Polyimidines. IV. The synthesis and polymerization of 3,3‐diphenyl‐6‐aminophthalide

Abstract

AbstractThe monomer 3,3‐diphenyl‐6‐aminophthalide was synthesized in a 20% yield by the following sequence of reactions: nitration of phthalimide, hydrolysis and dehydration to 4‐nitrophthalic anhydride, Friedel–Crafts reaction with benzene to 2‐benzoyl‐5‐nitrobenzoic acid, cyclization with thionyl chloride to the pseudoacid chloride, Friedel–Crafts reaction with benzene, and, finally, reduction of the nitro group to the amino function with Adams catalyst. Although the five‐substituted isomer is also possible, it was obtained in yields of only one‐fifth to one‐tenth of those for the 6‐substituted isomer. The 3,3‐diphenyl‐6‐aminophthalide underwent polymerization with difficulty to yield low‐molecular‐weight polyimidines (inherent viscosity up to 0.68 dl/g) in reasonable yields (32−88%). Because of the rigid character of the backbone and steric crowding, conditions for polymerization were rather severe: 1–2 days at 180–225°C in nitrobenzene or polyphosphoric acid or 350°C in a sealed tube. The addition of sand to the reactants in the sealed tubes caused an increase in yield and molecular weight. The polymers were subjected to thermogravimetric analysis in air and nitrogen. The temperatures at which a 10% weight loss occurred were as high as 440°C in air and 510°C in nitrogen. These stabilities were similar to those encountered for previously synthesized all‐aromatic polyimidines.