Preparation of a high molecular weight polypentafluorostyrene (PFS) and sulfonated polypentafluorostyrene (sPFS) is described in this paper. The high molecular weight PFS was obtained via classical emulsion polymerization reaction, which increased the molecular weight of PFS of about one order of magnitude higher than the one of the commercial polymer. The molecular weight is a crucial factor for the film forming properties of a polymer membrane, following the concept the higher-the better. Furthermore, we post-sulfonated the PFS by 100% (one sulfonic acid per styrene unit). This provided a polymer possessing very high ion-exchange capacity (IEC = 3.4 mmol g(-1)) with functional group coupled to the relatively flexible side units (phenyl rings). The phenyl rings are fluorinated which enhanced the nucleophilic substitution reaction (introduction of the functional groups) and reduced the pKa value of the resulting sulfonic acid (pKa = 2). The morphology of sPFS, studied by Small Angle Neutron and X-rays Scattering, revealed a high ordering of a nano-phase-separated system. All these factors raised the conductivity to one of the highest measured on polyelectrolyte (sigma = 36 mS cm(-1) at T = 160 degrees C, p(H2O) = 10(5) Pa). This value is one order of magnitude higher than the conductivity of Nafion 117 measured under the same conditions. (C) 2013 Elsevier B.V. All rights reserved.