title: Polymerization of ethylene by silica-supported dinuclear Cr(III) sites through an initiation step involving C-H bond activation.
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 Cr(II) and Cr(III) sites on silica. Whereas the Cr(II) material was a poor polymerization catalyst, the Cr(III) material was active. Poisoning studies showed that about 65 % of the Cr(III) 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)-Cr(III) species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr(III) O bonds.
authors: Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 2, CH-8093 Zürich (Switzerland).
authors: Matthew P
authors: MP
authors: Conley
authors: Murielle F
authors: MF
authors: Delley
authors: Georges
authors: G
authors: Siddiqi
authors: Giuseppe
authors: G
authors: Lapadula
authors: Sébastien
authors: S
authors: Norsic
authors: Vincent
authors: V
authors: Monteil
authors: Olga V
authors: OV
authors: Safonova
authors: Christophe
authors: C
authors: Copéret
keywords: CH activation; Phillips catalyst; chromium; ethylene polymerization; reaction mechanisms