Chiara Di Mauro

Samuel Malburet

Alain Graillot

Alice Mija

Recyclable, Repairable, and Reshapable (3R) Thermoset Materials with Shape Memory Properties from Bio-Based Epoxidized Vegetable Oils

Keywords: epoxidized vegetable oils, dynamic hardener, epoxy content, recyclability, reshapability, repairability, shape memory

The preparation of thermosets based on epoxidized vegetable oils (EVOs) involved a peculiar attention in recent years; however, most of them cannot be recycled once cross-linked. In the present work, epoxy thermosetting resins like-vitrimers with dynamic disulfide covalent bonds were prepared by copolymerizing twelve EVOs with 2,2′-dithiodibenzoic acid, as hardener. Here, we show for the first time the reprocessability, repairability, and recyclability properties of EVOs thermosets. The 3R abilities were evaluated in correlation with the EVO epoxy contents, which influence the final thermo-mechanical properties of the recycled material. The virgin versus recycled materials' comparison was studied by FT-IR, DSC, TGA, and DMA, also comparing their swelling ability and high gel content. The study investigates, in addition, the excellent shape memory properties of the reprocessed EVOs/disulfide materials.

■ INTRODUCTION

Global plastics production was around 335 million tons in 2016 producing 8.4 million tons of plastic waste. Only 2% from the total plastic waste worldwide was recycled, whereas in Europe the recycling was around 31%. [START_REF]Plastics-the Facts[END_REF][START_REF] Rahimi | Chemical recycling of waste plastics for new materials production[END_REF] Among the European Union countries, the waste management strategies vary significantly, [START_REF] Okan | Current approaches to waste polymer utilization and minimization: a review[END_REF] unfortunately conducting to large landfill accumulations with harmful effects on water, land, and all living species. To deal with these problems, in 2018, the European Commission promoted the design and production of plastic materials with repairability and recycling capacity and also the circular economy strategy. [START_REF] Strategy | A European strategy for plastics in a circular economy[END_REF] A new class of thermoset networks including dynamic covalent bonds has attracted special attention in recent years. These thermoset materials renamed covalent adaptive network (CAN) polymers by Bowman's group [START_REF] Bowman | Covalent Adaptable Networks: Reversible Bond Structures Incorporated in Polymer Networks[END_REF][START_REF] Kloxin | Covalent Adaptable Networks (CANs): A Unique Paradigm in Crosslinked Polymers[END_REF] demonstrated recycling capabilities as well as unchanged properties after reprocessing. This type of bond undergoes splitting/reforming exchanges under external stimuli such as temperature, light, pH, or pressure. There are a variety of dynamic covalent reactions allowing the CANs: transesterification, [START_REF] Rowan | Dynamic Covalent Chemistry[END_REF][START_REF] Montarnal | Silica-like malleable materials from permanent organic networks[END_REF][START_REF] Corbett | Dynamic Combinatorial Chemistry[END_REF][START_REF] Altuna | Epoxy Vitrimers: The Effect of Transesterification Reactions on the Network Structure[END_REF] Michael reactions, [START_REF] Shi | Reversible Michael addition of thiols as a new tool for dynamic combinatorial chemistry[END_REF] disulfides exchange, [START_REF] Jin | Recent advances in dynamic covalent chemistry[END_REF][START_REF] Rekondo | Catalyst-free room-temperature selfhealing elastomers based on aromatic disulfide metathesis[END_REF] Diels-Alder reactions, [START_REF] Sanyal | Diels-Alder Cycloaddition-Cycloreversion: A Powerful Combo in Materials Design[END_REF] or even physical bond exchange as the hydrogen-bonds exchange. [START_REF] Kolomiets | Double dynamers: molecular and supramolecular double dynamic polymers[END_REF][START_REF] Calama | Libraries of non-covalent hydrogen-bonded assemblies; combinatorial synthesis of supramolecular systems[END_REF] Based on transesterification reactions, Leibler et al. [START_REF] Montarnal | Silica-like malleable materials from permanent organic networks[END_REF] defined ″vitrimers″ as materials remaining cross-linked at high temperatures, showing a gradual change in viscosity [START_REF] Denissen | Vitrimers: permanent organic networks with glass-like fluidity[END_REF][START_REF] Jurowska | Vitrimers-the miracle polymer materials combining the properties of glass and plastic[END_REF] but concomitantly demonstrating fluidity. [START_REF] Denissen | Vitrimers: permanent organic networks with glass-like fluidity[END_REF][START_REF] Capelot | Catalytic Control of the Vitrimer Glass Transition[END_REF] These materials, like the common thermosets, have shown high resistance but also with recycling ability. The first vitrimers were synthesized through the reaction between diglycidyl ether of bisphenol A (DGEBA) and Pripol 1040, a mixture of C 18 fatty acids derivatives, containing dimers and trimers, in the presence of transesterification catalysts. [START_REF] Capelot | Catalytic Control of the Vitrimer Glass Transition[END_REF] Following this idea, several researches introduced a series of vitrimer systems formed by the incorporation of different exchangeable bonds, such as disulfide linkage. [START_REF] Rekondo | Catalyst-free room-temperature selfhealing elastomers based on aromatic disulfide metathesis[END_REF][START_REF] Lei | Room-Temperature Self-Healable and Remoldable Cross-linked Polymer Based on the Dynamic Exchange of Disulfide Bonds[END_REF][START_REF] Tsuruoka | Fusion of Different Crosslinked Polymers Based on Dynamic Disulfide Exchange[END_REF] Odriozola et al. [START_REF] De Luzuriaga | Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairayble and recyclable fiber-reinforced thermoset composites[END_REF] synthesized a reinforced polymeric composite made with an epoxy resin based on DGEBA and 4-AFD (4-aminophenyl disulfide). The obtained material showed excellent mechanical properties and also 3R functionality, i.e. recyclability, repairability, and reshapability. The relentless need for the use of sustainable materials has led to the growth of bio-based vitrimers consisting of isosorbide-derived epoxy (IS-EPO) cured with aromatic diamines containing disulfide bonds 4-AFD (4aminophenyl disulfide). This resin, devised by Ma et al., [START_REF] Ma | Bio-based epoxy vitrimers: Reprocessibility, controllable shape memory, and degradability[END_REF] showed shape memory properties and high degradability in alkaline solutions. Zhang et al. [START_REF] Zhou | Preparation of self-healing, recyclable epoxy resins and lowelectrical resistance composites based on double-disulfide bond exchange[END_REF] reacted a multifunctional epoxy monomer, based on bis(4-glycidyloxyphenyl) disulfide with 4-AFD, without a catalyst, producing a thermoset with short relaxation times (9 s) and a T g above 130 °C. The obtained thermosets were reprocessed, and the authors showed that the chemical and thermo-mechanical properties did not drop after three recycling cycles. Furthermore, the development of vitrimers focused on creating networks based on a double recycling synergy: the disulfides metathesis and the carboxylate transesterifications. Imbernon et al. [START_REF] Imbernon | Chemically crosslinked yet reprocessable epoxidized natural rubber via thermo-activated disulfide rearrangements[END_REF] cross-linked the ENR (epoxidized natural rubber) with an aliphatic dicarboxylic acid containing disulfides (dithiodibutyric acid), showing how the resin is able to reorganize at temperatures above 150 °C, recovering most of its mechanical properties after reprocessing. Similarly, Chen et al. [START_REF] Chen | Rapid Stress Relaxation and Moderate Temperature of Malleability Enabled by the Synergy of Disulfide Metathesis and Carboxylate Transesterification in Epoxy Vitrimers[END_REF] prepared an epoxy vitrimer by curing DGEBA with dithiodibutyric acid.

