Unexpectedly large entropic barrier controls bond rearrangements in vitrimers

Vitrimers are an emerging class of polymeric materials containing associative covalent dynamic bonds that make them recyclable by design. However, the fundamental mechanisms controlling their viscoelasticity remain poorly understood. Herein, we revealed that the density of dynamic covalent crosslinks has no influence on chain dynamics (beyond a weak change in the glass transition temperature), yet it strongly affects the linear viscoelasticity of vitrimers. Increasing the crosslink density induces a sol-gel transition consistent with predictions of classical gelation theory, demonstrating its applicability to vitrimers. The analysis of temperature-dependent bond rearrangement time reveals an unexpectedly large entropic barrier in the transition state that strongly slows down the bond exchange process despite its relatively low activation enthalpy. This insight explains quantitatively the unusually slow bond rearrangements in vitrimers and emphasizes the critical role of entropy in controlling the viscoelasticity of dynamic covalent networks, highlighting a fundamental shift in understanding relaxation processes.