Inverse Vulcanization-Induced Self-assembly of Polysulfide into Responsive Micelles and their Templated Hydrogel Adsorbent for Sr2+ Removal

Albeit sulfur-rich polymers have attracted increasing attention and become easily accessible, especially after the emergence of inverse vulcanization, a myriad of polysulfides suffer from limited water solubility due to the hydrophobic nature of sulfur and comonomers, hindering their use in water-related fields (e.g., metal remediation). Inverse vulcanization of water-soluble monomers still remains limited, let alone the understanding of its reaction mechanism in water and of its solution properties. In this study, sodium acrylate was employed to copolymerize with elemental sulfur in water to synthesize ionic polysulfides (SSAC) using organic bases as catalysts. A mechanism involving nucleophilic attack initiation, carbanion propagation, and water-involved hydrolysis termination is reported for the formation of SSACs with molecular weights of 1.1 to 1.7 kDa. There also exists possible side reactions of sulfur hydrolysis in alkaline water that produced sulfur oxoacid and its salts as byproducts. The ionic SSAC was demonstrated to be superhydrophilic and, more importantly, identified to self-assemble into stable negatively charged colloids with a hydrodynamic size of ∼130 nm and a critical micellar concentration of ∼10 mg/L. Then, the SSAC micelles were templated into a homogeneous sulfur-rich hydrogel (SSAC hydrogel) as physical cross-links via an ion-imprinting technique aided by sodium alginate. The hydrogel exhibits self-healing ability and outstanding adsorption capability to Sr²⁺ with a q_m of 134.4 mg/g and good robustness in a wide range of temperature (25 to 55 °C) and pH (4 to 10). Also, fast adsorption kinetics and good selectivity in the presence of Ca²⁺, Mg²⁺, and K⁺ with high concentrations were demonstrated.