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1. Sulfur-Containing Polymers Prepared from Fatty Acid-Derived Monomers: Application of Atom-Economical Thiol-ene/Thiol-yne Click Reactions and Inverse Vulcanization Strategies

   https://doi.org/10.3390/suschem1030015  

   Smith, Ashlyn D.; Tennyson, Andrew G.; Smith, Rhett C. (December 2020, Sustainable Chemistry)

   null (Ed.)

   This paper is review with 119 references. Approaches to supplant currently used plastics with materials made from more sustainably-sourced monomers is one of the great contemporary challenges in sustainable chemistry. Fatty acids are attractive candidates as polymer precursors because they can be affordably produced on all inhabited continents, and they are also abundant as underutilized by-products of other industries. In surveying the array of synthetic approaches to convert fatty acids into polymers, those routes that produce organosulfur polymers stand out as being especially attractive from a sustainability standpoint. The first well-explored synthetic approach to fatty acid-derived organosulfur polymers employs the thiol-ene click reaction or the closely-related thiol-yne variation. This approach is high-yielding under mild conditions with up to 100% atom economy and high functional group tolerance. More recently, inverse vulcanization has been employed to access high sulfur-content polymers by the reaction of fatty acid-derived olefins with elemental sulfur. This approach is attractive not only because it is theoretically 100% atom economical but also because elemental sulfur is itself an underutilized by-product of fossil fuel refining. The thiol-ene, inverse vulcanization, and mechanistically-related thiol-yne and classic vulcanization are therefore discussed as promising routes to access polymers and composites from fatty acid-derived precursors. 

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2. Expanding the thiol–X toolbox: photoinitiation and materials application of the acid-catalyzed thiol–ene (ACT) reaction

   https://doi.org/10.1039/D0PY01593H  

   Sutherland, Bryan P.; Kabra, Mukund; Kloxin, Christopher J. (March 2021, Polymer Chemistry)

   null (Ed.)

   The acid-catalyzed thiol–ene reaction (ACT) is a unique thiol–X conjugation strategy that produces S,X-acetal conjugates. Unlike the well-known radical-mediated thiol–ene and anion-mediated thiol-Michael reactions that produce static thioether bonds, acetals provide unique function for various fields such as drug delivery and protecting group chemistries; however, this reaction is relatively underutilized for creating new and unique materials owing to the unexplored reactivity over a broad set of substrates and potential side reactions. Solution-phase studies using a range of thiol and alkene substrates were conducted to evaluate the ACT reaction as a conjugation strategy. Substrates that efficiently undergo cationic polymerizations, such as those containing vinyl functional groups, were found to be highly reactive to thiols in the presence of catalytic amounts of acid. Additionally, sequential initiation of three separate thiol–X reactions (thiol-Michael, ACT, and thiol–ene) was achieved in a one-pot scheme simply by the addition of the appropriate catalyst demonstrating substrate selectivity. Furthermore, photoinitiation of the ACT reaction was achieved for the first time under 470 nm blue light using a novel photochromic photoacid. Finally, using multifunctional monomers, solid-state polymer networks were formed using the ACT reaction producing acetal crosslinks. The presence of S,X-acetal bonds results in an increased glass transition temperature of 20 °C as compared with the same polymeric film polymerized through the radical thiol–ene mechanism. This investigation demonstrates the broad impact of the ACT reaction and expands upon the diverse thiol–X library of conjugation strategies towards the development of novel materials systems. 

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3. Radical-disulfide exchange in thiol–ene–disulfidation polymerizations

   https://doi.org/10.1039/d2py00172a  

   Bongiardina, Nicholas J.; Soars, Shafer M.; Podgorski, Maciej; Bowman, Christopher N. (July 2022, Polymer Chemistry)

   Radical-disulfide exchange reactions in thiol–ene–disulfide networks were evaluated for several structurally distinct thiol and disulfide containing monomers. A new dimercaptopropionate disulfide monomer was introduced to assess how different disulfide moieties affect the exchange process and how the dynamic exchange impacts polymerization. The stress relaxation rate for the disulfides studied herein was highly tunable over a narrow range of network compositions, ranging from 50% relaxation over 10 minutes to complete relaxation over a few seconds, by changing the thiol–disulfide stoichiometry or the disulfide type in the monomer. The thiol/disulfide monomer pair was shown to have significant influence on how radical-disulfide exchange impacts the polymerization rate, where pairing a more stable radical forming thiol ( e.g. an alkyl thiol) with a less stable radical-forming disulfide ( e.g. a dithioglycolate disulfide) reduces the rate of the thiol–ene reaction by over an order of magnitude compared to the case where those two radicals are of the same type. The variations in rates of radical-disulfide exchange with dithioglycolate and dimercaptopropionate disulfides had a significant impact on stress relaxation and polymerization stress, where the stress due to polymerization for the final dimercaptopropionate network was about 20% of the stress in the equivalent dithiogylcolate network under the same conditions. These studies provide a fundamental understanding of this polymerization scheme and enable its implementation in materials design. 

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4. Systematic Investigation of Liquid Crystalline Elastomers Prepared by Thiol–Ene Photopolymerization

   https://doi.org/10.1021/acs.macromol.4c02423  

   Lewis, Kristin L; Phillips, Alexis T; Aye, Sarah S; Chen, Judy C; Hoang, Jonathan D; White, Timothy J (January 2025, Macromolecules)

   Liquid crystalline elastomers (LCEs) prepared via thiol−ene photopolymerization result in homogeneous distribution of molecular weight between cross-links. Numerous prior reports emphasize that LCEs are material actuators that undergo a thermomechanical response associated with an order−disorder transition. However, modern and widely utilized approaches to create LCEs result in heterogeneous networks. Theoretical examination suggests that network heterogeneity and high degrees of cross-linking cause a continuous association of strain with temperature, rather than a first-order, stepwise association. Alternatively, thiol−ene photopolymerization historically yields homogeneous polymers with tailorable cross-link densities. This report extends these prior studies to formulations, which are conducive to LCE preparation. Specifically, this examination copolymerizes a liquid crystalline dialkene mesogen with a tetrathiol cross-linker and dithiol chain extender via a purely thiol−ene polymerization. Notably, this composition is amenable to surface-enforced alignment. This contribution exploits the tunability of thiol−ene photopolymerization to emphasize the influence of cross-linking on the coupling of strain and temperature. 

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5. Photoswitchable Covalent Adaptive Networks Based on Thiol–Ene Elastomers

   https://doi.org/10.1021/acsami.1c22287  

   Cheng, Kezi; Chortos, Alex; Lewis, Jennifer A.; Clarke, David R. (January 2022, ACS Applied Materials & Interfaces)

   ABSTRACT: Covalent adaptive networks combine the advantages of cross-linked elastomers and dynamic bonding in a single system. In this work, we demonstrate a simple one-pot method to prepare thiol−ene elastomers that exhibit reversible photoinduced switching from an elastomeric gel to fluid state. This behavior can be generalized to thiol−ene cross-linked elastomers composed of different backbone chemistries (e.g., polydimethylsiloxane, polyethylene glycol, and polyurethane) and vinyl groups (e.g., allyl, vinyl ether, and acrylate). Photoswitching from the gel to fluid state occurs in seconds upon exposure to UV light and can be repeated over at least 180 cycles. These thiol−ene elastomers also exhibit the ability to heal, remold, and serve as reversible adhesives. KEYWORDS: covalent adaptive network, elastomer chemistry, click chemistry, self-healing, photoresponsive materials, adhesives 

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