Efficient ion separation from concentrated brines is critical for sustainable water resource utilization, zero-liquid discharge, and strategic ion recovery, yet it remains a formidable challenge for conventional ion-selective membranes (ISMs). Existing ISMs rely on weak ion-pore interactions, limiting their applicability to dilute solutions, whereas stronger binding designs often impose high diffusion barriers that suppress flux. Here, we report a supramolecular ISM in which 18-crown-6 (18C6) macrocycles are integrated into the one-dimensional nanochannels of covalent organic frameworks (COFs), forming a dual-channel architecture─supramolecular pathways for cations and macrocycle-separated free channels for anions. This design minimizes ion interference while enabling strong monovalent-ion recognition and rapid transport. As a result, the 18C6-COF membrane operates effectively in concentrated solutions, achieving high selectivity (SK+/Mg2+ = 254.7) and fast permeation (PK+ = 2403 mmol m-2 h-1), outperforming state-of-the-art ISMs. Experimental and simulation results show that monovalent cations migrate rapidly through the aligned 1D 18C6 channels via a knock-on-like process, even under strong ion-pore interactions, overcoming the traditional trade-off between binding affinity and transport kinetics. This study lays the groundwork for developing membranes with strong ion-channel interactions for high-concentration mixed-salt separation.
Polysaccharides exhibit remarkable stereochemical and regiochemical complexity, yet their natural heterogeneity produces differences in composition and material behavior that are difficult to control or predict. Here, we report a synthetic approach that overcomes these challenges through the controlled, cationic ring-opening polymerization of glucosamine-derived 2-oxazoline monomers, affording a new class of well-defined pseudo-polysaccharides, wherein each saccharide subunit is 1,2-N-linked through a nitrogen containing a pendant acyl group. Under optimized conditions using a benzyl-protected monomer (OBn-GlcOx) and a methyl tosylate initiator, polymerization proceeds to full conversion within 3 h at 75 °C with linear molecular weight growth, first-order kinetics, and low dispersity (D̵ ≤ 1.2), consistent with a controlled polymerization mechanism. The resulting polymers exhibit complete β-1,2-regio- and stereoselectivity, narrow molecular-weight distributions, and retention of chain-end functionality that enables chain extension to form diblock copolymers with 2-methyl-2-oxazoline. Following quantitative deprotection, the resulting polymer is water-soluble and both protected and nonprotected versions display markedly enhanced resistance to ultrasonic, acidic, and thermal degradation as compared to chitosan (mass loss ≤63% vs 93% under harshest conditions). These findings establish a synthetic route to stereoregular, amide-linked pseudo-polysaccharides with tunable physicochemical properties, expanding the accessible design space for well-defined, carbohydrate-based materials.

