On-demand reversible switching between dynamic covalent polymers and thermosetting polymers

Professor Xiaokong Liu's team at Jilin University proposed an "activation-quenching" strategy for dynamic covalent chemistry, achieving precise and controllable "on-off" switching of dynamic bond exchange reactions in dynamic covalent polymers (covalent adaptive networks or CANs). This strategy enables the polymer network to reversibly switch between a remodelable dynamic state and a highly stable thermosetting state on demand, effectively balancing the material's remodeling processability and thermal stability, and providing a novel approach to resolving the contradiction between sustainability and stability in dynamic covalent polymers.  The article was published as an open access research article in CCS Chemistry, the flagship journal of the Chinese Chemical Society. 

Background information:

Compared to thermoplastic polymers, thermosetting polymers possess superior mechanical properties, thermal stability, and solvent resistance, leading to their widespread application in human production and daily life. However, due to their permanently covalently cross-linked structure, they cannot be melt-processed or recycled, resulting in a severe environmental burden. Dynamically covalent polymers, based on dynamic covalent bonds, can be remodeled and recycled through thermally activated dynamic bond exchange reactions. However, thermally activated network reconstruction leads to a significant decrease in the mechanical properties and dimensional stability of the material at high temperatures, resulting in an inherent contradiction between sustainability and thermal stability, becoming a key bottleneck limiting the practical application of dynamically covalent polymers.

Highlights of this article:

1. A "quenching-activation" strategy for dynamic covalent chemistry is proposed.
Based on phenol-yne click chemistry, this study proposes a reversible "quenching" and "activation" method for dynamic exchange reactions through acid/base treatment. Deprotonated phenols (phenoxy anions) can dynamically exchange with phenol-yne adducts via an addition-elimination pathway. When the phenoxy anion is protonated by acid treatment, its nucleophilicity decreases, and the dynamic exchange reaction is "quenched." The protonated phenol can then be "reactivated" by base treatment, thus restarting the dynamic exchange reaction.

2. Enabled on-demand reversible switching between dynamic covalent polymers and thermosetting polymers.
This study utilized triphenol and diyne monomers to prepare a dynamic covalent polymer in an "activated" state via phenol-yne click chemistry catalyzed by an inorganic base (Cs₂CO₃ ). This dynamic covalent polymer was then "quenched" by acid treatment, transforming into a thermosetting polymer while completely retaining the original material's shape, size, and mechanical properties. The thermosetting polymer obtained after "quenching" can then be "activated" by alkali treatment to achieve solution recovery and re-transform into a dynamic covalent polymer.

3. It provides a novel approach to resolving the conflict between sustainability and stability in dynamic covalent polymers.
Dynamic covalent polymers in the "activated" state can be reshaped and recycled through heating, but they exhibit poor thermal stability, showing significant creep behavior at 80 °C . Thermosetting polymers obtained after "quenching" exhibit excellent thermal stability, with a 40 °C increase in glass transition temperature and a 70 °C increase in decomposition temperature , maintaining excellent creep resistance even at 160 °C . Therefore, thermosetting polymers in the "quenched" state can be applied to practical working conditions, while "activation" enables recycling, reshaping, and reuse.

Summary and Outlook:

In summary, this study achieved on-demand reversible switching between dynamic covalent polymers and thermosetting polymers. The developed material system integrates the advantages of dynamic covalent polymers, traditional thermoplastic polymers, and traditional thermosetting polymers, overcoming the inherent contradiction between the sustainability and thermal stability of dynamic covalent polymers, and laying an important foundation for the practical application of dynamic covalent polymers. This "quench-activation" strategy is also expected to be extended to functional material systems such as dynamic covalent liquid crystal networks and shape memory polymers, providing an effective way to resolve the contradiction between their thermally responsive performance and thermally induced network reconstruction.

The relevant research findings were published as a Research Article in CCS Chemistry. Minghao Wu, a doctoral student at Jilin University, China, is the first author of this paper, and Professor Xiaokong Liu  is the corresponding author. Professor Wei Zhang from the University of Colorado Boulder, USA, provided important guidance for this work. This research was supported by the Jilin Provincial Natural Science Foundation, the National Natural Science Foundation of China ( 22275069 , 22350011 ), and the National Key Research and Development Program of China ( 2023YFA1008804 ).

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.