Samantha Molina-Gutiérrez
email: samantha.molina-gutierrez@enscm.fr

Sara Dalle Vacche
email: sara.dallevacche@polito.it

Alessandra Vitale
email: alessandra.vitale@polito.it

Vincent Ladmiral
email: vincent.ladmiral@enscm.fr

Sylvain Caillol
email: sylvain.caillol@enscm.fr

Roberta Bongiovanni
email: roberta.bongiovanni@polito.it

Patrick Lacroix-Desmazes
email: patrick.lacroix-desmazes@enscm.fr

Photoinduced Polymerization of Eugenol-Derived Methacrylates

Keywords: biobased monomer, photoinduced-polymerization, eugenol-derived methacrylate

Biobased monomers have been used to replace their petroleum counterparts in the synthesis of polymers that are aimed at different applications. However, environmentally friendly polymerization processes are also essential to guarantee greener materials. Thus, photoinduced polymerization, which is low-energy consuming and solvent-free, rises as a suitable option. In this work, eugenol-, isoeugenol-, and dihydroeugenol-derived methacrylates are employed in radical photopolymerization to produce biobased polymers. The polymerization is monitored in the absence and presence of a photoinitiator and under air or protected from air, using Real-Time Fourier Transform Infrared Spectroscopy. The polymerization rate of the methacrylate double bonds was affected by the presence and reactivity of the allyl and propenyl groups in the eugenol-and isoeugenol-derived methacrylates, respectively. These groups are involved in radical addition, degradative chain transfer, and termination reactions, yielding crosslinked polymers. The materials, in the form of films, are characterized by differential scanning calorimetry, thermogravimetric, and contact angle analyses.

Introduction

The need for more environmentally friendly materials and processes has led to the development of suitable biobased building blocks to produce polymers [START_REF] Gandini | Progress of polymers from renewable resources: Furans, vegetable oils, and polysaccharides[END_REF]. However, the use of energy-efficient polymerization techniques is also paramount. Photoinduced polymerization is a suitable option, as it allows fast processes, low energy consumption, room temperature reactions, and solvent-free conditions with the concomitant reduction or elimination of volatile organic compounds (VOCs) [START_REF] Fouassier | Photopolymerization reactions under visible lights: Principle, mechanisms and examples of applications[END_REF]. Thanks to these advantages, it has found wide application in industrial processes. It is an established technique in the fields of coatings, inks, adhesives, and wood finishing [START_REF] Crivello | Photopolymer materials and processes for advanced technologies[END_REF]. Products from photopolymerization are present in everyday life, such as contact lenses [START_REF] Childs | Fabricating customized hydrogel contact lens[END_REF], filling for dental cavities [START_REF] Stansbuty | Curing dental resins and composites by photopolymerization[END_REF], and credit cards [START_REF] Lloret | Holographic lenses in an environment-friendly photopolymer[END_REF].

Photopolymerization processes are also characterized by spatial and temporal control, which means that they only occur in the irradiated area and they are stop-and-go reactions, i.e., they start and stop simply by switching on and off the light [START_REF] Corrigan | Seeing the light: Advancing materials chemistry through photopolymerization[END_REF]. Therefore, they are key reactions for the emerging additive manufacturing technologies [START_REF] Vitale | A unified approach for patterning via frontal photopolymerization[END_REF][START_REF] Ligon | Polymers for 3D printing and customized additive manufacturing[END_REF][START_REF] Zhang | 3D printing of photopolymers[END_REF]. For most applications, conventional radical reactions (proceeding via propagation of macromolecular radicals after initiation triggered by irradiation) are employed. Most monomers do not produce any radical initiating species with sufficiently high yields under light irradiation; thus, a photoinitiator (PI) may be required [START_REF] Crivello | Photopolymer materials and processes for advanced technologies[END_REF]. Most PIs form radicals either by homolytic cleavage or by hydrogen abstraction. Then, the radicals initiate the polymerization by reacting with the monomers. The most commonly used monomers in radical photopolymerization are acrylates and methacrylates. In search of sustainability, it is crucial to replace oil-based monomers with bio-based ones produced from renewable sources. Among the available biobased building blocks, some natural molecules can undergo autooxidation reactions, cyclization, isomerization, dimerization, and oligomerization in the presence of light [START_REF] Fertier | The use of renewable feedstock in UV-curable materials-A new age for polymers and green chemistry[END_REF][START_REF] Goldberg | Selective activation of C=C bond in sustainable phenolic compounds from ligninvia photooxidation: Experiment and density functional theory calculations[END_REF]. However, most of these require being suitably functionalized prior to photoinduced polymerization processes. The introduction of polymerizable functions on biobased building blocks is thus a crucial step.

Recently, the use of naturally occurring phenols, such as eugenol and eugenol-derivatives, has gained attention for producing biobased monomers, as they can be obtained by lignin depolymerization [START_REF] Khalil | Essential oil eugenol: Sources, extraction techniques and nutraceutical perspectives[END_REF][START_REF] Sakai | Formation of isoeugenol and eugenol during the cleavage of β-aryl ethers in lignin by alcohol-bisulfite treatment[END_REF][START_REF] Llevot | From lignin-derived aromatic compounds to novel biobased polymers[END_REF]. In addition, eugenol-derived monomers are attractive because they possess antioxidant, antiseptic, and antibacterial properties [START_REF] Kong | Advances in pharmacological research of eugenol[END_REF][START_REF] Da Silva | Eugenol derivatives: Synthesis, characterization, and evaluation of antibacterial and antioxidant activities[END_REF], which could be exploited in photopolymers for dentistry or food packaging [START_REF] Modjinou | Antibacterial and antioxidant bio-based networks derived from eugenol using photo-activated thiol-ene reaction[END_REF]. In particular, isoeugenol has a higher antibacterial activity than eugenol and is not genotoxic [START_REF] Munerato | Genotoxic effects of eugenol, isoeugenol and safrole in the wing spot test of Drosophila melanogaster[END_REF]. However, as many other biobased building blocks, eugenol and its derivatives do not possess functional groups that react readily through photoinduced polymerization. In addition, phenols scavenge free radicals and inhibit polymerization [START_REF] Lartigue-Peyrou | The use of phenolic compounds as free-radical polymerization inhibitors[END_REF][START_REF] Fujisawa | Action of eugenol as a retarder against polymerization of methyl methacrylate by benzoyl peroxide[END_REF]. Thus, suitable functional groups must be inserted to avoid this inhibition and promote polymerization.

Besides the functionalization of eugenol with epoxy groups for cationic photopolymerization [START_REF] Guzmán | New bio-based materials obtained by thiol-ene/thiol-epoxy dual curing click procedures from eugenol derivates[END_REF][START_REF] Guzmán | Preparation of new biobased coatings from a triglycidyl eugenol derivative through thiol-epoxy click reaction[END_REF][START_REF] Modjinou | Enhancement of biological properties of photoinduced biobased networks by post-functionalization with antibacterial molecule[END_REF], methacrylate functional groups have also been introduced in the molecule. By reacting to the allylic double bond with 3-mercaptopropionic acid and thiomalic acid (via thiol-ene chemistry) and then reacting to the resulting carboxylic acid product and phenol group of eugenol with glycidyl methacrylate, eugenol methacrylic derivatives were obtained. These monomers were then used in photoinduced copolymerization with AESO (acrylated epoxidized soybean oil) to produce biobased coatings [START_REF] Dai | Synthesis of eugenol-based multifunctional monomers via a thiol-ene reaction and preparation of UV curable resins together with soybean oil derivatives[END_REF]. Moreover, allyl-etherified eugenol-derivatives were copolymerized through thiol-ene reactions with pentaerythritol-based primary and secondary tetrathiol and with isocyanurate-based secondary trithiol, to prepare crosslinked polymers [START_REF] Yoshimura | Bio-based polymer networks by thiol-ene photopolymerizations of allyl-etherified eugenol derivatives[END_REF]. Similarly, allyl-etherified eugenol and linalool were copolymerized with trimethylolpropane tris(3-mercaptopropionate) to form crosslinked networks endowed with antioxidant and antibacterial properties [START_REF] Modjinou | Antibacterial and antioxidant bio-based networks derived from eugenol using photo-activated thiol-ene reaction[END_REF]. Later, a trifunctional allyl compound, tris(4-allyl-2 methoxyphenolyl) phosphate, was synthesized and reacted with thiols with two to four functionalities via thiol-ene chemistry and the influence of crosslink density on the different materials was studied [START_REF] Miao | Water-phase synthesis of a biobased allyl compound for building uv-curable flexible thiol-ene polymer networks with high mechanical strength and transparency[END_REF]. Thiol-ene chemistry was also employed to covalently attach eugenol through its allylic double bond to a limonene-derived polymer network and prepare antibacterial coatings [START_REF] Breloy | β-Carotene/limonene derivatives/eugenol: Green synthesis of antibacterial coatings under visible-light exposure[END_REF].

An easy two-step synthetic alternative to produce methacrylated eugenol derivatives was described in a previous work [START_REF] Molina-Gutiérrez | Eugenol: A promising building block for synthesis of radically polymerizable monomers[END_REF]. A methacrylate group was introduced in eugenol structures taking advantage of its phenol group. Ethoxy eugenyl methacrylate (EEMA), ethoxy isoeugenyl methacrylate (EIMA) and ethoxy dihydroeugenyl methacrylate (EDMA) (Figure 1) were synthesized and successfully employed in solution and emulsion polymerization [START_REF] Molina-Gutiérrez | Eugenol: A promising building block for synthesis of radically polymerizable monomers[END_REF][START_REF] Molina-Gutiérrez | Emulsion polymerization of dihydroeugenol-, eugenol-, and isoeugenol-derived methacrylates[END_REF].

Molecules 2020, 25, x 2 of 17 sufficiently high yields under light irradiation; thus, a photoinitiator (PI) may be required [START_REF] Crivello | Photopolymer materials and processes for advanced technologies[END_REF]. Most PIs form radicals either by homolytic cleavage or by hydrogen abstraction. Then, the radicals initiate the polymerization by reacting with the monomers. The most commonly used monomers in radical photopolymerization are acrylates and methacrylates. In search of sustainability, it is crucial to replace oil-based monomers with bio-based ones produced from renewable sources. Among the available biobased building blocks, some natural molecules can undergo autooxidation reactions, cyclization, isomerization, dimerization, and oligomerization in the presence of light [START_REF] Fertier | The use of renewable feedstock in UV-curable materials-A new age for polymers and green chemistry[END_REF][START_REF] Goldberg | Selective activation of C=C bond in sustainable phenolic compounds from ligninvia photooxidation: Experiment and density functional theory calculations[END_REF]. However, most of these require being suitably functionalized prior to photoinduced polymerization processes. The introduction of polymerizable functions on biobased building blocks is thus a crucial step.

