Crosslinking of polyurethane with boronic ester bonds: An impact of B–N coordination

In this contribution, we reported a new approach to crosslink linear polyurethane (PU) with dynamic boronic ester bonds. First, a linear PU was synthesized, which carries a plethora of 1,3-diol structural units. Second, two structurally similar diboronic acids were designed and synthesized, which contained amino and amido groups, respectively. Thereafter, the crosslinking between the linear PU and the diboronic acids was carried out via the generation of boronic ester bonds. Thanks to the crosslinking, the PU networks significantly displayed improved thermomechanical properties, contingent on types of crosslinkers. More importantly, the excellent reprocessing and shape memory properties were imparted to the PU networks, which were implemented by dynamic boronic ester bonds between 1,3-diol moieties of the linear PU and boronic acids of the crosslinkers. The dynamic exchange of boronic ester bonds is responsible for the reprocessing properties. For the shape memory properties, the PU networks featured the reprogramming behavior of original shapes via the dynamic exchange of boronic ester bonds. More importantly, the shape memory properties were also dependent on types of diphenylboronic acids, which was reflected with the difference in dynamicity of boronic ester bonds. For the PU networks with amino-containing crosslinker, the boronic ester bonds featured the B–N coordination. As a result, the boronic ester bonds had the increased dynamicity, which significantly affected the reprocessing properties.