Polymerization via Monomer Anions

The anionic polymerization of monomers with NH groups with strong bases does not occur via the addition of monomers to a growing macroanion, but via the addition of the monomer anion on a macromolecule. In the first step, a proton is abstracted by the strong base, for example, as in the reaction of an AT-carboxy anhydride with an alkoxide ion 

The activated monomer reacts with an intact monomer molecule, producing a dimer. 

In the subsequent transfer reaction, the monomer anion is regenerated, 0=c—N c=0 O NH—0=0 

This dimer, then, in the actual propagation reaction, is attacked at the amine end group by a monomer anion, and the chain is lengthened by one unit  

In this step, in contrast to that of (18-29), the ring of the growing macromolecule (here the dimer) is activated toward the monomer anion by an additional CO group ( ). Therefore, the monomer anion reacts preferentially with the polymer, not with the monomer, which still has to be converted to anionic form. This reaction step includes the regeneration of the monomer anion by the formation of the carbamine acid group [(18-31)], etc. 

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Polymerization via monomer anions without added activator behaves quite differently. In this case, theo>-amino acyl lactam activator is first formed in a slow reaction, and then, the polymer chain is started in a fast subsequent reaction. Further a>-amino acyl lactam molecules are formed continuously during the polymerization. Only when all base molecules react with monomer molecules before the other monomer molecules are consumed by formation of polymer chains is the degree of polymerization given by the ratio of monomer to base concentration. Since this is mostly not the case, there is no relationship between the ratio [monomer]/[base] and the degree of polymerization. Similar relationships occur for the polymerization of iV-carboxy anhydrides of a-amino acids with strong bases. 

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