Recently, the use of naturally occurring phenols, such as eugenol and eugenol-derivatives, has gained attention for producing biobased monomers, as they can be obtained by lignin depolymerization [START_REF] Khalil | Essential oil eugenol: Sources, extraction techniques and nutraceutical perspectives[END_REF][START_REF] Sakai | Formation of isoeugenol and eugenol during the cleavage of β-aryl ethers in lignin by alcohol-bisulfite treatment[END_REF][START_REF] Llevot | From lignin-derived aromatic compounds to novel biobased polymers[END_REF]. In addition, eugenol-derived monomers are attractive because they possess antioxidant, antiseptic, and antibacterial properties [START_REF] Kong | Advances in pharmacological research of eugenol[END_REF][START_REF] Da Silva | Eugenol derivatives: Synthesis, characterization, and evaluation of antibacterial and antioxidant activities[END_REF], which could be exploited in photopolymers for dentistry or food packaging [START_REF] Modjinou | Antibacterial and antioxidant bio-based networks derived from eugenol using photo-activated thiol-ene reaction[END_REF]. In particular, isoeugenol has a higher antibacterial activity than eugenol and is not genotoxic [START_REF] Munerato | Genotoxic effects of eugenol, isoeugenol and safrole in the wing spot test of Drosophila melanogaster[END_REF]. However, as many other biobased building blocks, eugenol and its derivatives do not possess functional groups that react readily through photoinduced polymerization. In addition, phenols scavenge free radicals and inhibit polymerization [START_REF] Lartigue-Peyrou | The use of phenolic compounds as free-radical polymerization inhibitors[END_REF][START_REF] Fujisawa | Action of eugenol as a retarder against polymerization of methyl methacrylate by benzoyl peroxide[END_REF]. Thus, suitable functional groups must be inserted to avoid this inhibition and promote polymerization.

Besides the functionalization of eugenol with epoxy groups for cationic photopolymerization [START_REF] Guzmán | New bio-based materials obtained by thiol-ene/thiol-epoxy dual curing click procedures from eugenol derivates[END_REF][START_REF] Guzmán | Preparation of new biobased coatings from a triglycidyl eugenol derivative through thiol-epoxy click reaction[END_REF][START_REF] Modjinou | Enhancement of biological properties of photoinduced biobased networks by post-functionalization with antibacterial molecule[END_REF], methacrylate functional groups have also been introduced in the molecule. By reacting to the allylic double bond with 3-mercaptopropionic acid and thiomalic acid (via thiol-ene chemistry) and then reacting to the resulting carboxylic acid product and phenol group of eugenol with glycidyl methacrylate, eugenol methacrylic derivatives were obtained. These monomers were then used in photoinduced copolymerization with AESO (acrylated epoxidized soybean oil) to produce biobased coatings [START_REF] Dai | Synthesis of eugenol-based multifunctional monomers via a thiol-ene reaction and preparation of UV curable resins together with soybean oil derivatives[END_REF]. Moreover, allyl-etherified eugenol-derivatives were copolymerized through thiol-ene reactions with pentaerythritol-based primary and secondary tetrathiol and with isocyanurate-based secondary trithiol, to prepare crosslinked polymers [START_REF] Yoshimura | Bio-based polymer networks by thiol-ene photopolymerizations of allyl-etherified eugenol derivatives[END_REF]. Similarly, allyl-etherified eugenol and linalool were copolymerized with trimethylolpropane tris(3-mercaptopropionate) to form crosslinked networks endowed with antioxidant and antibacterial properties [START_REF] Modjinou | Antibacterial and antioxidant bio-based networks derived from eugenol using photo-activated thiol-ene reaction[END_REF]. Later, a trifunctional allyl compound, tris(4-allyl-2 methoxyphenolyl) phosphate, was synthesized and reacted with thiols with two to four functionalities via thiol-ene chemistry and the influence of crosslink density on the different materials was studied [START_REF] Miao | Water-phase synthesis of a biobased allyl compound for building uv-curable flexible thiol-ene polymer networks with high mechanical strength and transparency[END_REF]. Thiol-ene chemistry was also employed to covalently attach eugenol through its allylic double bond to a limonene-derived polymer network and prepare antibacterial coatings [START_REF] Breloy | β-Carotene/limonene derivatives/eugenol: Green synthesis of antibacterial coatings under visible-light exposure[END_REF].

An easy two-step synthetic alternative to produce methacrylated eugenol derivatives was described in a previous work [START_REF] Molina-Gutiérrez | Eugenol: A promising building block for synthesis of radically polymerizable monomers[END_REF]. A methacrylate group was introduced in eugenol structures taking advantage of its phenol group. Ethoxy eugenyl methacrylate (EEMA), ethoxy isoeugenyl methacrylate (EIMA) and ethoxy dihydroeugenyl methacrylate (EDMA) (Figure 1) were synthesized and successfully employed in solution and emulsion polymerization [START_REF] Molina-Gutiérrez | Eugenol: A promising building block for synthesis of radically polymerizable monomers[END_REF][START_REF] Molina-Gutiérrez | Emulsion polymerization of dihydroeugenol-, eugenol-, and isoeugenol-derived methacrylates[END_REF]. Biobased polymers obtained in the form of homogeneous and transparent films are potentially interesting for industrial development and could find application in coatings, food packaging or Biobased polymers obtained in the form of homogeneous and transparent films are potentially interesting for industrial development and could find application in coatings, food packaging or dentistry. Therefore, in the present article, we investigate the photopolymerization of films of these eugenol methacrylates under irradiation in different conditions: with or without the radical photoinitiator and in the presence or in the absence of air. Moreover, the conversion of the methacrylic double bond of the three monomers as well as the conversion of the allylic (EEMA) or propenyl (EIMA) double bonds are monitored, and the properties of the polymers are tested.

Results and Discussion

Kinetic Monitoring of Photoinduced Polymerization of Eugenol-Derived Methacrylates

The photoinduced polymerization of ethoxy eugenyl methacrylate (EEMA), ethoxy isoeugenyl methacrylate (EIMA) and ethoxy dihydroeugenyl methacrylate (EDMA) was conducted by irradiating the monomers spread on a solid substrate in the form of films. Different experimental conditions were investigated. At first, the reactions were attempted in the absence of any photoinitiator (PI). Avoiding the use of PI is a crucial step in the development of new products for many real life applications (e.g., inks for food packaging, dental materials) as photoinitiators decompose into harmful species which can uncontrollably migrate [START_REF] Lalevée | Chapter 6. How to Design Novel Photoinitiators for Blue Light[END_REF]. Then, reactions were done in the presence of two different Norrish Type I photoinitiators. Azo-initiators, largely used in radical polymerization, can also be used as photoinitiators. However, they have been reported to have low efficiency compared to acyl photoinitiators [START_REF] Detrembleur | Synthetic and mechanistic inputs of photochemistry into the bis-acetylacetonatocobalt-mediated radical polymerization of n-butyl acrylate and vinyl acetate[END_REF]. Thus, Darocur 1173 and Irgacure 819 have been selected. As the reaction proceeds via a radical mechanism, the effect of oxygen was studied by irradiating the monomers either in the presence or absence of air. Experiments in the absence of air were carried out by covering the monomer films with a polypropylene (PP) film. This is a common strategy to protect polymerization samples from oxygen and reduce inhibition [START_REF] Ligon | Strategies to reduce oxygen inhibition in photoinduced polymerization[END_REF].

Photopolymerization Without Photoinitiator

The kinetics of the reactions of the eugenol-derivatives EDMA, EEMA, and EIMA were monitored by Real-Time FT-IR in transmission mode while they were exposed to a UV-light source (L9566-02A, 240 to 400 nm, 260 mW cm -2 ) [START_REF]Hamamatsu Spotlight Sources Lightningcure Series[END_REF] in the presence of air or in the absence of air. The band corresponding to the methacrylate double bond at 1638 cm -1 (C=C stretching vibration) [START_REF] Nyquist | Infrared study of vinyl acetate, methyl acrylate and methyl methacrylate in various solvents[END_REF], and the aromatic band at 1514 cm -1 (C-H aromatic in-plane bending) [START_REF] Chowdhry | Infrared and raman spectroscopy of eugenol, isoeugenol, and methyl eugenol: Conformational analysis and vibrational assignments from density functional theory calculations of the anharmonic fundamentals[END_REF] as reference, were monitored over the irradiation time. The conversion of the methacrylate double bonds (MDB) for EDMA, EEMA, and EIMA are presented in Figure 2. Simultaneously, the conversion of the allylic double bonds (ADB) from EEMA and propenyl double bonds (PDB) from EIMA were monitored using the bands at 995 cm -1 and 960 cm -1 respectively. The results comparing allylic and propenyl double bond conversion with regards to the presence or absence of air are plotted in Figure 3.

Molecules 2020, 25, x 3 of 17 dentistry. Therefore, in the present article, we investigate the photopolymerization of films of these eugenol methacrylates under irradiation in different conditions: with or without the radical photoinitiator and in the presence or in the absence of air. Moreover, the conversion of the methacrylic double bond of the three monomers as well as the conversion of the allylic (EEMA) or propenyl (EIMA) double bonds are monitored, and the properties of the polymers are tested.

Results and Discussion

Kinetic Monitoring of Photoinduced Polymerization of Eugenol-Derived Methacrylates

The photoinduced polymerization of ethoxy eugenyl methacrylate (EEMA), ethoxy isoeugenyl methacrylate (EIMA) and ethoxy dihydroeugenyl methacrylate (EDMA) was conducted by irradiating the monomers spread on a solid substrate in the form of films. Different experimental conditions were investigated. At first, the reactions were attempted in the absence of any photoinitiator (PI). Avoiding the use of PI is a crucial step in the development of new products for many real life applications (e.g., inks for food packaging, dental materials) as photoinitiators decompose into harmful species which can uncontrollably migrate [START_REF] Lalevée | Chapter 6. How to Design Novel Photoinitiators for Blue Light[END_REF]. Then, reactions were done in the presence of two different Norrish Type I photoinitiators. Azo-initiators, largely used in radical polymerization, can also be used as photoinitiators. However, they have been reported to have low efficiency compared to acyl photoinitiators [START_REF] Detrembleur | Synthetic and mechanistic inputs of photochemistry into the bis-acetylacetonatocobalt-mediated radical polymerization of n-butyl acrylate and vinyl acetate[END_REF]. Thus, Darocur 1173 and Irgacure 819 have been selected. As the reaction proceeds via a radical mechanism, the effect of oxygen was studied by irradiating the monomers either in the presence or absence of air. Experiments in the absence of air were carried out by covering the monomer films with a polypropylene (PP) film. This is a common strategy to protect polymerization samples from oxygen and reduce inhibition [START_REF] Ligon | Strategies to reduce oxygen inhibition in photoinduced polymerization[END_REF].

