A weak but inert hindered urethane bond for high-performance dynamic polyurethane polymers

Abstract

Covalent adaptable networks (CANs), which share the properties of both thermosets and thermoplastics at the same time, are desirable for many applications. Introducing bulky substituents is a feasible way to design dynamic covalent bonds for constructing CANs, as evidenced by the successful implementation in CANs based on hindered urea bonds (HUBs). However, the dynamicity induced by introducing bulky substituents always come with low bond energy, resulting in low mechanical strength and poor stability of the CANs. Herein, we designed a novel hindered urethane bond, which is weak in thermodynamic (Keq = 1701.23 L/mol at 25 °C) and inert in kinetic at low temperature, but stable in thermodynamic (Keq = 1.54 × 104 L/mol at 100 °C) and active in kinetic at high temperature (k-1 = 0.105 h−1 at 80 °C and 0.315 h−1 at 120 °C). As a result, the polyurethane based on it exhibits high mechanical properties (with Youngs’ modulus of 1011 ± 29 MPa and flexible modulus reached 1833 ± 50 MPa) and excellent reversibility (can be reprocessed at 60 °C under 100 kPa in 30 min and completely healed at 40 °C in 10 min). Moreover, unlike to many CANs based on hindered urea bonds, our dynamic polyurethanes are highly stable in humid environment or even water solutions due to the slow hydrolysis kinetics. Such high-performance dynamic polyurethane polymers are attractive for many applications.