New routes for the synthesis of high T-g thermally stable polymer foams with pore sizes in the nanometre regime have been developed. Foams were prepared by casting well-defined microphase-separated block copolymers comprising a thermally stable block and a thermally labile material. At properly designed volume fractions, the morphology provides a matrix of the thermally stable material with the thermally labile material as the dispersed phase. Upon thermal treatment, the thermally unstable block undergoes thermolysis generating pores, the size and shape of which are dictated by the initial copolymer morphology. Several labile blocks were surveyed including polystyrene, poly(alpha-methylstyrene) and several alpha-methylstyrene/styrene copolymers. Each of these polymers can unzip to its monomer upon heating; however, the rate is substantially slower for polystyrene. The copolymers were synthesized through either the poly(amic acid) precursor, followed by chemical imidization to the polyimide form, or the poly(amic alkyl ester) precursor followed by thermal imidization. The decomposition of the labile coblock was studied by thermogravimetric and dynamic mechanical analysis. Upon decomposition, the foams showed pore sizes in the nanometre regime along with the expected reduction in mass density.