Acids in Cationic Initiations

Complexation of Lewis acids with water is another case of formation of electrophiles that can initiate chain growth. MeXn represents a Lewis acid  

In the same manner, a reaction of a Lewis acid with an alkyl halide and subsequent initiation can be illustrated as follows  

In the above examples, the Lewis acid is actually the coinitiator and the water or the alkyl halide the initiators. 

Numerous studies confirm the need by many Lewis acids for other molecules, like water or alkyl halides, to form initiating ions. For instance, pure TICU will not initiate the polymerization of isobutylene. This led many to believe that none of the Lewis acids is capable of initiating cationic polymerizations of olefins by itself. Subsequent investigations, however, demonstrated that some strong Lewis acids are capable of initiating such polymerizations. 

Whether a Lewis acid is capable of initiating these polymerizations by itself was tested, with the aid of an early discovery. It has been known for some time that hindered bases like crowded pyridine derivatives exhibit specificity toward reactions with protons. Such bases might be used to discriminate between two types of initiating mechanisms encountered with Lewis acids. The base will not interfere with direct electrophilic additions of the Lewis acids to the monomer. On the other hand, it should prevent an initiation process by protons from taking place. If both pathways are operative, then the pyridine derivatives can only quench the protonic initiation and will offer a means of assessing the relative importance of each process. 

---