Delayed Thiol-Epoxy Photopolymerization: A General and Effective Strategy to Prepare Thick Composites

Base-catalyzed thiol-epoxy reactions: Energetic and kinetic evaluations

The mechanism of the base-catalyzed thiol-epoxide stage of the thiol-ene/thiol-epoxide curing process was investigated using quantum chemical tools. This study searched for conventional tertiary amines with low to medium basicity as initiators to control reaction rates and tailor industrial applications. Challenges arise from the stronger basicity of initiators, leading to an uncontrollable and short curing application period. This problem was put into quantitative data through kinetic and energetic studies for the first time. Furthermore, the base catalyst formulation of curing agents distinctively has a short pot life. More reactivity of terminal epoxy rings than internal ones was highlighted for the curing agents. It was revealed that the reactivity augments during the curing process while environmental polarity changes from higher to lower, which is one of the reasons that triggers an autocatalytic phenomenon. Electronegative atoms like fluorine on thiols significantly decrease the nucleophilicity of formed thiolate anion, enabling longer curing application.

Computational Study on Radical-Mediated Thiol-Epoxy Reactions

Radical-mediated thiol-epoxy reactions were elucidated for analyzing the overlap problem of the thiol-ene/thiol-epoxy systems using computational approaches. Nine epoxy model molecules were evaluated to mimic the chemical structures and reactivity of some industrial epoxy molecules. Modeling reaction mechanisms was conducted through density functional theory (DFT) calculations using the M06-2X/6-31+G(d,p) level at 1.0 atm and 298.15 K. An analog thiol-ene mechanism was proposed for radical-mediated thiol-epoxide reactions. Unlike the thiol-ene reactions, the addition reaction to epoxides is relatively slow (rate constants <10-4 M-1 s-1). However, the chain transfer, which paves the way for the overlapping of dual curing systems, is quite fast (rate constants >101 M-1 s-1). High stability of thiyl radicals, epoxy ring strain, and the instability of formed alkoxy radical from addition reaction were emphasized as the main driving forces for the reaction energetics and kinetics. Control of temperature and using certain thiols are strongly recommended to avoid curing step overlap based on the findings in this study.

Synthesis and Biodegradability of Tartaric Acid-Based Poly(ester-thioether)s via Thiol-Ene Click Polymerization

Using scandium triflate [Sc(OTf)₃] as a catalyst, chemoselective esterification of tartaric acids by 3-butene-1-ol was performed, and we produced three dialkene monomers: l-di(3-butenyl) tartrate (BTA), d-BTA, and meso-BTA. Thiol-ene polyaddition of these dialkenyl tartrates and dithiols including 1,2-ethanedithiol (ED), ethylene bis(thioglycolate) (EBTG), and d,l-dithiothreitol (DTT) proceeded in toluene at 70 °C under nitrogen to give tartrate-containing poly(ester-thioether)s (M_n, (4.2-9.0) × 10³; molecular weight distribution (M_w/M_n), 1.6-2.5). In differential scanning calorimetry, the poly(ester-thioether)s showed single T_gs between -25 and -8 °C. In biochemical oxygen demand (BOD) tests using activated sludge, poly(l-BTA-alt-EBTG) and poly(l-BTA-alt-ED) showed 32 and 8% biodegradability, which is comparable to that of similar l-malate-containing poly(ester-thioether)s (23 and 13% biodegradation, respectively). Notably, we observed enantio and diastereo effects on biodegradation because poly(l-BTA-alt-EBTG), poly(d-BTA-alt-EBTG), and poly(meso-BTA-alt- EBTG) showed different degradation behaviors during the biodegradation test (BOD/theoretical oxygen demand (TOD) values after 28 days, 32, 70, and 43%, respectively). Our findings provide insights into the design of biomass-based biodegradable polymers containing chiral centers.

Photobase-catalysed anionic thiol-epoxy click photopolymerization under NIR irradiation: from deep curing to shape memory

A photobase generator absorbing upconversion fluorescence can efficiently catalyze anionic thiol-epoxy click photopolymerization under 980 nm NIR light irradiation.

Streamlined concept towards spatially resolved photoactivation of dynamic transesterification in vitrimeric polymers by applying thermally stable photolatent bases

Through a well targeted design, vitrimers are able to reorganise their three-dimensional covalently crosslinked network structure by associative exchange reactions when the so-called topology freezing transition temperature (Tv) is exceeded....

Photoclick Chemistry: A Bright Idea

At its basic conceptualization, photoclick chemistry embodies a collection of click reactions that are performed via the application of light. The emergence of this concept has had diverse impact over a broad range of chemical and biological research due to the spatiotemporal control, high selectivity, and excellent product yields afforded by the combination of light and click chemistry. While the reactions designated as "photoclick" have many important features in common, each has its own particular combination of advantages and shortcomings. A more extensive realization of the potential of this chemistry requires a broader understanding of the physical and chemical characteristics of the specific reactions. This review discusses the features of the most frequently employed photoclick reactions reported in the literature: photomediated azide-alkyne cycloadditions, other 1,3-dipolarcycloadditions, Diels-Alder and inverse electron demand Diels-Alder additions, radical alternating addition chain transfer additions, and nucleophilic additions. Applications of these reactions in a variety of chemical syntheses, materials chemistry, and biological contexts are surveyed, with particular attention paid to the respective strengths and limitations of each reaction and how that reaction benefits from its combination with light. Finally, challenges to broader employment of these reactions are discussed, along with strategies and opportunities to mitigate such obstacles.

Introduction of Photolatent Bases for Locally Controlling Dynamic Exchange Reactions in Thermo-Activated Vitrimers

Vitrimers exhibit a covalently crosslinked network structure, as is characteristic of classic thermosetting polymers. However, they are capable of rearranging their network topology by thermo-activated associative exchange reactions when the topology freezing transition temperature (T_v ) is exceeded. Despite the vast number of developed vitrimers, there is a serious lack of methods that enable a (spatially) controlled onset of these rearrangement reactions above T_v . Herein, we highlight the localized release of the efficient transesterification catalyst 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) by the UV-induced cleavage of a photolatent base within a covalently crosslinked thiol-epoxy network. Demonstrated with stress relaxation measurements conducted well above the network's T_v , only the controlled release of TBD facilitates the immediate onset of transesterification in terms of a viscoelastic flow. Moreover, the spatially resolved UV-mediated photoactivation of vitrimeric properties is confirmed by permanent shape changes induced locally in the material.

Shape Memory Poly(β-hydroxythioether) Foams for Oil Remediation in Aquatic Environments

Shape memory poly(β-hydroxythioether) foams were produced using organobase catalyzed reactions between epoxide and thiol monomers, allowing for the rapid formation of porous media within approximately 5 min, confirmed using both rheology and physical foam blowing. The porous materials possess ultralow densities (0.022 g × cm^-3) and gel fractions of approximately 93%. Thermomechanical characterizations of the materials revealed glass transition temperatures tunable from approximately 50 to 100 °C, elastic moduli of approximately 2 kPa, and complete strain recovery upon heating of the sample above its glass transition temperature. The foams were characterized for their ability to take up oil from an aqueous multilayered ideal environment, revealing more than 2000% mass of oil (relative to the foam mass) could be collected. Importantly, while post-fabrication functionalization was possible with isocyanate chemistry followed by addition of hexadecanethiol or 3,3-bis(hexadecylthio)propan-1-ol, the oil collection efficiency of the system was not significantly enhanced, indicating that these materials, as porous media, possess unique attributes that make them appealing for environmental remediation without the need for costly modifications or manipulations.