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1. Command-destruct thermosets via photoinduced thiol-catalyzed β-scission of acyclic benzylidene acetals

   https://doi.org/10.1039/D0PY01006E  

   Walker, William D.; Sandoz, Michael J.; Roland, Samuel; Buster, Tony M.; Newman, John K.; Patton, Derek L. (November 2020, Polymer Chemistry)

   null (Ed.)

   Photoinduced thiol-catalyzed hydrogen abstraction and β-scission of acyclic benzylidene acetals is demonstrated as a new route to “command-destruct” polymer thermosets. Using this approach, we show that poly(thioether acetal) networks synthesized via thiol–ene photopolymerization rapidly degrade to alkyl benzoate byproducts when triggered with light, transitioning from solid to liquid within seconds. The light-driven construction and destruction processes, accessible via distinct differences in kinetics, are readily amenable for photopatterning, additive/subtractive manufacturing and wavelength-selective applications. 

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2. Hydrolytically degradable poly(β-thioether ester ketal) thermosets via radical-mediated thiol–ene photopolymerization

   https://doi.org/10.1039/c9py01082c  

   Alameda, Benjamin M.; Palmer, Travis C.; Sisemore, Jonathan D.; Pierini, Nicholas G.; Patton, Derek L. (October 2019, Polymer Chemistry)

   Thiol–ene photopolymerization was exploited for the synthesis of poly(β-thioether ester ketal) networks capable of undergoing complete degradation under acid and/or basic conditions. Using the design of four novel bisalkene diketal monomers, we demonstrate the ability to tune degradation profiles under acidic conditions with timescales dictated by the structure of the diketal linker, while hydrolysis of the β-thioether ester functionality dominates the degradation profile under basic conditions irrespective of the diketal structure. All four poly(β-thioether ester ketal) networks exhibited degradation behavior characteristic of a surface erosion process. The networks showed mechanical (low modulus) and thermomechanical properties (low T g ) typical of thiol–ene thermosets with minimal influence from the structure of the diketal linkage. To highlight the advantages of endowing a step-growth network with ketal linker chemistry, we demonstrated the ability to recover diketone precursors from the thermoset degradation by-products and recycle these compounds into building blocks for additional thermoset materials. 

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3. Bio-Based Thiol–ene Network Thermosets from Isosorbide and Terpenes

   https://doi.org/10.1021/acsmacrolett.4c00078  

   Prebihalo, Emily A; Johnson, Melody; Reineke, Theresa M (May 2024, ACS Macro Letters)

   Thermoset networks are chemically cross-linked materials that exhibit high heat resistance and mechanical strength; however, the permanently cross-linked system makes end-of-life degradation difficult. Thermosets that are inherently degradable and made from renewably derived starting materials are an underexplored area in sustainable polymer chemistry. Here, we report the synthesis of novel sugar- and terpene-based monomers as the enes in thiol–ene network formation. The resulting networks showed varied mechanical properties depending on the thiol used during cross-linking, ranging from strain-at-breaks of 12 to 200%. Networks with carveol or an isosorbide-based thiol incorporated showed plastic deformation under tensile stress testing, while geraniol-containing networks demonstrated linear stress–strain behavior. The storage modulus at the rubbery plateau was highly dependent on the thiol cross-linker, showing an order of magnitude difference between commercial PETMP, DTT, and synthesized Iso2MC. Thermal degradation temperatures were low for the networks, primarily below 200 °C, and the Tg values ranged from −17 to 31 °C. Networks were rapidly degraded under basic conditions, showing complete degradation after 2 days for nearly all synthesized thermosets. This library demonstrates the range of thermal and mechanical properties that can be targeted using monomers from sugars and terpenes and expands the field of renewably derived and degradable thermoset network materials. 

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4. Biobased and degradable thiol–ene networks from levoglucosan for sustainable 3D printing

   https://doi.org/10.1039/D2GC04185E  

   Porwal, Mayuri K.; Hausladen, Matthew M.; Ellison, Christopher J.; Reineke, Theresa M. (January 2023, Green Chemistry)

   Levoglucosan is a renewable chemical obtained in high yields from pyrolysis of cellulosic biomass, which offers rich functionality for synthetic modification and crosslinking. Here, we report the facile and scalable synthesis of a family of biobased networks from triallyl levoglucosan and multifunctional thiols via UV-initiated thiol–ene click chemistry. The multifunctional thiols utilized in this study can also be sourced from renewable feedstocks, leading to overall high bio-based content of the synthesized levoglucosan networks. The thermomechanical and hydrolytic degradation properties of the resultant networks are tailored based on the type and stoichiometric ratio of thiol crosslinker employed. The Young's modulus and glass transition temperature of levoglucosan-based networks are tunable over the wide ranges of 3.3 MPa to 14.5 MPa and −19.4 °C to 6.9 °C, respectively. The levoglucosan-based thermosets exhibit excellent thermal stability with Td,10% > 305 °C for all networks. The suitability of these resin formulations for extrusion-based 3D printing was illustrated using a UV-assisted direct ink write (DIW) system creating 3D printed parts with excellent fidelity. Hydrolytic degradation of these 3D printed parts via ester hydrolysis demonstrated that levoglucosan-based resins are excellent candidates for sustainable rapid prototyping and mass production applications. Overall, this work displays the utility of levoglucosan as a renewable platform chemical that enables access to tailored thermosets important in applications ranging from 3D printing to biomaterials. 

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5. Degradable polyanhydride networks derived from itaconic acid

   https://doi.org/10.1039/D0PY01388A  

   Sajjad, Hussnain; Lillie, Leon M.; Lau, C. Maggie; Ellison, Christopher J.; Tolman, William B.; Reineke, Theresa M. (February 2021, Polymer Chemistry)

   null (Ed.)

   The development of tunable and degradable crosslinked-polyanhydride networks from renewably derived itaconic anhydrides and multifunctional thiols is presented. Itaconic acid was initially converted to ethyl itaconic anhydride and isoamyl itaconic anhydride via a two-step synthetic procedure on hundred-gram scale with minimal purification. Dinorbornene-functionalized derivatives were prepared via cycloaddition chemistry, and photoinitiated thiol–ene polymerization reactions were explored using commercially available tetra- and hexa-functional thiols, all using solvent-free syntheses. The thiol–ene reaction kinetics of different monomer compositions were characterized by real-time Fourier transform infrared (RT-FTIR) spectroscopy, with the norbornene functionalized derivatives exhibiting the highest reactivity towards thiol–ene photopolymerizations. The thermal and mechanical characteristics of the thermosets were analyzed and the viscoelastic behavior was investigated by dynamic mechanical analysis to understand the influence of the ester functionality and choice of crosslinker on the material properties. The anhydride backbone was found to be susceptible to controlled degradation under physiologically-(phosphate-buffered saline) and environmentally-relevant (artificial seawater) testing conditions over a period of 60 days at 50 °C. This work demonstrates that itaconic acid may be a useful feedstock in the generation of degradable polyanhydride networks via thiol–ene photopolymerization. 

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