Semicrystalline Polyimides Starting from AB-Type Monomers

AB-type isomeric mixture of m - and p -monomethyl 4-(4‘-aminophenoxy)phthalate ( I ) and its pure m -monomethyl ester isomer ( II ) were successfully synthesized from 4-nitrophenol and 4-nitrophthalonitrile. The polyimide was prepared via a two-step method in which the direct polycondensation reactions were carried out in N -methylpyrrolidone (NMP) in the presence of diphenyl(2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonate (DBOP) as a condensation agent. AB-type crystalline polyimides, with a glass transition temperature ( T g ) of 260 °C, melting transition temperature ( T m ) of 528 °C, and 10% weight loss temperature ( T 10 ) of 575 °C, can be readily obtained by thermal imidization of poly(amic acid methyl ester) films. The polyimide films prepared by thermal imidization of the isomeric poly(amic acid methyl ester)s were tough, flexible, and transparent, whereas the film prepared from the pure m -monomethyl ester precursor was brittle and opaque, indicating that the constitutional isomerism of the precursors has a significant effect on the thermal imidization as well as properties of the resulting polyimide. Thermal imidization of the isomeric poly(amic acid methyl ester)s with different molecular weight resulted in polyimide films of essentially identical mechanical properties. X-ray diffraction measurements revealed that the polyimides had crystalline structure with a crystallinity of 20−31%.