Photopolymerization Without Photoinitiator

The kinetics of the reactions of the eugenol-derivatives EDMA, EEMA, and EIMA were monitored by Real-Time FT-IR in transmission mode while they were exposed to a UV-light source (L9566-02A, 240 to 400 nm, 260 mW cm -2 ) [START_REF]Hamamatsu Spotlight Sources Lightningcure Series[END_REF] in the presence of air or in the absence of air. The band corresponding to the methacrylate double bond at 1638 cm -1 (C=C stretching vibration) [START_REF] Nyquist | Infrared study of vinyl acetate, methyl acrylate and methyl methacrylate in various solvents[END_REF], and the aromatic band at 1514 cm -1 (C-H aromatic in-plane bending) [START_REF] Chowdhry | Infrared and raman spectroscopy of eugenol, isoeugenol, and methyl eugenol: Conformational analysis and vibrational assignments from density functional theory calculations of the anharmonic fundamentals[END_REF] as reference, were monitored over the irradiation time. The conversion of the methacrylate double bonds (MDB) for EDMA, EEMA, and EIMA are presented in Figure 2. Simultaneously, the conversion of the allylic double bonds (ADB) from EEMA and propenyl double bonds (PDB) from EIMA were monitored using the bands at 995 cm -1 and 960 cm -1 respectively. The results comparing allylic and propenyl double bond conversion with regards to the presence or absence of air are plotted in Figure 3. Figure 2 shows that the methacrylic double bonds (MDB) of all monomers can react upon light exposure even in the absence of any photoinitiator. This is not surprising, as it has been previously reported that (meth)acrylates could undergo photopolymerization without a photoinitiator due to self-initiation [START_REF] Wang | Self-initiated photopolymerization and photografting of acrylic monomers[END_REF][START_REF] Huang | Self-initiated photopolymerization of hyperbranched acrylates[END_REF][START_REF] Hoijemberg | Two routes towards photoinitiator-free photopolymerization in miniemulsion: Acrylate self-initiation and photoactive surfactant[END_REF][START_REF] Furutani | Initiator-free photopolymerization of common acrylate monomers with 254 nm light[END_REF]. Interestingly, the final conversion of MDB and the conversion rate (i.e., the slope of the conversion versus time curve) were different for the three monomers. The reactivity trend is as follows: EIMA >> EDMA > EEMA. EIMA was the most reactive monomer (higher slope, final conversion of 59%), while the conversion of EDMA and EEMA remained low reaching only 22% and 12% respectively at the end of the irradiation (Table 1). Conversion rates of EDMA and EEMA were much lower than that of EIMA. The different reactivities of the MDBs may be explained by the difference in the UV absorption spectra of the monomers (Figure S4). The monomers UV absorption curves overlap with the emission spectrum of the Hg lamp used as irradiation source [START_REF]Hamamatsu Spotlight Sources Lightningcure Series[END_REF]. The absorption of EIMA is significantly higher than that of EDMA and EEMA. EIMA is thus more likely to undergo faster self-initiation. To confirm that the monomers self-initiate due to UV absorption [START_REF] Wang | Self-initiated photopolymerization and photografting of acrylic monomers[END_REF], further polymerization experiments protected from air were performed under UV irradiation but using a filter to stop wavelengths below 365 nm. As expected, since the monomers absorb below 320 nm, no reaction was observed for any of the monomers. These experiments confirmed the hypothesis of self-initiation responsible for the polymerization occurring in the absence of PI.

Besides the methacrylic group, the reactivity of the allyl and propenyl groups, present in EEMA and EIMA respectively, were studied during photopolymerization processes protected from air (Figure 3). Indeed, allylic and propenyl double bonds can experience secondary reactions such as (degradative) chain transfer reactions (allylic hydrogen abstraction produce poorly-reactive highlystabilized radicals) and radical addition (cross-propagation) (Figure 4) [START_REF] Molina-Gutiérrez | Eugenol: A promising building block for synthesis of radically polymerizable monomers[END_REF][START_REF] Rojo | From natural products to polymeric derivatives of "eugenol": A new approach for preparation of dental composites and orthopedic bone cements[END_REF][START_REF] Zhang | Synthesis and characterization of methacrylated eugenol as a sustainable reactive diluent for a maleinated acrylated epoxidized soybean oil resin[END_REF][START_REF] Gazzotti | One-pot synthesis of sustainable high-performance thermoset by exploiting eugenol functionalized 1,3-dioxolan-4-one[END_REF]. The radicals formed from these secondary reactions can undergo further propagation or termination yielding branched and even crosslinked polymers (in the case of termination by combination). In the absence of air, the propenyl double bond (PDB) of EIMA was quite reactive and reached nearly the same conversion as the methacrylic double bonds (58%). On the other hand, the allylic double bond (ADB) of EEMA displayed a very low conversion (6%). For this monomer, a lower reaction rate and MDB conversion were obtained (Figure 2). EEMA allylic hydrogens can be abstracted and form highly stabilized radicals (main secondary reaction). This affects the propagation rate as the corresponding radicals become less reactive (degradative chain transfer). This effect was not seen for EIMA, implying that PDB reacts mainly though cross-propagation reactions between propenyl and methacrylic groups. Figure 2 shows that the methacrylic double bonds (MDB) of all monomers can react upon light exposure even in the absence of any photoinitiator. This is not surprising, as it has been previously reported that (meth)acrylates could undergo photopolymerization without a photoinitiator due to self-initiation [START_REF] Wang | Self-initiated photopolymerization and photografting of acrylic monomers[END_REF][START_REF] Huang | Self-initiated photopolymerization of hyperbranched acrylates[END_REF][START_REF] Hoijemberg | Two routes towards photoinitiator-free photopolymerization in miniemulsion: Acrylate self-initiation and photoactive surfactant[END_REF][START_REF] Furutani | Initiator-free photopolymerization of common acrylate monomers with 254 nm light[END_REF]. Interestingly, the final conversion of MDB and the conversion rate (i.e., the slope of the conversion versus time curve) were different for the three monomers. The reactivity trend is as follows: EIMA >> EDMA > EEMA. EIMA was the most reactive monomer (higher slope, final conversion of 59%), while the conversion of EDMA and EEMA remained low reaching only 22% and 12% respectively at the end of the irradiation (Table 1). Conversion rates of EDMA and EEMA were much lower than that of EIMA. The different reactivities of the MDBs may be explained by the difference in the UV absorption spectra of the monomers (Figure S4). The monomers UV absorption curves overlap with the emission spectrum of the Hg lamp used as irradiation source [START_REF]Hamamatsu Spotlight Sources Lightningcure Series[END_REF]. The absorption of EIMA is significantly higher than that of EDMA and EEMA. EIMA is thus more likely to undergo faster self-initiation. To confirm that the monomers self-initiate due to UV absorption [START_REF] Wang | Self-initiated photopolymerization and photografting of acrylic monomers[END_REF], further polymerization experiments protected from air were performed under UV irradiation but using a filter to stop wavelengths below 365 nm. As expected, since the monomers absorb below 320 nm, no reaction was observed for any of the monomers. These experiments confirmed the hypothesis of self-initiation responsible for the polymerization occurring in the absence of PI.

Besides the methacrylic group, the reactivity of the allyl and propenyl groups, present in EEMA and EIMA respectively, were studied during photopolymerization processes protected from air (Figure 3). Indeed, allylic and propenyl double bonds can experience secondary reactions such as (degradative) chain transfer reactions (allylic hydrogen abstraction produce poorly-reactive highly-stabilized radicals) and radical addition (cross-propagation) (Figure 4) [START_REF] Molina-Gutiérrez | Eugenol: A promising building block for synthesis of radically polymerizable monomers[END_REF][START_REF] Rojo | From natural products to polymeric derivatives of "eugenol": A new approach for preparation of dental composites and orthopedic bone cements[END_REF][START_REF] Zhang | Synthesis and characterization of methacrylated eugenol as a sustainable reactive diluent for a maleinated acrylated epoxidized soybean oil resin[END_REF][START_REF] Gazzotti | One-pot synthesis of sustainable high-performance thermoset by exploiting eugenol functionalized 1,3-dioxolan-4-one[END_REF]. The radicals formed from these secondary reactions can undergo further propagation or termination yielding branched and even crosslinked polymers (in the case of termination by combination). In the absence of air, the propenyl double bond (PDB) of EIMA was quite reactive and reached nearly the same conversion as the methacrylic double bonds (58%). On the other hand, the allylic double bond (ADB) of EEMA displayed a very low conversion (6%). For this monomer, a lower reaction rate and MDB conversion were obtained (Figure 2). EEMA allylic hydrogens can be abstracted and form highly stabilized radicals (main secondary reaction). This affects the propagation rate as the corresponding radicals become less reactive (degradative chain transfer). This effect was not seen for EIMA, implying that PDB reacts mainly though cross-propagation reactions between propenyl and methacrylic groups. Figure 2 shows that the methacrylate double bond conversions under air have different profiles compared to those of the polymerization protected from air. Changes in the curve slope are visible, signaling a noticeable variation of the speed of the reaction during the irradiation. This behavior is particularly clear for EIMA and to a lower extent for EEMA (Figure 2), but it is negligible for EDMA. The sigmoidal profile appearing in the curves is caused by the occurrence of two polymerization regimes. These are due to the formation of hydroperoxides in the presence of oxygen, as reported in literature [START_REF] Pynaert | Effect of reactive oxygen species on the kinetics of free radical photopolymerization[END_REF]. Indeed, radicals produced by irradiation of the monomers can react with oxygen according to Equations ( 1)- [START_REF] Crivello | Photopolymer materials and processes for advanced technologies[END_REF]. Figure 2 shows that the methacrylate double bond conversions under air have different profiles compared to those of the polymerization protected from air. Changes in the curve slope are visible, signaling a noticeable variation of the speed of the reaction during the irradiation. This behavior is particularly clear for EIMA and to a lower extent for EEMA (Figure 2), but it is negligible for EDMA. The sigmoidal profile appearing in the curves is caused by the occurrence of two polymerization regimes. These are due to the formation of hydroperoxides in the presence of oxygen, as reported in literature [START_REF] Pynaert | Effect of reactive oxygen species on the kinetics of free radical photopolymerization[END_REF]. Indeed, radicals produced by irradiation of the monomers can react with oxygen according to Equations ( 1)- [START_REF] Crivello | Photopolymer materials and processes for advanced technologies[END_REF].

