From homogeneous to supported metallocene catalysts

The mechanism of the monoalkene homogeneous polymerization with metallocene/methylalumoxane (MAO) system is reviewed with reference to structural and stereochemical control of the polymers as derived by the type of ligands around the transition metal (Zr and Ti). Major disadvantages of these homogeneous catalysts are the lack of morphology control and reactor fouling. Supported catalysts can allow to overcome these drawbacks and be utilized in cheaper bulk or gas-phase processes. Also they may permit the use of less MAO, while still offering facile control 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 catalysts and to characterize highly reactive species. In this paper attention is on the use of alumina, silica and zeolites for supporting metallocenes of Ti and Zr. The amount of metallocene which can be attached to the surface of these supports is much lower than expected on the basis of simple physical absorption. This seems to indicate that attachment 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 of the resulting heterogenized complexes. The formation of cationic species 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 presented to discuss the effect of support nature and surface features. (C) 1998 Elsevier Science B.V, All rights reserved.