NOVELTY - Use of soluble branched addition copolymer to form crosslinked hydrolytically stable membrane, is claimed. The soluble branched addition copolymer is cured after formation via addition polymerization process prior to formation of crosslinked membrane. The soluble branched addition copolymer is obtainable by addition polymerization process. The branched addition polymer comprises weight average molecular weight of 2000-1500000 Dalton. The branched addition copolymer comprises at least two chains which are covalently linked by a bridge other than at their ends.    USE - The soluble branched addition copolymer is useful: in formation of crosslinked hydrolytically stable membrane; or in the formation of an ion exchange membrane or film, where the membrane is used in the application areas comprising medical separation and diagnostics applications, industrial purification and separation, ion-exchange applications, desalination, water purification, gas separation, electrodialysis, pervaporation, fuel cells, energy generation, energy storage, filtration or sensors (all claimed).    ADVANTAGE - The soluble branched addition copolymer: exhibits less than 1% monomer impurity; and provides the film or membranes which exhibits electrical resistance of less than 5 ~0.cm2 (claimed).    DETAILED DESCRIPTION - Use of a soluble branched addition copolymer to form a crosslinked hydrolytically stable membrane, is claimed. The soluble branched addition copolymer is cured after formation via an addition polymerization process prior to formation of the crosslinked membrane. The soluble branched addition copolymer is obtainable by an addition polymerization process. The branched addition polymer comprises a weight average molecular weight of 2000-1500000 Dalton. The branched addition copolymer comprises at least two chains which are covalently linked by a bridge other than at their ends, where the chains comprise at least one ethylenically monounsaturated monomer. The bridge comprises at least one ethylenically polyunsaturated monomer. The polymer comprises a residue of a chain transfer agent and optionally a residue of an initiator. The mole ratio of polyunsaturated monomer(s) to monounsaturated monomer(s) is 1:100-1:3. The branched copolymer is hydrolytically stable so that the permselectivity and electrical resistance of the membrane does not change by more than 50% when subjected to hydrolysis at 25 degrees C for 48 hours and a pH of greater than 12. INDEPENDENT CLAIMS are also included for:    (1) an ion-exchange membrane comprising a cured branched addition copolymer and hardener comprising alpha - omega -diiodoalkanes comprising 1,4-diiodobutane, 1,5-diiodopentane, 1,6-diiodo hexane, 1,7-diiodoheptane, 1,8-diiodooctane or 1,10-diiododecane, alpha - omega -dibromoalkanes comprising 1,4-dibromobutane, 1,5-dibromopentane, 1,6-dibromo hexane, 1,7-dibromoheptane, 1,8-dibromooctane or 1,10-dibromodecane, alpha - omega -tertiaryalkydiaminoalkanes comprising N,N,N'N'-tetramethyl-1,6-diaminohexane, N,N'-diisopropylethylenediamine, 4,4'- bis(dimethylamino)benzophenone and bis(2-(dimethylamino)ethyl)ether or tolylene diisocyanate and hexamethylene diisocyanate; and    (2) the soluble branched addition copolymer.