New results with Phillips systems: surface compounds of transition metals, part XXXII

Abstract

The Phillips polyethylene process involves coordinatively unsaturated chromium sites as the catalyst, but unfortunately the path of the reaction is still unknown. Variation of the catalyst preparation procedure (including the use of other metals), the olefins and the polymerisation conditions have been used to obtain new information. Chromium - usually applied in the form of Cr(VI) - is reduced on the surface to the active oxidation state, preferably Cr(II), by the olefin itself, by the solvent or in a separate step by CO or aluminum alkyls. The reduction Cr(VI) → Cr(II) proceeds through a Cr(IV) intermediate. The activity of the centres is defined by their position on the surface of the support, especially by the proximity to surface OH groups (Cr IIA/Cr IIB concept). Similar systems are formed with other transition metals. With vanadium(III)/ethylene a primary 1:1 complex can be isolated; the ensuing polymerisation involves a very small part of the V(III) centres. Olefins with inner double bonds, or with branching near the double bond form oligomers. With molybdenum, the long-postulated surface Mo(II) species has now been identified by O 2 titration and characterised spectroscopically. Fluorescence measurements of the precursor surface Mo(VI) species allow differentiation of several sterically defined species. The reaction of the reduced system with olefins includes metathesis, showing the close relationship between Phillips and metathesis systems. With chromium catalysts, three experiments were performed: (a) the use of higher olefins, yielding comb-like polymers which are kept in solution during the whole reaction; (b) the use of higher temperatures, which allows the isolation of low molecular weight products; (c) IR reflectance spectroscopy of catalyst surfaces during the first moments of contact with olefin.