Our team reported the cross-linking study of two reference epoxidized vegetable oils, linseed oil (ELO) and soybean oil (ESO), with an aromatic carboxylic acid with disulfide bonds, by a highly selective copolymerization reaction. [START_REF] Tran | Chemical Reactivity and the Influence of Initiators on the Epoxidized Vegetable Oil/Dicarboxylic Acid System[END_REF][START_REF] Mauro | Building thermally and chemically reversible covalent bonds in vegetable oils based epoxy thermosets. Influence of epoxy-hardener ratio to promote recyclability[END_REF] The 10 times reprocessed thermosets maintained almost unchanged their thermo-mechanical properties and retained a T g value around 80 °C. [START_REF] Mauro | Enhancing the Recyclability of a Vegetable Oil-Based Epoxy Thermoset through Initiator Influence[END_REF] In another study, 12 cross-linked bio-based networks were prepared by reacting 12 EVOs with 2,2′dithiodibenzoic acid (DTBA). [START_REF] Mauro | Sustainable series of new epoxidized vegetable oils-based thermosets with chemical recycling properties[END_REF] In this work, we investigate their 3R ability (recyclability, repairability and reshapability) and shape memory behavior. Even if several studies have been conducted on polymers with 3R ability, [START_REF] De Luzuriaga | Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairayble and recyclable fiber-reinforced thermoset composites[END_REF][START_REF] Azcune | Aromatic disulfide crosslinks in polymer systems: Self-healing, reprocessability, recyclability and more[END_REF][START_REF] Casatorre | Reworkable crosslinked polythiourethanes with intrinsic injectability, repairability and recyclability[END_REF] to our knowledge, this is the first work on a large series of vegetable oil-based materials. FTIR, DSC, TGA, and DMA were used as techniques for the comparison between virgin EVO-based thermosets and recycled ones. A correlation was done between the swelling ability, the gel content, and the epoxy content of the EVOs monomers. Furthermore, the properties of the recycled thermosets and also the shape memory properties were explored.

■ MATERIALS AND METHODS

Materials. The epoxidized linseed and soybean oils, ELO (A) and ESO (B), were provided by Valtris Chemicals (England). The 10 new EVOs (epoxidized karanja oil, EKRNO; epoxidized castor oil, ECO; epoxidized St John's Wort oil, EJSWO; epoxidized peanut oil, EPO; epoxidized rapeseed oil, ERPO; epoxidized rose hip seed oil, ERHO; epoxidized safflower oil, ESFO; epoxidized grapeseed oil, EGRO; epoxidized camelina oil, ECMO; epoxidized hemp oil, EHO; and epoxidized perilla oil, EPLO) were provided by SPECIFIC POLYMERS (France) as references SP-3S-30-006.

These EVOs can be classified in three categories, in function of their nature and content in unsaturated fatty acids: i/category 1: EVOs from VO containing mainly mono-unsaturated fatty acids: EKRNO, ECO, ESJWO, EPO, and ERPO; ii/category 2: EVOs from VO containing mainly di-unsaturated fatty acids: ESO, ERHO, ESFO, EGRO, and EHO; and iii/category 3: EVOs from VO containing mainly tri-unsaturated fatty acids ECMO, ELO, and EPLO. The 2,2′dithiodibenzoic acid (DTBA) 95% hardener (E) and the imidazole (IM) initiator (99%) (F) were purchased from Sigma-Aldrich and were used as received. The chemical structures, reaction mechanism, and physico-chemical characteristics of the reagents are given in Table S1 and Figures S1 andS2.

Samples Preparation. The epoxy/hardener ratio was fixed at 1:1 according to Dusek et al. [START_REF] Mateǰka | Acid curing of epoxy resins. A comparison between the polymerization of diepoxide-diacid and monoepoxide-cyclic anhydride systems[END_REF] The epoxy monomer was heated at 80 °C, and consequently, the initiator was added at 1 wt %. Subsequently, the hardener was added and stirred at the same temperature for 10 min, placed into a silicone mold, and cured in oven. From the DSC study of reactivity presented in Table S2, the curing and post-curing protocols for the EVO thermosets were determined. [START_REF] Mauro | Sustainable series of new epoxidized vegetable oils-based thermosets with chemical recycling properties[END_REF] Recycling, Repairing, and Reshaping Procedures. The recycling was made by grinding the cross-linked thermoset. The obtained powder was pressed and heated between two Kapton films using a CARVER manual press. Temperature, pressure, and duration of the recycling were optimized and adapted to each resin: starting from 120 °C and 0.3 tons until 170 °C and 2 tons for high epoxy content (Table 1). Recycled specimens with rectangular dimensions of 30 × 7 × 2mm 3 were prepared for the thermo-mechanical characterizations. For the repairing and reshaping tests, the recycled EVO specimens obtained using the parameters given in Table 1 were prepared with dimensions 50 × 10 × 1 mm 3 .

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To demonstrate the repairing ability of thermosets, scratches have been produced on the reprocessed thermoset surface. The so-injured specimens were heated according to the recycling protocol (Table 1) for 5 min between the plates of the CARVER manual press, without applying any pressure.

The reshaping process was performed in a zigzag-shaped mold following the same conditions as for the repairing.

Analytical Methods. Differential Scanning Calorimetry (DSC). The thermosets glass transitions were evaluated with a Mettler-Toledo DSC 3 instrument equipped with STAR software. The DSC studies were performed in the temperature range of 80 to 180 °C, at a 10 °C/min heating rate. The samples (around 5-7 mg) were placed into 40 μL aluminum crucibles.

Fourier-Transform Infrared Spectroscopy (FT-IR). FT-IR analyses were done using a Thermo Scientific Nicolet iS50 FT-IR spectrometer with a deuterated L-aniline-doped triglycine sulfate (DLaTGS) detector in attenuated total reflectance (ATR) mode at 32 scans and 4 cm -1 resolution. The data were analyzed using OMNIC software, and the conversion percentage of the functional group was determined by eq 1:% =

where (A 0 ) and (A t ) are the area of absorbance peaks calculated at the initial time at the end of the curing protocol. The peak at 823 cm -1 was assigned to the oxirane C-O groups, while that at 1586 cm -1 is the reference band which belongs to the δ C-C of the aromatic signal from the DTBA hardener. Thermogravimetric Analyses (TGA). TGA measurements were performed with the help of a Mettler-Toledo TGA 2. To characterize the thermal stability, the samples were heated under air flow, 50 mL• min -1 at 10 °C•min -1 from 25 to 1000 °C.

The statistic heat-resistant index temperature (T s ), a characteristic of thermal stability, was calculated with eq 2:

T T T T 0.49 0.6 ( ) s 5% 30% 5% = •[ + • - ] (2)
where T 5% and T 30% are the temperatures at 5 and 30% weight loss, respectively. Dynamic Mechanical Analyses (DMA). DMA were carried out on a Mettler-Toledo DMA 1 instrument, equipped with STAR software, operated using the tension method. Elastic modulus values (E′) and damping factors (tan δ) were measured at a heating rate of 3 °C• min -1 from -80 to 170 °C and 1.0 Hz frequency. The relaxation, associated to the glass transition, was assigned at the maximum of the damping factor (tan δ = E″/E′).

Cross-linking Density. Cross-linking density of virgin and recycled thermosets was calculated from the elastic modulus value in the rubbery state by eq 3, according to Flory's theory: 34

E RT 3 ν = ′ ( 3
)
where E′ is the storage modulus of the thermoset in the rubbery plateau region at T g + 50 °C, R is the gas constant, and T is the absolute temperature in Kelvin.

Shape Memory. Shape memory properties were analyzed using rectangular specimens of recycled resin (10 × 5 × 1 mm 3 ). The specimen was heated at 90 °C for about 10 s, and then twisted manually to fix the temporary shape until room temperature is reached. Then, the helical sample was immersed in hot water (∼90 °C) to observe the shape recovery. A second permanent shape was tested: the sample was heated to 150 °C for 20 min, and by the help of a clamp, a helix shape was formed (second permanent shape). At the end the protocol, the twisted conformation was fixed at room temperature. Consequently, the specimen was heated at 90 °C for 10 s and deformed to a "C" shape and the temporary shape was fixed by cooling the sample at room temperature. Finally, the specimen was immersed in hot water to recover the second permanent shape.

Solvent Stability and Swelling Ratio (Q). The tests were studied in toluene for 48 h at room temperature in order to confirm the resistance of the reprocessed samples in the solvent. Specimens of dimensions 10 × 10 × 2 mm 3 were used to perform the test. The swelling ratio (Q m ) was calculated following eq 4:

Q w w w m s d s = - (4)
where w s and w d are the swelled mass and the swelled dry mass at equilibrium. Gel Content (GC %). Gel content % values were determined by immersing the samples (10 × 10 × 2 mm 3 ) in toluene. After 48 h, the insoluble fraction was removed and dried under vacuum at 50 °C. The GC % was calculated as follows:

w w GC% 0 = (5)
where w 0 and w are the initial mass and that after drying.

■ RESULTS AND DISCUSSIONS

The thermoset resins once cross-linked cannot be recycled.

