# Theoretical studies on anhydride dynamic covalent bond exchange mechanisms

## TL;DR

This paper explains how anhydrides enable recycling of plastics by studying their chemical bond exchange mechanisms under different conditions.

## Contribution

The study provides a detailed theoretical understanding of anhydride bond exchange mechanisms using DFT.

## Key findings

- The bond exchange barrier increases with temperature for both uncatalyzed and acid-catalyzed reactions.
- Acid catalysis significantly lowers the bond exchange barrier at lower temperatures.
- Anhydrides are suitable for high-temperature applications in covalent adaptable networks.

## Abstract

Anhydrides have been utilised to synthesize covalent adaptable networks (CANs), enabling the recycling of thermosets and immiscible plastic wastes due to their dynamic bond exchange properties. However, the exact bond exchange mechanism involving anhydrides remains unclear. Herein, we identify the bond exchange mechanism behind anhydride-based CANs through a comprehensive study of the anhydride bond exchange mechanism under uncatalysed and acid-catalysed reaction conditions using density functional theory (DFT) methods. Careful conformational samplings of structures and transition states have been performed to ensure accurate determination of the key rate-determining step. The bond exchange barrier increases from 44.1 kcal mol−1 at 25 °C to 52.8 kcal mol–1 at 200 °C for the uncatalysed route and from 25.9 kcal mol–1 to 33.0 kcal mol–1 over the same temperature range for acid-catalysed route. Our results implicate that anhydrides can be good dynamic covalent bond linker candidates in CANs for high temperature applications and acid catalysts may be used as mediators to assist low-temperature reprocessing and recycling.

Anhydrides are used in creating covalent adaptable networks for recycling thermosets and plastic wastes, yet their bond exchange mechanism is not fully understood. Herein, the authors elucidate this mechanism under uncatalysed and acid-catalysed reaction conditions using density functional theory, revealing that acid catalysis can reduce the bond exchange barrier at lower temperatures.