Hydrogen Transfer Polymerization
Anionic polymerization of unsubstituted acrylamide, catalyzed by strong bases, does not yield typical vinyl polymers. Instead, the product is a 1,3-adduct, poly(ẞ-alanine) [214]. Two alternate reaction paths were originally proposed [214]. These can be shown as follows:
Subsequently, studies of the rate of disappearance of acrylamide in dry sulfolane or pyridine with potassium-t-butoxide initiator led to the following proposed mechanism [215]:
Propagation proceeds in this manner [215]:
The propagating center is neither an ion nor a radical, but a carbon to carbon double bond at the end of the chain. The monomer anion adds to this double bond. This process is a step-growth polymerization and the monomer anion is called an activated monomer. Not all acrylamide polymerizations, initiated by strong bases, however, proceed by a hydrogen transfer process. Depending upon reaction conditions, such as solvent, monomer concentration, and temperature some polymerizations can take place through the carbon to carbon double bonds [216].
Cis and trans crotonamides can also polymerize by hydrogen transfer polymerization. Sodium t-butoxide in pyridine yields identical polymers from both isomers [217]. Also, hydrogen transfer polymerization of acrylamide with optically active, basic catalysts yields optically active polymers [218]. The reactions can be carried out in to luene, using optically active alcoholates of amyl alcohol. The initiating ability of the metal ions is in the following order, Na > Ba > Ca > Mg > Al[218].Optically active polyetherimide forms with optically active barium and calcium alcoholates, but not with the other cations [218]. In this reaction, however, the asymmetric synthesis takes place through an intermolecular hydrogen transfer rather than through an intramolecular hydrogen migration [218]:
