Self-Healing Materials Based on Boron Ester Bonds: A Review

Boron esters are reversible covalent bonds formed by the complexation of phenylboronic acid and diols. In aqueous solutions, the strength of boron ester bonds depends on the pH of the solution and the pKa of phenylboronic acid. When the pH of the solution is higher than the pKa of phenylboronic acid, boron ester bonds are easily formed. Conversely, when the pH of the solution is lower than the pKa of phenylboronic acid, the equilibrium tends towards free phenylboronic acid and diols. Due to the dynamic equilibrium between phenylboronic acid-diols and boron ester bonds, bond rearrangement can occur, leading to self-healing when the boron ester bonds at the damaged interface of the material rearrange. Additionally, under solvent-free conditions, direct ester exchange reactions can occur between boron ester bonds, allowing for network rearrangement. Many scholars at home and abroad have conducted in-depth research on boron ester bonds. For example, Guan et al. [46] studied the rate-tunable boron ester ester exchange reaction and utilized the principle of different kinetics of boron ester exchange reactions caused by the electronegativity difference of substituents on the adjacent positions of boron ester. They reacted two different diboronates with the adjacent diol grafted in the polymer to crosslink the polymer and studied the influence of the rate of exchange reaction on network rearrangement. Polymer systems with faster exchange reaction rates of boron ester exhibited excellent self-healing properties and good repeatability in processing. The mechanical properties of materials processed three times remained essentially unchanged. On the other hand, polymer systems with slower exchange rates of boron ester had almost no healing ability and poor repeatability in processing.