Abstract A polymerized ionic liquid (PIL) diblock copolymer with a long alkyl side-chain, poly(MMA- b -MUBIm-Br), was synthesized at various compositions from an ionic liquid monomer, (1-[(2-methacryloyloxy)undecyl]-3-butylimidazolium bromide) (MUBIm-Br), and a non-ionic monomer, methyl methacrylate (MMA). The PIL diblock copolymer was synthesized via post-functionalization from its non-ionic precursor PIL diblock copolymer, poly(MMA- b -BrUMA) (BrUMA = 11-bromoundecyl methacrylate), which was synthesized via the reverse addition fragmentation chain transfer (RAFT) polymerization technique. Differential scanning calorimetry reveals two distinct constant glass transition temperatures ( T g s) with a low PIL segment T g . These PIL block copolymers result in easily processable, flexible, transparent films with high mechanical strength. A high bromide ion conductivity of 64.85 mS cm −1 at 80 °C and 90% RH was measured for the PIL diblock copolymer with an ion exchange capacity (IEC) of 1.44 meq/g (23.3 mol% MUBIm-Br). Interestingly, this result was three times higher than its analogous PIL homopolymer (2.75 meq/g; 100 mol% MUBIm-Br) and an order of magnitude higher than a PIL block copolymer from a previous study with similar chemistry, similar IEC, higher water content, but shorter alkyl side-chain length. Ion conductivity did not scale as expected with water content, which is unusual for water-assisted ion transport (e.g., protons, hydroxide, chloride) in ion-containing polymers, and therefore suggests other mechanisms that impact ion transport in PIL block copolymers.