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 chemical and thermal transformation of the precursor polyimides having the tetrahydro[5]helicene unit. The illustrated diamines 4a and 4b were polymerized with two dianhydrides (6FDA and s-BPDA) to form high molecular weight polyimides 5a,b and 6a,b, containing the tetrahydro[5]helicene core. These precursor polyimides showed good solubilities in common solvents and good thermal properties. Chemical transformation in solution or in the film form could be effected by treatment with bromine at elevated temperatures. The resulting [5]helicene-containing polyimides showed a marked increase in thermooxidative stability. The onset temperatures for 5% weight loss in air were over 500 °C for the [5]helicene-based polyimides, about 70 °C higher than those for the precursor polyimides. Furthermore, the [5]helicene-containing polyimides retained the high tensile (Young's) moduli of the precursor polyimides, indicating that the transformation did not result in any chain cleavage or cross-linking. The transformation to yield [5]helicene-containing polyimides could also be effected by heating at 400 °C in air. The thermally induced transformation was supported by the synthesis and transformation of a model compound.