Synthesis and properties of novel soluble polyimides having an unsymmetric spiro tricyclic dianhydride unit

A novel spiro trialicyclic dianhydride, rel[1S,5R,6R]-3-oxabicyclo[3.2.1]octane-2,4-dione-6-spiro-3′-(tetrahydrofuran-2′,5′-dione) (DAn) was synthesized from itaconic anhydride (1) and cyclopentadiene via a Diels-Alder reaction, nitric acid oxidation, and the subsequent dehydration with acetic anhydride. The structure of DAn was determined by NMR spectra and single crystal X-ray diffraction. A series of soluble polyimides (PIs) having a spiro alicyclic unit in the main chain were prepared by the reactions of DAn with several diamines through a general two-step polymerization method. Polymerization temperature affected inherent viscosities of poly(amic acid)s. Chemically imidized PIs had the inherent viscosities in the range of 0.10–0.49 dL/g. Due to the unsymmetric spiro alicyclic structure, the PIs showed excellent solubility in many organic solvents. Glass transition temperatures of PIs ranged from 215 to 279°C and some of them showed no glass transition temperature. The 10% weight loss temperatures under nitrogen were in a range of 355–430°C. Films of PIs obtained by the thermal imidization were wholly transparent and colorless.