Polymerization of Ethylene by Silica‐Supported Dinuclear CrIII Sites through an Initiation Step Involving CH Bond Activation |
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Authors: | Matthew P Conley Murielle F Delley Georges Siddiqi Giuseppe Lapadula Sébastien Norsic Vincent Monteil Olga V Safonova Christophe Copéret |
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Institution: | 1. Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 2, CH‐8093 Zürich (Switzerland);2. UMR 5265 Laboratoire de Chimie, Catalyse, Polymères et Procédés (C2P2), LCPP team, Université de Lyon 1, CPE Lyon, CNRS, Bat308F, 43 Bd du 11 Novembre 1918, 69616 Villeurbanne (France);3. Paul Scherrer Institute, CH‐5232, Villigen (Switzerland) |
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Abstract: | The insertion of an olefin into a preformed metal–carbon bond is a common mechanism for transition‐metal‐catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal–carbon bond is not present in the precatalyst. The Phillips catalyst, CrO3 dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first Cr? C bond is formed remain unknown. We synthesized well‐defined dinuclear CrII and CrIII sites on silica. Whereas the CrII material was a poor polymerization catalyst, the CrIII material was active. Poisoning studies showed that about 65 % of the CrIII sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si–(μ‐OH)–CrIII species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at CrIII? O bonds. |
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Keywords: | C H activation chromium ethylene polymerization Phillips catalyst reaction mechanisms |
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