Mechanism of Stereoselective Placement

Coordination initiators perform two functions. First, they supply the species that initiates the polymerization. Second, the fragment of the initiator aside from the initiating portion has unique coordinating powers. Coordination of this fragment (which may be considered as 

Some chiral initiators have structures such that alternate monomer placements occur with opposite faces of the monomer to yield the syndiotactic polymer. This is syndioselective polymerization proceeding with catalyst site control and is usually observed only with some homogeneous initiators, both traditional Ziegler-Natta and metallocene. 

For polar monomers, heterogeneity is seldom a requirement for isoselective polymerization with traditional Ziegler-Natta initiators syndiotactic polymers are obtained only with the soluble initiators. Styrene and 1,3-dienes are intermediate in behavior between the polar and nonpolar monomer. These monomers undergo isoselective polymerization with both homogeneous and heterogeneous traditional Ziegler-Natta initiators. 

8-4 TRADITIONAL ZIEGLER-NATTA POLYMERIZATION OF NONPOLAR ALKENE MONOMERS 

Modification of an initiator system to increase activity has often come at the expense of stereoselectivity. The great utility of the Ziegler-Natta initiator system is the ability to change one or another of the components, or to add additional components (usually electron donors) to achieve very high stereoselectivity with high activity. The choice of the initiator components evolved in an empirical manner because of a less-than-complete understanding of the detailed structure of these initiators and the mechanism of their stereoselectivity. 

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