ChemSusChem, 2014, vol 7, 9, pp. 2621–2630
A series of copoly(arylene ether sulfone)s that have precisely two, three, or four quaternary ammonium (QA) groups clus- tered directly on single phenylene rings along the backbone are studied as anion-exchange membranes. The copolymers are synthesized by condensation polymerizations that involve either di-, tri-, or tetramethylhydroquinone followed by virtual- ly complete benzylic bromination using N-bromosuccinimide and quaternization with trimethylamine. This synthetic strategy allows excellent control and systematic variation of the local density and distribution of QA groups along the backbone. Small-angle X-ray scattering of these copolymers shows exten- sive ionic clustering, promoted by an increasing density of QA on the single phenylene rings. At an ion-exchange capacity (IEC) of 2.1 meq g 1, the water uptake decreases with the in- creasing local density of QA groups. Moreover, at moderate IECs at 20 8C, the Br conductivity of the densely functionalized copolymers is higher than a corresponding randomly function- alized polymer, despite the significantly higher water uptake of the latter. Thus, the location of multiple cations on single aromatic rings in the polymers facilitates the formation of a distinct percolating hydrophilic phase domain with a high ionic concentration to promote efficient anion transport, despite probable limitations by reduced ion dissociation. These find- ings imply a viable strategy to improve the performance of al- kaline membrane fuel cells.