Fluorinated cation exchange membrane and use thereof in electrolysis of alkali metal halide

A fluorinated cation exchange membrane which is made of a three component copolymer produced by copolymerizing a fluorovinylpolyether having the formula I

• cf.sub.2 .dbd.cf--o--cf.sub.2 --(cfxocf.sub.2).sub..alpha. --(cfx').sub..beta. --(cf.sub.2 ocfx").sub..gamma. --a
• wherein .alpha. is 0 to 3;
• .beta. is 0 to 6;
• .gamma. is 0 to 4 and either .alpha. or .gamma. is not 0;
• X, X' and X" respectively represent --F or a C.sub.1-5 perfluoroalkyl group;
• A represents --CN--, --COF, --COOH, --COOR.sub.1, --COOM or --CONR.sub.2 R.sub.3 ;
• R.sub.1 represents a C.sub.1-10 alkyl group preferably a C.sub.1-3 alkyl group;
• R.sub.2 and R.sub.3 respectively represents hydrogen atom or a C.sub.1-10 alkyl group;
• and M represents an alkali metal atom or a quaternary ammonium group, a fluorovinyl compound having the formula II
• cf.sub.2 .dbd.cf--(o).sub..delta. --(cfy).sub..gamma. --a'
• wherein .delta. is 0 to 1;
• .gamma. is 1 to 6;
• Y represents --F or a C.sub.1-5 perfluoroalkyl group;
• A' repesents --CN--, --COF, --COOH, --COOR.sub.1, --COOM or --CONR.sub.2 R.sub.3 ;
• and a fluorinated olefin having the formula III
• cf.sub.2 .dbd.czz'
• wherein Z and Z' respectively represent --H, --Cl, --F or --CF.sub.3, and if necessary hydrolyzing the copolymer wherein said copolymer comprises 1 to 30 mole % fluorovinylpolyether (I) and fluorovinyl compound (II) and wherein the molar ratio of fluorovinylpolyether (I) to fluorovinyl compound (II) is from 0.1 to 5.0.