Review by Benjamin R. Nelson and Bruce E. Kirkpatrick, University of Colorado Boulder,
Chemical and Biological Engineering
Editor’s Note: This new section is designed to feature concise reflections from graduate students on recent academic reports regarding the development of technologies in the UV/EB curing space. Each commentary highlights key findings on a specific subject, situating them within the broader scientific and industrial contexts and drawing attention to future potential impacts and implementations. By including emerging research voices, this feature aims to bridge the gap between academic innovation and industrial application.
Photocurable materials have long been dominated by vinyl-functional monomers because of their rapid radical and/or cationic polymerization kinetics, commercial availability and stability following polymerization. However, these materials also present many challenges, especially with the ever-growing demand for recyclable and reprocessable materials. Traditional chain-growth polymers typically are composed of carbon-carbon bonds in their backbone, which provide thermal and mechanical stability but also pose challenges for degradation and recycling. Incorporating non-carbon atoms into the polymer backbone can introduce dynamic or cleavable bonds and improve other physicochemical properties, such as refractive index or adhesive strength, enabling the creation of high-value polymers.
As a promising class of monomers for sustainable and functional polymer development, 1,2-dithiolanes rapidly are undergoing a renaissance in academic research on their polymerization into polydisulfide materials. Derived from biologically available precursors, such as lipoic acid and asparagusic acid, dithiolane polymers are both renewable and recyclable while facilitating low-cost and large-scale production (see Figure 1). As a cyclic disulfide, the high sulfur content of dithiolanes affords them a high refractive index, an attractive quality for optical materials, but without the odor concerns often associated with thiol monomers. This same sulfur content can be leveraged to impart tunable adhesive properties. Moreover, the ring strain inherent in dithiolanes as a five-membered heterocycle makes them primed for a variety of different polymerization mechanisms, creating opportunities for a diverse range of applications in functional materials, especially as photopolymer resins.
In a use case similar to current commodity engineering polymers, work by Zhang and coworkers in Matter utilized dithiolanes as a monomer for the creation of linear polydisulfides. 1 In this work, the authors demonstrated dual-closed-loop polymerization and recycling of these materials, using different ions for coordination with the carboxylic acid moiety of lipoic acid. This built upon their group’s previous work, improving control over mechanics (e.g., extensibility and modulus) by adding additional parameters (ion identity and concentration) to adjust these properties. Notably, these materials were polymerized through multiple mechanisms while maintaining their chemical recyclability back to repolymerizable monomer.
Beyond homopolymers, dithiolanes also have been shown to be effective building blocks in the creation of degradable copolymers by Albanese et al. in JACS. 2 Here, the authors copolymerized lipoic acid with a variety of vinyl monomers, yielding copolymers capable of degradation into oligomers via cleavage of disulfides and thioethers in the polymer backbone. As dithiolanes readily copolymerize with many vinyl monomers, a wide variety of material properties are achievable through rational selection of polymerizable reactive moieties and side groups, each of which plays a distinct role in determining the thermal, mechanical and degradative behavior of the resulting copolymer. These properties make these polymers particularly useful in controlled-release applications, as highlighted by the authors. Of note, Maes and colleagues, writing in Macromolecular Chemistry and Physics, identified that free-radical methacrylate polymerization effectively is orthogonal to the reactivity of lipoic acid, allowing for the construction of polymethacrylate networks containing responsive dithiolane residues. 3
Given their high disulfide content, dithiolanes also lend themselves particularly well to adhesive materials. Furthermore, the dynamicity of dithiolane-derived linear disulfides and their depolymerization back to cyclic monomer enables reversible adhesion. In Science, Pal et al. elegantly implemented lipoic acid as the primary component of surgical, structural and pressure-sensitive adhesives with long-term stability and on-demand degradation through functionalization of the carboxylic acid moiety of lipoic acid. 4 By installing a thiol-reactive endgroup, this modified lipoic acid induced network stabilization via thioester formation with free thiols at the end of polydisulfide chains. Due to the ease of chemical modification, these adhesives were conveniently tailored for a wide range of different substrates and applications.
Dithiolanes also have versatile applications in photopolymerizations. In the presence of an exogenous photoinitiator, rapid polymerization is observed in multifunctional dithiolane monomers, enabling light-based additive manufacturing. Machado et al. demonstrated this approach to 3D printing in Nature, creating materials capable of printing, depolymerization and subsequent reprinting. 5 The authors recently expanded on this concept in Polymer Chemistry, where the inherent blue light absorbance of dithiolanes was harnessed for initiator-free photopolymerization of network-forming macromonomers. 6 This initiator-free photopolymerization is especially useful in printing micron-scale features while retaining dynamic crosslink behavior.
Looking ahead, the industrial relevance of dithiolane-based polymers hinges on addressing challenges, such as ensuring mechanical robustness and achieving precise control over degradation. While their recyclability and high sulfur content suggest potential niche and high-value use cases in optics and specialty adhesives, improvements in toughness and thermal resistance could expand their role as commodity plastics. Controlled depolymerization strategies, such as stimuli-responsive endgroups to control self-immolation, may afford better regulation of recycling. Though these monomers diverge from many of the traditional carbon-carbon frameworks of most industrial polymers, they retain many aspects of chain growth polymerization and are readily copolymerizable, preserving familiar processing advantages of many current industrial polymers. As academic research continues to better understand and tune these polymerizations, dithiolanes are ripe with untapped industrial potential.
References
- Zhang, Q.; Deng, Y.; Shi, C. Y.; Feringa, B. L.; Tian, H.; Qu, D. H. Dual closed-loop chemical recycling of synthetic polymers by intrinsically reconfigurable poly(disulfides). In Matter, Cell Press: 2021; Vol. 4, pp 1352-1364.
- Albanese, K. R.; Morris, P. T.; Read de Alaniz, J.; Bates, C. M.; Hawker, C. J. Controlled-Radical Polymerization of alpha-Lipoic Acid: A General Route to Degradable Vinyl Copolymers. J Am Chem Soc 2023. DOI: 10.1021/jacs.3c08248 From NLM Publisher.
- Maes, S.; Scholiers, V.; Prez, F. E. D. Photo‐crosslinking and Reductive decrosslinking of Polymethacrylate‐based Copolymers Containing 1,2‐dithiolane Rings. In Macromolecular Chemistry and Physics, 2021; p 2100445.
- Pal, S.; Shin, J.; DeFrates, K.; Arslan, M.; Dale, K.; Chen, H.; Ramirez, D.; Messersmith, P. B. Recyclable surgical, consumer, and industrial adhesives of poly(α-lipoic acid). Science 2024, 385 (6711), 877-883. DOI: doi:10.1126/science.ado6292.
- Machado, T. O.; Stubbs, C. J.; Chiaradia, V.; Alraddadi, M. A.; Brandolese, A.; Worch, J. C.; Dove, A. P. A renewably sourced, circular photopolymer resin for additive manufacturing. Nature 2024. DOI: 10.1038/s41586-024-07399-9.
- Nelson, B. R.; Cione, J. T.; Kirkpatrick, B. E.; Kreienbrink, K. M.; Dhand, A. P.; Burdick, J. A.; Shields Iv, C. W.; Anseth, K. S.; Bowman, C. N. Multifunctional dithiolane monomers for multi-scale, recyclable light-driven additive manufacturing. Polymer Chemistry 2025. DOI: 10.1039/d5py00199d.