𝑅 ⋅ + 𝑂 2 → 𝑅𝑂 2 ⋅ (1) 𝑅𝑂 2 ⋅ + 𝑅𝐻 → 𝑅𝑂 2 𝐻 + 𝑅 ⋅ (2) 𝑅𝑂 2 𝐻 ℎ𝑣 → 𝑅𝑂 ⋅ + 𝐻𝑂 ⋅ (3)
R • + O 2 → RO 2 • (1) RO 2 • + RH → RO 2 H + R • (2) RO 2 H hv → RO • + HO • (3)
Once peroxyl radicals (RO 2 ) are formed (Equation ( 1)), hydroperoxides (RO 2 H) are generated by hydrogen abstraction (Equation ( 3)) [START_REF] Ligon | Strategies to reduce oxygen inhibition in photoinduced polymerization[END_REF]. The three monomers possess abstractable hydrogen atoms: bis-allylic hydrogens in EEMA (Ph-CH 2 -CH= CH 2 ), propenylic hydrogens in EIMA (Ph-CH= CH-CH 3 ), and benzylic hydrogens in EDMA (Ph-CH 2 -CH 2 -CH 3 ). Hydroperoxides react slowly, therefore oxygen is often described as an inhibitor of radical polymerization. The phenomenon is particularly severe in photopolymerization when monomers are irradiated in films. In such a situation, a large area is exposed to oxygen, and oxygen can be continuously replaced by diffusion at the surface of the reacting formulation. However, hydroperoxides can decompose, through continued irradiation, to produce new radicals (Equation ( 3)) that are able to trigger additional initiation and a second polymerization regime [START_REF] Pynaert | Effect of reactive oxygen species on the kinetics of free radical photopolymerization[END_REF]. Herein, IR analyses confirmed the hydroperoxides formation during the photopolymerization reactions carried out in the presence of air (Figures S12-S14).

Contrary to what was expected, all the monomers showed a higher MDB conversion in the presence of air than in the absence of air. During the first minute of irradiation, EEMA and EDMA displayed very similar MDB conversion under air or protected from air. However, conversion increased significantly at higher irradiation time in the presence of air. Specifically, for EEMA, MDB final conversion reached 66% under air (and only 12% when protected from air over the same irradiation time). For EDMA, the final conversion was 35% in the presence of air and 22% when protected from air (Table 1). Finally, for EIMA, the MDB conversion and conversion rates under air were always higher than in the absence of air from the onset of irradiation. Similarly to EEMA, after the first polymerization regime, the conversion rate of EIMA MDB increased (producing a second polymerization regime) and a final conversion of 86% was reached (59% in the absence of air).

Figure 3 shows that EEMA ADB are consumed up to 49% in the presence of air, while they are almost non-reactive in the absence of air. This can explain the high reactivity of the EEMA MDBs under air. Allylic double bonds undergo hydrogen abstraction leading to radicals that can react with oxygen to form peroxy radicals which scavenge oxygen and thus prevent the oxygen inhibition of MDB polymerization. The peroxy radicals can then form hydroperoxides that decompose to provide additional radicals for further MDB polymerization. In the case of EIMA, the conversion of PDB reached relatively high values both under air (68%) and while air-protected (58%) and was always higher under air, independent of the irradiation time (Figure 3 and Table 1). PDB can be consumed not only by the formation of peroxy and hydroperoxy radicals when oxygen is present but also by cross-propagation reactions (Figure 4). Noteworthy is that only a slightly higher consumption of PDB was observed in the polymerizations carried out under air compared to the polymerizations protected from it. This suggests that although hydroperoxides are formed under air, cross-propagation is the main secondary reaction.

In addition, higher conversion of the monomers under air can also be related to the formation of ozone induced by the UV irradiation at 242 nm and its subsequent photolysis into singlet oxygen ( 1 O 2 ) [START_REF] Schiff | Kinetics of ozone photochemistry[END_REF][START_REF] Matsumi | Photolysis of atmospheric ozone in the ultraviolet region[END_REF]. Singlet oxygen can react with the ADB and PDB of both EEMA and EIMA, again forming peroxy radicals and hydroperoxides which dissociate into other radicals [START_REF] Frimer | The reaction of singlet oxygen with olefins: The question of mechanism[END_REF][START_REF] Singleton | Mechanism of ene reactions of singlet oxygen. a two-step no-intermediate mechanism[END_REF][START_REF] Ghogare | Using singlet oxygen to synthesize natural products and drugs[END_REF][START_REF] Parker | Laser photochemistry of molecular oxygen[END_REF].

Styrene can polymerize in the absence of a photoinitiator, due its capacity to form charge-transfer complexes with oxygen [START_REF] Kodaira | Photopolymerization of styrene in the presence of oxygen. role of the charge-transfer complex[END_REF][START_REF] Kodaira | The role of the charge-transfer complex in the photocopolymerization of oxygen with styrene and α-methylstyrene[END_REF]. These complexes lead to the production of peroxides eventually leading to the production of radicals. Recently, Krueger et al. [START_REF] Krüger | Photoinitiated bulk and emulsion polymerization of styrene-evidence for photo-controlled radical polymerization[END_REF] concluded that in photoinitiator-free styrene polymerizations, oxygen reacts photochemically with styrene at the beginning of their polymerization reactions but that peroxides are not the sole source of radical formation. The photochemical radical generation via photo electron transfer (PET) requires a donor-acceptor pair. In the absence of oxygen, the PET between styrene-polystyrene leads to the generation of radical ions continuing the polymerization in the absence of PI. A similar process could occur in the case of isoeugenol. Moreover, the triplet state of isoeugenol derivatives has been suspected to produce singlet oxygen able to react with the double bond to form dioxetane, which can cleave to produce aldehydes [START_REF] Kodaira | The role of the charge-transfer complex in the photocopolymerization of oxygen with styrene and α-methylstyrene[END_REF][START_REF] Gellerstedt | Light-induced oxidation of lignin. The behaviour of structural units containing a ring-conjugated double bond[END_REF]. Nonetheless, no increment or change was noticed in the bands at 2827 cm -1 and 2725 cm -1 , corresponding to the Fermi resonance characteristic of aldehydes. Hence, the consumption of PDB does not follow this pathway here.

To avoid the absorption of light by the monomers and the possible formation of ozone, further polymerization experiments under air were performed using a 355-375 nm bandpass filter. Unsurprisingly, no reaction was observed for any of the monomers, since neither monomer homolytic cleavage nor ozone production (both leading to radicals) occur at this longer irradiation wavelength.

In conclusion, the eugenol-derived methacrylates can photopolymerize in the absence of the photoinitiator both in the presence or absence of air as long as the irradiation wavelengths are short (from 220 to 355 nm). The presence of oxygen (while irradiating at short wavelengths <365 nm) leads to higher conversion of the methacrylic, allylic, and propenyl double bonds of the eugenol-derived methacrylates as a consequence of the production of hydroperoxides and their decomposition. The presence of ADB and PDB causes secondary reactions such as allylic hydrogen abstraction and cross-propagation which could lead to branched or crosslinked structures.

Photopolymerization with Photoinitiator

Experiments proceeded with the use of common photoinitiators. Darocur 1173 was added to the monomers at 2% wbm (weight based on monomer). It is a Norrish Type I photoinitiator that undergoes homolytic cleavage to produce two carbon-centered radicals (Figure 5 and Figure S5). Experiments proceeded with the use of common photoinitiators. Darocur 1173 was added to the monomers at 2% wbm (weight based on monomer). It is a Norrish Type I photoinitiator that undergoes homolytic cleavage to produce two carbon-centered radicals (Figures 5 andS5). The evolutions with irradiation time of the MDB conversions of the three monomers for the photopolymerizations carried out in the presence and in the absence of air using Darocur 1173 are shown in Figure 6. The evolutions with time of the conversions of ADB and PDB in the same conditions are displayed in Figure 7. The comparison of these data with those observed for polymerizations carried out in the absence of the photoinitiator demonstrates, as expected, that the PI accelerates the polymerization.

In the absence of air, the conversion of EDMA MDB was fast and reached 100%. The polymerization rates of the difunctional methacrylates, EEMA and EIMA, were slower. EEMA MDB conversion reached 66%, whereas that of EIMA MDB reached 100% although at a lower rate than EDMA (Table 1). In the case of EEMA, radicals were presumed to be consumed by the allyl groups (degradative chain transfer), even to a small extent, to form highly stabilized radicals that resulted in a lower polymerization rate and ultimately in termination reactions limiting the conversion. No increment or appearance of the band at 960 cm -1 corresponding to the propenyl double bonds was observed, suggesting that the isomerization of EEMA into EIMA does not occur under these experimental conditions. In addition, the conversion of allylic double bonds to propenyl double bonds would lead to the decrease of the 995 cm -1 peak area (corresponding to allylic double bond), which did not occur, as conversion was <10%. In the case of EIMA, the PDBs were consumed up to 56% (Table 1) most likely via cross-propagation (vide supra). This cross-propagation slightly slows down the polymerization but does not prevent the quantitative conversion of EIMA MDB. Moreover, as discussed above, EIMA has a higher absorption than EEMA and forms propagating species by itself, thus enhancing the conversion.

In the presence of air, the polymerization rates were lower than those observed for polymerizations carried out in the absence of air. This decrease of the polymerization rate was likely caused by oxygen inhibition. EDMA was strongly inhibited by air and presented the lowest MDB conversion (61%). EIMA and EEMA were less affected and reached high conversions: 92% and 81%, The evolutions with irradiation time of the MDB conversions of the three monomers for the photopolymerizations carried out in the presence and in the absence of air using Darocur 1173 are shown in Figure 6. The evolutions with time of the conversions of ADB and PDB in the same conditions are displayed in Figure 7. The comparison of these data with those observed for polymerizations carried out in the absence of the photoinitiator demonstrates, as expected, that the PI accelerates the polymerization.

