Catalyst Speciation during <i>ansa</i>-Zirconocene-Catalyzed Polymerization of 1-Hexene Studied by UV-vis Spectroscopy—Formation and Partial Re-Activation of Zr-Allyl Intermediates <xref rid="fn1-polymers-503797" ref-type="fn">†</xref>

• Main Authors: Valentina N. Panchenko, Dmitrii E. Babushkin, John E. Bercaw, Hans H. Brintzinger
• Format: Article
• Language: English
• Published: MDPI AG 2019-05-01
• Series: Polymers
• Subjects: polymerization catalysis; UV-vis spectroscopy; catalyst speciation; zirconocene-allyl cations; de-activation; re-activation
• Online Access: https://www.mdpi.com/2073-4360/11/6/936

Catalyst speciation during polymerization of 1-hexene in benzene or toluene solutions of the catalyst precursor SBIZr(&#956;-Me)₂AlMe₂⁺ B(C₆F₅)₄^&#8722; (SBI = <i>rac</i>-dimethylsilyl-bis(1-indenyl)) at 23 &#176;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-&#963;-polyhexenyl cations arising from SBIZr-Me⁺ (formed from SBIZr(&#956;-Me)₂AlMe₂⁺ by release of AlMe₃) by repeated olefin insertions, while the other one is proposed to consist of SBIZr-&#951;³-allyl cations of composition SBIZr-&#951;³-(1-R-C₃H₄)⁺ (R = n-propyl), formed by &#963;-bond metathesis between SBIZr-Me⁺ and 1-hexene under release of methane. At later reaction stages, all zirconocene-&#963;&#8722;polymeryl cations appear to decay to yet another SBIZr-allyl species, i.e., to cations of the type SBIZr-&#951;³-(x-R-(3-x)-pol-C₃H₃)⁺ (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-&#963;&#8722;polymeryl cations within a few seconds, presumably by initial 1-hexene insertion into the &#951;¹- isomer, followed by repeated additional insertions, while the initially formed, less crowded allyl cations, SBIZr-&#951;³-(1-R-C₃H₄)⁺ appear to remain unchanged. Implications of these results with regard to the kinetics of zirconocene-catalyzed olefin polymerization are discussed.