Redefining Friedel-Crafts Porous Polymers: Complex Architecture from a Multifaceted Linker and Subsequent Engineering into Cation Exchanger

Friedel–Crafts porous polymers have conventionally been depicted as simple networks of aromatic units connected by aliphatic bridges. However, this structural representation fails to explain several inherent properties, such as marked hydrophilicity, strong visible-light absorption, and unexpectedly high product yields. The lack of commercial adoption further reflects the challenge in tuning material performance when the underlying chemical structure remains ambiguous. In this work, we overturn this simplistic model by revealing a more complex architecture arising from the multifaceted role of the linker reagent. Using elemental analysis, IR, NMR, X-ray photoelectron spectroscopy, and gas chromatography–mass spectrometry, we demonstrate that during AlCl₃-catalyzed Friedel–Crafts polymerization, CH₂Cl₂ serves not only as a crosslinker but also as a versatile molecular precursor, generating abundant one-carbon side groups and extended conjugated domains. This refined structural framework resolves the long-standing structure–property anomalies and redefines the chemical landscape of this widely studied polymer family. Building on this insight, we developed a metal-free solid-state thermal oxidation process that efficiently converts these side groups into carboxyl units under ambient air. The resulting carboxyl-rich materials exhibit exceptional cation-exchange capacities, comparable to those of commercial weak-acid cation-exchange resins, yet with negligible swelling during operation and minimal raw material costs. This work not only revises the chemical identity of a classic polymer family but also establishes a scalable and cost-effective design strategy toward sustainable separation and water purification technologies.