Molecules 2020, 25, x 8 of 17 respectively (Table 1). As previously discussed, reactions with oxygen can lead to the formation of hydroperoxides, which decompose, causing a second polymerization regime. The corresponding sigmoidal curve is observed quite clearly for EEMA, but not for EIMA. In addition, the conversion of PDB was higher than that of ADB (76% and 64% respectively, Figure 7). PDBs were also highly consumed when protected from air (56%), while ADBs were not (7%). This may be because crosspropagation is the dominant reaction in the consumption of PDBs (only a fraction might be consumed by hydrogen abstraction or hydroperoxide formation). In contrast, in the case of EEMA, for which bis-allylic H-abstraction and radical termination dominate, the absence of air limits the overall polymerization. However, in the presence of air, hydroperoxide dissociation provides the necessary radicals to continue EEMA MDB polymerization. The effect of the monomer light absorption and the possible formation of ozone on the kinetics of polymerization remained to be investigated. Thus, a bandpass filter centered at 365 nm, preventing monomer light absorption and ozone formation (<242 nm) was again used to irradiate the formulations. Experiments with Darocur 1173 were performed and results are shown in Figures 8 and9. In the absence of air, the conversion of EDMA MDB was fast and reached 100%. The polymerization rates of the difunctional methacrylates, EEMA and EIMA, were slower. EEMA MDB conversion reached 66%, whereas that of EIMA MDB reached 100% although at a lower rate than EDMA (Table 1). In the case of EEMA, radicals were presumed to be consumed by the allyl groups (degradative chain transfer), even to a small extent, to form highly stabilized radicals that resulted in a lower polymerization rate and ultimately in termination reactions limiting the conversion. No increment or appearance of the band at 960 cm -1 corresponding to the propenyl double bonds was observed, suggesting that the isomerization of EEMA into EIMA does not occur under these experimental conditions. In addition, the conversion of allylic double bonds to propenyl double bonds would lead to the decrease of the 995 cm -1 peak area (corresponding to allylic double bond), which did not occur, as conversion was <10%. In the case of EIMA, the PDBs were consumed up to 56% (Table 1) most likely via cross-propagation (vide supra). This cross-propagation slightly slows down the polymerization but does not prevent the quantitative conversion of EIMA MDB. Moreover, as discussed above, EIMA has a higher absorption than EEMA and forms propagating species by itself, thus enhancing the conversion.

In the presence of air, the polymerization rates were lower than those observed for polymerizations carried out in the absence of air. This decrease of the polymerization rate was likely caused by oxygen inhibition. EDMA was strongly inhibited by air and presented the lowest MDB conversion (61%). EIMA and EEMA were less affected and reached high conversions: 92% and 81%, respectively (Table 1). As previously discussed, reactions with oxygen can lead to the formation of hydroperoxides, which decompose, causing a second polymerization regime. The corresponding sigmoidal curve is observed quite clearly for EEMA, but not for EIMA. In addition, the conversion of PDB was higher than that of ADB (76% and 64% respectively, Figure 7). PDBs were also highly consumed when protected from air (56%), while ADBs were not (7%). This may be because cross-propagation is the dominant reaction in the consumption of PDBs (only a fraction might be consumed by hydrogen abstraction or hydroperoxide formation). In contrast, in the case of EEMA, for which bis-allylic H-abstraction and radical termination dominate, the absence of air limits the overall polymerization. However, in the presence of air, hydroperoxide dissociation provides the necessary radicals to continue EEMA MDB polymerization.

The effect of the monomer light absorption and the possible formation of ozone on the kinetics of polymerization remained to be investigated. Thus, a bandpass filter centered at 365 nm, preventing monomer light absorption and ozone formation (<242 nm) was again used to irradiate the formulations. Experiments with Darocur 1173 were performed and results are shown in Figures 8 and9.

monomers (which do not absorb at 365, see Figure S4). Moreover, the irradiance decreases because of the filter (78 mW.cm -2 UVA). However, contrary to the experiments carried out without filter, EIMA showed lower MDB conversion than EEMA. This means that EIMA UV light absorption and cleavage (responsible for the reaction in the absence of PI) contribute to the formation of reactive species. Moreover, the consumption of the PDBs reached 40% (Table 1), while ADB consumption remained low (6%). The comparison of the results of the polymerizations irradiated with and without filter using Darocur 1173 (Norrish Type I photoinitiator) suggests that the monomer absorptions play an important role in their reactivity, especially for EIMA. Moreover, in the presence of air, reactions of peroxides and hydroperoxides, ozone formation and photolysis to singlet oxygen, contribute to the polymerization mechanism.

In a final study, a passband filter centered at 365 nm and another Norrish type I PI with high absorption at longer wavelengths (absorption in the UVA region), Irgacure 819 [START_REF] Eibel | Choosing the ideal photoinitiator for free radical photopolymerizations: Predictions based on simulations using established data[END_REF] were used. The results obtained in the different conditions (with and without air) are gathered in the SI (Figures S16 andS17). In this case, the potential cleavage of the methacrylates was prevented by the pass band filter and oxygen inhibition or the production of hydroperoxides was avoided by protecting the samples from air. In addition, the flux of radicals had been raised by using Irgacure 819, which has a higher molar extinction coefficient and quantum yield than Darocur 1173 in the UVA region [START_REF] Eibel | Choosing the ideal photoinitiator for free radical photopolymerizations: Predictions based on simulations using established data[END_REF]. A behavior similar to that of Darocur 1173 was observed. The experiments executed in different conditions (with or without initiator, in the presence and absence of air, with or without filter) revealed that EDMA polymerization was always strongly inhibited in the presence of air. On the contrary, the presence of the pending allylic (EEMA) or propenyl (EIMA) double bonds could produce a second polymerization regime due to dissociation of hydroperoxides (formed in-situ in the presence of air under shorter (<320 nm) wavelength irradiation). It was also shown that the dominant reaction mechanism for PDB is cross-propagation rather than hydrogen abstraction or hydroperoxide formation, as they were consumed to a high extent even in the absence of air. The polymerization of EIMA was the least affected by air.

Polymers Characterization

Properties of the polymers prepared by photoinduced polymerization in the presence of Darocur 1173, both in the presence and absence of air, were measured (Table 2). The polymerization conditions (i.e., use of a UV irradiation spectrum from 320 to 390 nm and of Darocur 1173 as PI) were selected to guarantee high conversions. Polymers obtained from EDMA had a linear structure and were soluble (gel content ≈ 0%). In contrast, polymers from EEMA and EIMA were crosslinked and completely insoluble (gel content = 100%), suggesting that the unreacted functional groups potentially present (when the conversion was not quantitative, as reported in Table S2) were dangling from the network and that no free oligomer or monomer were present. The glass transition temperature did not vary much between the samples irradiated in the presence or in the absence of In the presence of air, almost no conversion of MDB could be measured for EEMA and EDMA (conversion <10%, Figure 8 and Table 1). Similar results were observed for EEMA ADB with a conversion close to zero (2%, Figure 9 and Table 1). In the presence of the filter, the production of radicals by cleavage of the photoinitiator was strongly diminished and the scarce quantity of radicals could quickly be quenched by oxygen, while no peroxides nor hydroperoxides could be generated [START_REF] Ligon | Strategies to reduce oxygen inhibition in photoinduced polymerization[END_REF]. Nevertheless, the considerable consumption of both EIMA MDB and PDB (39% and 58% respectively, Table 1 was observed in spite of the presence of oxygen. This may be explained by the formation of charge-transfer complexes of EIMA with oxygen (as reported for styrene [START_REF] Kodaira | Photopolymerization of styrene in the presence of oxygen. role of the charge-transfer complex[END_REF][START_REF] Kodaira | The role of the charge-transfer complex in the photocopolymerization of oxygen with styrene and α-methylstyrene[END_REF]) which lead to the production of radicals and allow propagation.

In the absence of air, both the polymerization rate and the final conversion increased significantly. MDB conversion followed the trend: EDMA (94%) > EEMA (74%) > EIMA (65%) (Table 1). The polymerization rate was lower than that observed for the reaction carried out using light, including shorter irradiation wavelengths (i.e., without filter). This may be explained by a lower radical production both from Darocur 1173 (which absorbs weakly at 365 nm) and from the monomers (which do not absorb at 365, see Figure S4). Moreover, the irradiance decreases because of the filter (78 mW.cm -2 UVA). However, contrary to the experiments carried out without filter, EIMA showed lower MDB conversion than EEMA. This means that EIMA UV light absorption and cleavage (responsible for the reaction in the absence of PI) contribute to the formation of reactive species. Moreover, the consumption of the PDBs reached 40% (Table 1), while ADB consumption remained low (6%).

The comparison of the results of the polymerizations irradiated with and without filter using Darocur 1173 (Norrish Type I photoinitiator) suggests that the monomer absorptions play an important role in their reactivity, especially for EIMA. Moreover, in the presence of air, reactions of peroxides and hydroperoxides, ozone formation and photolysis to singlet oxygen, contribute to the polymerization mechanism.

In a final study, a passband filter centered at 365 nm and another Norrish type I PI with high absorption at longer wavelengths (absorption in the UVA region), Irgacure 819 [START_REF] Eibel | Choosing the ideal photoinitiator for free radical photopolymerizations: Predictions based on simulations using established data[END_REF] were used. The results obtained in the different conditions (with and without air) are gathered in the SI (Figures S16 andS17). In this case, the potential cleavage of the methacrylates was prevented by the pass band filter and oxygen inhibition or the production of hydroperoxides was avoided by protecting the samples from air. In addition, the flux of radicals had been raised by using Irgacure 819, which has a higher molar extinction coefficient and quantum yield than Darocur 1173 in the UVA region [START_REF] Eibel | Choosing the ideal photoinitiator for free radical photopolymerizations: Predictions based on simulations using established data[END_REF]. A behavior similar to that of Darocur 1173 was observed.

The experiments executed in different conditions (with or without initiator, in the presence and absence of air, with or without filter) revealed that EDMA polymerization was always strongly inhibited in the presence of air. On the contrary, the presence of the pending allylic (EEMA) or propenyl (EIMA) double bonds could produce a second polymerization regime due to dissociation of hydroperoxides (formed in-situ in the presence of air under shorter (<320 nm) wavelength irradiation). It was also shown that the dominant reaction mechanism for PDB is cross-propagation rather than hydrogen abstraction or hydroperoxide formation, as they were consumed to a high extent even in the absence of air. The polymerization of EIMA was the least affected by air.

