The structure and reactivity of supported Ziegler—Natta catalysts in olefin polymerization
Polimery 1997, No 10, 616
Conditions of formation were established of active sites based either on isolated complexes of a transition metal covalent-bonded to the oxide carrier surface (Si-O-TiCl3) or on clusters of a transition metal compound (nTiCl3/SiO2). The composition and the structure of the active sites were studied by mass spectrometry of the products of temperature-programmed desorption, X-ray analysis, EPR spectroscopy, polarography and gravimetric methods. The activities and stereo specificities of these types of catalysts in gas-phasepropylene polymerization were found to be substantially different. The formation of atactic and isotactic polypropylene is assumed to be associated with the presence of EPR-active Ti (III) and EPR-silent Ti (III). The g-factors of EPR signals are ofimportance, too. At the same time the activities of the catalysts based on isolated covalent-bonded complexes of Ti and on the clusters of TiCl3 do not differ greatly in ethylene polymerization. In these two cases the structural changes occurring in the vicinity of the active centers do not affect significantly the reactivity of the Ti-C bonds.