The isomeric diamine monomer 5(6)-amino-1-(4-aminophenyl)-1,3,3-trimethylindane (DAPI) was successfully synthesized via the dimerization of a-methylstyrene followed by nitration and reduction. High molecular weight, soluble polyimides were synthesized via ester-acid solution imidization techniques and had glass-transition temperature values ranging from 247 to 369 degrees C. The polymers were soluble in common organic solvents because of the asymmetric and nonplanar nature of DAPI and displayed good short-term thermal stability by thermogravimetric analysis, as shown by their 5% weight-loss values above 500 degrees C in air. The DAPI/(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) polyimide also showed 2-h thermal stability at 400 degrees C under nitrogen, despite the partial aliphatic character. Refractive index values as low as 1.571 were observed for DAPI/GFDA, which allows an estimated dielectric constant of 2.47 to be derived. The permeation of O-2 and N-2 was conducted on thin dense films. The bulky, bent, and isomeric nature of DAPI imparted film-for ming membranes that permitted high O-2 permeability. In combination with 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), DAPI had a good combination of O-2 permeability and O-2/N-2 selectivity values of 2.8 Ba and 7.3, respectively. The polymerization method utilized to facilitate the cyclization of DAPI/BTDA to the polyimide affected the final thermal properties of the resulting polymer. The chemical imidization of DAPI/BTDA generated a polyimide with a glass-transition temperature value of 311 degrees C and a 5% weight-loss value in air of 451 degrees C. However, thermal and eater-acid imidization routes yielded an increase in the thermal properties. The ester-acid solution imidization of DAPI/BTDA produced a polymer glass-transition temperature value of 333 degrees C and a 5% weight-loss value of 525 degrees C in air.