Room-temperature healable, recyclable and mechanically super-strong poly(urea-urethane)s cross-linked with nitrogen-coordinated boroxines

Mechanically robust polyurethanes and polyureas with excellent healing and recycling capacities have an extended service life, and can reduce raw material consumption and environmental pollution. In this work, healable, recyclable and mechanically super-strong poly(urea-urethane)s (PUUs) are conveniently fabricated via cross-linking isocyanate-terminated poly(tetramethylene ether glycol) (PTMEG) with nitrogen-coordinated boroxines (NCBs). The tripodal NCBs generate three-dimensionally cross-linked PUUs (denoted as NCB-PUUs) with a high cross-linking density. Meanwhile, hydrogen bonds and phase-separated PTMEG segments can serve as physical cross-linkers to further strengthen NCB-PUUs. As a result, the NCB-PUUs exhibit a tensile strength of ∼47 MPa and a toughness of ∼190 MJ m−3. Because of the high reversibility of NCBs and hydrogen bonds, the NCB-PUUs are capable of efficiently healing and recycling at room temperature with the assistance of ethanol or a water/ethanol mixture to restore their original mechanical strength. NCB-PUUs can be conveniently deposited on solid surfaces by dip-coating and spraying methods to serve as protecting coatings. The NCB-PUU coatings have firm adhesion on solid surfaces and can be doped with organic dyes to display diverse colors. More importantly, the NCB-PUU coatings can heal deep and wide cuts and scratches for multiple times in a given region.