NOVELTY - A sulfonimide-bearing compound is new.    USE - For use in producing polyphosphazene useful in making e.g. fuel cell membrane.    ADVANTAGE - Improved polymer properties, e.g. increased flexibility and decreased water swelling, are achieved with membranes formed from blends of polyphosphazene and other polymer.    DETAILED DESCRIPTION - A sulfonimide-bearing compound of formula (I) is new.    HOC6H4SO2NR1SO2Rf (I)    R1 = Li, K, H, or Na; and    Rf = 1-8C perfluoroalkyl.    INDEPENDENT CLAIMS are also included for:    (A) a compound of formula ROC6H4SO2NMSO2Rf (II);    (B) a compound of formula ROC6H4SO2NR1SO2Rf (III);    (C) preparation of alkali phenoxy sulfonimide functionalized polyphosphazene by:    (1) reacting polyphosphazene of formula (NPCl2)n with alkali oxide derivative, e.g. sodium p-methyl phenoxide, lithium p-methyl phenoxide, or potassium p-methyl phenoxide to produce a first intermediate of formula (NP(Cl)x(OC6H4)2-x)n;    (2) reacting the first intermediate with second alkali salt of formula R1OC6H4SO2NR1SO2Rf to produce a second intermediate of formula (NP(OC6H4SO2NR1SO2Rf)x(OC6H4CH3)y(Cl)2-x-y)n; and    (3) reacting the second intermediate with a third alkali salt, e.g. H3CC6H4ONa, NaOC6H5, NaOC6H4CF3, LiOC6H4CH3, LiOC6H5, LiOC6H4CF3, H3CC6H4OK, KOC6H5, or KOC6H4CF3 to produce an alkali phenoxy sulfonimide functionalized polyphosphazene of formula (NP(OC6H4SO2NR1SO2Rf)x(OC6H4CH3)2-x)n; and    (D) preparation of sulfonamide functionalized polyphosphazene homopolymer of formula (NP(OC6H4SO2NR1SO2Rf)2)n by reacting (NPCl2)n with R1OC6H4NR1SO2Rf at 60-200 degrees C and ambient pressure to 12 bar for 12-40 hours;    (E) preparation of phenoxy sulfonimide functionalized polyphosphazene copolymer of formula (NP(ZR2)x(ZC6H4SO2NR1SO2Rf)2-x)n by reacting (PNCl2)n with first amount of a compound of formula R3R2, with a second amount of a compound of formula R3C6H4SO2NHSO2Rf at 60-200 degrees C to produce a reaction product; and reacting the reaction product with R3R2 at 60-200 degrees C and 3.5-4 bar;    (F) a blend of sulfonamide functionalized polyphosphazene comprising sulfonimide functionalized polyphosphazene and another polymer, e.g. PTFE, polyvinylidene fluoride (PDVF), polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), polystyrene (PS), polybutadiene (BR), polyvinylidene chloride (PVDC), polymethyl methacrylate (PMMA), polyvinyl alcohol (PVOH), polyvinyl acetate (PVA), polyphenylene oxide (PPO), polyether ether ketone (PEEK), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polycarbonate (PC), polyether sulfone, polybenzimidazole (PBI), polydimethyl siloxane, polyphenylene sulfide (PS), polypyrrole, polyphenylene, polyaniline, poly(bis(pentoxy)phosphazene), poly(bis(phenoxy)phosphazene), poly((methoxyethoxyethoxy)(m-methyl phenoxy)phosphazene), styrene-acrylonitrile copolymer (SAN), acrylonitrile-butadiene-styrene terpolymer (ABS), or ethylene-methacrylic acid copolymer;    (G) a composition comprising sulfonimide functionalized polyphosphazene polymer, and additive, e.g. carbon black, graphite, platinum, ruthenium, silica, montmorillonite, clay, titanium dioxide, zirconium oxide, phosphoric acid, phosphotungstic acid, silicomolybdic acid, phosphomolybdic acid, salts such as CF3SO2NLiSO2CF3, hexaphenoxycyclotriphosphazene, di(m-methylphenoxy)tetra(trifluoroethoxy)cyclotriphosphazene, plasticizers such as methanol, ethanol and hexane, and cross-linkers such as diamine;    (H) a membrane comprising crosslinked sulfonimide functionalized polyphosphazene; and (I) a fuel cell comprising the membrane.    R = 1-5C alkyl;    R1 = Li, H, K and Na;    Rf = 1-8C perfluoroalkyl;    M = H, Li, K, Na, and/or R'3NH+;    R3 = NaO, LiO. KO, NH2;    R' = 1-5C alkyl;    n = at least 3;    R2 = -CH2CH3, -C6H4CH3, -CH2CH2OCH2CH2OCH3, -CH2CH2OTHP, C6H4COOPr, -CH2CF3, -CH2CF2OCF2CF2OCF3, -C6H4CF3, or -C6F5;    THP = tetrahydropyranyl;    Z = O or NH; and    R3 = -NaO, -LiO, -KO, and/or NH2.