the use of certain catalysts provide for an improved process for the preparation of soluble polyimides , polyamides , and polyamideimides . the catalysts are alkali metal salts of formula mor , wherein r represents alkyl or aryl and m represents an akali metal . the improved process comprises reacting organic diisocyanates with polycarboxylic compounds consisting of tetracarboxylic acids or the intramolecular dianhydrides thereof , tricarboxylic acids or the monoanhydrides thereof , dicarboxylic acids , and mixtures thereof , in the presence of said catalysts . the polymers are obtained in solution at low reaction temperatures and short reaction times thereby avoiding side - reactions which otherwise would be detrimental to polymer molecular weight and ultimate polymer properties .