Eliminating creep in vitrimers using temperature-resilient siloxane exchange chemistry and N-heterocyclic carbenes

Abstract

This study explores a novel N-heterocyclic carbene-mediated siloxane exchange mechanism, laying the foundation for designing covalent adaptable networks (CANs) with high temperature stability (>200 °C) for dynamic covalent chemistry. Small molecule siloxane compounds, obtained by hydrosilylation reactions, are used to demonstrate siloxane-exchange via a mechanism supported by density functional theory. The proposed mechanism presents an equilibrium, at elevated temperatures, between an imidazolium salt and its free carbene form, which is the catalytically active species. Following this mechanistic insight, a tetra-substituted ester-terminated siloxane cross-linker was synthesized and cured with a commercial amine hardener. The ensuing ester–amine reaction yields thermally stable, non-dynamic amide bonds, thereby enhancing material stability. The resulting CANs exhibit rapid stress relaxation at elevated temperatures and demonstrate successful recycling through compression molding without any significant loss of material properties. Remarkably, the synthesized material showcases high creep resistance, even up to 150 °C, indicating the benefits of having a thermally reversible catalyst system for siloxane activation. This ground-up design of dynamic chemistry and material synthesis not only presents innovative material design but also suggests avenues for exploring thermally stable, fast-exchanging and yet creep-resistant CANs.