FROM HOMOGENEOUS TO SUPPORTED METALLOCENE CATALYSTS

The mechanism of the monoalkene homogeneous polymerization with metall ocene/methylalumoxane (MAO) system is reviewed with reference to struc tural and stereochemical control of the polymers as derived by the typ e of ligands around the transition metal (Zr and Ti). Major disadvanta ges of these homogeneous catalysts are the lack of morphology control and reactor fouling. Supported catalysts can allow to overcome these d rawbacks and be utilized in cheaper bulk or gas-phase processes. Also they may permit the use of less MAO, while still offering facile contr ol of polymer properties by ligand variation. Binding of well-defined transition metal complexes onto carefully prepared inorganic surfaces; is certainly a very attractive approach to create new heterogeneous c atalysts and to characterize highly reactive species. In this paper at tention is on the use of alumina, silica and zeolites for supporting m etallocenes of Ti and Zr. The amount of metallocene which can be attac hed to the surface of these supports is much lower than expected on th e basis of simple physical absorption. This seems to indicate that att achment takes place at specific sites of the surface. In any case the amount of hydroxyl groups seems to play a determining role as shown by structural analysis and catalytic activity of olefin polymerization o f the resulting heterogenized complexes. The formation of cationic spe cies on the inorganic surface was proposed in a very analogous way as proposed in solution. Results concerning homo- and copolymerization of monoalkenes with different supported metallocene complexes are presen ted to discuss the effect of support nature and surface features. (C) 1998 Elsevier Science B.V, All rights reserved.