TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preparation (claimed): A (1,3)dioxolane-containing compound of formula (2) is heat-decomposed, to obtain perfluoro(2-methylene-1,3-dioxolane) derivative (3). Preferred Method: The (1,3)dioxolane-containing compound (2) is produced by epoxidizing fluorosulfonyl group-containing compound of formula: FO2S-R1-CH=CH=R2-SO2F (1), forming a dioxolane ring, and fluorinating.R1,R2=1-8C alkylene optionally having etheric oxygen atom between carbon atoms.Some or all of hydrogen atoms in alkylene group is optionally substituted by fluorine atoms. Each of -R1-SO2F and -R2-SO2F is 2-fluorosulfonyl-tetrafluoroethoxy group-substituted 1-3C alkylene group.    TECHNOLOGY FOCUS - POLYMERS - Preferred Methods: The perfluoro(2-methylene-1,3-dioxolane) derivative (3) and at least one polymerizable monomer copolymerizable with the derivative (3) are polymerized, to obtain fluorosulfonyl group-containing polymer (A). The fluorosulfonyl group in the fluorosulfonyl group-containing polymer is subjected to an alkali hydrolysis and acid treatment, to obtain a polymer (B) having sulfonate groups or sulfonic acid groups. Preferred Units: The polymer (A) comprises a fluorosulfonyl group-containing monomer unit (a) of formula (3a), and a monomer unit (b). The sulfonic acid group-containing polymer (B) contains a sulfonic acid group-containing monomer unit (c) of formula (5a), and a monomer unit (d). Preferred Properties: The polymer (A) has molecular weight of 5x 103 to 5x 106. The content of monomer unit (a) contained in the polymer (A) is 0.1-99.9 mol%. The polymer (B) has molecular weight of 5x 103 to 5x 106. The content of monomer unit (c) contained in the polymer (B) is 0.1-99.9 mol%. SPECIFIC COMPOUNDS - The perfluoro(2-methylene-1,3-dioxolane) derivative is chosen from 5 specific disclosed derivatives, such as compounds of formula (I-V).    EXAMPLE - Fluorosulfonyl group-containing compound (in g) (304) of formula (a) and potassium fluoride (3.36) were mixed in a flask, under nitrogen atmosphere, and heated in an oil bath. At 87 degrees C, fluoro compound (CF3CF2CF2OCF(CF3)COF) was distilled. The reaction mixture was maintained at 110 degrees C for 2 hours, cooled, and distilled under reduced pressure, to obtain fluorosulfonyl group-containing compound (176) of formula (b) having boiling point of 117-144 degrees C. Glass beads having particle size of 105-125 mu m were filled into a tube reactor having inner diameter of 1.6 mu m, and heated at 325 degrees C. A gas mixture comprising 99 mol% of nitrogen gas and 1 mol% of vaporized gas of the obtained compound (b) preliminary heated in the raw material line, was introduced from the bottom of the tube reactor. Further, at the top end of the tube reactor, a dry ice trap was set up. In such a state, the gas mixture was supplied for 2 hours, and reacted. The resulting crude liquid was distilled, to obtain perfluoro(2-methylene-1,3-dioxolane) derivative (a mixture of anti-form and syn-form) of formula (I) having boiling point of 99-102 degrees C.