Zirconocene-Catalyzed Propene Polymerization: A Quenched-Flow Kinetic Study

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Abstract
The kinetics of propene polymerization catalyzed by ansa-metallocenes were studied using quenched-flow techniques. Two catalyst systems were investigated, (SBI)ZrMe2/AliBu3/[Ph3C][CN{B(C6F5)3}2] (1:100:1) at 25.0 °C and (SBI)ZrCl2/methylalumoxane at 40.0 °C (Al:Zr = 2400:1) (SBI = rac-Me2Si(1-Indenyl)2). The aims of the study were to address fundamental mechanistic aspects of metallocene-catalyzed alkene polymerizations, catalyst initiation, the quantitative correlation between catalyst structure and the rate of chain propagation, and the nature of dormant states. One of the most important but largely unknown factors in metallocene catalysis is the distribution of the catalyst between dormant states and species actively involved in polymer chain growth. Measurements of polymer yield Y versus reaction time t for propene concentrations [M] = 0.15−0.59 mol L-1 and zirconocene concentrations in the range [Zr] = (2.38−9.52) × 10-5 mol L-1 for the borate system showed first-order dependence on [M] and [Zr]. Up to t ≈ 1 s, the half-life of catalyst initiation is comparable to the half-life of chain growth; that is, this phase is governed by non-steady-state kinetics. We propose a rate law which takes account of this and accurately describes the initial rates. Curve fitting of Y(t) data provides an apparent chain growth rate constant kpapp on the order of 103 L mol-1 s-1. By contrast, the evolution with time of the number-average polymer molecular weight, which is independent of the concentration of catalyst involved, leads to a kp which is an order of magnitude larger, (17.2 ± 1.4) × 103 L mol-1 s-1. The ratio kpapp/kp = 0.08 indicates that under the given conditions only about 8% of the total catalyst is actively engaged in chain growth at any one time. The system (SBI)ZrCl2/methylalumoxane is significantly less active, kpapp = 48.4 ± 2.7 and kp = (6 ± 2) × 102 L mol-1 s-1, while, surprisingly, the mole fraction of active species is essentially identical, 8%. Evidently, the energetics of the chain growth sequence are strongly modulated by the nature of the counteranion. Increasing the counteranion/zirconium ratio from 1:1 to 20:1 has no influence on catalyst activity. These findings are consistent with a model of closely associated ion pairs throughout the chain growth sequence. For the borate system, propagation is ∼6000 times faster than initiation, while for the MAO catalyst, kp/ki ≈ 800. Polymers obtained at 25 °C show 0.1−0.2 mol % 2,1-regioerrors, and end-group analysis identifies 2,1-misinsertions as the main cause for chain termination (66%), as compared to 34% for the vinylidene end groups. The results suggest that 2,1-regioerrors are a major contributor to the formation of dormant species, even at short reaction times.

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