Vitrimers, due to their nature, can undergo reversible crosslinks through bond cleavage and reformation/exchange, guaranteeing a future benefit from economic and environmental point of view. Leibler et al. [START_REF] Capelot | Metal-Catalyzed Transesterification for Healing and Assembling of Thermosets[END_REF] displayed this behavior in the case of DGEBA/acid or DGEBA/anhydride networks using transesterification catalysts (zinc acetate (Zn(OAc) 2 ), triazabicyclodecene (TBD), or triphenylphosphine (PPh 3 )). Other authors, as Odriozola et al. [START_REF] Rekondo | Catalyst-free room-temperature selfhealing elastomers based on aromatic disulfide metathesis[END_REF][START_REF] De Luzuriaga | Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairayble and recyclable fiber-reinforced thermoset composites[END_REF][START_REF] Azcune | Aromatic disulfide crosslinks in polymer systems: Self-healing, reprocessability, recyclability and more[END_REF] revealed the possibility to obtain reprocessed thermoset and elastomer materials made with disulfide cross-linkers. Apart from their work, a few studies were dedicated on the properties of the recycled materials, especially those of EVO thermosets. As previously shown, [START_REF] Tran | Chemical Reactivity and the Influence of Initiators on the Epoxidized Vegetable Oil/Dicarboxylic Acid System[END_REF][START_REF] Mauro | Building thermally and chemically reversible covalent bonds in vegetable oils based epoxy thermosets. Influence of epoxy-hardener ratio to promote recyclability[END_REF] the bio-based thermosets prepared with ELO or ESO and dicarboxylic acid as a dynamic disulfide cross-linker, can be used to produce innovative networks with recycling abilities due to the synergy of two recycling mechanisms: disulfide exchange and transesterification reactions (Figure 1). Several authors [START_REF] Denissen | Vitrimers: permanent organic networks with glass-like fluidity[END_REF][START_REF] Capelot | Catalytic Control of the Vitrimer Glass Transition[END_REF][START_REF] De Luzuriaga | Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairayble and recyclable fiber-reinforced thermoset composites[END_REF] proved that the recyclability requires temperatures above T g , usually higher

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than the topology freezing transition temperature (T v ), to allow the network to flow and to guarantee a successful recycling.

The processing conditions for the synthesis of thermosetting resins are in strong relationship with their epoxy content as shown in Table 1 and schematized in Figure 2 for the series of 12 EVOs resins. As presented in Table 1, thermosets from category 1 need milder recycling conditions due to a lower number of cross-linking nodes that are spaced, correlated to lower T g , and so favoring an easier rearrangement during the reprocessing protocol. In contrast, in the EVOs with higher epoxy content, as for the thermosets derived from category 3, the high cross-link density required stronger conditions to recombine the network.

The temperature, pressure and time necessary to obtain recycled thermosets were compared with those applied for DGEBA dynamic networks. Odriozola et al. [START_REF] De Luzuriaga | Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairayble and recyclable fiber-reinforced thermoset composites[END_REF] used a reprocessing at 200 °C, 100 bars for 5 min for the DGEBA/ 4-AFD vitrimer. The same cross-linker, 4-AFD, was used by Ma et al. [START_REF] Ma | Bio-based epoxy vitrimers: Reprocessibility, controllable shape memory, and degradability[END_REF] to test the recyclability of IS-EPO (isosorbidederived epoxy) by heating the samples at 100 °C for 1 h (no pressure specification). Zhang et al. [START_REF] Zhou | Preparation of self-healing, recyclable epoxy resins and lowelectrical resistance composites based on double-disulfide bond exchange[END_REF] reprocessed the bis(4glycidyloxyphenyl) disulfide/4-AFD thermoset at 180 °C for 20 min and 200 bars. Chen et al. [START_REF] Chen | Rapid Stress Relaxation and Moderate Temperature of Malleability Enabled by the Synergy of Disulfide Metathesis and Carboxylate Transesterification in Epoxy Vitrimers[END_REF] showed that the system DGEBA/DTDA (4,4′-dithiodibutyric acid) catalyzed by 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD) produced a vitrimer that can be recycled at 100 °C for 1 h with recovering of thermomechanical properties at the end of the four cycles. The aliphatic chains of DTDA and the presence of DGEBA were reported as responsible for the milder recycling conditions compared to those used by Leibler et al. [START_REF] Imbernon | Chemically crosslinked yet reprocessable epoxidized natural rubber via thermo-activated disulfide rearrangements[END_REF] for the recycling of ENR (epoxidized natural rubber)/DTDA vitrimer at 180 °C for 40 min and 8 tons of pressure.

■ MECHANICAL RECYCLING

Investigation of the Recycled Thermosets. To investigate how the recycling affected the structure of the thermoset network, FTIR comparative studies were performed on uncured thermosetting resins, on the virgin thermoset materials and the recycled one. Starting with the FTIR results, the copolymerization conversions were calculated according to eq 1. Figure S3 and Table S3 present and summarize the comparison for all the studied materials.

The results presented in Figure S3 highlight that the absorption of epoxy functions located at 823 and 847 cm -1 disappeared in both virgin and recycled thermosets. The FTIR spectra do not show sensible difference between the virgin and recycled thermosets.

Moreover, as shown in Figure 3, ELO thermosets display high copolymerization conversion for both the virgin and the recycled materials (with 99 and 98% of conversion). Thereafter, the virgin and recycled EKRNO and EPLO thermosets show a constant conversion (before and after the reprocessing), while the EHO thermosets present the lowest copolymerization percentage, decreasing after the recycling from 78 to 75%. The other reprocessed thermosets show conversions superior to 90%, with a small drop after recycling: EJSWO (from 97 to 96%), ERPO (from 94 to 92%), and EGRO (from 92 to 91%). Figure 3 summarizes the results of calculated epoxy conversions in function of the kind of EVOs and their epoxy content.

The reprocessed thermosets were analyzed also by dynamic DSC studies to determine the glass transition before and after recycling to evaluate how the recycling affected the materials properties. Figure 4A and Table S4 display the obtained and compared results. We can notice a small decrease of T g values after the recycling process in the majority of EVO thermosets, except the ERPO/DTBA system that kept the same glass transition value after the reprocessing. The most affected is the system with perilla oil, EPLO/DTBA resin, and the thermosets of category 3, showing a decreasing of T g from 91 to 84 °C. Hu et al. [START_REF] Ma | Bio-based epoxy vitrimers: Reprocessibility, controllable shape memory, and degradability[END_REF] reported also a decrease of the T g for the isosorbidederived epoxy systems cured with 4,4′-disulfanediyldianiline attributed to some loss in disulfide linkages. In our case, the decrease of T g can be associated to a higher amount of DTBA (so in disulfide linkages and ester linkages) in the formulation with EPLO that has the higher epoxy content. In consequence, it is possible that not all the dynamic bonds were reformed during the reprocessing due to the high cross-link density of this category of EVOs. The other reprocessed resins present a reduction with only 1-2 °C, as EKRNO and ESJWO (from 17 to 16 °C), ESO (from 45 to 43 °C), EGRO (from 50 to 48 °C), or ECO (from 30 to 29 °C). This result can be attributed to the lower network reticulation facilitating the bond reorganization and therefore the maintenance of mechanical properties after recycling. Similar results were obtained by Chen et al. [START_REF] Chen | Castor oil derived poly(urethane urea) networks with reprocessibility and enhanced mechanical properties[END_REF] for castor oil/4-aminophenyl disulfide polyurethane networks.

The analysis of thermo-mechanical behavior, performed by DMA, shows that each recycled sample (Figure S5) displays a unique and clear α transition. In Figure S4, the comparison of

tan δ(°C) E′ G (MPa) a E′ R (MPa) b ν(mmol•cm -3 ) c EVO/DTBA V R V R V R V R EKRNO 34 ± 1 2 8±

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the tan δ and storage moduli, E′, for virgin and recycled resins are illustrated and the results are displayed in Table 2. Figure 4B confronts the values of tan δ corroborated also with the epoxy content. From this figure, we can notice that the tan δ values of recycled samples do not follow the same trend with the corresponding T g results presented in Figure 4A. As an example, the tan δ of reprocessed ESJWO, EPO, ERPO, ESO, and ESFO increase after the recycling procedure, consistent with the results obtained for the ELO reference system. [START_REF] Mauro | Enhancing the Recyclability of a Vegetable Oil-Based Epoxy Thermoset through Initiator Influence[END_REF] In contrast, surprisingly, the other reprocessed resins exhibit a drop of tan δ values; the more evident decrease being measured in the case of EPLO recycled resin versus virgin resin. Moreover, the amplitudes of the damping factor show a sensible increase after the recycling, especially for reprocessed resins with higher epoxy content. Also, all the reprocessed resins exhibited higher E′ in the glassy state than the virgin thermosets. Higher moduli in both the glassy state and at room temperature were observed by Memon et al. [START_REF] Memon | Vanillin-Based Epoxy Vitrimer with High Performance and Closed-Loop Recyclability[END_REF] for the recycled vitrimers obtained by bio-based vanillin diglycidyl ether and petroleum-based counterpart (DGEBA) cross-linked with isophorone diamine (IPDA) in the presence of ethylene glycol diglycidyl ether as reactive diluent. The authors explained that the degree of hardening of the two epoxy networks would increase slightly after heat press treatment. Recycled EVO thermosets of category 3 exhibited, in general, higher moduli than the virgin resins. The analysis of the cross-linking density applied using Flory's theory [START_REF] Flory | Principles of polymer chemistry[END_REF] shows that after the reprocessing procedure, the cross-link densities of EVOs with higher epoxy content suffer a reduction, in contrast with EVOs with low epoxy content that have a constant or even increased crosslinking. As example, EPLO virgin resin has the higher crosslinking and performance properties, showing after recycling a loss of properties and a decrease in cross-linking from 3.0 to 0.82 mmol•cm -3 , according to the previous considerations regarding the difficulty of recycling high reticulated thermosets.

