266007 Robust Determination of the Kinetic Mechanism and Rate Constants for the Polymerization of 1-Hexene with a Series of Zirconium Phenolate Catalysts
The study of single-site catalysts for olefin polymerization is particularly attractive due to the possibility of directly correlating the physical properties of the resulting polymer to structural features of the catalyst.1 Based on this correlation, a catalyst structure may be manipulated to yield specific polymeric architectures. However, it is only through quantitative kinetics that these types of correlations can be drawn and exploited for the rational design of new single-site catalysts.2
The prevailing practice in the field is to measure catalyst activity and bulk properties of the resulting polymer, such as Mw and PDI.3While these methods have been vital in the screening and discovery of novel catalytic systems, they lead to incomplete and often incorrect picture of the contribution of each elementary step to the overall catalytic polymerization. The next level of quantitative kinetic analysis is to extract rate constants by isolating the various elementary steps. This approach is (1) ultimately futile because it will inevitably fail for systems where the elementary steps are interconnected and (2) misses an opportunity to exploit the information contained in the molecular weight distribution.
As opposed to attempting to isolate each elementary step, we simultaneously solve a set of differential rate equations based on mass action kinetics for all the species present in the reaction mixture (monomers, chains of every length and of every stoichiometric form, and every possible form of the catalytic species) according to the postulated overall mechanism. This treatment affords the concentrations of all species as well as the MWD of polymer chains at every time point during the course of the reaction.
The approach is used to determine the rate constants for each elementary step for the Salan catalysts [Zr(ONTHFO)Bn][BnB(C6F5)3] (1), [Zr(ONNMe2O)Bn][BnB(C6F5)3] (2), [Hf(ONTHFO)Bn][BnB(C6F5)3] (3). The catalytic polymerization is found to include initiation, propagation with misinsertion followed by recovery, and monomer-independent chain transfer from both primary and secondary polymer chains. While catalysts 1 and 2 follow the same overall mechanism and display comparable rate constants for initiation and propagation, 2 exhibits chain transfer rate constants that are two orders of magnitude faster than those observed for catalyst 1. Catalyst 3 is structurally identical to 1, however, we observe a significant change in the monomer consumption rate of Catalyst 1 as compared to that of Catalyst 3, where the former propagates 30 times faster than the latter. We find the Hf catalyst to produce significantly more vinyl end groups, primarily as consequence of the lower propagation rate.
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