Conventional synthesis of polyimides includes high-temperature (160-350 degrees C) imidization of poly(amic acid)s. In the present work, imidization has been carried out at much lower temperatures (40-160 degrees C). 1,2,4,5,-cyclohexanetetracarboxylic dianhydride (HPMDA) or pyromellitic dianhydride (PMDA) was polymerized with an aromatic diamine, 4,4'-diaminodiphenylmethane (DDPM), to give poly(amic acid) s, which were then imidized chemically. Imidization was more than 90% complete even at the very low imidization temperature of 40 degrees C. It was found that the imidization occurs in two steps: an initial rapid cyclization and a subsequent slower cyclization. The activation energy for the rapid process was determined to be 4.3 kJ/mol, and that of the slower process, 4.8 kJ/mol. As the imidization temperature decreases, the transmittance of the resulting polyimides tends to gradually increase, the cutoff wavelength decreases and the color becomes pale. A partially aliphatic polyimide based on HPMDA and DDPM prepared at 40 degrees C yielded thin films that were highly transparent and colorless, and had good flexibility, solubility and thermal stability. The polyimide films prepared in this study may be good candidates for flexible, transparent plastic substrates in the display industry. (C) 2015 Wiley Periodicals, Inc.