ACID-CATALYZED REACTIONS IN POLYIMIDE SYNTHESIS

Melts of aromatic carboxylic acids are found to be excellent reaction media for 1-pot high molecular weight polyimide synthesis from diamine s and tetracarboxylic acid dianhydrides. No reversible reaction of pol yamic acids (PAA) formation was observed. The effect of the reactivity equalization was observed for low-and high reactive diamines in acid media. The intrinsic acid catalysis of the imidization reaction was sh own to take place also in polycyclization of PAA in concentrated solut ions in amic solvents. It is found that the dependence of relative imi dization rate (% conv./min) vs. AA/N-MP ratio for model low molecular and oligomeric amic acids (AA) in N-MP at 140-150 degrees C possesses a sharp maximum near the molar ratio 1:1, the imidization rate at the point of the maximum being an order of magnitude higher than that for diluted solutions. A scheme is proposed which includes the opportunity of two reaction channels to occur: a usual one (I) and a catalytic on e (II). In diluted solutions and in solid phase experiments with easy evacuation of volatile products, the role of catalytic channel II is l ow. To the contrary, in high concentrated solutions or in solid phase experiments under the conditions exluding volatile products evacuation , the catalytic channel becomes the key one. It is proposed that the c atalytic reaction proceeds via the common acid catalysis mechanism, th e solvent and water playing the role of co-catalysts, probably through the mechanism of ionic dissociation of AA or hydrogen-bound complex A A-solvent. It is shown that the water released in the course of solid phase imidization of phtalamic acid at 140 degrees C under the conditi ons where vaporization is impossible causes a sharp autocatalytic effe ct after initial 20%-conversion period to obtain entirely imidizied pr oduct.