The Role of Carbene Complexes in the Phillips Olefin Polymerization (Surface Compounds of Transition Metals, Part XXXI (1))

Abstract

Coordinatively highly unsaturated chromium(II) on silicagel surface — the typical active site for the catalytic olefine polymerization in the Phillips process — reacts at higher temperatures with olefins under <CH₂> transfer. This reaction can be rationalized by a sequence olefin-complex → carbene complex →metallacyclobutane; these intermediates allow to explain as well the observed isomerization reactions as start- and propagation steps of the polymerization. An irreversible transition from the metallacyclobutane to π-allyl-Cr(IV)-complexes terminates the activity of the sites, yielding alkanes and H₂ as reduction products.- Experiments at lower temperatures (< 120°C) show that this model is not appropriate to explain the “usual” polymerization, since oligomers with ±<CH₂> are not found and carbene complexes — made by reaction of the surface Cr(II) with diazocompounds — react with olefins to cyclopropane derivatives. Therefore we have to conclude that the carbene complex model may play a realistic role only in the higher temperature range, causing a redox termination of the catalytic reaction.