Ziegler-Natta and Related Polymerizations of Alkenes

77See P. Pino and R. Miilhaupt, Angew. Chem. Int. Ed. Engl., 1980,19, 857, for a brief history of early work in transition-metal catalyzed polymerization. 

Although the Cossee mechanism accounted for much of the experimental data related to Z-N polymerization, it was a bold proposal because 1,2-inser-tions into M-C bonds of early transition metal complexes were unknown at the time. Metal alkyls, should they form by 1,2-insertion into an M-C bond, ought to have a high propensity to undergo loss of a (3-hydrogen by 1,2-elimination. Thus, 

79There are indications that even catalysts that appear to be soluble are actually present as small aggregate particles. 

After many insertions, chain termination occurs  

Nevertheless, the paucity of evidence for direct M-C alkene insertion was a nagging problem associated with the Cossee mechanism. Several years later, 

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