Polymerizations of Lactams
Polymerizations of lactams produce important commercial polymers. The polymerization reactions, therefore, received considerable attention. Lactam molecules polymerize by three different mechanisms: cationic, anionic, and a hydrolytic one (by water or water releasing substances). The lactam ring is strongly resonance stabilized and the carbonyl activity is low. Nevertheless, the ring-opening polymerizations start with small amounts of initiators through trans-acylation reactions. Fairly high temperatures, however, are needed, often above 200C. In all such reactions, one molecule acts as the acylating agent or as an electrophile while the other one acts as a nucleophile and undergoes the acylation. Generally, the initiators activate the inactive amide groups causing them to react with other lactams through successive transamidations that result in formations of polyamides. Both acids and bases catalyze the transamidation reactions. The additions of electrophiles affect increases in the electrophilicity of the carbonyl carbon of the acylating lactam. The nucleophiles, on the other hand, increase the nucleophilic character of the lactam substrate (if they are bases).
All initiators can be divided into two groups. To the first one belong strong bases capable of forming lactam anions by removing the amide proton. This starts the anionic polymerization reaction. To the second one belong active hydrogen compounds capable of protonating the amide bond and thereby affecting cationic polymerization [114]. Side reactions are common in lactam polymerizations. Their nature and extent depends upon the concentration and character of the initiators, the temperatures of the reactions, and the structures of the lactams. When cationic polymerizations of lactams are initiated by strong acids, strongly basic amidine groups can be produced. These groups bind the strong acids, inactive the growth centers, and decrease the rate of polymerization. Use of strong bases to initiate polymerizations of lactams possessing at least one a-hydrogen also result in side reaction. Compounds form that decrease the basicity of lactams and polyamides and slow the polymerizations. Also, side reactions give rise to irregular structures, namely branching. The ring-opening polymerization reactions depend upon thermodynamic and kinetic factors, and on the total molecular strain energies of the particular ring structures. Six-membered d-Valero lactam is the most stable ring structure and most difficult to polymerize. Also, presence of substituents increases the stability of the rings and decreases the ability to polymerize.
