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

Area of Science:

• Materials Science
• Polymer Chemistry
• Organic Chemistry

Background:

• The structure-property relationship is crucial in materials science, especially for macromolecular solids.
• Dynamic covalent bonds (DCBs) in hydrogels offer advanced properties like self-healing and recyclability compared to static covalent bonds.

Purpose of the Study:

• Investigate how substitutional differences in dual DCB imine boronic ester crosslinkers and matrix pH affect hydrogel properties.
• Explore the formation and impact of 3-amino-benzoxaborole heterocycles.
• Demonstrate closed-loop end-of-life (EOL) management pathways for hydrogels.

Main Methods:

• Synthesized hydrogels using ortho- and para-imine boronic esters with varying pH.
• Characterized hydrogel properties, including elasticity and crosslinking density.
• Evaluated self-healing and recycling capabilities of the hydrogels.

Main Results:

• Discovered the formation of 3-amino-benzoxaborole heterocycles from 2-formylphenylboronic acid, which enhanced hydrogel elasticity.
• Hydrogels with ortho-imine boronic esters showed greater elasticity than those with para-imine boronic esters, despite lower crosslinking density.
• Achieved hydrogel reprocessing with at least 90% recovery of rheological properties through self-healing.
• Established a fully circular recycling pathway, recovering starting materials and achieving over 100% recovery of rheological properties.

Conclusions:

• The formation of 3-amino-benzoxaborole heterocycles is a key factor in enhancing hydrogel elasticity.
• The study demonstrates the potential for designing hydrogels with superior mechanical properties and complete circularity.
• Complete closed-loop EOL pathways were successfully designed, promoting sustainable materials management.