Abstract
Since the discovery that transition metals salts mixed with organoaluminum reagents catalyze the polymerization of ethylene to crystalline polyethylene, organo-metallic complexes and reagents have played a major role in the polymer industry [1]. Over the past 20 years a tremendous amount has been learned about the structures and mechanisms of reactions of complexes related to those proposed to be active in these systems [2]. In the related area of olefin metathesis and ring-opening metathesis polymerization (ROMP), metal carbenes and metallacycles were proposed intermediates, and over the past few years a number of complexes with these structures that will catalyze the olefin metathesis reaction have been prepared and studied [3]. In contrast to the ill-defined classical catalysts based on Ziegler-type catalysts, these are living polymerization systems. This was first observed using Tebbe-type reagents [4].

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