Intermediates for fluorinated cation exchange membrane

A novel fluorinated cation exchange membrane containing carboxylic acid groups and sulfonic acid groups, both in the form of a specific pendant structure, the carboxylic acid groups being at least 20% on one surface of the membrane and gradually decreasing toward the innerside of the membrane, which membrane is useful in electrolysis of an aqueous alkali metal halide solution with advantageously stable performance for a long term under more severe operational conditions than those conventionally used. The membrane can be prepared from a novel copolymer of a fluorinated olefin with a novel sulfur containing fluorinated vinylether of the formula: wherein k is 0 or 1, l is an integer of 3 to 5, Z is -S- or -SO2- and R is C1-C10 alkyl, an aryl, C1-C10 perfluoroalkyl.