Polymethacrylimide (PMI) foam was obtained by two steps:(1)a expandable tablet was prepared via
bulk copolymerization of monomers mixture;(2)polymethacrylimide (PMI) foam was produced by heating the expandable tablet at 190 for 1 hour. Fourier transform infrared (FTIR),1 H-NMR,dynamic mechanical thermal analyses (DMTA) and differential scanning calorimetry(DSC) was used to investigate the effects of thermal treatment in copol ymer structures, meanwhile,the molecule structure transformation during foaming process was studied. The results in dicate that some adjacent AN and MAA monomeric units transformed into six membered imide ring structures and some of the adjacent MAA monomeric units changed into six membered anhydride ring structures,furthermore,some imide ring structure is formed due to the reaction between NH3(the decomposition products of blowing agents) and an hydride ring structures;insolubility of after heated copolymer suggested crosslinking structure is generated;the peak at  10. 50 in 1H-NMR spectrum is related to imide ring (—CO—NH—CO—) proton. Scanning electron microscope (SEM) image shows a closed cell foam with fine cell structures is obtained.