Polyaddition Synthesis Using Alkyne Esters for the Design of Vinylogous Urethane Vitrimers

TLDR

Vitrimers are covalent adaptable networks that combine thermoset stability with reshapeability, and vinylogous urethane (VU) variants are attractive but traditionally synthesized by acetoacetate–amine polycondensation that releases water and creates porosity defects. This study demonstrates that alkyne esters can replace acetoacetates as building blocks for VU vitrimers via polyaddition with amines, yielding water‑free, defect‑free materials. Polyaddition of alkyne esters with amines forms VU bonds whose formation and dynamic exchange were probed by small‑molecule reactions and rationalized through density‑functional theory calculations. The resulting water‑free VU vitrimers exhibit material properties comparable to acetoacetate‑based vitrimers, while the alkyne‑ester approach also enables a one‑pot, three‑component curing of VU epoxy vitrimers with tunable viscoelastic behavior through monomer ratio adjustment.

Abstract

Vitrimers are a subclass of covalent adaptable networks which introduce reshapeability and recyclability in thermoset materials while maintaining a high degree of chemical resistance and dimensional stability. Vitrimer materials based on vinylogous urethane (VU) chemistry have drawn a lot of attention in this area. Classically, these are obtained by the polycondensation polymerization of acetoacetate and amine monomers. Unfortunately, this also releases water, often leading to porosity defects in the initially obtained non-reprocessed cross-linked materials. Here, we demonstrate that alkyne esters (AE) can be used as alternative building blocks for VU vitrimers by a polyaddition polymerization with amines, leading to water-free formulations and straightforward access to defect-free cured VU vitrimer materials. The bond formation and dynamic bond exchange was also studied by small molecule reactions, further rationalized by a computational (DFT) approach. The resulting water-free VU vitrimers display similar material properties compared to vitrimers based on acetoacetates, although also some differences are seen, which can be related to a minor amide-bond forming side reaction. Furthermore, by use of this novel AE approach, polyaddition curing of VU epoxy vitrimers can easily be prepared in a one-pot three-component method, combining AE, amine, and epoxy monomers. This study shows that AE monomers can be used as an easy drop-in method to obtain processable epoxy materials with tunable viscoelastic properties, where the viscous flow behavior can in principle be fully tuned by varying the monomers' ratios and compositions.