Process for preparing a highly selective fluorinated anion exchange membrane

A process for preparing a heterogeneous anion exchange membrane having enhanced current efficiency and reduced reverse diffusion of protons, which comprises:

• (a) treating from about 1 to 80% of the thickness of a carboxylate membrane made of a copolymer having repeating units of the formula;
• ##STR12## wherein X is F or CF.sub.3, l is an integer of from 0 to 5, m is 0 or 1, n is an integer of from 1 to 5, and p/q is from 4 to 16, and R is a lower alkyl group, with a mineral acid which is effective as an acid catalyst for hydrolysis;
• or a base which is effective as a base catalyst for hydrolysis;
• and
• (b) reacting the treated membrane with
• (i) a lower dialkylamine of the formula:
• ##STR13## wherein R.sup.1 and R.sup.2 are lower alkyl groups;
• or (ii) a diamine of the formula:
• ##STR14## wherein R.sup.6 is a hydrogen atom or a lower alkyl group, each of R.sup.7 and R.sup.8 is a lower alkyl group, or R.sup.6 and R.sup.7 together form a polyethylene group of the formula (CH.sub.2).sub.a, wherein a is an integer of 2 or 3;
• or
• (iii) a diamine of the formula:
• ##STR15## wherein R.sup.9 is a hydrogen atom or a lower alkyl group, each of R.sup.10 and R.sup.11 is a lower alkyl group, or R.sup.9 and R.sup.10 together form a polymethylene chain of the formula CH.sub.2).sub.2, wherein c is an integer of 2 or 3, and b is an integer of from 3 to 7;
• to thereby form a carboxylic acid amide membrane, which is treated with a reducing agent to form a corresponding amine membrane, which is then alkylated with a lower alkyl group to form a quaternary ammonium group-containing membrane.