Polymers Characterization

Properties of the polymers prepared by photoinduced polymerization in the presence of Darocur 1173, both in the presence and absence of air, were measured (Table 2). The polymerization conditions (i.e., use of a UV irradiation spectrum from 320 to 390 nm and of Darocur 1173 as PI) were selected to guarantee high conversions. Polymers obtained from EDMA had a linear structure and were soluble (gel content ≈ 0%). In contrast, polymers from EEMA and EIMA were crosslinked and completely insoluble (gel content = 100%), suggesting that the unreacted functional groups potentially present (when the conversion was not quantitative, as reported in Table S2) were dangling from the network and that no free oligomer or monomer were present. The glass transition temperature did not vary much between the samples irradiated in the presence or in the absence of air except for poly(EDMA) prepared by irradiation under air. In this case, the presence of oligomers or unreacted monomer plasticized the resulting polymer and reduced its T g . The obtained T g s were in agreement with the results previously obtained with polymers prepared by emulsion polymerization [START_REF] Molina-Gutiérrez | Emulsion polymerization of dihydroeugenol-, eugenol-, and isoeugenol-derived methacrylates[END_REF]. The TGA results showed that the starting degradation temperatures of the polymers were always higher than 230 • C (Figures 10 and11). Polymerization carried out in the presence of air led to crosslinked poly(EEMA) with higher decomposition temperatures, due to a higher consumption of ADBs. A slightly lower degradation temperature was registered for poly(EIMA) prepared in the presence of air but both polymers (produced under air or in the absence of air) exhibited complex profiles, indicating complex polymeric architectures. Their glass transition temperatures (ranging from 8 • C and 58 • C) as well as their degradation temperatures (above 230 • C) make these materials suitable for application in coatings.

Materials and Methods

Materials

Ethoxy eugenyl methacrylate (EEMA), ethoxy isoeugenyl methacrylate (EIMA), ethoxy dihydroeugenyl methacrylate (EDMA) monomers (Figure 1) were synthesized as described in a

Materials and Methods

Materials

Ethoxy eugenyl methacrylate (EEMA), ethoxy isoeugenyl methacrylate (EIMA), ethoxy dihydroeugenyl methacrylate (EDMA) monomers (Figure 1) were synthesized as described in a The water and hexadecane contact angles (Table 2) indicated that the wettability of all the polymers were independent of the structure. The polymers were almost hydrophobic and displayed moderate oleophilicity.

Materials and Methods

Materials

Ethoxy eugenyl methacrylate (EEMA), ethoxy isoeugenyl methacrylate (EIMA), ethoxy dihydroeugenyl methacrylate (EDMA) monomers (Figure 1) were synthesized as described in a previous article from our group [START_REF] Molina-Gutiérrez | Eugenol: A promising building block for synthesis of radically polymerizable monomers[END_REF]. Toluene (>98%) was purchased from Sigma-Aldrich (Milano, Italy). The radical photoinitiators 2-hydroxy-2-methylpropiophenone (tradename Darocur 1173) and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (tradename Irgacure 819), kindly given by Badische Anilin und Soda Fabrik (BASF) (Ludwigshafen, Germany) were used as received. Darocur 1173 and Irgacure 819 structures are presented in Figure 12.

Photoinduced Polymerization of Eugenol Derived Methacrylates

Samples Preparation

To monitor the photopolymerization kinetics of each monomer, a mixture of monomer and PI at 2% wbm (weight based on monomer) was spread over a silicon wafer using a rod coater, forming a film with a thickness of 10 µ m. Samples were irradiated up to 9 min either under air or protected from air with a 30 μm-thick polypropylene (PP) film.

Kinetics Monitoring

Photopolymerization was monitored using Real-Time Fourier Transform Infrared (FT-IR) spectroscopy on a Nicolet iS50 spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). The spectra were acquired in transmission mode, in the 650-4000 cm -1 range, with 1 scan per spectrum and a resolution of 4 cm -1 .

A high-pressure mercury-xenon lamp Lightning Cure LC8 from Hamamatsu equipped with a flexible light guide was used as UV-light source (L9566-02A, 220 to 600 nm) [START_REF]Hamamatsu Spotlight Sources Lightningcure Series[END_REF] and an EIT Powerpuck ® II radiometer (EIT, LLC., Leesburg, VA, USA) was used to measure the UV irradiance. The samples were irradiated with 260 mW cm -2 (sum of UVA, UVB, UVC, UVV). In some experiments, light was filtered using a A9616-07 filter (Hamamatsu, Shizuoka, Japan) with a transmittance wavelength of 355-375 nm (centered at 365 nm) The filtered light had and intensity of 78 mW cm -2 (UVA).

Conversion Determination

The methacrylate double bonds (MDB) conversion was monitored using the band at 1638 cm -1 , the allylic double bonds (ADB) conversion was monitored using the band at 995 cm -1 , and the propenyl double bonds (PDB) conversion was determined using the 960 cm -1 band (Figure S1) [START_REF] Yoshimura | Bio-based polymer networks by thiol-ene photopolymerizations of allyl-etherified eugenol derivatives[END_REF][START_REF] Chowdhry | Infrared and raman spectroscopy of eugenol, isoeugenol, and methyl eugenol: Conformational analysis and vibrational assignments from density functional theory calculations of the anharmonic fundamentals[END_REF][START_REF] Silverstein | Spectrometric Identification of Organic Compounds[END_REF][START_REF] Wang | Rapid evaluation and quantitative analysis of eugenol derivatives in essential oils and cosmetic formulations on human skin using attenuated total reflectance-infrared spectroscopy[END_REF]. Each conversion was calculated using the following Equation (4):

𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛% 𝑡=𝑥 = 100 × (1 - 𝐴 𝑡=𝑥 𝑅𝑒𝑓. 𝐴 𝑡=𝑥 𝐴 𝑡=0 𝑅𝑒𝑓. 𝐴 𝑡=0 ) ( 4
)
Where A is the absorbance of the IR band of the functional group monitored during irradiation;

Photoinduced Polymerization of Eugenol Derived Methacrylates

Samples Preparation

To monitor the photopolymerization kinetics of each monomer, a mixture of monomer and PI at 2% wbm (weight based on monomer) was spread over a silicon wafer using a rod coater, forming a film with a thickness of 10 µm. Samples were irradiated up to 9 min either under air or protected from air with a 30 µm-thick polypropylene (PP) film.

Kinetics Monitoring

Photopolymerization was monitored using Real-Time Fourier Transform Infrared (FT-IR) spectroscopy on a Nicolet iS50 spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). The spectra were acquired in transmission mode, in the 650-4000 cm -1 range, with 1 scan per spectrum and a resolution of 4 cm -1 .

A high-pressure mercury-xenon lamp Lightning Cure LC8 from Hamamatsu equipped with a flexible light guide was used as UV-light source (L9566-02A, 220 to 600 nm) [START_REF]Hamamatsu Spotlight Sources Lightningcure Series[END_REF] and an EIT Powerpuck ® II radiometer (EIT, LLC., Leesburg, VA, USA) was used to measure the UV irradiance. The samples were irradiated with 260 mW cm -2 (sum of UVA, UVB, UVC, UVV). In some experiments, light was filtered using a A9616-07 filter (Hamamatsu, Shizuoka, Japan) with a transmittance wavelength of 355-375 nm (centered at 365 nm) The filtered light had and intensity of 78 mW cm -2 (UVA).

Conversion Determination

The methacrylate double bonds (MDB) conversion was monitored using the band at 1638 cm -1 , the allylic double bonds (ADB) conversion was monitored using the band at 995 cm -1 , and the propenyl

Conclusions

Three eugenol-derived methacrylates (EDMA, EEMA, EIMA) were polymerized via photopolymerization without a photoinitiator and with two Norrish Type I photoinitiators (Darocur 1173 and Irgacure 819), under air or without air. Their polymerization behavior under the different conditions was described. The monomers were shown to polymerize in the absence of a photoinitiator, especially in the presence of oxygen, due to self-initiation and oxidation reactions. In the presence of air, EIMA showed the highest conversions in any of the conditions studied. The second polymerization regimes, due to the formation and photolysis of hydroperoxides, were observed upon irradiation at a wavelength shorter than 365 nm in air with or without PI. This effect was clearly visible for EIMA and EEMA. Moreover, both allylic and propenyl double bonds were reactive in the presence of air. In addition, PDBs were shown to be predominantly polymerized via cross-propagation reactions while ADBs were mainly consumed under air via hydrogen abstraction and hydroperoxides formation. In the absence of air and using PI, EDMA reached the highest conversions. To eliminate the self-initiation of the monomers as well as the formation of hydroperoxides, a 365 nm passband filter and air-protected conditions were used. Under these conditions, the polymerization rate followed the order EDMA > EEMA > EIMA. EEMA displayed a significant reduction of the propagation rate, due to the formation of highly stabilized bis-allylic radicals. EIMA exhibited a lower MDB conversion, due to cross-propagation with the PDB. The polymers properties indicated that their use in applications in coatings and in dentistry could be envisaged.

Figure 1 .

 1 Figure 1. Eugenol-derived methacrylates.

Figure 1 .

 1 Figure 1. Eugenol-derived methacrylates.

Figure 2 .

 2 Figure 2. Methacrylate double bond (MDB) conversion of eugenol-derived monomers versus irradiation time in the absence of photoinitiator.

Figure 2 .

 2 Figure 2. Methacrylate double bond (MDB) conversion of eugenol-derived monomers versus irradiation time in the absence of photoinitiator.

Figure 3 .

 3 Figure 3. Allylic (ADB) and Propenyl double bond (PDB) conversion of eugenol-derived monomers versus irradiation time in the absence of photoinitiator.

Figure 3 .

 3 Figure 3. Allylic (ADB) and Propenyl double bond (PDB) conversion of eugenol-derived monomers versus irradiation time in the absence of photoinitiator.

Figure 4 .

 4 Figure 4. Scavenging and cross-propagation reactions on allylic and propenyl double bonds of eugenol-derived methacrylates.

Figure 4 .

 4 Figure 4. Scavenging and cross-propagation reactions on allylic and propenyl double bonds of eugenol-derived methacrylates.

Figure 5 .

 5 Figure 5. Homolytic cleavage under light of Darocur 1173.

Figure 5 .

 5 Figure 5. Homolytic cleavage under light of Darocur 1173.

Figure 6 .

 6 Figure 6. Methacrylate double bond (MDB) conversion of eugenol-derived monomers versus irradiation time in the presence of Darocur 1173.

Figure 6 .

 6 Figure 6. Methacrylate double bond (MDB) conversion of eugenol-derived monomers versus irradiation time in the presence of Darocur 1173.

Figure 6 .

 6 Figure 6. Methacrylate double bond (MDB) conversion of eugenol-derived monomers versus irradiation time in the presence of Darocur 1173.

Figure 7 .

 7 Figure 7. Allylic (ADB) and propenyl double bond (PDB) conversion of eugenol-derived monomers versus irradiation time in the presence of Darocur 1173.

Figure 7 .

 7 Figure 7. Allylic (ADB) and propenyl double bond (PDB) conversion of eugenol-derived monomers versus irradiation time in the presence of Darocur 1173.

Figure 8 .

 8 Figure 8. Methacrylate double bond (MDB) conversion of eugenol-derived monomers with irradiation time in the presence of Darocur 1173, irradiation under λ = 365 nm.

Figure 8 .

