Unexpectedly large entropic barrier controls bond rearrangements in vitrimers

Abstract

• •
  Entropic barriers control bond exchange in vitrimers
• •
  Extremely suppressed numbers of activated states cause slow bond rearrangements
• •
  Calling attention to the critical role of entropic barrier in dynamic processes

Vitrimers are an emerging class of recyclable polymers that could help address the global plastic waste challenge while enabling advanced functionalities such as self-healing and shape reconfiguration. However, their broader adoption has been hindered by a fundamental gap in understanding why their bond rearrangements—and thus flow and reprocessing—are often unexpectedly slow. Our work identifies the key missing factor: a large entropic barrier. While previous studies focused mainly on enthalpic energy barriers, we show that the rarity of suitable molecular configurations dramatically limits how often associative dynamic covalent bond exchange can occur. This insight resolves a long-standing puzzle and provides a clearer, more complete picture of how vitrimers relax and flow.

Looking ahead, this discovery opens new opportunities to design recyclable polymers with precisely controlled mechanical properties and processing behavior by tuning not only bond strength but also molecular configurational freedom. In the short term, our findings offer practical guidance for improving vitrimer performance in applications such as reprocessable plastics, coatings, and adhesives. In the longer term, recognizing the importance of entropic barriers—often overlooked in chemistry and materials science—may reshape how scientists think about reaction rates and dynamics across a wide range of systems, from soft materials to complex chemical processes. By enabling more predictive design of sustainable and high-performance materials, this work has the potential to contribute to advances in environmentally responsible manufacturing and next-generation functional materials.

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.

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.

1. Download: Download high-res image (228KB)
2. Download: Download full-size image