Cationic Group 4 metallocene complexes and their role in polymerisation catalysis: The chemistry of well defined Ziegler catalysts

Cationic alkyl complexes of Group 4 metallocenes of the type [MCp(2)R](+) (M = Ti, Zr or Hf, Cp = C2H5) have been recognised as the catalytically active species in metallocene-based olefin polymerisation catalysts. These highly electrophilic 14-electron species possess a very complex chemistry in which the formation of temporarily dormant stabilised adducts plays a dominant role. Cationic metal alkyls of this kind are found to be extremely active polymerisation catalysts, with high stereoselectivities and the potential to produce numerous previously inaccessible polymeric materials. A detailed understanding of the chemistry of these species promises to lead to a new generation of well defined polymerisation catalysts. Metallocene-based catalysts already play an increasing role in major industrial polymerisation processes.