testable predictions. DFT is not sufficiently accurate to predict absolute rates, but selectivities, molecular weights, and relative abundances can often be predicted more accurately because they involve ratios of rates. Relative abundances are particularly important because the evidence for proton transfer hinges upon the assignment of infrared (IR) peaks at 3,640 and 3,605 cm to Si(OH)Cr–R intermediates (1). According to the proposed mechanism, the total abundance of these species is