AbstractIt is well known that cation exchange membranes, having a very thin layer of a cationic polyelectrolyte on the membrane surface, have preferential permselectivity for monovalent cations in a monovelent‐divalent cations system. We studied the relationship between preferential permselectivity and molecular structure of the cationic polyelectrolyte. Grafted poly(4‐vinyl‐N‐methylpyridinium‐iodide) was used and was compared with poly(4‐vinyl‐N‐methylpyridiniumiodide). The backbone polymers were poly(styrene‐co‐p‐benzylstyrene) and poly(benzyl), onto which 4‐vinylpyridine was grafted by anionic polymerization and then quaternized with CH3I. The grafted poly(4‐vinyl‐N‐methylpyridinium‐iodide) is effective in making the cation exchange membrane preferentially permselevtive for Na+‐ Ca2+system and is more preferable than poly(4‐vinyl‐N‐methylpyridinium‐iodide) in terms of electric resistance of the membrane. However, the relationship between the molecular structure of the cationic polyelectrolyte and the durability of the
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