Stereoselectivity, Kinetics, and Mechanism

The mechanism of Ziegler-type polymerization has not only to explain rate expressions found by kinetic measurements, but also the structure of the polymer. The structure and the molecular-weight distribution of the polymers are a record of what happened during the polymerization reaction. What is to be explained may be summarized by discussing propylene as a monomer. 

On using vanadium-based homogeneous catalysts, polymers consisting of syndiotactic stereo-blocks and stereo-irregular blocks are obtained 

It must be noted that the approach of the monomer to the reactive metal-carbon bond should occur from the less hindered side. The greater stability of the trans complex should arise because the substituted carbon atom of the last unit is involved in the formation of the four-membered ring, so that reaction is hindered. The structure of copolymers of deuteropropylene from cis- and trans-(U,)-propylene proved that, even in syndiotactic polymerization, cis-addition to the double bond occurs, because with 1,2-disubstituted monomers, structures of ery-thro and threo di-syndiotactic polymers degenerate (see Fig. 4). 

If the metal (M) has very bulky substituents, the second insertion step is impossible. With smaller substituents, the monomer always approaches the reactive metal-carbon bond from the same side, the customary situation if M is part of a disturbed crystal lattice, leading to isotactic polymerization (see Fig. 5). 

Pino et al. (II) summarized the attempts made to produce optically 

---