Thiol–Ene Photopolymerization for Highly Selective Anion Exchange Membranes with Sulfonium and Quaternary Ammonium Fixed Charges

Ion exchange membranes (IEMs) with high charge concentration and low water uptake are essential for efficient electrodialysis of hypersaline brines; yet, achieving both properties simultaneously remains challenging. Here, we employed thiol–ene photopolymerization to synthesize anion exchange membranes (AEMs) that exhibit low water uptake while maintaining high ion exchange capacities. The fabrication process involved a “click” reaction between hexane-1,6-dithiol and two olefin-containing components: poly(1,2-butadiene) as a hydrophobic backbone and triallylamine to provide sites for fixed positive charges. Postmodification with iodomethane converted tertiary amines to quaternary ammonium groups and some sulfides to tertiary sulfonium groups. The inherent chemical compatibility of the reactants enabled solvent-free processing, forming highly concentrated reaction mixtures before ultraviolet (UV)-initiated cross-linking. By varying the poly(1,2-butadiene) to triallylamine ratio, we achieved control over fixed charge content, cross-linking density, and water uptake. The hydrophobic thiol–ene network yielded low water uptake (0.150–0.329 g water/g dry polymer) and high charge concentrations (7.0–10.7 eq/L) with low hydration numbers (5.2–7.9 water molecules per fixed charge), aided by the relatively hydrophobic tertiary sulfonium groups. The best-performing membranes exceeded previous trade-off relationships, exhibiting high counterion/co-ion selectivities of ∼500 in 1 m NaCl with ∼2× greater conductivity than similarly selective membranes, while maintaining osmotic water permeabilities in line with existing AEMs of similar water uptake. This work establishes an approach for fabricating high-performance AEMs with a combined high charge concentration and low water uptake, advancing the development of membranes for hypersaline brine treatment.