(324d) Composition and Structures of Active Sites in Mixed-Metal Ziegler-Natta Catalysts

Deposition of butylmagnesium reagents onto a porous silica results in a butylmagnesium-modified support for which the grafting stoichiometry can be deduced based on the known number of accessible hydroxyl groups. The results show that a significant fraction of the grafting occurs on siloxane rings that are opened with consequent alkylation of the silica surface. TiCl₄ and/or VOCl₃ can be attached to these ?passivated' silica surfaces by non-protolytic grafting, to yield the immediate precursors of certain Ziegler-Natta catalysts that give particularly narrow molecular weights and are capable of making ethylene/alpha-olefin copolymers with good stretchability and puncture resistance for cast film applications. The nature of the transition metal sites (nuclearity, oxidation state, ligand complement) has been studied using a combination of elemental analysis, XPS, EPR, XAS, and analysis of volatile organic products.

For systems containing only one transition metal on the butylmagnesium-modified silica support, the Ti or V content is 20 - 40% higher than the Mg content, requiring that some Mg sites react with more than one equivalent of the transition metal. For bimetallic (V-Ti) systems, the Ti content is much higher than the V content, even though equal amounts of each metal chloride were added simultaneously to the butylmagnesium-modified silica. The higher reactivity of TiCl₄ compared to VOCl₃ leads to non-random mixed-metal active sites. XPS and EPR confirm partial reduction to Ti(III) and V(IV), via reactions of the butyl groups, while EXAFS is consistent with attachment via bridging chlorides.

Treatment of the butylmagnesium-modified silica with HCl eliminates butyl groups on magnesium and results in a very different reactivity towards the transition metal complexes. The Ti and V contents are much lower than the Mg content, even when the transition metal halides are initially present in excess. Bridging chloride ligands are not observed by EXAFS, suggesting that V and Ti react primarily with regenerated hydroxyl sites. The ramifications for ethylene polymerization activity and the microstructure of the resulting polymer will be discussed.