A series of copolyimides were prepared from various diamines (polysiloxane and isophorone units) with aromatic tetracarboxylic dianhydrides via a two-step (thermal imidization) method. The monomers and polymers were produced in high yields, and the copolyimides containing Si-O-C bonds and asymmetric meta catenation in the polymer backbone exhibited good solubility. The glass-transition temperatures (T-g's) of all the copolyimides were found to be 201-262 and 215-258degreesC by differential scanning calorimetry (DSC) and dynamic mechanical analysis, respectively. Thermogravimetric analyses indicated that the polymers were fairly stable up to 502-578degreesC (10 wt % loss in N2) and 490-574degreesC (10 wt % loss in air). The char yields at 800degreesC in N2 and air atmospheres were 26-59 and 20-53%, respectively. The copolymerization results, determined with 1H-NMR and DSC, indicated a random copolymer. (C) 2003 Wiley Periodicals, Inc.