Dissolution of trifunctional aziridine compounds, 2,2-bishydroxymethylbutanol-tris[3-(1-aziridinyl)propionate] (3AZ) and tetramethylolmethane-tri-β-aziridinylpropionate (3AZOH), in water initiates a ring-opening polymerization and successful yields the corresponding network polymers via cationic polymerization. The polymerization of 3AZ induced phase separation and produced porous polymers under a wide range of monomer concentrations and polymerization temperatures. The phase separation rate in the 3AZ/water system was estimated by quantifying the turbidity by means of light transmission where transmittance decreased with an increase in the content of phase-separated materials. The rate increased with an increase in reaction temperature. The 3AZ porous polymers showed characteristic surface morphologies, which were formed by connected particles with diameters of about 4-5 μm. The porous polymers were not breakable by the compression test under 50 N. The Young's modulus of the 3AZ porous polymers increased with an increase in polymerization temperature, which may be accounted for by cross-linking through the formation of quaternary ammonium salt formed by a termination reaction. The 3AZ porous polymer absorbed various solvents.