Corradini site, Ziegler-Natta catalysts

The stereoselective behaviour of heterogeneous Ziegler-Natta catalysts is caused by the chirality of the catalytic active site but not by chiral atoms in the growing chain. The model devised by Corradini et al. [282] can explain the experimental results relative to the first insertion of a chiral a-olefin into an initial Ti-CH3 bond [377,378], i.e. the absence of enantioselectivity (discrimination between re and si insertions) but the presence of diastereoselectivity... 

There is no single mechanism for Ziegler-Natta polymerization because of the variety of catalyst and co-catalyst systems as well as the different phases in which the reaction may take place. The process of stereoregular polymerization can be understood tfom the mechanism of initiation and propagation as the monomer is inserted at the polymerization site on the catalyst surface. The detailed mechanism for achieving stereospecificity is an active area of research (Corradini and Busico, 1989, Tait and Watkins, 1989), but some general principles may be learned tfom the simple Ziegler-Natta catalysts (Allcock and Lampe, 1981). 

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