 8 Figure 8. Methacrylate double bond (MDB) conversion of eugenol-derived monomers with irradiation time in the presence of Darocur 1173, irradiation under λ = 365 nm. Molecules 2020, 25, x 10 of 17

Figure 9 .

 9 Figure 9. Allylic (ADB) and propenyl double bond (PDB) conversion of eugenol-derived monomers with irradiation time in the presence of Darocur 1173, irradiation under λ = 365 nm.

Figure 9 .

 9 Figure 9. Allylic (ADB) and propenyl double bond (PDB) conversion of eugenol-derived monomers with irradiation time in the presence of Darocur 1173, irradiation under λ = 365 nm.

Figure 10 .

 10 Figure 10. Thermogravimetric analysis of the different eugenol-derived methacrylates polymers from polymerization with Darocur 1173 under air (1° derivative).

Figure 11 .

 11 Figure 11. Thermogravimetric analysis of the different eugenol-derived methacrylates polymers from polymerization with Darocur 1173 protected from air (1° derivative).

Figure 10 .

 10 Figure 10. Thermogravimetric analysis of the different eugenol-derived methacrylates polymers from polymerization with Darocur 1173 under air (1 • derivative).

Figure 10 .

 10 Figure 10. Thermogravimetric analysis of the different eugenol-derived methacrylates polymers from polymerization with Darocur 1173 under air (1° derivative).

Figure 11 .

 11 Figure 11. Thermogravimetric analysis of the different eugenol-derived methacrylates polymers from polymerization with Darocur 1173 protected from air (1° derivative).

Figure 11 .

 11 Figure 11. Thermogravimetric analysis of the different eugenol-derived methacrylates polymers from polymerization with Darocur 1173 protected from air (1 • derivative).

  The radical photoinitiators 2-hydroxy-2-methylpropiophenone (tradename Darocur 1173) and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (tradename Irgacure 819), kindly given by Badische Anilin und Soda Fabrik (BASF) (Ludwigshafen, Germany) were used as received. Darocur 1173 and Irgacure 819 structures are presented in Figure12.

Figure 12 .

 12 Figure 12. Photoinitiators (a) Darocur 1173 (b) Irgacure 819.

Figure 12 .

 12 Figure 12. Photoinitiators (a) Darocur 1173 (b) Irgacure 819.

Table 1 .

 1 Methacrylate (MDB), allylic (ADB) and propenyl (PDB) double bond conversions under different conditions of irradiation.

				Conversion (%)
	Monomer	Condition	220-600 nm	365 nm
			Without PI	Darocur 1173	Darocur 1173	Irgacure 819
	EDMA MDB		22	100	94	96
	EEMA MDB	Air protected	12	66	74	76
	EIMA MDB		59	100	65	78
	EEMA ADB		6	7	6	3
	EIMA PDB		58	56	40	12
	EDMA MDB		35	61	8	8
	EEMA MDB	Under air	66	81	0	7
	EIMA MDB		86	92	39	40
	EEMA ADB		49	64	2	9
	EIMA PDB		68	76	58	30
	Molecules 2020, 25, x					5 of 17

Table 1 .

 1 Methacrylate (MDB), allylic (ADB) and propenyl (PDB) double bond conversions under different conditions of irradiation.

				Conversion (%)
	Monomer	Condition	220-600 nm Without PI Darocur 1173	365 nm Darocur 1173 Irgacure 819
	EDMA MDB		22	100	94	96
	EEMA MDB		12	66	74	76
	EIMA MDB	Air protected	59	100	65	78
	EEMA ADB		6	7	6	3
	EIMA PDB		58	56	40	12
	EDMA MDB		35	61	8	8
	EEMA MDB		66	81	0	7
	EIMA MDB	Under air	86	92	39	40
	EEMA ADB		49	64	2	9
	EIMA PDB		68	76	58	30
	2.1.2. Photopolymerization with Photoinitiator

Table 2 .

 2 Thermal properties, gel content, and contact angle of homopolymers produced with Darocur 1173.

	Monomer	Polymerization Condition	Gel Content (%)	T g ( • C)	T d5% ( • C)	Contact Angle DI Water ( • )	Contact Angle Hexadecane ( • )
	EDMA	with air	2	8	236	92	24
		no air	3	23	269	84	30
	EEMA	with air	100	35	298	89	33
		no air	98	34	294	85	34
	EIMA	with air	100	56	246	85	24
		no air	100	58	258	82	25

Table 2 .

 2 Thermal properties, gel content, and contact angle of homopolymers produced with Darocur 1173.

	Monomer	Polymerization Condition	Gel Content (%)	Tg (°C)	Td5% (°C)	Contact Angle DI Water (°)	Contact Angle Hexadecane (°)
	EDMA	with air	2	8	236	92	24
		no air	3	23	269	84	30
	EEMA	with air	100	35	298	89	33
		no air	98	34	294	85	34
	EIMA	with air	100	56	246	85	24
		no air	100	58	258	82	25

Table 2 .

 2 Thermal properties, gel content, and contact angle of homopolymers produced with Darocur 1173.

	Monomer	Polymerization Condition	Gel Content (%)	Tg (°C)	Td5% (°C)	Contact Angle DI Water (°)	Contact Angle Hexadecane (°)
	EDMA	with air	2	8	236	92	24
		no air	3	23	269	84	30
	EEMA	with air	100	35	298	89	33
		no air	98	34	294	85	34
	EIMA	with air	100	56	246	85	24
		no air	100	58	258	82	25

Acknowledgments: Samantha Molina-Gutiérrez would like to thank Parnian Kianfar, Gustavo González, and Giuseppe Trusiano for their support during the development of this work. This manuscript is in honor of the 50 year anniversary of the French Polymer Group (Groupe Français des Polymères-GFP).

double bonds (PDB) conversion was determined using the 960 cm -1 band (Figure S1) [START_REF] Yoshimura | Bio-based polymer networks by thiol-ene photopolymerizations of allyl-etherified eugenol derivatives[END_REF][START_REF] Chowdhry | Infrared and raman spectroscopy of eugenol, isoeugenol, and methyl eugenol: Conformational analysis and vibrational assignments from density functional theory calculations of the anharmonic fundamentals[END_REF][START_REF] Silverstein | Spectrometric Identification of Organic Compounds[END_REF][START_REF] Wang | Rapid evaluation and quantitative analysis of eugenol derivatives in essential oils and cosmetic formulations on human skin using attenuated total reflectance-infrared spectroscopy[END_REF]. Each conversion was calculated using the following Equation (4):

Where A is the absorbance of the IR band of the functional group monitored during irradiation; Ref A is the absorbance of the band of the aromatic ring (C-C stretching) taken as a reference (1540 cm -1 to 1490 cm -1 ).

Absorbances were estimated as the area of the vibrational bands under examination. Data were processed using OMNIC software (Madison, WI, USA). All curves were smoothed using the Savitzky-Golay method with 20 points window and second polynomial order. For the determination of EEMA methacrylate double bonds conversion, an approximation was made as peaks corresponding to the allylic and methacrylate double bond superimposed at circa 1638 cm -1 . The calculation details are explained in SI (Figures S2 andS3 and Table S1).

Characterization Methods

Samples for thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and gel content were prepared by coating a glass slide with 200 µm films and irradiating it for 10 min using a DYMAX 5000 EC UV flood lamp (Dymax Corporation, Torrington, CT, USA) in the range of 320 to 390 nm with an intensity on the sample of 156 mW cm -2 (UVA and UVV).

IR spectra to determine the conversion were acquired on a Thermo Scientific Nicolet 6700 FTIR apparatus (Madison, WI, USA) in the 650-4000 cm -1 range, with 32 scan per spectrum and a resolution of 4 cm -1 (using attenuated total reflectance technique, ATR) on both faces of the film: the one exposed to the atmosphere and the one in contact with the glass slide.

Ultraviolet and visible spectroscopy (Figures S4 andS5): UV-Vis spectra were recorded on a Varian Cary 50 Scan spectrophotometer (Victoria, Australia) from 200 to 800 nm with a scan rate of 4800 nm min -1 .

Thermogravimetric Analysis: TGA was performed on 5-10 mg samples on a TGA Q50 apparatus from TA Instruments (New Castle, DE, USA) from 20 • C to 580 • C, in an aluminum pan, at a heating rate of 20 • C min -1 , under nitrogen.

Differential Scanning Calorimetry: (Figures S6-S11) DSC measurements were performed on 10-15 mg samples, under nitrogen atmosphere, with a Netzsch DSC 200 F3 instrument (Selb, Germany) using the following heating/cooling cycle: first cooling ramp from room temperature (ca. Gel content measurements: The gel content of the polymers was measured by placing approximately 30-50 mg of polymer in a Teflon pocket which was subsequently immersed in 10 mL of toluene for 24 h, then dried in a ventilated oven at 50 • C for 4 h. The gel content was calculated based on the initial (W i ) and final (W f ) polymer mass according to Equation [START_REF] Stansbuty | Curing dental resins and composites by photopolymerization[END_REF].

Supplementary Materials: The following are available online.

Conflicts of Interest:

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.


Alexandra E Usenka

Vladimir V Pankov

Vaibhav Vibhu

Aurelien Flura

Jean-Claude C Grenier

Jean-Marc M Bassat

Temperature programmed oxygen desorption and sorption processes on Pr2-XLaxNiO4+δ nickelates

come

Introduction

Due to their high oxygen ion diffusivity and electronic conductivity at high temperature, the mixed ionic-electronic conducting (MIEC) lanthanide nickelates Ln2NiO4+ with K2NiF4-type structure can be used as solid oxide fuel cell (SOFC) cathodes, separation membranes or oxygen sensors (1 6).

Oxygen exchange with atmosphere, i.e. desorption-sorption processes, obviously depends on the crystallographic structure as well as on the oxygen non-stoichiometry of the oxides. Temperature programmed desorption (TPD) followed by solid electrolyte coulometry/potentiometry gives valuable data on the oxygen exchange processes: temperature at which oxygen releases (desorption), stoichiometry changes and structure transition occur (7). Interestingly it also allows estimating the nature of desorpted oxygen species with respect to their mobility (8).

To investigate oxygen TPD and ionic transport in MIEC oxides being induced by a chemical potential gradient, it is convenient to use the multifunctional solid electrolyte (7 13). It acts both as independent and supplementary technique to thermo-gravimetry, gaseous chromatography, X-ray and neutron diffraction in a wide range of temperature and oxygen partial pressure (8). A devices application for investigation of various compounds has been reported by Vashook et al. (8). described in the work of Teske (7), where properties of perovskite-like cuprates Y Ba Cu O were investigated. For example, the time dependence of oxygen exchange gave useful informations about oxygen diffusion in the materials. Spectra of temperature programmed desorption and sorption (TPD) detected the oxygen variation with temperature. According to the authors, TPD spectra of Y Ba Cu O could be associated with series of structural and conductivity transitions.