In contrast, some recycled EVO thermosets, such as ECO, ESJWO, EPO, ESO, and ERHO, exhibited higher values compared with the virgin materials. This result can be explained probably by the fact that under reprocessing conditions (temperature and pressure) an easier rearrangement of the network's structure occurred favoring the increasing in cross-links density.

The thermal stabilities were investigated by thermogravimetric analysis under an air atmosphere by comparing virgin and recycled EVOs resins. The obtained results with the temperature at 5% weight loss (T 5% ) are displayed in Figure 5 and summarized in Table 3 and Table S4. These results indicate that the recycling does not affect the thermal stabilities of the thermosets, as also presented in Figure S5 where the thermograms of virgin and recycled resins are almost similar.

For the ELO reference, no degradation was observed after the recycling protocol; a T 5% = 275 °C being measured for the virgin and reprocessed thermosets. [START_REF] Mauro | Enhancing the Recyclability of a Vegetable Oil-Based Epoxy Thermoset through Initiator Influence[END_REF] Similar results were obtained for other EVOs [START_REF] Mauro | Sustainable series of new epoxidized vegetable oils-based thermosets with chemical recycling properties[END_REF] The thermal properties reported by Ma et al. [START_REF] Ma | Bio-based epoxy vitrimers: Reprocessibility, controllable shape memory, and degradability[END_REF] for the isosorbide-derived epoxy/4-AFD thermoset showed a decrease Table 3. T 5% by TGA Analysis for Virgin Thermosets (V) and Reprocessed (R) Materials

T 5% , °C T 5% , °C EVOs/ DTBA V R EVOs/ DTBA V R EKRNO 285 ± 1 275 ± 1 ERHO 275 ± 1 275 ± 1 ECO 275 ± 1 275 ± 1 ESFO 271 ± 1 265 ± 1 ESJWO 290 ± 1 285 ± 1 EGRO 265 ± 1 265 ± 1 EPO 285 ± 1 275 ± 1 ECMO 265 ± 1 265 ± 1 ERPO 285 ± 1 282 ± 1 EHO 265 ± 1 265 ± 1 ESO 271 ± 1 271 ± 1 EPLO 265 ± 1 265 ± 1
of the T 5% from 269 °C for the virgin thermoset to 263 °C for the 1st recycling until reaching 250 °C at the end of the third recycling. The authors explained that the obtained T 5% decreases due to the increasing of dangling chains induced by the loss of S-S or C-C bonds. Repairability and Reshapability. Figure 6A displays the aspect of damage in the surface of the recycled EKRNO/ DTBA/IM resin and that it is possible to repair the scratch caused in the material by applying the temperature used for the recycling, for 5 min, to obtain a fully recovered surface. Odriozola et al. [START_REF] De Luzuriaga | Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairayble and recyclable fiber-reinforced thermoset composites[END_REF] repaired completely the scratch performed in a specimen based on DGEBA/4-AFD, applying 200 °C heating and pressure and demonstrating that the DGEBA network prepared using a diamine cross-linker without disulfide bonds (diethyltoluenediamine) was unsuccessful to be repaired. The dynamic disulfide networks allow the reprocessing, giving to the chains the ability to relax at elevated temperature and moreover to modify their shape.

The reshaping of some thermosets was reported for disulfide-based materials [START_REF] De Luzuriaga | Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairayble and recyclable fiber-reinforced thermoset composites[END_REF][START_REF] Zhou | Preparation of self-healing, recyclable epoxy resins and lowelectrical resistance composites based on double-disulfide bond exchange[END_REF][START_REF] Azcune | Aromatic disulfide crosslinks in polymer systems: Self-healing, reprocessability, recyclability and more[END_REF] as for transesterification mechanisms [START_REF] Montarnal | Silica-like malleable materials from permanent organic networks[END_REF][START_REF] Capelot | Metal-Catalyzed Transesterification for Healing and Assembling of Thermosets[END_REF] or for imine [START_REF] Zhao | Recyclable and Malleable Epoxy Thermoset Bearing Aromatic Imine Bonds[END_REF] networks. The results of thermoforming applied for the recycled ESJWO/DTBA is shown in Figure 6B. A CARVER manual press using a zigzag-shaped mold was used, applying heating to 150 °C that was kept for 5 min. Reshapability appears occur easier in recycled thermosets of the categories 1 and 2, while the repairability gave excellent results for all recycled EVOs. Shape Memory. Finally, the shape memory abilities were tested for prepared recycled thermosets. In Figure 7, the results obtained with the EPO recycled thermoset are presented. To test the material shape memory, the sample was twisted to a temporary shape and recovered to their original shape. Figure 7a shows the initial, permanent strip shape n°1. This specimen was twisted manually after heating at 90 °C for 10 s and immediately giving the helix shape (Figure 7b), and then the twisted strip reached the temporary shape at room temperature. Afterward, immersing the strip in hot water (∼90 °C), the original flat permanent shape n°1 was recovered (Figure 7c). Consequently, to reach the permanent shape n°2, the strip was twisted and maintained with a clamp to keep constant the shape and located in oven at 150 °C for 20 min (Figure 7d) and then was cooled to room temperature. After heating at 90 °C for 10 s, a new temporary "C" shape was given to the specimen (Figure 7e). At the end, a new immersion in hot water led to reach the helix permanent shape n°2 again (Figure 7f).

However, all the recycled thermoset materials exhibited shape memory ability. Due to the high T g of the recycled EVO resins from category 3, higher temperatures are necessary to reach the temporary and consequently the permanent shape n°1

. It was found that there are required temperatures with ∼30 °C above the T g of the material to obtain satisfactory shape memory results. Some thermosetting polymers exhibited shape memory properties comparable with EVOs/DTBA reprocessed resins. Williams et al. [START_REF] Altuna | Shape memory epoxy vitrimers based on DGEBA crosslinked with dicarboxylic acids and their blends with citric acid[END_REF] reported shape memory properties of DGEBA cross-linked with citric acid/sebacic acid thermosets, demonstrating that when T > T g , the sample went back to the permanent shape. Zhang et al. [START_REF] Liu | Eugenol-Derived Biobased Epoxy: Shape Memory, Repairing, and Recyclability[END_REF] made an epoxy vitrimer based on eugenol epoxide cross-linked with succinic anhydride, explaining that the shape changing is reversible and can be repeated several times due to dynamic exchange reactions. As for EVOs/DTBA systems, Fang et al. [START_REF] Fang | Reconfigurable, Reprocessable Thermoset Shape Memory Polymer with Highly Tunable Topological Rearrangement Kinetics[END_REF] showed the synergy of two nets based on dynamic bonds (urethane and hindered urea) incorporated into a poly(urea urethane) network using hexamethylene diisocyanate (HMDI) and a mixture of a diol and a triol cross-linker. The authors reported multiple shape memory effects and recovery at a temperature of 50 °C.

The use of materials with these characteristics is growing exponentially in various fields of application, especially in the biomedical field as internal medical devices in contact with tissue and body fluids as reported by El Feninat et al. [START_REF] El Feninat | Shape Memory Materials for Biomedical Applications[END_REF] Chemical Stability. Our team [START_REF] Mauro | Sustainable series of new epoxidized vegetable oils-based thermosets with chemical recycling properties[END_REF] demonstrated that the EVO-based thermosets can be chemically recycled selecting appropriate solvents and conditions. However, both virgin and reprocessed EVO-thermosets exhibited excellent solvent resistance. Toluene was selected as solvent to study the solvent stability of the recycled EVOs materials. The experiment was performed immersing the specimens for 48 h at room temperature, and the results are displayed in Figure 8.

