Evidence for a chromasiloxane ring size effect in Phillips (Cr/SiO2) polymerization catalysts

The ambient temperature reaction of CrO2Cl2 with silica, followed by mild heating to induce formation of uniform grafted silylchromate diesters, was studied as a function of the silica pretreatment temperature. The reactivity of the resulting chromate sites toward ethylene is qualitatively different: those formed on the silica pretreated at 200 °C are incapable of initiating polymerization, while those formed on silicas pretreated at 450 and 800 °C spontaneously induce polymerization with kinetic profiles closely resembling that of the calcined Phillips catalyst (CrOx/SiO2). Comparison of their X-ray absorption spectra suggests subtle differences in the chromate–support interactions, which can be interpreted in terms of changes in the chromasiloxane ring size distribution. The unstrained sites favored on the highly hydroxylated silica surface are consistent with 8-membered chromasiloxane rings formed by attachment of the CrO2 fragment to non-vicinal hydroxyls, while the strained sites on highly dehydroxylated silica surfaces are suggested to be 6-membered chromasiloxane rings created from vicinal hydroxyls located on adjacent silicon atoms. Simple computational models for these sites predict changes in the vibrational spectra and the XANES that are consistent with experimental observations.