Charge cancellation polymerization

The authors suggest that in a charge cancellation polymerization, propagation will be assisted by zwitterion association. Association of monomeric units has been advanced as an explanation for abnormally fast stereoregular polymerization of acrylic acid in bulk or polar solvents53. ... 

Zwitterions capable of charge cancellation polymerization can be formed in situ by the interaction of suitable molecules. Saegusa61,62 has used this method to produce a wide range of alternating copolymers. For instance a quantitative yield of alternating copolymer is obtained when equimolar amounts of 2-oxazoIine and /3-propiolactone are mixed in an aprotic solvent at room temperature. 

Charge cancellation polymerizations must be considered as special cases. Some degree of charge separation already exists in acetonitrile oxide but a base is required to initiate its polymerization. Although linked by a conjugated system ionisation is essentially complete in cyclic sulfonium zwitterion monomers. No initiator is required and monomeric units add at both ends of the p>olymer chain. The initial step in the general scheme above is best considered as part of monomer synthesis. 

Both Saegusa63 and Schmidt52 have shown that macrocycles are more likely to be formed when charge cancellation polymerizations are carried out in solvents of low dielectric constant. The equilibrium concentration of cyclic ion pairs will increase at the expense of linear ion pairs and free ions if the dielectric constant of the medium falls. Naturally under these circumstances there is a relative increase in the rate of macrocycle formation. 

During vinyl polymerizations, the zwitterion concentration is usually very small. Ion pairing of type (ii) would be entropically very unfavourable. However, the formation of macrocycles during charge cancellation polymerizations suggests that at least in the case... 

Frequently, macrocycles form a substantial proportion of the product of charge cancellation polymerizations. De Sarlo et al.7I) have shown that if such products have accessible donor atoms they can function as macrocyelic ligands. Other macrocycle forming monomers with donor atoms or groups would be of value. 

The last few years have seen the synthesis of zwitterions which polymerize by the charge cancellation mechanism foreshadowed by Swarc. To varying degrees these monomers do not exhibit a unique propagation reaction, charge cancellation can occur between monomers, chain and monomer, or chain and chain. However, there can be no doubt about the nature of the chains and one novel reaction predicted for zwitterions, macrocycle formation, has been shown to occur. 

This review deals with polymerizations initiated by a molecule, in which at least one author believes macrozwitterions are formed. The majority are of the simple addition type and are subdivided into vinyl, carbonyl, and strained ring. The other types are all charge cancellations. Monomers are either preformed zwitterions, highly polarised molecules (i.e. CH3CNO) or zwitterions are formed in situ by nucleophile/electrophile pairs. 

Although similar grating cancellation and revelation behaviors have been observed in other PR polymeric composites, they are attributed either to the trap s intercommunication or to the residual ionic motion [101-103]. However, the complementary gratings in the present study are formed by the space-charge field of two types of photogenerated charge carriers. 

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