At the end of the test, the recycled thermosets were weighted and, in accordance with eq 4, the determined swelling ratio, Q, did not show a mass loss. The results are plotted in Figure 9A, displaying that the recycled thermosets provided by EVOs with low epoxy content have a lower swelling ratio compared to the virgin ones; while moving from category 1 to 3 and increasing the epoxy content, the reprocessed specimens showed an increased swelling ratio compared with the virgin resins. The recycled ERHO resin is the single case of category 2, which shows a decrease in swelling ability. These results can be explained by evolution of cross-linking decreases after recycling. As previously shown, the EVOs with low epoxy content had increased values of cross-linking density after recycling. Therefore, in this case, the swelling ratio is lower in the recycled resins. For EVOs with high epoxy content, the network density decreases after recycling, so the ratio Q is increased.

The gel content (GC) is a fundamental parameter closely interconnected with the cross-link density: a high gel content indicates a high density of cross-links. [START_REF] Saithai | Effects of different epoxidation methods of soybean oil on the characteristics of acrylated epoxidized soybean oil-co-poly(methyl methacrylate) copolymer[END_REF] Figure 9B displays the GC % of the thermosets and recycled materials, analyzed at the end of 48 h in toluene. All the EVO thermosets have a high gel content % from 95.8 to 99.8% for the virgin ones and from 95.3 to 99.9% for the reprocessed ones, which indicates that EVOs are well cured by DTBA, even after reprocessing. Higher GC % values were observed for the thermosets from category 3: the results are in good agreement with fully bio-based thermosets based on epoxidized sucrose soyate (ESS) and citric or malic acids in different solvents (ethanol, 2-propanol, 1-propanol and 1,4-dioxane). [START_REF] Ma | Effect of solvents on the curing and properties of fully bio-based thermosets for coatings[END_REF] The authors showed a gel content range from 97.3 to 99.6% using acetone and performing the experiment at room temperature for 24 h. The same authors reported a GC between 94.4 and 99.7% for ESS cross-linked with several natural acids in the presence of water to assist the reaction and to solubilize the acids (citric, DL-malic, L-tartaric, malonic, oxalic, and glutaric acid) using an extra catalyst or toxic compounds .

45

GC % higher than 99% were obtained applying the same conditions by Ozawa and Shibata [START_REF] Ozawa | Reprocessable bismaleimide-diamine thermosets based on disulfide bonds[END_REF] for the reprocessable disulfide-containing bismaleimide-amine thermosets prepared by Michael addition reaction of 4,4′-bismaleimidodiphenylmethane, 4-aminophenyl disulfide (APDS), and Jeffamine ED-600. Yu et al. [START_REF] Yu | Vanillin-based degradable epoxy vitrimers: Reprocessability and mechanical properties study[END_REF] reported vanillin-based epoxy vitrimers prepared by reacting the mono-glycidyl structure of vanillin with an isophorone diamine hardener. The authors found GC ratios up to 91.3%, and after the first reprocessing, the GC increased to 94.5% to reach 96.2% for the third time reprocessed vitrimers. Tan et al. [START_REF] Tan | Interpenetrating polymer network structured thermosets prepared from epoxidized soybean oil/ diglycidyl ether of bisphenol A[END_REF][START_REF] Tan | Curing Characteristics and Thermal Properties of Epoxidized Soybean Oil Based Thermosetting Resin[END_REF] reported ESO/methylhexahydrophthalic anhydride vitrimer obtained using 2-ethyl-4-methylimidazole as catalyst GC values between 87 and 95.9%, in function of the catalyst percentage, and the authors compared the values with ESO/DGEBA blends obtaining 98% for 50:50 mixture.

As shown in Figure 9, for the virgin EVO thermosets from category 1, lower GC % values were obtained. After reprocessing, these values decreased, in accordance with the cross-linking density decreasing. The low swelling ratio value and high gel content % obtained for ECO-virgin thermoset were confirmed even after recycling. [START_REF] Mauro | Sustainable series of new epoxidized vegetable oils-based thermosets with chemical recycling properties[END_REF]

■ CONCLUSIONS

In this work, 12 thermosets based on EVOs cured with a disulfide containing diacid hardener have been recycled and the properties have been compared with those of virgin resins. The synergy of the mechanisms, disulfide exchange and transesterification reaction, guaranteed complete mechanical recycling for all materials. Thermosets from category 1, with low epoxy content, have shown greater facility of reprocessing. Also, these materials showed a lower decreasing of crosslinking, mechanical properties, and a better solvent resistance compared with the thermosets from the third category, made using VOs with tri-unsaturated fatty acids. The 3R skills have been confirmed following repairability and reshapability tests. Finally, excellent shape memory properties have been observed for the recycled materials, as vitrimers, opening to a category of renewable and sustainable materials.

■ ASSOCIATED CONTENT

* sı Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsabm.0c01199. Chemical reagents; curing and post-curing conditions for the selected EVOs; and FT-IR, T g , TGA, DMA analysis, and gel content % comparison between virgin and reprocessed thermosets (PDF)

Figure 1 .

 1 Figure 1. Network rearrangement mechanisms obtained by dual exchange reactions: disulfide exchange and transesterification reactions.

Figure 2 .

 2 Figure 2. Mechanical reprocessing of ESFO/DTBA thermoset by hot pressing.

Figure 3 .

 3 Figure 3. Epoxy conversion % for the virgin and recycled materials, determined by FTIR, in function of EVO epoxy content.

Figure 4 .

 4 Figure 4. T g values determined by (A) dynamic DSC and (b) tan δ obtained by DMA for the virgin and the reprocessed thermosets. Correlation with the EVOs epoxy content.

  after reprocessing: ECO and ERHO (275 °C); ESO (271 °C); and EGRO, ECMO, EHO, and EPLO (265 °C).

Figure 5 .

 5 Figure 5. analysis: comparison of thermal stabilities, T 5% , of virgin EVO thermosets and reprocessed ones versus epoxy content.

Figure 6 .

 6 Figure 6. (A) Repairability experiments of the EKRNO/DTBA and (B) reshapability of ESJWO/DTBA recycled thermosets.

Figure 7 .

 7 Figure 7. Shape memory and permanent shape for the EPO/DTBA recycled thermosetting resin: (a) permanent shape n°1, (b) temporary shape after heating and twisting, (c) permanent shape n°1 after recovering, (d) permanent shape n°2, (e) temporary shape after heating, and (f) recovered permanent shape n°2.

Figure 8 .

 8 Figure 8. Solvent stability of reprocessed EVOs thermosets in toluene after 48 h at room temperature.

Figure 9 .

 9 Figure 9. (A) Swelling ratio Q and (B) gel content % at the end of the 48 h of immersion in toluene in function of the epoxy content for the virgin and recycled materials.

Table 1 .

 1 Reprocessing Conditions and Visual Aspect of EVOs/DTBA Thermosets

Table 2 .

 2 DMA Results Obtained for Virgin (V) and Reprocessed Materials (R)

  UniversitéCote d'Azur, Institut de Chimie de Nice, UMR CNRS 7272, Nice Cedex 2 06108, France; orcid.org/ 0000-0001-5208-5956; Email: Alice.Mija@univ-cotedazur.fr Authors Chiara Di Mauro -UniversitéCote d'Azur, Institut de Chimie de Nice, UMR CNRS 7272, Nice Cedex 2 06108, France Samuel Malburet -SPECIFIC POLYMERS, Castries 34160, France; orcid.org/0000-0002-4224-9520 Alain Graillot -SPECIFIC POLYMERS, Castries 34160, France Complete contact information is available at: https://pubs.acs.org/10.1021/acsabm.0c01199 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was supported by the ECOXY project funded by the Bio-Based Industries Joint Undertaking under European Union Horizon 2020 research and innovation program (grant agreement no. 744311).

