Soluble and Colorless Polyimides from Bicyclo[2.2.2]octane-2,3,5,6-tetracarboxylic 2,3:5,6-Dianhydrides

Two tetracarboxylic dianhydrides with a polyalicyclic structure, bicyclo[2.2.2]octane-2-endo,3-endo,5-exo,6-exo-tetracarboxylic 2,3:5,6-dianhydride (5a) and bicyclo[2.2.2]octane-2-exo,3-exo,5-exo,6-exo-tetracarboxylic 2,3:5,6-dianhydride (5b), were synthesized, and polycondensations of the dianhydrides with aromatic diamines were carried out in well-purified N,N-dimethylacetamide (DMAc) at 85−105 °C under N2. The resulting poly(amic acid)s as precipitated possessed inherent viscosities in the range 0.5−0.1 dL/g. Some of the poly(amic acid)s formed flexible and tough films after curing. Polyimides were also obtained by a thermal solution imidization method where the DMAc solutions of poly(amic acid)s were heated at reflux temperature for 6 h. All of the examined polyimides were soluble at room temperature in aprotic and protic polar solvents such as DMAc and m-cresol. All of the polyimides showed excellent thermal stability with no significant weight loss up to approximately 400 °C, and the 5% weight loss temperatures in N2 were over 450 °C. The polyimides had glass transition temperatures over 380 °C except for the polyimides using 1,3-bis(4-aminophenoxy)benzene as an aromatic diamine. The flexible polyimide films possessed a tensile modulus range of 1.5−2.6 GPa, a tensile strength range of 52−96 MPa, an elongation at break of 3−11%, and a dielectric constant range of 3.2−3.7. These films exhibited cutoffs at wavelengths shorter than 320 nm and were entirely colorless. Normalized transparencies in the visible region (400−780 nm) were over 86% and, notably, the film prepared from 5a and 4,4‘-diaminodiphenyl ether had an excellent transparency of 94%. Colorlessness and transparency of the film at room temperature were maintained up to 200 °C when the film was heated in air and to 400 °C in N2. The polyimide of which the end group (the aromatic amino group) was capped using acetic anhydride was almost colorless even when heated at 300 °C in air.