Synthesis and Metal Ion Adsorption Properties of a Dense Triazole Polymer Carrying Cysteine Residues

ABSTRACT Copper(I)‐catalyzed azide–alkyne cycloaddition (CuAAC) is promising as a reaction to synthesize functional polymers consisting of 1,2,3‐triazole units. Recently, we reported a series of dense 1,2,3‐triazole polymers of 4‐azido‐5‐hexynoic acid derivatives. Herein, we designed a new water‐soluble dense 1,2,3‐triazole polymer carrying amino acid residues in the side chains, that is, poly( N ‐(4‐azido‐5‐hexynoyl)cysteine) (poly(AC)), bearing L‐cysteine as a ligand for metal ions. CuAAC polymerization of a protected monomer followed by deprotection and neutralization yielded the sodium salt of poly(AC) (poly(AC)Na). The adsorption capacity of poly(AC)Na with Cd 2+ was investigated with a colorimetric assay. Furthermore, the interactions for poly(AC) with group 12 metal ions (Zn 2+ , Cd 2+ , and Hg 2+ ) were investigated using isothermal titration calorimetry (ITC) and transmittance measurements. Utilizing the colorimetric assay data, the Cd 2+ adsorption capacity per unit weight of poly(AC)Na was evaluated to be 4–5 mmol g −1 , comparable to those of Cd 2+ adsorbents reported previously. The ITC data demonstrated that the interactions of poly(AC)Na with Zn 2+ , Cd 2+ , and Hg 2+ were predominantly exothermic. Based on the ITC data, apparent dissociation constants of interactions were roughly estimated to be in the order of 10 −5 –10 −4 M, which are comparable to those for metal‐binding biomolecules. The transmittance data indicated that the poly(AC)Na/CdCl 2 system underwent phase separation.