Cross-linked cyclopolymers from pH-responsive diallyl amine salts for environmental remediation

This article reviews the synthesis of crosslinked polyzwitterions (CPZs) (poly-carboxybetaines, -phosphonobetaines, and -sulfobetaines) having multiple pH-responsive centers as well as hydrophobic motifs. The synthesis follows the Butler cyclopolymerization protocol involving a multitude of diallylammonium salts along with crosslinkers and their copolymerization with SO₂ and maleic acid. The CPZs have been transformed into crosslinked anionic polyelectrolytes (CAPEs) under the influence of pH. CPZs/CAPEs were effectively used as sorbents to remove priority toxic metal ions and organic dyes (cationic and anionic) from their aqueous solutions and industrial wastewater. Many amphiphilic polymers incorporating a few mol% hydrophobic monomers have shown enhanced efficacies in removing organic dyes. Previous works involving expensive sorbents with limited stability led us to design stable pH-responsive resins due to the lack of labile organic motifs. The presence of pH-responsive chelating motifs thus helped to trap priority metal contaminants. While most of the previous studies involved the removal of toxic metal ions in high-concentration regimes, the studies in low-concentration regimes remain challenging. The current review deals with multipurpose pH-responsive resins with chelating motifs, positive and negative charge centers, and hydrophobes to entrap organic contaminants. Most of the resins in the current review efficiently reduced toxic metal ion concentrations in aqueous and industrial wastewater in a short time to ppb and sub-ppb levels, thereby making them safe for human health. Changing the backbone to have positive or negative charge centers has led to the sequestration of respective anionic and cationic dyes. Quite a few of the resins were found to be super adsorbents for the fast removal of dyes, with maximum adsorption capacities for methylene blue being in the range of 2109–2609 mg g⁻¹. Future works are expected to focus on immobilizing cyclopolymers on magnetic nanoparticles.