Imide-to-benzoxazole rearrangement in ortho substituted poly (4-4′-diphenylene pyromellitimide)s

Ortho substituted poly(4,4'-diphenylene pyromellitimide)s were prepared from pyromellitic dianhydride and 3,3'-dihydroxybenzidine or 3,3'-dimethoxybenzidine. Thermal cyclodehydration of the corresponding poly(amic acid)s, PAA, led to the formation of ortho-hydroxy- or o-methoxy-polyimides, while catalytic imidization in the presence of aliphatic anhydrides was accompanied by acylation of the OH groups resulting in the formation of lateral acetoxy substituents. The direction of the latter reaction was controlled by the acidity of dehydration agents and the use of a catalyst. Imidization in the presence of acetic anhydride and pyridine led to poly[(3,3'-diacetoxy-4,4'-diphenylene)pyromellitimide] and the use of trifluoroacetic anhydride resulted in the formation of the corresponding polyisoimide with ortho-trifluoroacetoxy groups. This polyisoimide was completely soluble in amide solvents above 60degreesC. The polymers were studied by FTIR spectroscopy, TGA, and WAXS. It was found that ortho substituents such as OH and methoxy groups could react with the imide cycle above 350degreesC causing its rearrangement to benzoxazole. The formation of notable amounts of benzoxazole was also observed for o-acetoxy polyimides.