True Closed-Loop Recyclable Hydrogels Enabled by Imine Boronic Ester Crosslinking

The relationship between molecular structure and macroscopic function is a foundational principle in materials science, in which subtle molecular variations produce pronounced differences in strength, stiffness, and elasticity of macromolecular solids. In hydrogels, replacing static covalent bonds with dynamic covalent bonds (DCBs) creates newfound capabilities, including self-healing and recyclability. Herein, substitutional differences in dual DCB imine boronic ester crosslinkers, together with matrix pH, influencing hydrogel properties are investigated. A comparison of ortho- and para-imine boronic esters showed the formation of 3-amino-benzoxaborole heterocycles in hydrogels derived from 2-formylphenylboronic acid. Tautomerization to the heterocycle significantly enhanced hydrogel elasticity, despite a lower crosslinking density than in hydrogels formed with 4-formylphenylboronic acid. Two closed-loop end-of-life (EOL) management pathways are also demonstrated. Reprocessing through self-healing is accomplished, with hydrogels regaining at least 90% of their original rheological properties. A fully circular recycling pathway is also established, recovering all starting materials for reuse, with recycled hydrogels achieving over 100% recovery of rheological properties. Overall, the presence of previously undisclosed 3-amino-benzoxaborole structures is demonstrated, expanding the understanding of formylphenylboronic acids in polymeric materials, and complete closed-loop EOL pathways are designed to inspire greater focus on full EOL processes in materials circularity.