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

Catalyst speciation during polymerization of 1-hexene in benzene or toluene solutions of the catalyst precursor SBIZr(μ-Me)(2)AlMe(2)(+) B(C(6)F(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)(2)AlMe(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(3)H(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(3)H(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(3)H(4))(+) appear to remain unchanged. Implications of these results with regard to the kinetics of zirconocene-catalyzed olefin polymerization are discussed.