Chemical Imidization Study by Spectroscopic Techniques. 2. Polyamic Acids

Chemical imidization of polyamic acid based on 1,5-diaminonaphthalene and a dianhydride was studied by UV−visible and FT-IR ATR techniques under various reaction conditions, such as polyamic acid concentration, solvent condition (dry vs humidity-exposed NMP), catalysts of tertiary amines with different degrees of steric crowding and base strength, and the concentration of the acetic anhydride and pyridine. With acetic anhydride and pyridine, the reaction was very similar to that of the model amic acids, with the simultaneous formation of bisimide groups and bisisoimide groups, followed by isomerization of bisisoimde groups to bisimide groups, which is very sensitive to the dryness of the solvent. The reaction with polymer films supported a similar trend. Chemical imidization seems to proceed by nucleophilic catalysis by tertiary amines, depending on the steric crowding as well as the base strength. Reducing steric crowding in the catalyst facilitates the formation of an acylammonium (or pyridinium) cation. The adjacent positive charge on the nitrogen makes the acyl group more electrophilic, making it easier for the amic acid anion to attack and form the mixed anhydride intermediate. The catalyst can further accelerate the reaction by deprotonating the mixed anhydride if the tertiary amine is not sterically crowded. The anionic mixed anhydride quickly cyclizes to give an imide group and an isoimide group. N-Methylpyrrolidine and triethylenediamine were found to make the reaction at least 50 times faster than pyridine, which is the most commonly used industrial catalyst, without changing the molecular weight and its distribution. This investigation shows that the acidity of the original amic acid and the acid produced during the reaction have a profound impact on the products and their ratio, which can be understood based on the mechanistic scheme proposed for the model amic acid. It is suggested that a careful choice of polyamic acid concentration, dehydrating agent, and catalyst may lead to greater control over the reaction and the polymer properties.