NOVELTY - The ion exchange membrane comprises a polymeric microporous substrate and a cross-linked ion transferring polymeric layer disposed on the substrate. The cross-linked ion transferring polymeric layer comprises a polymerization product of a functional monomer, cross-linking monomer and a low value reactivity ratio (r2/rs) monomer. The low value reactivity ratio monomer includes a cross linking monomer. A thickness of the polymeric microporous substrate is 20-255 microns.    USE - The ion exchange membrane is useful in an electrochemical separation device of a water treatment system (claimed), which is useful for treating water and other liquids containing dissolved ionic species, as a semiperaieable or selectively permeable ion exchange or bipolar membrane, and for desalination of seawater by electrochemical methods, where electrochemical separation device is useful are for producing create ultrapure water for pharmaceutical and semiconductor industries, as power generating sources in reverse electrodialysis, as separators in fuel cells, for recovery of metal ions in the electroplating and metal finishing industries and in food and beverage industry.    ADVANTAGE - The ion exchange membrane can be economically fabricated with low electrical resistance, high permselectivity and chemical stability, long service life and good mechanical strength and dimensional stability so that seawater can be economically desalinated with less energy consumption.    DETAILED DESCRIPTION - The ion exchange membrane comprises a polymeric microporous substrate and a cross-linked ion transferring polymeric layer disposed on the substrate. The cross linked ion transferring polymeric layer comprises a polymerization product of a functional monomer, cross linking monomer and a low value reactivity ratio (r2/rs) monomer. The low value reactivity ratio monomer includes a cross linking monomer. A thickness of the polymeric microporous substrate is 20-255 microns. The cross-linked ion transferring polymeric layer is formed by the polymerization of 2-sulfoethylmacrylate, the low value monomer and the cross-linking monomer. The low value monomer has a reactivity ratio of less than 1.3. The membrane has an apparent permselectivity of 88% and resistivity of less than 1.5 Omega -cm-2. The ion transferring polymeric layer comprises a cross-linked anion transferring polymer. The ion exchange membrane includes the polymerization product of a first monomer having a reactivity ratio (r1) and a second monomer having a reactivity ratio (r2). A reactivity ratio product of the first monomer and the second monomer is less than 0.3 or 0.9-1.1. An INDEPENDENT CLAIM is included for a method for making an ion exchange membrane.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of an electrochemical test unit.    Electrochemical test unit (100)    Electrodes (110)    Reference electrodes. (120)