benzimidazole polycarbodiimides polyphosphazines etc and interpolymers including block interpolymers containing repeating units from the above such as terpolymers of acrylonitrile vinyl bromide sodium salt of para sulfophenylmethallyl ethers and grafts and blends containing any of the foregoing typical substituents providing substituted polymers include halogens such as fluorine chlorine and bromine hydroxyl groups lower alkyl groups lower alkoxy groups monocyclic aryl lower acyl groups and the like selection of the asymmetric separation membrane for improved gas separations may be made on the basis of heat resistance solvent resistance mechanical strength of the asymmetric separation membrane as well as other factors indicated by the operating conditions for selective permeation as long as the asymmetric separation membrane has the prerequisite relative separation factors in accordance with the invention for at least one pair of gases in making asymmetric hollow filament separation membranes a wide variety of spinning conditions may be employed these methods are disclosed for example in the preparation of polysulfone hollow filaments by cabasso et al in research and development of ns 1 and related polysulfone hollow fibers for reverse osmosis desalination of seawater supra in addition methods taught for preparing the porous separation membrane as disclosed in u s pat no 4 230 463 are appropriate for preparing the preformed asymmetric separation membranes which when contacted with effective amounts of a volatile lewis acid result in the improved asymmetric gas separation membranes having enhanced separation factors one polymeric material utilized in gas separations is polysulfone.