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1. Chemical recycling of poly(thiourethane) thermosets enabled by dynamic thiourethane bonds

   https://doi.org/10.1039/d0py01050b  

   Huang, Sijia; Podgórski, Maciej; Han, Xun; Bowman, Christopher N. (November 2020, Polymer Chemistry)

   null (Ed.)

   Recycling of polyurethanes is largely infeasible due to the harsh reprocessing conditions and associated risks of side reactions and degradation whereas polymer networks incorporating dynamic covalent bonds represent an attractive approach to the design of recyclable materials. Here, we report findings on the dynamic nature of thiourethanes, and their application as a new class of recyclable analogs of urethane materials. A series of small molecule experiments was initially conducted to determine the equilibrium constant and exchange reaction kinetic constant for the thiol–isocyanate reaction. Furthermore, incorporating those thiourethane moieties into a cross-linked network resulted in thermoset materials that are readily depolymerized to liquid oligomers. The resultant oligomers can be re-crosslinked to thiourethanes without any loss of performance nor change in mechanical properties (peak stress of 25 MPa with max strain of 200%). Moreover, the recycled thiol oligomers from thiourethane network polymers could potentially be transformed into other materials with mechanical properties that exceed those of the initial, pristine thiourethane materials. Overall, the ease with which these polythiourethanes are polymerized, recycled and reformulated gives a new direction and hope in the design of sustainable polymers. 

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2. 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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3. Facile Synthesis of Diverse and Functional Nanostructures Derived from a Polyhomocysteine-Based Redox-Responsive Block Copolymer

   https://doi.org/10.1021/acsmacrolett.5c00270  

   Bickle, Molly S; Zhao, Bowen; Zhang, Xiao; Fu, Shiwei; Zhang, Fuwu (September 2025, ACS Macro Letters)

   Not AvaStimuli-responsive polypeptides offer unique advantages for biomedical applications due to their biocompatibility, degradability, and structural tunability. In this study, we report the synthesis of innovative redox-responsive polypeptide-based diblock copolymers consisting of functional disulfide-containing homocysteine derivatives and hydrophobic γ-benzyl-l-glutamate segments via sequential ring-opening polymerizations. The polymerization kinetics revealed that the polymerizations were well-controlled with living characteristics, resulting in diblock copolymers PHcy-b-PBLG with narrow molecular weight distributions. The resulting functional-hydrophobic diblock copolymers were further converted to a variety of pendant chains via thiol–disulfide exchange reactions, yielding amphiphilic polymers with tunable surface charges. These disulfide-linked materials readily self-assembled into nanoparticles in aqueous environments with hydrophobic PBLG forming the core and redox-sensitive PHcy forming the shell. The redox-responsive nanoparticles displayed a narrow size distribution, excellent colloidal stability, and excellent biocompatibility. The disulfide bonds within the polymer backbone confer redox sensitivity, allowing potential cleavage in reducing environments. Owing to their tunable surface functionality, redox-responsiveness, and biocompatibility, this platform provides a versatile route to engineer responsive nanostructures for biomedical applications, for example, positively charged nanoparticles toward nucleic acid delivery.ilable 

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4. 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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5. 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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