(428f) Structure of the Cr(II) Active Sites in a Model Phillips' Polyethylene Catalyst, Cr/SiO2, Prepared by Reduction of Grafted CrO2Cl2

The Phillips (CrO_x/SiO₂) catalyst is of primary importance in commercial production of high density polyethylene. The Cr(II) sites widely believed to be the direct precursors of the propagating centers are presumed to be di-coordinated with two silanolate ligands derived from the silica surface, although there is no direct evidence for this coordination geometry. Moreover, this single proposed structure does not explain why only a small fraction of the Cr(II) sites participate in ethylene polymerization, or the very high polydispersity of the polyethylene that they generate. Starting from our recently characterized, uniform grafted chromate sites prepared via the ambient temperature reaction of CrO₂Cl₂ with silica followed by mild heating, we obtained the corresponding Cr(II) sites by CO reduction. The coordination environment of Cr, studied by X-ray absorption spectroscopy, contains additional siloxane ligands that mediate binding of both CO and ethylene. The reactivity in ethylene polymerization appears to depend on these hitherto 'invisible' ligands. Proposed structural differences for the immediate precursors of polymerization and oligomerization sites are inferred.