Homogeneous Stereospecific Cationic Polymerizations

The mechanism proposed by Bawn and Ledwith (1962) postulates the existence of an sp configuration for the terminal carbon in the growing chain due to the attendant gegen ion, and especially in low-dielectric solvents. They also point out that the structure of the alkyl vinyl ethers, with the exception of the ethyl and isopropyl members, will be subject to steric shielding of one side of the double bond, i.e.. 

This blocks one mode of double-bond opening and assists stereoregulation. This conclusion is supported by the lack of any crystalline polymer in the product when the ethyl and isopropyl groups are used where no blocking is possible. 

The formation of a loose six-membered ring is thought to stabilize the growing carbonium ion in the reaction so that the only route for monomer approach is past the counterion. 

A four-centered cyclic-transition state is involved in the propagation stage leading to the insertion of a monomer imit between the catalyst and the chain end, with subsequent regeneration of the cychc stmctnre. An alternative transition state, proposed by Cram and Kopecky, has a similar bnt more rigid stmctnre. 

---