Differences between insertions of ethylene into metallocene and non-metallocene ethylene polymerization catalysts

Density functional theory (DFT) calculations at ?B97XD/SDD level have been conducted to study the differences between the insertions of ethylene into the non-metallocene octahedral group 4 catalysts and those into metallocene group 4 catalysts. The study shows that both can be described by the Cossess mechanim, but are different energetically. The latter have energy profiles involving stable p complexes between ethylene and catalysts prior to insertion and the insertion transition states (TSs) being lower than reactants, whereas the p complexes involved in the former are significantly less stable than their counterparts in the latter, and the insertion TSs are higher than or close to the reactants. The energy decomposition analyses on the activation energies indicate that the octahedral catalysts exert more significant steric effects than the metallocene catalysts, which are responsible for the differences. The cis arrangements of two chloride atoms in the precursors of metallocene catalysts were considered to be crucial for their catalytic activity, but the precursors of octahedral catalysts with either cis or trans arrangements of two chloride atoms exhibit activity. Nevertheless, the effective catalysts generated from octahedral precursors with cis chloride atoms have lower activation barriers than those from precursors with trans chloride atoms. Copyright (C) 2012 John Wiley & Sons, Ltd.