the subject of this invention is a novel polymeric composition useful in membrane technology such as microfiltration , nanofiltration , ultrafiltration , reverse osmosis and gas separation . the composition is formed by taking a known condensation polymer such as polyether sulfone , polysulfone or a polyarylether sulfone and modifying it to change the molecular weight distribution . the useful polymers of the invention will have an unmodified number average molecular weight of about 63 , 000 or less and weight fraction of molecules with a molecular weight of 50 , 000 or less in the range of 30 to 35 %. utilizing fractional precipitation or other known techniques for concentrating a high molecular weight fraction of a polymer the polymers are modified to have a number average molecular weight of a least 59 , 000 or a weight fraction of molecules with a molecular weight of 50 , 000 or less of not more than 19 %. superior resistance to cracking and other forms of membrane degradation results from using the polymer compositions of the present invention in filtration applications .