Cationic-initiated chain polymerization polyisobutylene

Some alkene monomers can be polymerized by a cationic initiator, as well as by a radical initiator. Cationic polymerization occurs by a chain-reaction pathway and requires the use of a strong protic or Lewis acid catalyst. The chain-carrying step is the electrophilic addition of a carbocation intermediate to the carbon-carbon double bond of another monomer unit. Not surprisingly, cationic polymerization is most effective when a stable, tertiary carbocation intermediate is involved. Thus, the most common commercial use of cationic polymerization is for the preparation of polyisobutylene by treatment of isobutylene (2-mcthylpropene) with BF3 catalyst at -80 C. The product is used in the manufacture of inner tubes for truck and bicycle tires. 

Preparations of macro-initiators or telechelic polymers by cationic methods have been executed primarily by polymerizing isobutylene in the presence of a co-initiator that also functions as a chain transfer agent. A typical reaction sequence is shown in Scheme 1, outlining the synthesis of difunctional polyisobutylene (PIB), which is then used to initiate the polymerization of a-methyl styrene (ffi-MS) to produce an A-B-A type block copolymer. By similar methods, polyisobutylenes with phenol, phenyl, cyclopentadiene, and olefin termini have been synthesized. 

The type of monomers suitable for cationic polymerization are those containing an electron-donating substituent such as 1,1-dialkyl, alkene, alkoxy, and phenyl that stabilize the propagating cationic centers. Successftil industrial examples include polyisobutylene and its copolymers with dienes such as butyl mbber. In ionic polymerization, initiator is conventionally called a catalyst. However, by definition, catalyst and initiator are two different types of reagents. Catalyst takes part in reactions but can be removed from the final product if necessary. On the other side, initiator molecules or their fragments become a part of the produced chains after polymerization. In cationic polymerization, a single catalyst is not sufficient and a cocatalyst is required. Typical catalysts are Lewis acids such as BF3, AICI3, and TiCU that must be used with a protonic cocatalyst such as H2O and methanol ... 

Anionic and cationic polymerizations are often associated. Feldthusen et al. [5f] prepared copolymers containing linear and star-shaped blocks a living polyisobutylene chain was prepared by cationic polymerization, its ends were converted into 2,2-diphenylvinyl groups, then metallated and used as initiators of the ferf-butyl methacrylate anionic polymerization. 

---