Methoxy Substitution Enhances Metal Ion Binding Capacity of the Water-Soluble Spiropyran Copolymer

Spiropyran-containing polymers have exhibited metal ion binding capabilities in organic solvents, but their limited solubility in water has restricted their application. To address this challenge, two water-soluble spiropyran copolymers were synthesized by copolymerizing spiropyran acrylate (SPA) or methoxy-functionalized spiropyran (SPOCH₃) monomers with hydrophilic oligo(ethylene oxide) monoacrylate monomers (OEOA). The impacts of methoxy substitution on copolymer photoisomerization and metal ion complexation properties were elucidated. The two copolymers (SPA-co-OEOA and SPOCH₃-co-OEOA) retained the photoresponsive property in water. Methoxy substitution enhanced the stability of the ring-opened merocyanine (MC) isomer, with further aggregation of zwitterionic MC through electrostatic interactions. Consequently, only 21% of the MC reverted to the ring-closed isomer under visible light. UV–vis absorbance spectra demonstrated the formation of the MC-M²⁺ complex in ethanol with Cu²⁺, Zn²⁺, or Ca²⁺. Moreover, it was evident that methoxy groups contributed additional binding sites for metal ion complexation. In addition to qualitative characterization, the influence of methoxy substitution on the Cu²⁺ complexation in pure water was further quantified. The results showed that SPOCH₃-co-OEOA complexed with 74% Cu²⁺ (C₀ = 2.0 mg/L), which was 1.7 times higher than that of SPA-co-OEOA. The estimated pK_a values were 3.9 and 4.2 for SPA-co-OEOA and SPOCH₃-co-OEOA, respectively. The protonation of MC to MCH⁺ reduces the available phenolate sites for metal ion binding, elucidating the observed loss in Cu²⁺ removal capacity for both copolymers under the acidic condition. This work advances the fundamental understanding of the effects of the functional group on the photoresponsive and metal ion complexation behaviors of water-soluble spiropyran copolymers, which is essential toward the development of spiropyran-based materials for selective removal and/or recovery of metal ion(s) in aqueous settings.