TECHNOLOGY FOCUS - POLYMERS - Preferred Process: The polymer (I) comprises recurring units derived from at least one fluorinated functional monomer chosen from: sulfonated perfluoroolefin of formula F2C=CF(CF2)n-SO2X' (IIa), sulfonated perfluorovinylethers of formula F2C=CF(CF2)m-SO2X' (IIb), sulfonated perfluoroalkoxy vinylethers of formula F2C=CF(O-CF2-CF(R1))w-O-CF2-(CFR2)y-SO2X' (IIc), and/or a sulfonated aromatic (per)fluoroolefins of formula F2C=CF(Ar-SO2X') (IId). The polymer (I) is chosen from tetra-fluoroethylene (TFE) copolymers comprising recurring units derived from at least one functional per(halo)fluoromonomer chosen among sulfonated per-fluorovinylethers of formula (IIb) and/or sulfonated perfluoroalkoxy vinylethers of formula (IIc). The polymer(I) is in the form of powder, and has an average particle size of 20-200 mu m. The medium (M) is chosen from: perfluoropolyethers of formula: B1-O-(CF(CF3)CF2O)b1(CFXO)b2-B2 (IIIa); perfluoropolyethers of formula: L1-O-(CF(CF3)CF2O)c1(C2F4O)c2(CFX)c3-L2 (IIIb); perfluoropolyethers of formula: D1-O-(C2F4O)d1-(CF2O)d2-D2 (IIIc); or perfluoropolyethers of formula: G1-O-(CF2CF2CF2O)g1-(CF2CF2CH2O)g2-(CF2CF2CHFO)g3-G2 (IIId).n=0-6;X'=halo, or -O-M+ (preferably F);M+=cation selected from H+, (NH4)+, K+, Li+, and/or Na+;m=1-10;w=0-2;R1 and R2=F, Cl or a 1-10C per-fluoroalkyl (optionally substituted with ether oxygens);y=0-6;Ar=3-15C (hetero)aromatic moiety (preferably perfluorinated);X=F or CF3;B1, B2, L1, L2, D1, D2, G1 and G2=CF3, C2F5 or C3F7;b1, b2, c1-c3, d1 and d2=integer having value of greater than or equal to 0;g1-g3=integer having value of greater than or equal to 0 (preferably g2 and g3 are 0).When b1 and b2 both, or d1 and d2 both are different from 0, then the different recurring units are statistically distributed along the chain; and when at least two of c1-c3, or at least two of g1-g3 are different from 0, then the different recurring units are statistically distributed along the chain. Preferred Compounds: The fluoropolymer (P1) exhibits a release of fluoride anion of less than (7.5x 10-4)% with respect to the total amount of fluorine of the polymer (P1), when submitted to a Fenton test where a sample of polymer (P1) in its acid form is contacted for 16 hours at 40 degrees C with an aqueous solution of hydrogen peroxide (3%) comprising 200 ppm of bivalent iron ions. EXAMPLE - A mixture of demineralised water (11.5 l); the monomer of formula F2C=CF-O-CF2CF2-SO2F (980 g); and a solution of CF2ClO(CF2CF(CF3)O)n(CF2O)mCF2COOK in water with an average molecular weight of 521 and the ratio n/m=10 (5 wt.%, 3100 g) in the autoclave was heated to 60 degrees C. An aqueous solution of potassium persulfate (6 g/l, 150 ml) was added. The pressure was maintained at 13.5 Bar (absolute pressure) by adding gaseous tetrafluoroethylene (TFE). After adding TFE (1000 g) in the reactor, the monomer F2C=CF-O-CF2CF2-SO2F (175 g) was added. Then the sulfonated comonomer was added stepwise following TFE consumption: thus, 175 g of the monomer F2C=CF-O-CF2CF2-SO2F was added every 200 g of TFE fed to the autoclave. The reaction was interrupted after 233 minutes by stopping the stirring, cooling the mixture and reducing the internal pressure by venting the TFE. A total of 4000 g of TFE were fed. The latex was kept under nitrogen bubbling for 16 hours to strip away residual monomers from the polymerization. The produced latex was found to possess 28.5 wt.% of solids. The latex was coagulated by freezing and thawing and the recovered coagulated polymer powder was washed with water and dried to obtain a semi-crystalline fluoropolymer (A1) having -SO2F functional groups (6 kg). The polymer was evaluated for unstable -COF end-group content. It was observed that the polymer (A1) had unstable end-groups of -COF in an amount of 14 mmol/kg. A dispersion (3 wt.%, 500 g) of the polymer (A1) was prepared by suspending polymer (A1) (15 g) in Galden D80 (RTM: perfluoropolyether). The dispersion was introduced into a reactor equipped with mercury vapor ultraviolet (UV) lamp (150 W). A mixture of nitrogen/fluorine gases (2 Nl/hour) was fed into the suspension under UV irradiation for 10 hours at 25 degrees C. The residual fluorine was vented to obtain a stabilized polymer (A2). A sample of the stabilized polymer (A2) was analyzed to determine the residual unstable end-groups. The total amount of residual end groups of -COF type was found to be less than 0.05 mmol/kg polymer. A specimen from the hydrolyzed membrane made from the polymer (A2) was submitted to the Fenton test and was found to release an amount of fluoride anions of less than 7.0x 10-4% (F- released on total fluorine).