	■ AUTHOR INFORMATION Corresponding Author
	Alice Mija -■ ABBREVIATIONS EVO epoxidized vegetable oil; ELO epoxidized linseed oil;
	EKRNO epoxidized karanja oil; ECRO epoxidized cardanol
	oil; ECO epoxidized castor oil; EPO epoxidized peanut oil;
	ERPO epoxidized rapeseed oil; ESO epoxidized soybean oil;
	ERHO epoxidized rose hip seed oil; ESFO epoxidized
	safflower oil; EGRO epoxidized grapeseed oil; ECMO
	epoxidized camelina oil; EHO epoxidized hemp oil; EPLO
	epoxidized perilla oil; IM imidazole; DTBA 2,2′-dithiodiben-
	zoic acid; 4-AFD 4,4′-aminophenyl disulfide; DTDA 4,4′-
	dithiodibutyric acid; DGEBA diglycidyl ether of bisphenol A;
	FT-IR Fourier transform infrared spectroscopy; DSC differ-
	ential scanning calorimetry; DMA dynamic mechanical
	analysis; TGA thermogravimetric analysis; DTG derivative
	thermogravimetric analysis; T g glass transition temperature;
	GC % gel content %

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Abraham Chemtob

Bao Tram

Vu Ngoc

Julien Pinaud

Patrick Lacroix-Desmazes

Valérie Héroguez

Photochemically Activated Thiol-Michael Reactions via a Photolatent N-Heterocyclic Carbene

come   L'archive ouverte pluridisciplinaire

INTRODUCTION

Organo-mediated polymerizations are characterized by the use of small organic molecules acting as highly active initiators or catalysts. Many classes of organic activators have been well known for some years, most of which being Brønsted/Lewis acids or bases. [1] The most prominent examples are sulfonic acids for initiating anionic chain polymerization or cyclic guanidine for catalyzing polyurethane step polymerization. [2][3] A renewed interest is emerging in this field with the primary objective of replacing conventional metal or organometallic compounds to reduce cost, toxicity, and sensitivity towards air or oxygen. In a few cases, organic activators can even drive higher polymerization rates through activation of monomer or polymer chain-ends. This extends the possibility of reaction at ambient temperature or polymerization of a broader scope of monomers. To overcome these new challenges, N-heterocyclic carbenes (NHCs) are one of the most promising organocatalysts.

NHCs are neutral divalent carbon species including only six valence electrons. Four are involved in σ-bonds with generally two adjacent nitrogen atoms, and two non-bonding electrons remain at the central carbon atom. [4] High (Brønsted/Lewis) basicity, strong nucleophilicity and structural versatility are the main attributes of NHCs. The field of NHC-mediated polymerizations has been extensively investigated and has rapidly expanded over the past ten years. The main successful examples include anionic ring-opening chain-growth polymerization (lactones, [START_REF] Matyjaszewski | Polymer science: A comprehensive reference[END_REF] lactams, [START_REF] Naumann | [END_REF] epoxides, [7] or cyclosiloxanes [8] ) and to less extent some step-growth polymerization (polyurethane, [9] transesterification, [10] benzoin condensation [11,12] ).

While NHCs are highly effective in various polymerization reactions paradigms, the downside is their high sensitivity towards air and water, which necessitates handling under rigorously anhydrous conditions using standard glove box or Schlenk techniques. In situ preparation of NHCs has been the alternative strategy to circumvent this problem. The most general methods include the deprotonation or reduction of imidazolium salt, [13] but this method requires the addition of an external reactant and a mixing stage to ensure homogenization. More practical and viable industrially are latent NHCs, where free NHCs are created after application of an external stimulus, such as heat or radiation. An ideal protected NHC is robust and can be stored indefinitely outside an inert atmosphere. Therefore, the catalyst may be weighed out on bench utilizing normal methods, and storable one-component mixtures containing both monomer and catalyst may be prepared to reduce cost, and ease processing in particular for cross-linking reactions. Today, the development of stable, easy-to-prepare-and-handle latent NHC complexes that are readily activated under the reaction conditions is a very important issue to extend the use of NHC in academic and industrial laboratories. To date, thermal dissociation of NHC adducts (NHC-CO2, NHC-alcohol, NHC-hydrogen carbonates, NHC-metal complexes) is the most widely used mode of releasing free NHCs on demand. [14] As major achievements, the use of imidazol(in)ium carboxylate (NHC-CO2) has proved an efficient approach for the ring-opening polymerization of cyclic esters and the synthesis of epoxy/anhydride thermoset. [3,15,16] However, it is very challenging to control CO2 adduct activity and dissociation rate, and preserve latency at ambient temperature.

A major step forward, then, would be to develop a NHC photogenerator, i.e. activable upon irradiation, and using readily available starting materials on a large scale. We envisioned that a photolatent NHC would have distinctive advantages compared to a thermally latent analogue: release of free NHC at ambient temperature (considering that many NHC-mediated polymerizations can be conducted without heating), higher temporal resolution in the order of seconds, and spatial resolution opening novel applications such as photolithography. Recently, we described a NHC photogenerating system activable at 365 nm around a simple concept. It consists of 2-isopropylthioxanthone (ITX) as photosensitive species and 1,3-bis(mesityl)imidazolium tetraphenylborate (IMesH + BPh4 -) acting as NHC protected form. As described in Scheme 1, the photochemical reaction proceeds in two consecutive steps: after radiation absorption, thioxanthone triplet excited state is photoreduced by the borate anion, subsequent proton transfer takes place from imidazolium cation to produce the expected NHC, 1,3-bis(mesityl)imidazol-2-ylidene (IMes). [17] To the best of our knowledge, it was the first photolatent NHC system. In 2013, Bielawski et al. developed a NHC exhibiting only a photoswitchable activity, [18] in the sense that change from visible light to UV caused a cyclization reaction, resulting in a NHC-alcohol devoid of activity. Scheme 1. Photochemical mechanism for NHC (IMes) formation from bicomponent mixture IMesH + BPh4 -/ITX In order to demonstrate the utility of this photolatent catalysis, its activity was evaluated in thiol-Michael reactions for small molecule synthesis first, then polymer synthesis via step-growth polymerization. Thiol-Michael addition polymers have found a plethora of applications ranging from polymer modifications of biological systems to hydrogels. [19] As illustrated in Scheme 2, this reaction involves the use of catalytic amount of base/nucleophile to facilitate the reaction between a thiol and an electron deficient vinyl group to yield a thioether. In 2012, Bowman et al. demonstrated that imidazole derivatives showed better catalytic activity than conventional tertiary amine for the thiol-Michael reaction. [20] However, the activity of NHC imidazoline and imidazolidine species, which are the most common NHCs, have never been examined. All thiol-X reactions are generally highly dependent of the pKa of the thiol along with its structure. Compared to a conventional base, a strong nucleophile such as NHC as catalyst is likely to accommodate a broader range of thiols and Michael substrates while driving high reaction rates. In this report, a systematic approach has been used to study NHC-catalyzed thiol-Michael reactions. The feasibility of photoNHC catalysis in simple coupling reaction with small molecules was firstly assessed, then the synthesis of cross-linked polymer was investigated (see monomer list in Table 1). EXPERIMENTAL SECTION Chemicals 1,3-bis(mesityl)imidzolium tetraphenylborate (IMesH + BPh4 -) was synthesized following an established procedure. [17] 2-isopropylthioxanthone (ITX, 97 %), methyl acrylate (MA, 99 %), 1,3bis(mesityl)imidzolium chloride (IMesH + Cl -), sodium tetraphenylborate (NaBPh4), butyl acrylate (BuA, 99%), pentaerythritol tetrakis(3-mercaptopropionate) (PETMP, 95 %), 2,2,6,6tetramethylpiperidie 1-oxyl free radical (TEMPO, 98%), N,N-bis-(2,6-dimethylphenyl)carbodiimide (CDI) were purchased from Sigma-Aldrich. 1,6-hexanediol diacrylate stabilized with MEHQ (HDDA, 98 %), 1-hexanethiol (98 %), butyl thiolglycolate (98 %) were purchased from TCI Europe. All chemicals reagents were obtained commercially and used without further purification. Tetrahydrofuran (THF), acetonitrile (ACN) were purchased from Carlo Erba. Acetonitrile-d 3 and THF-d 8 were purchased from Sigma-Aldrich. All solvents were stored in molecular sieves.

ITX IMesH + BPh 4 -

Protocols used for thiol-Michael reactions

All samples display a stoichiometric amount of thiol and acrylate functions.

1-component system.

