Cationic polymerization initiation

Gandini,A. and Cheradame, H. Cationic Polymerization. Initiation with Alkenyl Monomers. Vol. 34/35, pp. 1—289. 

Kennedy and co-workers10 studied model cationic polymerization initiation and termination. They determined the effect of halogens in f-BuX and MeX on the rate of reaction between f-BuX and Me3Al. The pseudo second order rate constant decreased (Table 1) as ... 

Gcwdini, A. The Behaviour of Furan Derivatives in Polymerization Reactions. Vol. 25, pp. 47-96. Gandini, A. and Cheradamc, H. Cationic Polymerization. Initiation with Alkenyl Monomers. Vol. 34/35, pp. 1-289. 

Rashkov, I. B., and Gitsov, I., Cationic polymerization initiated by intercalation compounds of Lewis acids. II. Initiating ability and mechanism of action of the initiators, J. Polym. 

Fig ure 2.7 Cationic polymerization initiated by Bronsted-Lowry acid a) initiation, b) propagation, c) termination... 

The actual species responsible for cationic polymerizations initiated by ionizing radiation is not established. The most frequently described mechanism postulates reaction between radical-cation and monomer to form separate cationic and radical species subsequently, the cationic species propagates rapidly while the radical species propagates very slowly. The proposed mechanism for isobutylene involves transfer of a hydrogen radical from monomer to the radical-cation to form the r-butyl carbocation and an unreactive allyl-type radical ... 

Cationic polymerization initiated by ionizing radiation is markedly different from other cationic polymerizations in that the propagating species is a free ion remote from a counterion. Overall electrical neutrality is maintained by electrons trapped by the monomer. 

The expressions (Eqs. 5-34 and 5-42) for Rp in cationic polymerization point out one very significant difference between cationic and radical polymerizations. Radical polymerizations show a -order dependence of Rp on while cationic polymerizations show a first-order depenence of Rp on R,. The difference is a consequence of their different modes of termination. Termination is second-order in the propagating species in radical polymerization but only first-order in cationic polymerization. The one exception to this generalization is certain cationic polymerizations initiated by ionizing radiation (Secs. 5-2a-6, 3-4d). Initiation consists of the formation of radical-cations from monomer followed by dimerization to dicarbo-cations (Eq. 5-11). An alternate proposal is reaction of the radical-cation with monomer to form a monocarbocation species (Eq. 5-12). In either case, the carbocation centers propagate by successive additions of monomer with radical propagation not favored at low temperatures in superpure and dry sytems. 

Unlike cationic polymerization initiated by a conventional catalyst, the propagating species in the present system would bear different type of counter-ion or would be much more free. The counter-anion obtained in this entirely organic system would be large and unstable. The problem of the counterion in charge transfer ionic polymerization certainly requires further study. 

Cham transfer -m cationic polymerization [INITIATORS - CATIONIC INITIATOPH] (Vol 14)... 

---