NOVELTY - Polymer composition (I) comprises structural units derived from an aromatic halosulfonyl isocyanate compound (A).    USE - (I) is useful in: an article, which is a fiber such as many fiber defining a membrane or hollow fiber, a sheet or a film; the composite membrane; the membrane, which is useful in the water treatment system and an ultrafiltration membrane, where the membrane is a reverse osmosis membrane; in the bioseparation apparatus; and in the separation unit. The water treatment system is useful in a desalination unit (all claimed). (I) is useful in the separation membrane and ion exchange membrane. The membrane is useful: in liquid filtration, polarity-based chemical separations, pervaporization, gas separation, industrial electrochemistry such as chlor-alkali production and electrochemical applications, and super acid catalysts; and as a medium in enzyme immobilization.    ADVANTAGE - (I) provides improved membranes that have combination of high selectivity, flux and chemical tolerance in addition to being efficient and economical. (I) provides membranes having superior hydrophilicity, and high crosslinking density with improved solute rejection against both inorganic and organic materials, water flux, and mechanical durability.    DETAILED DESCRIPTION - Polymer composition (I) comprises structural units derived from an aromatic halosulfonyl isocyanate compound of formula ((O=C=N)n-Ar1-(S(=O)2-X)m) (A).    Ar1 = 3-40C aromatic radical (which is free of aliphatic C-H bonds);    X = halo;    m = 2-5; and    n = 1-5.    INDEPENDENT CLAIMS are included for:    (1) a composite membrane comprising (I), where (I) is located on at least one side of a porous support material;    (2) a membrane comprising (I);    (3) a water treatment system, comprising the membrane, where the membrane is a filtration membrane that passes a flow of water from an aqueous solution of relatively high solute concentration to a solution of relatively low solute concentration in response to a pressure differential across the membrane;    (4) a bioseparation apparatus comprising the membrane, where the bioseparation apparatus is capable of separating one biological fluid component from another biological fluid component; and    (5) a separation unit comprising many hollow fiber membranes, where at least one of many membranes comprises a membrane formed from (I).