TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The fluorinated acid polymer has a fluorinated olefin backbone and pendant groups selected from fluorinated ether sulfonate, fluorinated ester sulfonate and/or fluorinated ether sulfonamide. The sulfonimide is of formula -(SO2-Rf-SO2-NH)n-. The fluorinated acid polymer comprises a fluorinated polymer backbone and a side chain of formula -OR15-SO2-NH-(SO2-NH-SO2-NH)a-SO2R16. The fluorinated acid polymer is selected from a copolymer of 1,1-difluoroethylene and 2-(2,2-difluoro-2-(trifluoromethyl)allyloxy)-1,1,2,2-tetrafluoroethanesulfonic acid; and/or copolymer of ethylene and 2-(2-(1,2,2-trifluorovinyloxy)-1,1,2,3,3,3-hexafluoropropoxy)-1,1,2,2-tetrafluoroethanesulfonic acid. The fluorinated acid polymer is a homopolymer or copolymer of monomers of formula (III). The polymer or copolymer is derived from at least one monomer of formula (IV) or (V), where the polymer can be converted to the acid form following polymerization. The fluorinated acid polymer is a homopolymer or copolymer derived from monomers of formula (VI). The fluorinated acid polymer is of formula -((CF2-CF2)a-(CF2-CF(O)((CF2-CF(R16)-O)z-(CF)c-SO2-NH-(SO2-(CF2)d-SO2-NH)e-SO2R16))n- or of formula (VII) and (VIII).Rf=fluorinated alkylene, fluorinated heteroalkylene, fluorinated arylene or fluorinated heteroarylene;n=4;R15=fluorinated alkylene or fluorinated heteroalkylene;R16=fluorinated alkyl or fluorinated aryl;a=0-4;b=1-5;R13=OH or NHR14;R14=alkyl, fluoroalkyl, sulfonylalkyl or sulfonylfluoroalkyl;W1=(CF2)b, (CF2)b, S(CF2)b or (CF2)bO(CF2)b;a, z, c, d and e=1-3;x=0-2;R17-R20=halogen, alkyl, 1-10C alkoxy, Y1, C(Rf'')(Rf'')OR21, R4Y1 or OR4Y1 (optionally substituted by T1) or H;T1=halogen or ether oxygen;Y1=COE2, SO2E2 or sulfonimide;R21=H or acid-labile protecting group;Rf''=1-10C fluoroalkyl;Rf''+Rf''=(CF2)y;y=2-10;R4=alkylene (optionally substituted by T1);E2=OH, halogen or OR5;andR5=alkyl (optionally substituted by T1).Provided that at least one of R17-R20 is Y1, R4Y1 or OR4Y1. EXAMPLE - No suitable example given.