Alicyclic Dianhydrides and Polyimides Containing Double-Bond Bridged Cyclohexanes and Investigations on Their Retro Diels–Alder Reaction

In this study, two alicyclic dianhydrides, bicyclo[2.2.2]oct-7-ene-2-exo,3-exo,5-exo,6-exo,7-methyltetracarboxylic-2,3:5,6-dianhydride (MBTA) and bicyclo[2.2.2]oct-7-ene-2-exo,3-exo,5-exo,6-exo,7,8-dimethyltetracarboxylic-2,3:5,6-dianhydride (DMBTA), containing methyl substituents on the double-bond bridged cyclohexane structures were successfully synthesized by Diels–Alder reaction. The chemical structures of these dianhydrides with exo-exo configuration were fully characterized by NMR spectroscopy and X-ray single crystal diffraction. Alicyclic polyimides were prepared from these dianhydrides and diamines, such as 4,4′-oxydianiline (4,4′-ODA) and 2,2′-bis(trifluoromethyl)benzidine (TFMB). The introduction of methyl substituents on the double-bond bridges exhibits enhancements in various properties compared with their nonsubstituted analogs. These polyimides with high molecular weights can form flexible, transparent, and colorless films with superior optical transmittance at wavelength 400 nm (T400) of 86–88% and yellow indices (b*) of 0.32–0.87. They also showed high thermal stability with the degradation temperature at 5% weight loss (Td5%) of 423–447 °C in nitrogen and glass transition temperature (Tg) of 372–417 °C. These polyimides are good candidates as substrates for flexible electronic application. These polyimides also showed excellent solubility in organic solvents. Especially, PI-MBTA/TFMB and PI-DMBTA/TFMB are soluble even in low-boiling-point solvents such as cyclopentanone (CPN), implying the potential application as negative photosensitive polyimides (PSPIs) using CPN as the developing solvent. The double bonds of the bridges of MBTA and DMBTA cannot be hydrogenated with hydrogen pressure of 1–4.5 kg cm–2 using palladium on carbon as the catalyst at room temperature for 1–5 days. This could be attributed to the steric hindrance of the methyl substituents on the double bond. From the results of model compounds and the addition of strong dienophile (maleic anhydride), we conclude that the double-bond bridged cyclohexane system cannot undergo retro Diels–Alder reaction at 360 °C.