Chapter 6 Active Centers in Transition Metal Catalysts and Mechanisms of Polymerization Reactions

Abstract

This chapter describes the current understanding of the structure of active centers in various transition metal catalysts and mechanisms of alkene polymerization reactions. In the case of stereospecific catalysts, it is reasonable to separate two mechanistic issues of the polymerization reactions, the chemistry, and the stereospecificity. Two types of information are presented; one derived from experimental data and another from theoretical analysis. All alkene polymerization reactions with transition metal catalysts are, in the mechanistic sense, particular cases of one general type of reaction, carbometallation— that is, insertion of the C═C bond of an alkene molecule into the metal–carbon bond. Three distinct types of catalysts based on metallocene complexes are known: (1) The early catalysts, combinations of titanocene complexes and organoaluminum chlorides. (2) Kaminsky—Sinn catalysts, combinations of metallocene complexes and MAO. (3) Catalysts containing alkylated metallocene complexes and ion-forming cocatalysts. Detailed investigations of the three groups of catalysts have shown that the same active species are present in all of them, metallocenium cations containing a transition metal–carbon bond.