NOVELTY - Method of forming an ionomeric membrane (A) comprising an aryl-sulfonyl unit (I), comprises: reacting a polymer comprising an aryl unit in chlorosulfonic acid to form a first precipitate comprising a polymer comprising an aryl chlorosulfonyl unit (II) and dissolving in a polar aprotic solvent to form a first solution; forming a polymeric membrane from the first solution; and reacting the obtained polymer with a nucleophilic compound to form a base polymer having (I).    USE - (A) is useful in a fuel cell (claimed).    ADVANTAGE - (I) is an improved proton conductive polymer membrane that maintains robust mechanical properties and high ionic conductivity at a wide range of humidity conditions.    DETAILED DESCRIPTION - Method of forming an ionomeric membrane comprising an aryl-sulfonyl unit (I) having a structure of formula (-(E1(SO2-R1)-P1-Q1-P2)i-) comprises: reacting a polymer comprising an aryl unit having a structure of formula (-(E1-P1-Q1-P2)i-) in chlorosulfonic acid to form a first precipitate comprising a polymer comprising an aryl chlorosulfonyl unit having a structure of formula (-(E1(ClSO2)-P1-Q1-P2)i-) (II) and dissolving in a polar aprotic solvent to form a first solution; forming a polymeric membrane from the first solution, the polymeric membrane including the polymer comprising a polymer unit having (II); and reacting the polymer comprising a polymer unit having (II) with a nucleophilic compound to form a base polymer having (I).    P1, P2 = absent, -O-, -S-, -SO-, -SO2-, -CO-, -NH-, -NR2- or -R3-;    R1 = -OR5, -NR5, -X, -NHR5Z2, -OR5Z2 or -NHSO2CF3;    R2 = 1-25C alkyl or 1-25C aryl;    R3 = 1-25C alkylene, 1-25C perfluoroalkylene or 1-25C arylene;    R4 = trifluoromethyl, 2-35C alkyl, 1-25C perfluoroalkylene, 1-25C aryl, or another E1 group;    R5 = 1-25C alkyl or 1-25C aryl;    E1 = an aromatic-containing moiety (preferably phenyl, phenanthrenyl, biphenyl, naphthalenyl, diphenyl ether moiety, (1,1',4',1'')terphenyl, 9,9-diphenyl-9H-fluorenyl, dimethyl-diphenyl-silanyl, phenylsulfanylbenzenyl, (1-phenylcyclohexyl)benzenyl, 1,4-diphenoxy-benzenyl, benzenesulfonylbenzenyl, 1,4-diphenoxybenzenyl, diphenylmethanone moiety, diphenylphosphorylbenzenyl, (2,2,2-trifluoro-1-phenyl-1-(trifluoromethyl)ethyl)benzenyl or ((E)-2-phenylprop-1-enyl)benzenyl);    Z2 = -SO3H, -PO3H, -OH or -NH2;    X = -OH, halo, ester or -NH-S(=O)2-R4;    i = 1-10000; and    Q1 = a fluorinated cyclobutyl moiety.