INTRODUCTION OF A [5]HELICENE UNIT IN POLYIMIDES BY CHEMICAL AND THERMAL TRANSFORMATIONS OF PRECURSOR POLYMERS

Several [5]helicene-containing polyimides have been synthesized by che mical and thermal transformation of the precursor polyimides having th e tetrahydro[5]helicene unit. The illustrated diamines 4a and 4b were polymerized with two dianhydrides (6FDA and s-BPDA) to form high molec ular weight polyimides 5a,b and 6a,b, containing the tetrahydro[5]heli cene core. These precursor polyimides showed good solubilities in comm on solvents and good thermal properties. Chemical transformation in so lution or in the film form could be effected by treatment with bromine at elevated temperatures. The resulting [5]helicene-containing polyim ides showed a marked increase in thermooxidative stability. The onset temperatures for 5% weight loss in air were over 500 degrees C for the [5]helicene-based polyimides, about 70 degrees C higher than those fo r the precursor polyimides. Furthermore, the [5]helicene-containing po lyimides retained the high tensile (Young's) moduli of the precursor p olyimides, indicating that the transformation did not result in any ch ain cleavage or cross-linking. The transformation to yield [5]helicene -containing polyimides could also be effected by heating at 400 degree s C in air. The thermally induced transformation was supported by the synthesis and transformation of a model compound.