Mechanistic Insights into Chromium-Catalyzed Ethylene Trimerization

Abstract

The kinetics of ethylene trimerization by a chromium N-phosphinoamidine (Cr-(P,N)) precatalyst activated by modified methylaluminoxane (MMAO) has been investigated by high-pressure NMR techniques. An in-depth kinetic analysis of this metallacyclic mechanism has been conducted. It was found that an intermediate in the trimerization catalytic cycle, proposed in this study as the chromium alkenyl hydride species, degrades into a polymer active site where this degradation step is independent of ethylene concentration and is first order in catalyst. Additionally, we report that at least one of the first two ethylene coordination steps must be reversible in order to predict the features of the monomer consumption profiles. The reaction order in ethylene is dependent on the reversibility of the ethylene coordination steps. The observation of these details of the mechanism explains many of the challenges inherent in the examination of this and similar catalyst systems and emphasizes the usefulness of operando high-pressure NMR studies and a quantitative kinetic modeling approach in the study of such systems.