A new route to high-T-g, thermally stable polyimide foams has been developed. Foams were prepared by casting microphase-separated graft copolymers comprising a thermally stable main chain, polyimide, and a thermally labile graft, poly(ctl-methylstyrene). The copolymer compositions were designed so that the thermally labile block would be the dispersed phase. This can unzip to its monomer upon heating, and the decomposition product diffuses out of the him, leaving pores embedded in a matrix of the thermally stable component. The copolymers were synthesized through either the poly(amic acid) precursor, followed by chemical cyclodehydration to the imide form, or the poly(amic alkyl ester) precursor followed by thermal imidization. The decomposition of the alpha-methylstyrene in the block copolymer was studied by thermogravimetric, dynamic mechanical and thermomechanical analyses. Mild decomposition conditions were required to avoid rapid depolymerization of the alpha-methylstyrene and excessive plasticization of the polyimide matrix. The foams showed pore sizes with diameters ranging from less than 20 nm to over 1 mu m, depending upon the synthetic route employed, and the reduction in the mass density was generally consistent with the starting composition.