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.

The chemical kinetics for a series of three zirconium amine bis(phenolate) catalysts for poly 1-hexene polymerization have been examined as a function of temperature. Detailed modeling of the experimental data has yielded the activation parameters for many of the reaction rate constants, including those for propagation, initiation, chain transfer, and monomer misinsertion and recovery. While existing literature is sparse, the results herein generally agree with previously published values; specifically, for the propagation rate constant, the magnitude of the enthalpy of activation is low (12 kcal mol −1 and below), and the magnitude of the entropy of activation is moderate (up to −27 cal mol −1 K −1 ). With regard to the remaining rate constants, Arrhenius behavior is observed in most cases despite the complexity of the temperature dependence in the two-step adsorption/insertion kinetics. The rate expression for these reactions approaches an Arrhenius form in certain limiting cases of the relative elementary rate constants. The reaction rate data are compared against these limiting cases, leading to the conclusion that docking, with an early transition state, is rate limiting for propagation and placing bounds on the interpretation of most of the remaining constants. The challenges encountered in assigning temperature-dependent rate constants for the polymerization reactions are indicative of both the significant complexity in modeling a reaction in which thousands of species are present and the extreme care that is required in generating reproducible data.