Nonclassical chain polymerization

Evidence has now been presented that indicates that the above compound behaves as a carbocationic polymerization initiator for styrene, W-vinylcarbazole, vinyl ethers, and isobutylene. The mechanism of initiation and polymerization of these monomers by such metallocene complexes is still being investigated. It was suggested by Wang et al. [53], that the mechanism of carbocationic polymerization of such olefins by the above complex would involve coordination of the olefins, as shown below, in a nonclassical p -fashion, with the metal-olefin. This interaction is stabilized by a complementary borate-olefin interaction. The next step in the polymerization process by this mechanism, then involves attack on the carbocationic centers of the metal ions-activated olefin molecules by secondary olefin monomers, followed by chain growth [53] ... 

Cations are kinetic chain carriers in cationic polymerizations. Such cations may be, for example, carbocations or oxonium ions. All electrophilic carbon atoms may be described as carbocations. These can be classified as carbenium ions (trivalent carbocations) and as carbonium ions (carbocations with coordination numbers of four or five). Carbenium ions such as, for example, R3C are classical carbocations. Carbonium ions such as, for example, R5C or R5C2 are nonclassical ions. 

---