Porous polymeric films are promising materials for the production of ultralow-dielectric constant materials. A high porosity polyimide thin film was prepared via the phase separation of a polyimide precursor in an N,N-dimethylacetamide (solvent)/2-(diethylamino)ethyl methacrylate/photoinitiator system. A novel technique involving high-pressure CO2 (5 MPa) gas injection was used to form CO2-2-(diethylamino)ethyl methacrylate zwitterion salt and induce the immediate phase separation and solvent droplet formation (within 60 s) of a wet precursor film on a metal substrate. The film was exposed to UV light through quartz windows for 30 s to polymerize the 2-(diethylamino)ethyl methacrylate while maintaining a constant CO, pressure. The cured thin film containing numerous pores with an average diameter of approximately 1 mu m +/- 1.0 mu m was treated at 320 degrees C for 1 h under a continuous flow of nitrogen. The obtained film was 30 mu m thick and exhibited pores with an average diameter of approximately 1 mu m +/- 0.9 mu m. The ultralow-k level minimum relative dielectric constant for the optimal polyimide film was 1.536, and the porosity was 74% with open porous structure.