A means of generating high temperature polymer foams which leads to pore sizes in the nanometre regime has been developed. Foams were prepared by casting block copolymers comprising a thermally stable block and a thermally labile material, such that the morphology provides a matrix of the thermally stable material with the thermally labile material as the dispersed phase. Upon a thermal treatment the thermally unstable block undergoes thermolysis, leaving pores where the size and shape are dictated by the initial copolymer morphology. Multiblock and triblock copolymers, comprising rigid, semi-rigid and flexible polyimide matrices with either poly(propylene oxide) or poly(methyl methacrylate) as the thermally labile coblocks, were prepared. The copolymer synthesis was carried out through the poly(amic alkyl ester) precursor to the polyimide since this precursor is stable and allows for isolation and characterization prior to imidization. Microphase-separated morphologies were observed for all copolymers irrespective of block type or length by bath dynamic mechanical and small-angle X-ray scattering techniques. Upon decomposition of the propylene oxide or methyl methacrylate coblock, reductions in the film thickness and the total integrated scattering were found for those copolymers derived from rigid and semi-rigid polyimide matrices, thus indicating a collapse of the foam as it was being formed. Conversely, copolymers based on the flexible polyimide produced stable foams upon decomposition of the labile coblocks.