Imidization of polypromellitamic acid based on 4,4′‐methylenedianiline

Imidization of polyamic acid prepared from pyromellitic dianhydride and 4,4′‐methylenedianiline was investigated at 373–493 K. At a certain degree of imidization, depending on the reaction temperature, the reaction is significantly slowed down. For this reason, imidization was divided into two phases, a fast and a slow one, and the rate constants were calculated from the kinetic equation of a first‐order reaction. At imidization temperature above 443 K the activation enthalpy of the reaction decreases, probably due to the thermal decomposition of complexes of o ‐carboxybenzamide groups with residues of the solvent. At higher imidization temperatures also the activation entropy of imidization decreases; this is probably caused by the higher rigidity of the film because of evaporation of the solvent which acts as a plasticizer.