Multiblock Copolymer Anion-Exchange Membranes Derived from Vinyl Addition Polynorbornenes

Block copolymers have shown promise in ion-exchange membranes as they can phase separate into well-defined nanostructures which promote transport. Herein, a systematic study of multiblock copolymers containing cationic blocks is presented (diblock up to pentablock), and these were contrasted against a statistical copolymer. A series of vinyl addition polynorbornene anion-exchange membranes were prepared by copolymerization of 5-n-hexyl-2-norbornene and 5-(4-bromobutyl)-2-norbornene, followed by conversion of the halide to a trimethylammonium group. The hydroxide conductivities of all synthesized block copolymers were higher than the statistical copolymer, with the tetra and pentablock copolymers being the most conductive. The higher conductivity of the multiblocks is likely a combination of the increased surface-to-volume ratio (smaller domain sizes) improving the connectedness of ionic domains. Water uptake of the block copolymers was also dependent on the number and order of blocks. Copolymers with ionic blocks at one chain end took up more water than those where the ionic segments were confined to the chain interior. Finally, a method was developed to attach alkaline-stable tetraaminophosphonium cations to the bromo-functionalized statistical and pentablock polynorbornene. Interestingly, the synthesized phosphonium polymers had double the water uptake of their ammonium counterparts, which was attributed to the larger occupied volume of the phosphonium as compared to the ammonium group.