Development of fundamental kinetic models of single-site olefin polymerization with a focus on [rac-(C2H4(1- indenyl)2)Zr(Me)][MeB(C6F5)3] catalyzed polymerization of 1-hexene
Single-site catalysis is an active research area due to the commercial importance of the materials produced and the single-site nature of the catalysts which are conducive to fundamental studies. The ability to rationally design new single-site catalysts requires fundamental understanding of the polymerization mechanism, including robust determination of the values of the associated rate constants. This task is difficult because the comprehensive mechanism is unknown a priori for a specific catalyst system. While a number of studies have kinetically probed parts of the mechanism, these studies do not result in a consistent mechanistic interpretation and robust determination of the rate constants. In contrast, it is shown that comprehensive, quantitative kinetic modeling based on mass action kinetics coupled with a rich multi-response data set (i.e. the time evolution of monomer concentration, molecular weight distribution (MWD), vinyl groups, active-site count etc. obtained from a batch polymerization) is an essential tool for determining full mechanistic detail and obtaining robust rate constants. To aid the application of this method, computer-aided tools were developed to facilitate the generation, solution and optimization of the kinetic models. The strength of comprehensive quantitative kinetic modeling is highlighted for the polymerization of 1-hexene catalyzed by rac-(C 2H4(1-Ind)2)ZrMe2 activated with B(C 6F5)3. While extensive studies have been published on this catalyst system, the previously acknowledged kinetic mechanism is unable to predict the MWD. However, it was found to be possible to predict the entire multi-response data set (including the MWDs) using a kinetic model featuring a catalytic event that renders approximately 50% of the catalyst inactive for the duration of the polymerization. This finding has significant implications regarding the behavior of the catalyst and the polymer produced. In addition, robust values of rate constants were obtained for future use in developing predictive structure-activity relationships for catalyst design. This example catalyst system illustrates the ability of comprehensive kinetic modeling to elucidate mechanistic detail raising the question of whether this new found mechanism is potentially relevant to other single-site polymerization catalysts, where it has hitherto gone undetected as a result of incomplete kinetic modeling.
Delgass, Purdue University.
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