Membranes　with　high　ion　conductivity　and　selectivity　are　important　for　vanadium　redox　flow　batteries.　Herein,　densely　quaternized　anion　exchange　membranes　based　on　quaternary　ammonium　functionalized　octa-benzylmethyl-containing　poly(fluorenyl　ether　ketone)s　(QA-OMPFEKs)　were　prepared　from　the　(i)　condensation　polymerization　of　a　newly　developed　octa-benzylmethyl-containing　bisphenol　monomer　via　Ullmann　coupling,　(ii)　bromination　at　the　benzylmethyl　sites　using　N-bromosuccinimide,　and　(iii)　quaternization　of　the　bromomethyl　groups　using　trimethylamine.　The　QA-OMPFEK-20　with　an　ion　exchange　capacity　(IEC)　of　1.66　mmol　g−1　exhibited　a　higher　SO42−　conductivity　(9.62　mS　cm−1)　than　that　of　the　QA-TMPFEK-40　(4.82　mS　cm−1)　at　room　temperature,　which　had　a　slightly　higher　IEC　of　1.73　mmol　g−1　but　much　lower　QA　density.　The　enhanced　SO42−　conductivity　of　QA-OMPFEK-20　was　attributed　to　the　ion-segregated　structure　arising　from　the　densely　anchored　QA　groups,　which　was　validated　by　SAXS　observation.　Furthermore,　the　QA-OMPFEK-20　showed　much　lower　VO2+　permeability　(1.24×10−14　m2　s−1)　than　QA-TMPFEK-40　(5.40×10−13　m2　s−1)　and　Nafion　N212　(5.36×10−12　m2　s−1),　leading　to　improved　Coulombic　and　energy　efficiencies　in　Vanadium　redox　flow　batteries　(VRFBs).　Therefore,　the　Ullmann　coupling　extension　is　a　valuable　approach　for　the　development　of　high　performance　anion　exchange　membranes　for　VRFBs.　©　2018,　Science　China　Press　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.