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)_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.
Additional Information© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Received: 26 April 2019; Accepted: 24 May 2019; Published: 29 May 2019. (This article belongs to the Special Issue Catalytic Polymerization) Helpful discussions of this work with Vladimir A. Zakharov, Evgenii P. Talsi and Konstantin P. Bryliakov (Boreskov Institute of Catalysis) are gratefully acknowledged. For help with the use of research equipment we thank Werner Röll (†, Universität Konstanz). Recordings and analyses of polymer gel-permeation chromatograms by Lars Bolk (Universität Konstanz) and of NMR and UV-vis spectra by Michael C. Haibach (California Institute of Technology) are acknowledged and appreciated by the authors. Valuable help with this work was provided by gifts of several catalyst samples by Dr. M. Ringwald (MCAT GmbH) and by the gift of a sample of SBIZr(Me)-CH_2SiMe_3 by Manfred Bochmann and Simon J. Lancaster (University of East Anglia). Thanks for an extensive introduction to the use of the Multi Curve Resolution-Alternative Least Square program and for adapting this software to the needs of the present project are due to Roma Tauler and to Joaquim Jaumot (Universitat de Barcelona). Finally, the authors wish to thank Clark R. Landis (University of Wisconsin-Madison) for an in-depth discussion of the results and conclusions presented above. This communication is dedicated to Professor Gerhard Fink on the occasion of his 80th birthday in recognition of his many fruitful contributions to the field of polymerization catalysis. This research work was funded by Ministry of Science and Higher Education of the Russian Federation (project АААА-А17-117041710087-9) and by Deutsche Forschungsgemeinschaft (grant Br 510/15-1). Author Contributions: conceptualization, V. N. P., D. E. B., H. H. B.; preparation of samples, UV-vis and NMR spectroscopy, V. N. P., D. E. B., J. E. B.; writing—original draft preparation, H.H.B.; writing—review and editing, J. E. B; funding acquisition, D. E. B., H. H. B., J. E. B.. The authors declare no conflict of interest.
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