Mechanically Robust Polyurethane Vitrimers with B–N-Coordinated Boronic Ester Bonds

The reaction of 4-(hydroxymethyl)phenylboronic acid with triethanolamine was carried out to synthesize a boronic ester diol featuring boron–nitrogen (B–N) coordination. This diol served as a chain extender, allowing B–N-coordinated boronic ester bonds to be introduced into polyurethane (PU) networks. Toward this end, we first synthesized a series of α,ω-dihydroxyl polyurethane telechelics with α,ω-dihydroxypropyl poly(dimethylsiloxane) as the polymeric diol. By varying the molar ratio of the boronic ester diol to 1,4-butane diol, the α,ω-dihydroxyl polyurethane telechelics were synthesized with variable contents of the dynamic covalent bonds. Thereafter, the reaction of α,ω-dihydroxyl PU telechelics with 3-isocyanatopropyl triethoxysilane was carried out to gain α,ω-ditriethoxysiloxane PU telechelics. The latter were readily cross-linked via the sol–gel process to afford the mechanically robust PU networks, with Young’s modulus and tensile strength as high as 302.7 and 35.4 MPa, respectively. More importantly, the reprocessing properties were imparted to the PU networks. Compared to the control PU network, the reprocessing temperature was depressed to 120 °C. In the meantime, the shape memory properties of PU networks were significantly improved through the introduction of dynamic covalent bonds. Furthermore, the shape memory networks featured reconfigurability with the introduction of B–N-coordinated boronic ester bonds.