Unexpectedly large entropic barrier controls bond rearrangements in vitrimers

Vitrimers can rearrange under stimuli (e.g., heat, pH, or light) without degrading network integrity, enabling exceptional properties like recyclability, self-healing, and weldability. Yet, their viscoelastic behavior remains poorly understood. Our detailed studies revealed that increasing dynamic crosslink density at fixed molecular weight between crosslinking does not affect chain dynamics but strongly alters linear viscoelasticity of vitrimers, inducing a sol-gel transition consistent with predictions of classical gelation theory. In contrast, lowering molecular weight between crosslinks significantly increases vitrimers’ T_g and slows drastically segmental relaxation. Surprisingly, their bond rearrangement time remains largely unaffected, particularly at elevated temperatures, demonstrating a clear decoupling between bond rearrangement and segmental mobility. Detailed analysis reveals the bond rearrangement time can exhibit either a stronger or weaker temperature dependence than segmental relaxation, depending on the distance of observed temperature range from T_g. Finally, the temperature-dependent analysis of bond rearrangement time reveals an unexpectedly large entropic barrier, strongly slowing bond exchange process despite its relatively low activation enthalpy. These findings emphasize the crucial role of entropy in vitrimer dynamics and offer new insights into the thermodynamic and kinetic control of dynamic covalent networks.