A new route has been developed for the synthesis of thermally stable polymer foams with high glass transition temperatures. Compositionally asymmetric microphase-separated block copolymers were prepared where the minor component, poly(alpha-methylstyrene), is thermally labile and the major component, polyimide, is thermally stable. The minor component can unzip, to its monomer upon heating, and the decomposition products diffuse out of the him leaving behind pores embedded in a matrix of the thermally stable component. The addition of homopolymer of the thermally labile component to the block copolymer offered one means to increase the volume fraction of voids while maintaining a discrete cell structure. Mild decomposition conditions were required to avoid rapid depolymerization of the alpha-methylstyrene and subsequent plasticization of the polyimide matrix. The foams showed pore sizes with diameters ranging from 40 nm to just over 1 mu m, depending on the copolymer used in the mixture, along with the expected increase in porosity.