Boron-containing polymers have many
applications resulting from
their prominent properties. Organoboron species with reversible B–O
bonds have been successfully employed for the fabrication of various
self-healing/healable and reprocessable polymers. However, the application
of the polymers containing boronic ester or boroxine linkages is limited
because of their instability to water. Herein, we report the hydrolytic
stability and dynamic covalent chemistry of the nitrogen-coordinating
cyclic boronic diester (NCB) linkages, and a new class of vitrimers
based on NCB linkages is developed through the chemical reactions
of reactive hydrogen with isocyanate. Thermodynamic and kinetic studies
demonstrated that NCB linkages exhibit enhanced water and heat resistance,
whereas the exchange reactions between NCB linkages can take place
upon heating without any catalyst. The model compounds of NCBC-X1
and NCBC-X2 containing a urethane group and urea group, respectively,
also showed higher hydrolytic stability compared to that of conventional
boronic esters. Polyurethane vitrimers and poly(urea-urethane) vitrimers
based on NCB linkages exhibited excellent solvent resistance and mechanical
properties like general thermosets, which can be repaired, reprocessed,
and recycled via the transesterification of NCB linkages upon heating.
Especially, vitrimers based on NCB linkages presented improved stability
to water and heat compared to those through conventional boronic esters
because of the existence of N → B internal coordination. We
anticipate that this work will provide a new strategy for designing
the next generation of sustainable materials.