Catalyst Speciation During ansa-Zirconocene-Catalyzed Polymerization of 1-Hexene Studied by UV-vis Spectroscopy—Formation and Partial Re-Activation of Zr-Allyl Intermediates

Abstract

Catalyst speciation during polymerization of 1-hexene in benzene or toluene solutions of the catalyst precursor SBIZr(μ-Me)_2AlMe_2^+ B(C_6F_5)_4^− (SBI = rac-dimethylsilyl-bis(1-indenyl)) at 23 °C is studied by following the accompanying UV-vis-spectral changes. These indicate that the onset of polymerization catalysis is associated with the concurrent formation of two distinct zirconocene species. One of these is proposed to consist of SBIZr-σ-polyhexenyl cations arising from SBIZr-Me^+ (formed from SBIZr(μ-Me)_2AlMe_2^+ by release of AlMe_3) by repeated olefin insertions, while the other one is proposed to consist of SBIZr-η^3-allyl cations of composition SBIZr-η^3-(1-R-C_3H_4)^+ (R = n-propyl), formed by σ-bond metathesis between SBIZr-Me^+ and 1-hexene under release of methane. At later reaction stages, all zirconocene-σ−polymeryl cations appear to decay to yet another SBIZr-allyl species, i.e., to cations of the type SBIZr- η^3-(x-R-(3-x)-pol-C_3H_3)^+ (pol = i-polyhexenyl, x = 1 or 2). Renewed addition of excess 1-hexene is proposed to convert these sterically encumbered Zr-allyl cations back to catalytically active SBIZr-σ−polymeryl cations within a few seconds, presumably by initial 1-hexene insertion into the η^1- isomer, followed by repeated additional insertions, while the initially formed, less crowded allyl cations, SBIZr-η^3-(1-R-C_3H_4)^+ appear to remain unchanged. Implications of these results with regard to the kinetics of zirconocene-catalyzed olefin polymerization are discussed.

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