High-performance butyl methacrylate/acrylamide/acrylic acid (BMA/AM/AA) copolymers, being the precursor of PMI foam, were synthesized by free radical bulk copolymerization. Transformation of the molecular structures of AM/AA copolymers, BMA/AM copolymers and BMA/AM/AA copolymers was deeply investigated by use of 13C and 1H NMR, FTIR, DSC, TGA, DMA and mechanical property test. Results confirmed the occurrence of BMA/AM/AA copolymers. It tends to indicate the cyclization and cross-linking reactions during heat treatment, which would further induce the cyclic structures and cross-linking structures. A higher treatment temperature would result in a higher degree of cyclization and cross-linking. These findings confirmed the transformation of the molecular structures during heat treatment, which may be the key to improving the properties of BMA/AM/AA copolymers.