| Summary: | Catalyst speciation during polymerization of 1-hexene in benzene or toluene solutions of the catalyst precursor SBIZr(μ-Me)<sub>2</sub>AlMe<sub>2</sub><sup>+</sup> B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub><sup>−</sup> (SBI = <i>rac</i>-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<sup>+</sup> (formed from SBIZr(μ-Me)<sub>2</sub>AlMe<sub>2</sub><sup>+</sup> by release of AlMe<sub>3</sub>) by repeated olefin insertions, while the other one is proposed to consist of SBIZr-η<sup>3</sup>-allyl cations of composition SBIZr-η<sup>3</sup>-(1-R-C<sub>3</sub>H<sub>4</sub>)<sup>+</sup> (R = n-propyl), formed by σ-bond metathesis between SBIZr-Me<sup>+</sup> 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<sup>3</sup>-(x-R-(3-x)-pol-C<sub>3</sub>H<sub>3</sub>)<sup>+</sup> (pol = <i>i</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<sup>1</sup>- isomer, followed by repeated additional insertions, while the initially formed, less crowded allyl cations, SBIZr-η<sup>3</sup>-(1-R-C<sub>3</sub>H<sub>4</sub>)<sup>+</sup> appear to remain unchanged. Implications of these results with regard to the kinetics of zirconocene-catalyzed olefin polymerization are discussed.
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