Some mechanisms of olefin polymerization by Ziegler catalysts

The nature of the intermediates leading to insertion is uncertain. There are those who propose a direct role for the ligands about the transition metal [89], e.g. 

Other workers suggest that there is no direct role of the metal ligands, other than steric, and they envisage mechanism where two or more coordination sites on the central transition metal are used [90a]  

Of course, with different catalysts it is not improbable that both types of mechanisms may be found. 

The polymerization of a-olefins by Ziegler catalysts leads to many 

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