A borosilicate NMR tube was charged with IMesH + BPh4 -(5.3 mg, 0.008 mmol, 2 eq), ITX (1 mg, 0.004 mmol, 1 eq) and TEMPO (1 mg, 0.006 mmol, 1.5 eq). In a control experiment, the mixture IMesH + BPh4 -/ITX was replaced by free IMes (2.5 mg, 0.008 mmol, 2 eq). PETMP (122 mg, 0.25 mmol, 62.5 equiv.) and HDDA (113 mg, 0.5 mmol, 125 equiv.) were added to the NMR tube. Acetonitrile-d 3 (0.03 M, relative to IMesH + BPh4 -, 0.3 mL) was added, and the NMR tube was capped with a rubber septum, degassed in N2 atmosphere (1 min) and irradiated for a variable duration (1-5 min) in a LED strip photochemical reactor at room temperature. As sketched in Figure 1, the LED reactor was constructed by winding a LED strip (365 nm, Lightingwill, SMD352, 60 LEDs/m) around a quartz cylinder (internal diameter: 5.5 cm, length: 20 cm). 5 coils of LED were assembled, leading to a total length of 86 cm. The NMR tube was introduced in the axis of the quartz cylinder. The distance between the tube surface and the coiled LED array was 2.25 cm. At this distance, the estimated irradiance was 6.5 mW cm -2 at 365 nm. 2-component system. a borosilicate NMR tube 1 was charged with IMesH + BPh4 -(5.3 mg, 0.008 mmol, 2 eq), ITX (1 mg, 0.004 mmol, 1 eq) and TEMPO (1 mg, 0.006 mmol, 1.5 eq) with acetonitrile-d 3 (0.2 mL) and capped with a rubber septum, degassed in N2 atmosphere (1 min) and irradiated for a duration of 1-5 min in a LED strip photochemical reactor at room temperature. The asirradiated tube was added to a NMR tube 2 containing a mixture of PETMP (122 mg, 0.25 mmol, 62.5 eq), HDDA (113 mg, 0.5 mmol, 125 equiv) dissolved in acetonitrile-d 3 (0.1 mL). Prior to the addition of pthe hotogenerated NHC, the tube 2 was degassed (N2, 1 min) and capped with a rubber septum.

CHARACTERIZATION

Fourier Transform Infrared Spectroscopy (FTIR) analysis was performed using a Nicolet iS50 equipped with a horizontal accessory. The FTIR spectra were treated using OMNIC 9.8 software. The spectra were recorded by a deuterated triglycine sulfate (DTGS) detector with a KBr beam splitter. All kinetics were monitored at a scan rate of one scan per 1.5 s. In a solution thiol-Michael photopolymerization experiment, a two-component formulation (see previous section) was introduced in a 5 mL Sulzer Mixpac TM mixer containing two separate cartridges: one for the monomers (HDDA/PETMP) and the second for as-irradiated IMesH + BPh4 -/ITX/TEMPO (5 min). This double syringe enabled to mix both components together and introduce the reactive medium in a demountable IR liquid sandwich cell consisting of two CaF2 windows separated by a lead Teflon spacer (1 mm). The spacer and windows form a sealed liquid sample compartment with a steady path length. Our configuration uses 38.5×19.5×4 mm rectangular windows, the inlet of the front plate allows introduction of the reactive mixture without disassembling the cell, before the onset of gelation. FTIR analysis was triggered just after the IR cell filling. The conversion of thiol and acrylate functional groups were assessed approx. 10 s after mixture preparation by monitoring the gradual disappearance of the IR bands centered at 2580 cm -1 (νSH stretching mode) and 1620 cm -1 (νC=C stretching mode) respectively. In a variant process referred to as bulk polymerization, the reaction mixture coming out from the double syringe was cast on a 25 mm diameter circular KBr window.

Carbodiimide-NHC adduct formation.

A borosilicate NMR tube was charged with IMesH + BPh4 -(10 mg, 0.015 mmol, 2 eq) and ITX (2 mg, 0.008 mol, 1 eq). THF-d 8 (0.03 M, relative to IMesH + BPh4 -, 0.5 mL) was added and the NMR tube was capped with a rubber septum and irradiated for a variable duration (30 sec -10 min) with a LED waveguide (Hamamatsu spot light source LC-L1V3). 12 mg (2 eq) of N,N-bis-(2,6-dimethylphenyl)carbodiimide (CDI) was then added into the NMR tube to have a 1:1 stoichiometric ratio with IMesH + BPh4 -. The reaction media immediately changed color from yellow solution to orange. The formation of the soluble zwitterionic adduct IMes-CDI was confirmed by 1 H-NMR spectroscopy and its yield determined.

Gel content measurement of the polymer formed after thiol-Michael polymerization was performed after different irradiation times (30 s -5 min). After 10 min storage, the polymers were dried at 70°C under vacuum at least 8 h to remove all volatile compounds (solvent, monomer), weighed and soaked in THF solvent overnight. The solid products were filtrated, dried and weighed again to observe any change of mass.

𝐺𝑒𝑙 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 = 𝑊 1 * 100 𝑊 2
𝑊 1 is the weight of the insoluble fraction after washing, and 𝑊 2 is the weight of the original polymer after photopolymerization and drying.

Nuclear Magnetic Resonance (NMR).

NMR spectra were recorded on a Varian Mercury Plus 300 MHz NMR spectrometer operating at 25 °C in acetonitrile-d 3 . VNMRJ software was used for proton analysis. All chemical shifts were reported in ppm relative to acetonitrile solvent.

Optical pyrometry. The use of pyrometry to follow photopolymerization reactions was developed by Crivello et al. [21] In this study, we used a thermometer Ebro TFN-530 probe to follow the change of temperature in a 2-component system. The probe was dipped in the solution of monomers. Temperature measurement started after addition of photogenerate IMes to PETMP/HDDA mixture. Change of temperature value every 5 s was reported until a steady temperature close ambient temperature was reached.

RESULTS AND DISCUSSIONS I. Photogeneration of NHC

Organoborates have been well established as one-electron reducing agents (electron donors) in a variety of thermal and photochemical redox processes. [22] In a recent example, we used tetraphenylborate imidazolium salt (IMesH + BPh4 -) in combination with the thioxanthone derivative ITX as NHC photogenerating system [17] in which the key steps involve an initial photoinduced electron transfer and subsequent proton transfer (Scheme 1). Another well-known example proceeding through the same mechanism is ITX/tertiary amine serving as free radical initiators in photopolymerization of acrylates. [23] Electron transfer is induced by photo-excitation of ITX because its shift to triplet excited state ( 3 ITX * ) drives an increase of oxidant properties. Therefore, NHC generation can be controlled by radiation at 365 nm where only ITX absorbs while the imidazolium salt behaves as stable and protected NHC form. In our system, use of 1,3-bis(mesityl)imidazolium tetraphenylborate (IMesH + BPh4 -) enables the formation of the well-known NHC: 1,3dimesitylimidazol-2-ylidene also referred to as IMes. To determine the yield of NHC photogenerated, IMes was converted into a stable adduct by addition of carbodiimide into the as-irradiated IMesH + BPh4 -/ITX (see experimental procedure). Moisture-stable "NHC-CDI" adducts or zwitterionic-type amidinates can be well identified by 1 H-NMR spectroscopy (not given), the quantity of adduct being the mirror of the quantity of IMes photogenerated. The IMes-CDI adduct was achieved by the direct treatment of the corresponding CDI with a IMes in a 1:1 stoichiometric ratio in THF at room temperature. [24] Control experiments with free IMes validated this process as relatively precise titration method (± 2%). A maximum yield of 32% was reached after 10 min irradiation (65 mW cm -2 ). Longer irradiation led to decomposition of NHC formed, presumably through electron transfer from IMes (a strong nucleophile) to 3 ITX * .

II.

Thiol-Michael reaction

II.1 Coupling reaction with small molecules

To demonstrate the concept of photoNHC-catalyzed thiol-Michael reaction, PETMP and BuA (see Table 1 for structures) were used respectively as thiol and ene reactant in a model non-crosslinking system. The mixture was found to be stable in acetonitrile for at least 24 h even under radiation exposure. As preliminary experiment, addition of free IMes (0.5 mol% with respect to monomers) resulted in 100% of conversion (SH and acrylate) after only 2 min, illustrating the efficiency of NHC organocatalyst for thiol-Michael reaction at ambient temperature. Then, reaction of PETMP/BuA (62.5/250 equiv.) was attempted in an NMR tube containing acetonitrile-d 3 using a mixture of IMesH + BPh4 -/ITX/TEMPO (2/1/1.5 equiv.). TEMPO was incorporated into the photogenerating system to inhibit the radicals produced by the photolysis mechanism. Scheme 1 shows indeed that both NHC and radicals are produced simultaneously, in addition excited ITX is able to abstract H from thiol to from thiyl radicals. In our case, TEMPO aims to prevent thiol-ene reaction and chain-growth homopolymerization of acrylates. In the monocomponent system, no reaction proceeded without irradiation, supporting photolatency of the photogenerating system. After 5 min of irradiation (6.5 mW cm -2 ), about half thiol and ene reactive functions have been consumed (Figure 2). Control experiments in absence of TEMPO or with higher TEMPO content (3 equiv.) were carried without significant change in reaction kinetics, which agrees with a minimal role played by the radicals in the polymerization progress. The quantity of TEMPO in solution was thus fixed at 1.5 equiv. in the rest of the study. Nevertheless, the major challenge is ascertaining whether thiol-Michael occurs, and if it does, its extent relative to thiol-ene radical process that results in the same final product. While the synchronous conversion of thiols and acrylates rules out the possibility of homopolymerization, radical mediated thiol-ene is not completely without ambiguity since TEMPO may not scavenge all radicals. The strongest evidence to support a NHC-catalyzed thiol-Michael addition was to proceed with a 2-step procedure: IMesH + BPh4 -/ITX/TEMPO was first irradiated, then transferred to PETMP/BuA reactive mixture to be sure that short-lived radicals do not interfere.

