Preparation of permselective composite membrane

A process for producing a permselective composite membrane, which comprises

• (1) dissolving an imidazolone ring-containing aromatic polymer in an aqueous liquid medium containing a water-miscible amine having a pka of more than 5.0 in a concentration of at least 5% by weight to form a solution containing the polymer in a concentration of 0.01 to 20.0% by weight, said polymer comprising at least 30 mole %, based on the entire recurring units, of at least one recurring unit of the following formula ##STR114## wherein A.sub.1 represents an aromatic group having a valence of (3+l), the dashed line represents the presence or absence of a bond, and the two nitrogen atoms and W.sub.1 are all bonded to the nuclear carbon atoms of the aromatic group A.sub.1 ;
• A.sub.2 represents an aromatic group having a valence of (2+m);
• Y is --O-- or ##STR115## in which R.sub.1 is a hydrogen atom or a monovalent hydrocarbon residue;
• and the two nitrogen atoms bonded to A.sub.1 are bonded to the ring carbon atoms at the orthoposition of the aromatic group A.sub.1 ;
• l and m are identical or different and each represent an integer of 0 to 3;
• when Y is ##STR116## and one of W.sub.1 groups is bonded to the ring carbon atom at the ortho-position of the aromatic group A.sub.1 together with Y, the W.sub.1, Y and A.sub.1 can form a 5-membered ring same as ##STR117## in which all symbols are the same as defined above;
• and when W.sub.1 does not form the 5-membered ring, W.sub.1 and W.sub.2 are identical or different and represent at least one group selected from substituted nitrogen-containing groups derived from a primary amino group;
• (2) coating the resulting polymer solution on a microporous substrate, and
• (3) drying the coating to form an ultrathin film of the polymer on the microporous substrate.