NOVELTY - Producing ion-conductive polymer membrane for fuel cells, comprises (a) reacting substituted ethylene polymers (I) with sulfonic acid derivatives (II), in presence of a organocuprate-compound to form sulfonic acid groups-containing ethylene polymers (III), and (b) forming a membrane from the polymer (III), or (i) reacting alkoxy compounds with sulfonic acid derivatives (V), in presence of organocuprate-compound, (ii) converting the compound (VI) to a salt (VII), and reacting (VII) with substituted ethylene polymers (I), and (iii) forming a membrane from the polymer (VIII).    USE - The method is useful for producing ion-conductive polymer membrane for fuel cells.    ADVANTAGE - The method produces ion-conductive polymer membrane in a cost-efficient manner.    DETAILED DESCRIPTION - Producing ion-conductive polymer membrane for fuel cells, comprises (a) reacting substituted ethylene polymers of formula (-(C(R1)(R2)-C(R3)(R4-X1))n-) (I) with sulfonic acid derivatives of formula (X2-R5-SO3Z) (II), in presence of a organocuprate-compound to form sulfonic acid groups-containing ethylene polymers of formula (-(C(R1)(R2)-C(R3)(R4-R5-SO3Z))n-) (III), and (b) forming a membrane from the polymer (III), or (i) reacting alkoxy compounds of formula (R7-O-R6-X3) (IV) with sulfonic acid derivatives of formula (X2-R5-SO3Z) (V), in presence of organocuprate-compound to form sulfonic acid derivatives-containing alkoxy compounds of formula (R7-O-R6-R5-SO2Z) (VI), (ii) converting the compound (VI) to a salt of formula (MO-R6-R5-SO2Z) (VII), and reacting (VII) with substituted ethylene polymers (I) to form sulfonic acid groups-containing ethylene polymers of the formula (-(C(R1)(R2)-C(R3)(R4-O-R6-R5-SO3Z))n-) (VIII), and (iii) forming a membrane from the polymer (VIII).    X1, X2 = halo;    R1, R2, R3 = H or 1-6 alkyl;    R4, R5 = phenyl, 1-12C alkyl, 1-12C alkyl ether, 1-12C fluoroalkyl, 1-12C fluoroalkyl ether, 7-30C arylalkyl, 7-30C phenylalkyl, 7-12C aryl, -(CH2)r, or -(CF2)p-O-(CF2)q-;    R6 = phenyl, 1-12C alkyl, 1-12C alkyl ethers or 1-12C fluoroalkyl;    Z = H or metal;    n = 10-500; and    p, q, r = 2-10.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of the fuel cell.    Proton exchange membrane fuel cell (10)    Ion-conducting polymer membrane (12)    Cathode catalyst layer (14)    Anode catalyst layer (16)    Conductive plate (20)