Copolymers of methyl methacrylate(MMA) with N,N-dimethylaminopropylacrylamide (DMA) were heated at various temperatures, and their thermal crosslinking was investigated. A copolymer with a compsition of [MMA]/[DMA] = 3 became completely insoluble in chloroform when heated at 180 degrees C for 5 minutes or at 150 degrees C for 20 minutes. Other copolymers containing less than 5 mol % DMA also crosslinked when heated at 170 degrees C for 30 minutes. A mixture of polyMMA with N,N-dimethylaminopropylbenzamide also crosslinked when heated at 170 degrees C. A copolymer of styrene with DMA also crosslinked when heated at above 140 degrees C although the crosslinking was much slower than that of poly(MMA-co-DMA). The formation of imide linkages is proposed as the crosslinking mechanism.