Advancing Polymer Reycling: Polydithioacetals as Entropy-Driven Solutions for a Circular Polymer Economy

Significance 

The global plastic waste crisis is worsening due to the rapid growth of the plastic industry and the lack of sustainable end-of-life management strategies, highlighting the urgent need for recyclable polymers. The current traditional recycling methods often degrade the polymer quality, leading to downcycling. Chemical recycling to monomers, which allows the synthesis of new polymers with quality comparable to the virgin material, presents a promising solution to achieve a circular economy in the polymer sector. Ring-opening polymerization (ROP) has been identified as a viable route for creating polymers that can undergo reversible ring-closing depolymerization (RCD) to regenerate cyclic monomers. However, traditional ROP is predominantly driven by enthalpic considerations, reliant on the relief of ring strain in small cyclic monomers. This process faces limitations in terms of the range of suitable monomers and the conditions under which depolymerization and repolymerization can efficiently occur. The concept of entropy-driven ROP (ED-ROP), which utilizes large, strainless macrocycles as monomers, introduces a significant shift in the approach to designing recyclable polymers. ED-ROP is characterized by an increase in conformational entropy during polymerization, leading to a favorable overall entropy change (ΔS_p > 0). This process is thermodynamically favored at higher temperatures, which is advantageous for applications where thermal stability is crucial.

A new study published in Angewandte Chemie International Edition conducted by Postdoctoral fellow Dr. Lasith S. Kariyawasam, Julian F. Highmoore, and led by Assistant Professor Ying Yang from the Department of Chemistry at University of Nevada, Reno addressed a critical aspect of modern polymer science: the development of recyclable polymers through the lens of sustainability. The researchers developed a new class of chemically recyclable polymers known as polydithioacetals (PDTAs), synthesized through a straightforward reaction between 3,4,5-trimethoxybenzaldehyde and various alkyl dithiols in the presence of an acid catalyst. This process yielded pristine PDTAs characterized by their ease of synthesis and the potential for functionalization through the choice of dithiol and benzaldehyde derivatives. The synthesized PDTAs were found to be odorless, transparent, and exhibited soft material properties with glass transition temperatures ranging from -6 to +9 °C.

A key finding of the authors’ study is their demonstration of the chemically recyclable nature of PDTAs. This was achieved through a two-step process involving RCD to generate macrocyclic monomers from the polymer chain, followed by ED-ROP to reform the PDTA polymers. This reversible process showcases the potential of PDTAs in circular polymer economies, where the ability to revert polymers back to their monomeric forms and repolymerize them is crucial for sustainable materials management.

The researchers also highlighted the innovative use of ED-ROP, where polymerization is driven by an increase in conformational entropy rather than the traditional enthalpic considerations seen in conventional ROP processes. This approach is advantageous because it allows polymerization to occur at higher temperatures, enhancing the thermal stability of the resulting polymers in the presence of residual catalyst. Moreover, they demonstrated high conversions for both the depolymerization and repolymerization processes, indicating the efficiency and viability of the ED-ROP mechanism. An important aspect of the PDTAs studied is their thermal reprocessability, which is enabled by acid-catalyzed dithioacetal exchange reactions. This property allows the crosslinked PDTA networks to be reprocessed at elevated temperatures without losing their structural integrity. Additionally, the networks retain their ability to be depolymerized into macrocyclic monomers in the presence of a solvent, which can then be repolymerized to regenerate the crosslinked network, demonstrating a closed-loop recycling process.

The authors’ findings provided a foundation for the development of new recyclable polymer materials that align with the principles of a sustainable circular economy. The demonstrated recyclability and reprocessability of PDTAs, coupled with their ease of synthesis and functionalizability, open up new avenues for the design of environmentally friendly materials. Future research could explore the scalability of this approach, the development of PDTAs with enhanced mechanical properties, and the application of this technology in various industrial sectors.

According to the authors, PDTAs offer several advantages over traditional polymers, including ease of synthesis, functionalizability, and inherent recyclability. The ability to undergo ED-ROP at ambient temperatures facilitates efficient recycling processes, potentially transforming the landscape of polymer manufacturing and waste management. Moreover, the introduction of aromatic groups or other structural modifications can enhance the mechanical properties and thermal stability of PDTAs, broadening their applicability in various domains. The research on PDTAs and their recyclability through ED-ROP represents a significant advancement in the pursuit of sustainable polymer technologies. Future studies could explore the scalability of this approach, the development of more diverse monomer systems, and the integration of PDTAs into commercial products. Addressing the challenges related to solvent use in depolymerization and enhancing the mechanical properties of the resulting polymers will be crucial for realizing the full potential of this innovative platform. In conclusion, the new work of Assistant Professor Ying Yang and colleagues introduces polydithioacetals as a promising new class of chemically recyclable polymers, facilitated by entropy-driven ring-opening polymerization. This study opens new avenues for the development of sustainable materials critical for addressing the global plastic waste challenge.

Advancing Polymer Reycling: Polydithioacetals as Entropy-Driven Solutions for a Circular Polymer Economy - Advances in Engineering

About the author

Dr. Lasith S. Kariyawasam is as a postdoctoral scholar in the research group led by Professor Ying Yang at the University of Nevada, Reno. His primary research focus centers on the development of chemically recyclable dithioacetal polymers. Lasith earned his B.Sc. (Hons) degree in Chemistry from the University of Colombo, Sri Lanka, in 2014. He continued his academic journey by obtaining a Ph.D. in Organic Chemistry from Miami University (Ohio) in 2020, working under the guidance of Professor C. Scott Hartley. His doctoral dissertation delved into the utilization of common reagents, specifically carbodiimides, to drive carboxylic acid precursors out of equilibrium through the formation of corresponding transient anhydrides.

About the author

Julian F. Highmoore is currently a PhD student in Prof. Ying Yang’s research group at the University of Nevada, Reno. Their primary research focus is on the development of chemically recyclable dithioacetal-based covalently adaptable networks and metallopolymers. Julian earned their B.S in Chemistry from Clarkson University, New York, in 2021 under the purview of Prof. Devon Shipp where their research focused mainly on thiolactone chemistry under the mentorship of, then PhD student, Dr. Yongneng Wu.

About the author

Dr. Ying Yang earned her PhD in Materials Science and Engineering at Clemson University under the guidance of Prof. Marek Urban. She then had a postdoc appointment in Prof. Urban’s group working on self-healing materials. She joined the University of Nevada, Reno as an assistant professor in the Chemistry department in 2019. Her lab focuses on designing highly recyclable polymers and bioinspired materials.

Reference

Kariyawasam LS, Highmoore JF, Yang Y. Chemically Recyclable Dithioacetal Polymers via Reversible Entropy-Driven Ring-Opening Polymerization. Angew Chem Int Ed Engl. 2023;62(26):e202303039. doi: 10.1002/anie.202303039.

Go to Angew Chem Int Ed Engl.

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