SPONTANEOUS CYCLODEHYDRATION OF N-(O-AMINOPHENYL) AMIC ACIDS

The model reaction between phthalic anhydride and 1,2-diaminobenzene w as studied under conditions analogous to those of the low-temperature polycondensation of aromatic dianhydrides with bis(o-diamines) in amid e solvents followed by thermal cyclodehydration in condensed state to form ladder polyimidazopyrrolones (polypyrrones). The intermediate N-( o-aminophenyl)phthalamic acid was found to undergo spontaneous cyclode hydration to give N-(o-aminophenyl) phthalimide and 2-(o-carboxy-pheny l) benzimidazole. The reaction occurred at ambient temperature in the presence of water or alcohol without using a dehydration agent. The yi eld of imide-amine and/or carboxy-benzimidazole depended on the temper ature of the condensation reaction. Temperatures below 0 degrees C app eared to favor the formation of the carboxy-benzimidazole. Thermal cyc lization of N-(o-aminophenyl) phthalamic acid passed through the same intermediates: imide-amine and carboxy-benzimidazole. The former conve rted to corresponding pyrrone above 200 degrees C, while the secondary cyclization of the latter started above 250 degrees C. Spontaneous cy clodehydration was also observed for polyamide acid-amine precursors o btained by low-temperature polycondensation of pyromellitic dianhydrid e with 3,3'-diaminobenzidine. The prepolymer solutions in DMF diluted with water at room temperature turned to a gel after 48-72 h. A spectr oscopic study of the resulting polymers indicated the presence of sign ificant amounts of imide cycles. (C) 1998 John Wiley & Sons, Inc.