Tuning the Viscosity Profile of Ionic Vitrimers Incorporating 1,2,3-Triazolium Cross-Links

Vitrimers are dynamic polymer networks with unique viscoelastic behavior combining the best attributes of thermosets and thermoplastics. Ionic vitrimers are a recent class of dynamic materials, where 1,2,3-triazolium cross-links are reshuffled by trans-N-alkylation exchange reactions. Comparison of dynamic properties with a selection of vitrimers relying on different exchange reactions highlights the particularly high viscous flow activation energies of trans-N-alkylation reactions, thus providing an enhanced compromise between fast reprocessing at moderately high temperatures and low creep at service temperature. Varying the [monomer]/[cross-linker] ratio in the initial formulation of these 1,2,3-triazolium-based networks affords a fine tuning of their viscosity profiles. Confrontation of rheometry and X-ray photoelectron spectroscopy data allows the correlation of variations in chemical composition with changes in the covalent exchange dynamics. This unprecedented approach enables the proposition of a dissociative two-step mechanism for the trans-N-alkylation of 1,2,3-triazoliums initiated by a nucleophilic attack of the 1,2,3-triazolium cross-links by the iodide counteranion, yielding uncrosslinking by de-N-alkylation. Subsequent rapid re-N-alkylation of the formed 1,2,3-triazole by surrounding iodide-functionalized dangling chains affords exchange of the cross-link position. This study highlights that strictly associative exchange reactions are not compulsory to induce vitrimer behavior, and may pave the way to a much wider variety of vitrimers relying on conventional reversible covalent reactions.