There is often a trade-off between mechanical properties and the dynamic nature of self-healing. In this work, we report the design and synthesis of a polymer containing thermodynamically stable whilst kinetically labile coordination complex to address this conundrum. The Hbimcp (2,6-bis((imino)methyl)-4-chlorophenol) ligand we used is a kind of alterdentate ligand, which provides two equivalent imine-N donor centers. However, due to the steric hindrance, the two equivalent imine-N donor centers can not coordinate with the same metal ion (Zn(II) in our study) at the same time and therefore the two coordination atoms are alternative and interchangeable. The Zn-Hbimcp coordination bond has a relatively large association constant (2.2 * 10¹¹) but also undergoes fast and reversible intra- and inter-molecular ligand exchange processes. The as-prepared Zn(Hbimcp)₂-PDMS polymer is highly stretchable (up to 2400 % strain) with a high toughness of 29.3 MJ m^-3, and autonomously self-heal at room temperature. Control experiments by varying the metal ions and metal-to-ligand molar ratios showed that the optimal combination of its bond strength and bond dynamics is responsible for the material’s mechanical toughness and self-healing property. This molecular design concept points out a promising direction for the preparation of self-healing polymers with excellent mechanical properties. We further show this type of polymer can be potentially used as energy absorbing material for car crash protection, sportswear, helmet liners, shockproof pad, armored clothing. It can also be potentially used as self-healing and self-adaptable electronic materials for biological interfaces.

a. The structure of polymer complex Zn(Hbimcp)<sub>2</sub>-PDMS; b. Strain–stress curves of a film healed at room temperature (25 °C) for different lengths of time show an increase of the stretching ability when the film is allowed to heal for a longer time; c. Possible energy dissipation process of [Zn(Hbimcp)<sub>2</sub>]<sup>2+</sup>; d. The calculation of energy-absorbing efficiency. e. Optical images of the composite sponge under compression, healing, and recovery processes.

a. The structure of polymer complex Zn(Hbimcp)₂-PDMS; b. Strain–stress curves of a film healed at room temperature (25 °C) for different lengths of time show an increase of the stretching ability when the film is allowed to heal for a longer time; c. Possible energy dissipation process of [Zn(Hbimcp)₂]²⁺; d. The calculation of energy-absorbing efficiency. e. Optical images of the composite sponge under compression, healing, and recovery processes.

Jiancheng Lai

Postdoc, Stanford University

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