N-[4-(Azidocarbonyl)phenyl] maleimide (3) and N-[4-(N'-phenoxycarbonylamino)phenyl]maleimide (5) were polymerized and copolymerized with methyl methacrylate (MMA) or acrylic acid (AA) by initiation with AIBN. The resultant polymers were to have aromatic isocyanato groups when heated. In the differential scanning calorimetry measurement, the polymers from monomer 3 showed a strong and broad exotherm around 130 degrees C. Thermogravimetric analysis also showed a weight decrease corresponding to the elimination of nitrogen at that temperature. The formation of isocyanato groups was confirmed by infra-red measurement with varying the cell temperature. The coupling reaction of the polymer with N-4-(nitrophenyl)-L-prolinol, resulting in the formation of urethane bonds, was carried out in a quantitative yield. The crosslinked films from copolymers of 5 and MMA or AA were obtained when heated above 200 degrees C. The cured films were insoluble in common organic solvents. With the copolymers with AA it was expected that carboxylic groups would react with isocyanato groups generated from urethane units when heated. The copolymer was degraded above 150 degrees C. After curing, its thermal stability improved dramatically, no weight loss being observed up to 300 degrees C.