Pyrrole-Bearing Porous Network Polymers Synthesized via Halogen-Bond-Assisted Radical Solid-Phase Polymerization for Highly Efficient and Selective Adsorption of Lithium Ions from Model Seawater Reverse Osmosis (SWRO) Brine

Methylene pyrrole oligomers (MPOs) with multiple vinyl groups and a calix[4]pyrrole (CP) macrocycle with multiple vinyl groups were synthesized and used as monomers in halogen-bond (XB)-assisted radical solid-phase polymerization (SPP) to generate network polymers. The monomers were cocrystallized with additional XB linker molecules, and the obtained cocrystals were polymerized in the solid phase. Because of the alignment of the monomers in the cocrystals, the adjacent vinyl groups were close enough to undergo radical polymerization effectively, enabling the formation of highly cross-linked network polymers that are hardly obtainable in liquid (solution) phase polymerization. After removal of the linker, the obtained porous polymers served as highly efficient and selective adsorbents of lithium ion (Li + ) from model seawater reverse osmosis (SWRO) brine at a low Li concentration (0.34 mg L –1 ), adsorbing up to 21 mg of Li + (2.1 wt %) per 1 g of adsorbent, which is the highest record Li + adsorption capacity in the field of Li + adsorption from SWRO brine.