Figure 3 is a functional group conversion plot as a function of time after different irradiation times (1, 2 and 3 min) of the IMesH + BPh4 -/ITX/TEMPO mixture. Much faster conversion rates were observed. 1 and 2 min irradiation were not sufficient to reach full conversion during irradiation, and there was a gradual increase of thiol and acrylate conversion. Because of the initial nucleophile presence after irradiation, it is actually expected that the thiol-Michael reaction continues in the dark even after cessation of irradiation. Such living character is instrumental in radiation curing to allow conversions of unexposed areas and reach higher conversions. The fact that both reactive functions are consumed approx. at the same rate is consistent with a thiol-Michael reaction. The slight imbalance between thiol and acrylate conversion was assigned to SH/C=C stoichiometry deviation. With 1 min irradiation, 90% conversion was achieved, while full conversions were obtained for longer irradiation times. Low NHC yield after 1 min irradiation can account for the lower NHC yield. Our results illustrate that an increasing amount of NHC species may be generated with longer irradiation time. Because a 2-step process is easier to understand since the identities of the catalytic species are much better established, the rest of the reactions were performed with this methodology. Using PETMP/BuA as model mixture, the effect of the thiol and acrylate structure on thiol-Michael reactivity was examined. In a typical experiment, IMesH + BPh4 -/ITX/TEMPO was irradiated 2 min then introduced into the thiol/acrylate mixture, and conversions determined by 1 H-NMR after 10 min storage. As shown in Table 2, the reaction is negligibly influenced by the structure and functionality of the acrylate. By contrast, thiol structure results in a much stronger effect. An increasing reactivity order was found for alkanethiol (hexanethiol, runs 4-6) > thiolpropionate (PETMP, runs 1-3) > thiolacetate (butyl thioglycolate, runs 7-9). In this latter case, yield was only 31.6% and 13.1%, with MA and HDDA respectively. These results differ from last works of Chan et al. [25] who demonstrated a reverse in thiol reactivity order using amines and amidines as catalysts.

A B C

Because NHC-catalyzed thiol-Michael reactions proceed via a nucleophile-catalyzed pathway, it is likely that a change in reactivity may happen.

Table 2. Conversion after 10 min of reaction with photoNHCs followed by 1 H-NMR specroscopy. Thiol/Ene functions 250/250 equiv. was used. IMesH + BPh4 -/ITX/TEMPO 2/1/1.5 equiv. in acetonitrile-d 3 ([IMesH + BPh4 -] = 0.03 M, V = 0.3 mL).

II.2 Cross-linking reactions

Our model cross-linking system contains PETMP and HDDA and a 2-component methodology has been implemented in THF via the addition of photogenerated NHC. Control experiment proved that the monomer mixture was stable. Additionally, the reaction did not occur in the presence of acetic acid suggesting that NHC acts as organocatalyst. In contrast, immediate gelation takes place after addition of the as-irradiated IMesH + BPh4 -/ITX/TEMPO mixture (5 min, 6.5 mW cm -2 ) to PETMP/HDDA. Interestingly, the reaction rate was not affected by the change of TEMPO amount in the system, which supports again the prominence of a nucleophile-mediated thiol-Michael reaction. The obtained polymer turned out to be insoluble in various organic solvents, suggesting that a crosslinking reaction occurred.

Obviously, the high reactivity of this system makes the monitoring of the cross-linking kinetics challenging. To overcome this issue, a double syringe was used where the two compartments were filled by the monomer mixture and the catalytic system. By this way, it has been possible to fill a IR liquid cell and follow the reaction kinetics of a solution polymerization by rapid-scan FTIR spectroscopy (see experimental section for details). As can be seen in Figure 4, both thiol and acrylate are consumed at the same rate, which is consistent with a thiol-Michael addition reaction mechanism. The IR spectra show the consumption of νSH stretching mode (2550 cm -1 ) and νC=C stretching mode (1620 cm -1 ) exploited to create the kinetic curves for two intermediate times t=0 and t=5 min (still partial conversion). At 5 min, the conversion was at 85 %. A complete conversion can be reached in approx. 10 min. To monitor reaction progress, gel contents were measured for different irradiation times of IMesH + BPh4 -/ITX/TEMPO mixture (30 s -5 min, Figure 5). While a limited gel content of 70% was found for 30 seconds irradiation, longer exposure to UV radiation led to a completely insoluble crosslinked structure. Because of the high exothermicity of thiol-Michael cross-linking reaction, we attempted to use pyrometry to evaluate the change of temperature during the photoinduced polymerization of HDDA/PETMP catalyzed by IMesH + BPh4 -/ITX. 20 s after addition of photogenerated IMes (5 min irradiation), temperature reaches a maximum at 38°C (Figure 6). The reaction is reproducible and higher maximum temperature can be obtained upon adding a higher concentration of photoNHC catalysts. For this reason, coupling of real-time infrared spectroscopy with optical IR pyrometry is a useful method for simultaneously monitoring the chemical conversion and the temperature.

Run

CONCLUSION

The utility of a photolatent NHC organocatalyst for the photochemical activation of thiol-Michael reaction has been demonstrated. Using a two-step procedure to prevent adventitious radical-mediated thiol-ene reaction, the NHC molecule IMes was first photogenerated in solution with a mixture of IMesH + BPh4 -/ITX (2/1 equiv.), then introduced into a stoichiometric thiol/ene reactant mixture (250 equiv. of ene and thiol function). 5 min irradiation of the NHC photogenerating system enabled a complete reaction of the PETMP/BuA mixture in the matter of minutes. This results supports the efficient photogeneration of Imes, that was previously established in control experiments. In addition to coupling reactions between small molecules, the synthesis of cross-linked polymers was also reported via a step-growth polymerization mechanism. Using the same photocatalyst system, we proved that the tetrathiol/diacrylate mixture PETMP/HDDA achieved full conversion in less than 1 min through FTIR spectroscopy. Nevertheless, further efforts are required to study thiol-Michael reactivity with a broader range of ene and thiol monomers. We foresee that the photolatent IMes can be a potential candidate for the organocatalysis of transesterification reactions or other thiol-X reactions such as thiol-isocyanate with many advantages including easier handling of chemicals better temporal and spatial control of the reaction.

Scheme 2 .

 2 Scheme 2. Nucleophile (Nu)-catalyzed mechanism of thiol-Michael addition reaction.

Figure 1 .

 1 Figure 1. LED photoreactor built by winding a 365 nm LED strip around a quartz cylinder. NMR tube is placed at the center.

Figure 2 .

 2 Figure 2. Conversion time plot for thiol-Michael reaction in acetonitrile-d 3 of PETMP/BuA (62.5/250 equiv.) using IMesH + BPh4 -/ITX/TEMPO (2/1/1.5 equiv). A one-pot procedure was used.

Figure 3 .

 3 Figure 3. Conversion time plot of thiol and acrylate functions of PETMP/BuA (62.5/250 equiv.) after addition of an irradiated mixture based on IMesH + BPh4 -/ITX/TEMPO (2/1/1.5 equiv.) following a two-step procedure. Different irradiation times were used: A: 1 min, B: 2 min and C: 5 min. Acrylate conversion Thiol conversion. The conversions were determined by 1 H-NMR spectroscopy.

Figure 4 .

 4 Figure 4. Monomer conversion plot as a function of time for thiol and acrylate functions in IR liquid cell (spacer thickness =1 mm) for solution photopolymerization of PETMP/HDDA: 62.5/125 equiv. (component 1), IMesH + BPh4 -/ITX/TEMPO: 2/1/1.5 equiv. (component 2) in THF ([IMesH + BPh4 -] = 0.013 M, V = 0.6 mL). Irradiation time: 5 min. thiol conversion, acrylate conversion.

Figure 5 .

 5 Figure 5. Increase of gel content as a function of irradiation time.

Figure 6 .

 6 Figure 6. Temporal change of temperature in the course of photoNHC-catalyzed polymerization of PETMP/HDDA.

Table 1 .

 1 Chemical structures of thiol and alkene monomers

THIOL Hexanethiol Butyl thioglycolate Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) ACRYLATE Methyl acrylate (MA) Butyl acrylate (BuA) 1,6-Hexandiol diacrylate (HDDA)