Cationic polymerization spontaneous chain termination

The chain carriers in chain-growth polymerization may be anions or cations rather than free radicals. Such ionic polymerization shares many features with free-radical polymerization, but differs in one important respect Since ions of the same charge sign repel one another, spontaneous binary termination by reaction of two chain carriers with one another cannot occur. In fact, the reaction may run out of monomer with chain carriers still intact. 

The key feature distinguishing anionic (and cationic) from free-radical polymerization is the absence of spontaneous binary termination and has already been mentioned. Unless chain transfer occurs, polymer chains keep growing until all monomer is used up. At that stage, the polymer still carries reactive centers [65] —it is said to be a "living polymer" [66,67]—, and polymerization can be started anew by addition of further monomer. Block copolymers can be synthesized from a living polymer by addition of a different monomer [68,69]. 

Interest in anionic polymerizations arises in part from the reactivity of the living carbanionic sites4 7) Access can be provided to polymers with a functional chain end. Such species are difficult to obtain by other methods. Polycondensations yield ro-functional polymers but they provide neither accurate molecular weight control nor low polydispersity. Recently Kennedy51) developed the inifer technique which is based upon selective transfer to fit vinylic polymers obtained cationically with functions at chain end. Also some cationic ring-opening polymerizations52) without spontaneous termination can yield re-functional polymers upon induced deactivation. Anionic polymerization remains however the most versatile and widely used method to synthesize tailor made re-functional macromolecules. 

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