PURPOSE: To obtain functional polymer film capable of changing selective separation function for gas, liquid, and solute optionally by the irradiation with light or radiant rays by using a polymer consisting essentially of 2,3-epithiopropyl ester of acrylic acid expressed by the specified formula.

CONSTITUTION: A monomer expressed by the formula, for example, 2,3- epithiopropyl methacrylate and a comonomer such as methacrylic acid, etc. are dissolved in a solvent such as 1,4-dioxane, etc., and the monomer and the comonomer are copolymerized in the presence of a radical initiator such as α,α-azobisisobutyronitrile, etc. The copolymer is purified and redissolved in dioxane, etc. to prepare a soln. and a film of the copolymer is prepd. by casting. The film thus obtd. is crosslinked between the polymer molecules, if necessary by irradiating with light or radiant rays. Thus, three-dimensional net work structure is formed. Several functional groups such as ion exchange group may be fixed to the film.