SULFONIC ACID GROUP-CONTAINING POLYMER, FLUOROSULFONYL GROUP-CONTAINING POLYMER, AND LIQUID COMPOSITION

To provide a sulfonic acid group-containing polymer which is capable of forming a low hydrogen permeable membrane, which shows a high ion exchange capacity, and which shows a proper softening temperature. The sulfonic acid group-containing polymer has units u1 represented by the following formula u1, units u2 represented by the following formula u2, and units u3 based on tetrafluoroethylene,in the formula u1, RF1 and RF2 are each independently a C1-3 perfluoroalkylene group, and Z+ is a hydrogen ion, a metal ion, or an ammonium ion, -[CF2-CF(CF2O-Rf1)]- Formula u2in the formula u2, Rf1 is a perfluoroalkyl group which may contain a SO3-Z+ group.