Recent Developments in Stereoregulation by Ziegler-Natta Polymerization of α-Olefins

Since its discovery a great deal of work has been done for understanding the mechanism of Ziegler-Natta polymerization of α-olefins. In spite of this almost 25 years old activity many points do not appear entirely clarified and firmly established. Indeed these catalysts join unique and very interesting properties to a very large complexity. Thus the field is under constant development and also the aspects generally accepted (1) are often put in debate. For istance the α-olefin insertion in a transition metal-carbon bond, considered as one of the fixed step of the mechanism, has been questioned very recently by proposing (2) a mechanism proceeding via a 1,2 hydrogen shift from the α-carbon of the growing chain and formation of metallocycle and carbene intermediates. Even if this last proposal does not explain end group nature, particularly when using AlPh3, large predominance of primary insertion in isotactic polymerization and different role of growing chain end and metal complex in the steric control, it however indicates that several problems are still open as discussed in other chapters.