A supramolecular complex between an ionic monomer 3‐sulfopropyl methacrylate (SPMAK) and crown ether 18‐crown‐6 (18C6) has been employed to prepare a strong anionic cylindrical polyelectrolyte brush poly(potassium 3‐sulfopropyl methacrylate) (PSPMAK) by atom transfer radical polymerization (ATRP) in polar solvent dimethyl sulfoxide (DMSO). This strategy solved the problem of the solubilities of the incompatible hydrophobic poly‐initiator and hydrophilic ionic monomer. The formation of the PSPMAK brush is well proven by 1H NMR, aqueous gel permeation chromatography (GPC), dynamic light scattering (DLS), static light scattering (SLS), atomic force microscopy (AFM), and cryogenic transmission electron microscopy (cryo‐TEM) measurements. Cleavage of the side chains and further analysis reveal that the initiating efficiency of the polymerization is as low as 0.35.
