Polycrystal Monolayer Membranes of Covalent Organic Frameworks for Osmosis‐Driven Pharmaceutical Purification

Crystalline covalent organic framework (COF) membranes with well-defined architectures are critical for sustainable separation technologies. However, conventional synthetic routes relying on straightforward monomer condensation often fall short in producing COF membranes with superior crystallographic order. Here, the ambient-condition synthesis of COF-LZU1 polycrystal monolayer membranes is reported via imine-exchange chemistry. By employing aniline as a monofunctional modulator, the crystallization pathway is shifted from direct imine formation to a kinetically regulated imine-exchange process. This modulation suppresses rapid nucleation and mitigates both intra- and intercrystalline defects, yielding structurally distinctive membranes composed of polycrystal monolayers. The method is compatible with various substrates and enables facile fabrication of COF-LZU1 polycrystal monolayer separation membranes. Membrane morphology and crystallinity can be fine-tuned by adjusting the volumes of aniline and acetic acid. The optimized COF-LZU1 membrane performs exceptional molecular sieving in osmosis-driven separation, achieving efficient purification of pharmaceuticals from their precursors with operational stability over 500 h. This work provides a powerful strategy for developing unambiguously crystalline COF membranes and reveals the significance of crystallinity in advancing COF-based membrane technologies.