Special Reactions for Formation of Polyamides
There are occasional reports in the literature on use of special reactions to form polyamides. One is a synthesis via enamines. In this case, diisocyanates are condensed with cyclopentanone enamines of morpholine or of piperidine [77]:
The molecular weights of the polymers decrease when the ring sizes of the ketone components increase. Excess diisocyanate yields branched and cross-linked polymers. The enamine units in the polymers can be hydrolyzed with formic acid to the corresponding ketones [77]. A similar reaction was reported from aromatic isocyanates and imidazoles, where 1,4 tetramethylene-N,N0-di imidazoles were reacted with aromatic polyisocyanates to form thermoset polyamides [45]:
Another route to polyamides is via the Ritter reaction [78]:
When aromatic di nitriles are used, high melting polymers with good thermal stability form [78]. Direct polycondensation of various dicarboxylic acids with diamines is possible [81] under mild conditions by using a catalytic system of an enol phosphite in the presence of imidazole. One such enol phosphite is diethyl,1-methyl-3-oxo-1-butenyl phosphite. Polymers with inherent viscosities of 1–0.25 form. Among the organic bases, imidazole is most effective [84]. The reaction is applicable to both aliphatic and aromatic dicarboxylic acids and diamines.
Direct polycondensation of a diamine with dicarboxylic acid was carried out in supercritical carbon dioxide in preparation of nylon 4,6 [84].
This nylon was reported to melt at 210C.
