Polycondensation zwitterion

Many researchers have investigated the use of amines and alcohols as initiators for the ROP of lactones. As a rule, amines and alcohols are not nucleophilic enough to be efficient initiators, and it is then mandatory to use catalysts to perform the polymerization successfully. Nevertheless, highly reactive p-lactones exhibit a particular behavior because their polymerization can be initiated by nucleophilic amines in the absence of any catalyst. As far as tertiary amines are concerned, the initiation step implies the formation of a zwitterion made up of an ammonium cation and a carboxylate anion, as shown in Fig. 20. Authors coined the name zwitterionic polymerization for this process [80]. Nevertheless, this polymerization is not really new because the mechanism is mainly anionic. Interestingly, Rticheldorf and coworkers did not exclude the possibility that, at least at some stage of the polymerization, chain extension takes place by step-growth polycondensation [81]. 

Until recently periodic copolymers (consisting of more than two monomers) have been a more or less theoretical domain, treating special kinds of polycondensations rather than polymerizations. Not long ago, donor—acceptor complexes (often zwitterions) composed of two or more compounds (monomers) were discovered, homopolymerizing as a whole to yield periodic polymers. 

Poly(pivalolactone) is the polyester of hydroxy pivalic acid. Pivalo-lactone is used as monomer since hydroxy pivalic acid does not polycondense to high molar masses. Living zwitterions are formed with tributyl phosphine as initiator and these add on further pivalolactone ... 

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