Polycombination of zwitterions

A special case of donor-acceptor interactions occurs when two monomers come into contact such that one exhibits a signicant donor and the other an acceptor character. The donor-acceptor complex then has the structure of a zwitterion resulting from the combination of radical ions after electron transfer from donor to acceptor 

When the equilibrium (83) is shifted sufficiently to the right, zwitterion concentration in the medium is so high that the end groups of opposite charge react before the monomer has had a chance to become attached to a suitable ion. A strictly alternating copolymer is formed (see also Chap. 5, Sect. 5.8) 

In systems where the equilibrium is shifted less to the right of eqn. (83), monomer addition to a suitable end can also occur. As the addition rates of monomers MA and MD are generally different, the product need not have a strictly alternating structure. 

These copolymerizations start spontaneously without the addition of initiator. They were discovered by Saegusa et al. [306]. Originally only a few monomer pairs were known which yielded zwitterions by way of DA complexes. Representatives of MD are 

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