A new high-performance foam plastic was prepared via MAA,AN and AM radical bulk polymerization,and the corresponding foaming and heat treatment.The important reaction mechanism of in-situ cyclization taking place in the course of the expandable copolymer foaming and heat treament was discussed,and then was validated via optical microscope,infrared adsorption spectrum(FTIR) and differential thermal analysis(DSC).The results reveal that there exist weak exothermic peak of 149.17℃ and strong endothermic peak of 270.85℃ in the DSC curves of MAA/AN/AM copolymer after foaming and heat treament(160℃/2h→200℃/2h).The peak temperature of DTG curve is 175.87℃,and the weightlessness rate is lesser at 276.58℃ in the TG curve.In the case of FTIR,—OH absorption peak at 930-970cm^-1 and 1480cm^-1 are weakened,and C—N absorption peak of imide is enhanced.>C=O absorption peak at 1700cm^-1 happens excursion towards the low frequency field,and the second absorption peak enhances.—C≡N absorption peak at 2240cm^-1 are weakened,while a new —C=N— absorption band appears.All these datum reveal that in-situ cyclization has taken place,and some rigid ringed structures appear in the copolymer molecule chains,for example,hexahydric imide rings and trapeziform structures.