"Dynamic Covalent Bonds in Boronic Ester Self-Healing Polymers"
Over the last decade, self-healing materials have become attractive for numerous applications, ranging from electronic skin to recyclable rubbers. The ability to self-heal physical damage in polymeric materials through physical or chemical processes has brought increased interest in smart technologies.(1) Recently, scientists have synthesized self-healing materials that employ dynamic covalent motifs that have been shown to improve mechanical properties (i.e. Young’s modulus, yield strength) through enhancing thermodynamic stability and elasticity. However, dynamic covalent bonds such as those made by thiol coupling or Diels-Alder reactions generally cannot self-heal under ambient conditions.
Recent studies have shown that boronic esters are promising cross-linking functional groups in self-healing polymers due to their dynamic covalent bonding at room temperature: bond making and breaking is highly reversible. Utilizing boronic esters in a novel series of polyurethane elastomers resulted in enhanced stretchability and self-healing properties by increasing cross-linking density.(2) Similar improvements to materials properties were obtained when boronic esters were crosslinked to epoxidized natural rubber (3) and polyethylene materials.(4) In both cases, this particular choice in crosslinker led to enhanced mechanical (i.e. tensile strength, Young’s modulus) and rheological properties as measured by dynamic mechanical analysis. The self-healing behaviors illustrated in these systems highlight the potential opportunities for polymers cross-linked by boronic esters to be used in advanced biomedical devices and coatings.(5)
(1) Zheng N, Xu Y, Zhao Q, Xie T. Dynamic covalent polymer networks: A molecular platform for designing functions beyond chemical recycling and self-healing. Chem. Rev. 2021, 121, 1716-1745.
(2) Yang Y, Du FS, Li Z. Highly stretchable, self-healable, and adhesive polyurethane elastomers based on boronic ester bonds. ACS Appl. Polym. Mater. 2020, 2, 5630-5640.
(3) Chen Y, Tang Z, Liu Y, Wu S, Guo B. Mechanically robust, self-healable, and reprocessable elastomers enabled by dynamic dual cross-links. Macromolecules. 2019, 52, 3805-3812.
(4) Wang Z, Gu Y, Mingyu M, Liu Y, Chen M. Strengthening polyethylene thermoplastic through a dynamic covalent networkings additive based on alkylboron chemistry. Macromolecules. 2021, 54, 1760.
(5) Sumerlin BS, Cash J. Room Temperature Self-Healing Polymers Based on Dynamic-Covalent Boronic Esters. Marcomolecules. 2015, 48, 2098-2106.