Semicrystalline polyimides starting from AB-type monomers

AB-type isomeric mixture of m- and p-monomethyl 4-(4 ' -aminophenoxy)phthal ate (I) and its pure m-monomethyl ester isomer (II) were successfully synth esized from 4-nitrophenol and 4-nitrophthalonitrile. The polyimide was prep ared via a two-step method in which the direct polycondensation reactions w ere carried out in N-methylpyrrolidone (NMP) in the presence of diphenyl(2, 3-dihydro-2-thioxo-3-benzoxaz (DBOP) as a condensation agent. AB-type cryst alline polyimides, with a glass transition temperature (T-g) of 260 degrees C, melting transition temperature (T-m) of 528 degreesC, and 10% weight los s temperature (T-10) of 575 degreesC, can be readily obtained by thermal im idization of poly(amic acid methyl ester) films. The polyimide films prepar ed by thermal imidization of the isomeric poly(amic acid methyl ester)s wer e tough, flexible, and transparent, whereas the film prepared from the pure m-monomethyl ester precursor was brittle and opaque, indicating that the c onstitutional isomerism of the precursors has a significant effect on the t hermal imidization as well as properties of the resulting polyimide. Therma l imidization of the isomeric poly(amic acid methyl ester)s with different molecular weight resulted in polyimide films of essentially identical mecha nical properties. X-ray diffraction measurements revealed that the polyimid es had crystalline structure with a crystallinity of 20-31%.