The selective separation of alkaline salts from aqueous media is of fundamental importance in chemistry. It is, for instance, critical to the production of commodity materials (e.g., bromine and potassium) from high-salt sources, such as the Dead Sea and the Great Salt Lake[1] and, on a very different scale, to the regulation of taste[2] and the maintenance of osmotic balance in cells.[3] Materials that could allow for such separations are thus of potential interest in a wide range of applications.[4] Polymeric systems are particularly attractive in this regard because they are generally easy to isolate from solutions or mixtures.