Refs (9,10) described the operating modes of the OSEC device that are required to build p T diagrams and to investigate the thermodynamic properties of mixed nonstoichiometric oxides. For instance, a tentative analysis of oxygen non-stoichiometry and electrical conductivity of the binary strontium cobalt oxide SrCoOx has been performed in ref. (11). The authors demonstrated strong evidence of correlation for maxima in TPD spectra (within the temperature range 500 and the relationship of a -disorder transition of the cubic high-temperature phase.

Regarding nickelates with K2NiF4

type structure, oxygen non-stoichiometry combined with electrical conductivity measurements were performed on La SrxNiOy and Pr SrxNiOy (3,12,13). TPD spectra allowed concluding about the existence of weakly bonded oxygen, capable to reversibly exchange with the gas phase at temperature as low The present work is devoted to the investigation of oxygen TPD in Pr2-xLaxNiO4+ powders by coulometric solid electrolyte technique, i.e.

Experimental

High purity powders were prepared via the citrate-nitrate route (modified Pechini method) starting from the precursors Pr6O11 (Aldrich chem, 99.9%), La2O3 (Strem Chemical, 99.99%) and Ni(NO3)2 6H2O (Acros Organics, 99%). The lanthanide oxides (Pr6O11 and La2O3) were prebecause of their highly hygroscopic nature. After the combustion step of the nitrate-citrate oxides was confirmed by X-ray diffraction using a PANanalytical X'pert MPD diffractometer with Cu-Further details are given in ref. (14).

The room temperature oxygen over-stoichiometry of the synthesized oxides was determined either by iodometric titration (6) or by thermo-gravimetry analysis (TGA) measurements performed under Ar/5%H2 flow with a SETARAM setup (MTB 10-8 balance). Both values (table I) are in very good agreement. Oxygen desorption-sorption processes with atmosphere were studied on Pr2-xLaxNiO4+ 900 required value of oxygen partial pressure set by coulometry dosing was 50 Pa. Argon was used as a carrier gas and air was used as a comparison gas. The considered parameters were the time and temperature dependences of both the titration current (I2 in Fig. 1) and / or the oxygen over-stoichiometry. To simplify further discussions, the dependence will be called TPD spectra (7), the spectra data concerning not only desorption but also oxygen sorption. coulometric regime. The measurement system consists of two identical solid electrolyte cells, each one using a tube made of 8 mol. % Y2O3 stabilized ZrO2 (8YSZ). Each cell has a pair of oxygen pumping (Ii, pump zone) and potential measurement (Ui, gauge zone) electrodes. Porous Pt partly covering areas of inner and outer walls of YSZ tube ensures the electrical contacts and serves as ele trode components. . The cells allow stabilizing a steady-state gas flow with a given oxygen concentration by presetting the electrode voltage (Ui) at a known temperature. The voltage is controlled by feedback adjustment of the coulometric titration current (Ii) between the oxygenpumping electrodes.

The experimental reactor and the cells are connected by stainless steel capillaries. If the reactor with a sample is placed between two such cells, the oxygen partial pressure can vary in a wide range (8). The temperature of the reactor can be selected independently of the cells temperature.

Any oxygen partial pressure p(O2) in the in-flowing gas (Ar) can be modified in cell 1 by coulometric dosing with O2 to reach the required value p(O2)'. Cell 2 keeps the required value of oxygen partial pressure p(O2)'' after the flowing gas has just left the reactor by pumping out or pumping in O2. If no oxygen exchange between the flowing gas and the sample takes place in the reactor, no change in oxygen partial pressure (p(O2)''= p(O2) ') is observed and titration current I2 keeps constant value. If oxygen desorption occurs, oxygen partial pressure rises in the gas flow of the reactor (p(O2) ''> p(O2) ') and cell 2 pumps out the O2 excess to the value p(O2 -out procedure is detected through the decrease of I2 (titration current) if U2 is kept constant. If the sample absorbs O2, oxygen partial pressure decreases in the gas flow of the reactor (p(O2) ''< p(O2) ') and cell 2 pumps in O2 till reaching the value p(O2)' = p(O2) ''. The pumping-in procedure is detected through the increase of I2 if U2 is kept constant.

All oxygen exchange in the reactor should be accompanied by a deviation of the coulometric titration current (I2,t) from the (I2, base) value, if U2 is kept constant. The oxygen exchanged mass m (O2)) may be calculated according to the law:

(

Knowing the mass and chemical composition of the investigated sample, the TPD spectra can be readily built.

Prior to the coulometric measurements, powders of Pr2-xLaxNiO4+ were flowed at following temperature cycle was then applied in order to build the TPD spectra: i) heating ii) isothermal plateau iii) cooling fterwards, each heating-cooling cycle performed for coulometric measurements was carried out in Ar atmosphere with p(O2) = 50 Pa at flow rate 48 52 ml/min. The duration times will be detailed in the following when necessary.

TGA measurements were used to compare oxygen over-stoichiometry defined with

Results and discussion

Fig. 2 shows the TPD spectra recorded for five compositions (x = 0, 0.5, 1.0, 1.5, 2) of the Pr2-xLaxNiO4+ powders, while Table II gathers their oxygen stoichiometry content. In the same atmosphere, plots of TPD spectra are almost similar for La2NiO4. 16 and Pr2-xLaxNiO it significantly differs for Pr2NiO4.26, showing some peculiarities which will be discussed below. Prior to further detailed consideration of the spectra, one should pay attention to the titration curves of the samples recorded under cooling conditions. At the beginning of the cooling, and from maximum temperature down to 290 , an oxygen sorption is observed. The heated samples are oxygen deficient while they exhibit an oxygen overstoichiometry at room temperature before heating. Due to presence of oxygen in the carriergas, they start to absorb O2 when cooling the powder, which can be seen from values of the titration current which are higher than the base current. Oxygen sorption stops at 290 , resulting in a quick current dropping to the base current value. Such a thermal behavior is the same for all the samples and will not be more discussed below.

TPD of La2NiO4.16

Two main desorption peaks are visible on the TPD spectrum of La2NiO4.16, located around 240 is chosen to refer on it); one can conclude that under the considered experimental conditions, there are two regions of very intensive and continuous . The second maximum of the titration current cannot be accurately determined, because it is stopped when heating at 900 (12) also observed two desorption maxima in the TPD spectrum of La2NiO4.15 powder in argon flow with lower oxygen partial pressure (10 Pa) in the temperature cycle 20 1050 with those of ref. (12) leads to conclude that the real location of the second maximum takes place at a temperature highe The oxygen desorption occurs with an apparent constant rate during heating La2NiO4.16 in the range 400

i.e. between the two previous peaks.

The absorption of O2 during cooling represents a diffuse maximum, i.e. a broad peak in the TPD spectrum (Fig. 2) Table II, one can see a difference between the initial and final over-stoichiometry of the powders. Because the material is prepared under air, but here cycled under lower pO2, the amount of desorbed oxygen while heating is larger than the amount of absorbed oxygen while cooling, as expected: for instance the initial oxygen over-stoichiometry of La2NiO is 0.16, the final one being 0.124. The influence of the oxygen content in the gas flow can be confirmed by the data reported in ref. (12) for the same material: after cycling under p(O2) = 10 Pa, the final oxygen over-stoichiometry is 0.06.

TPD of Pr2-xLaxNiO

In Fig. 2, two sharper maxima of oxygen desorption (compared to those of LNO) are visible in the TPD spectra of Pr2-xLaxNiO (x 0). More especially the second one becomes sharper and sharper when the Pr amount increases. In addition, at increasing Pr content, the start of the first oxygen desorption as well as the first maximum itself (which corresponds to the maximum speed of oxygen release) are shifted to lower temperatures (Table III) compared to La2NiO4. 16. The same conclusion on the temperature range of the second oxygen desorption maximum can be drawn. It should be also noticed that the released oxygen amount heating increases with the increase of Pr content in Pr2-xLaxNiO . This is clearly visible from the value of the over-stoichiometry and after heating (see and Table II) and the maxima values of oxygen desorption (Fig. 2). The amount of released oxygen depends on the initial overstoichiometry of the oxides, which in its turn depends on the Pr content, provided the preparation conditions of Pr2-xLaxNiO are the same. It is also well-known that the oxygen over--earth cation: the smaller the radius of the rare-earth cation, the higher the oxygen content is. For La, the value is reported to be 0.16 0 0.29 (14 19). In the Pr2-xLaxNiO increases, as expected. Very interestingly, the TPD spectrum of Pr2NiO4.26 drastically differs from the previous spectra (x < 2). Four sharp maxima (peaks) of oxygen desorption are observed at the (labeled 1, 2, 3, 4 in Fig. 3). Two prominent peaks (compared to the spectra of PLNO are located at 370 (labeled 2 and 3, Fig. 3).

Besides the four mentioned maxima in the TPD spectrum of Pr2NiO4.26, our data (labeled 5 and 6, the existence and resolution were confirmed in experimental series with l TPD Spectra Comparison Our TPD spectrum is in rather good agreement with that reported for Pr2NiO by Sadykov et al. (20). Due to different oxygen partial pressure and inert atmosphere used in our experiments, the observed maxima logically shift to lower temperatures compared to ref. 20. The number of peaks and their position vs. temperature in the spectrum reflect the complexity of the dynamical change in the crystal lattice and, as a result, the oxygen ions mobility (connected to their position in the crystal lattice and most likely to the migration/diffusion pathways) while heating. A direct observation of the crystal lattice dynamics and an exact identification of the oxygen positions at atomic scale in nonequilibrium conditions under heating is limited by in situ existing experimental techniques, which makes difficult the interpretation of the TPD spectrum only on the basis of literature data.

According to the available data in literature (6,20), the peaks observed in the temperature range 210 located in Pr2O2 layers (NaCl-type structure) but without any link with the possible lowering of the mean praseodymium charge (which would be between 3+ and 4+), as supposed by Sadykov et al. (20). Indeed, the charge is only 3+ in Pr2NiO whatever the oxygen over-stoichiometry in air at T = 25 by XANES studies (21). Combination of experimental and modelling data concerning interstitialcy / interstitial diffusion mechanisms from (20, 22 25) allows the following suggestions: the peaks 1 3, 5, 6, depicted in Fig. 3 could be likely attributed to mobile i) and reflect different dynamic positions of Oi in crystal lattice under heating at T= ~ 400 , corresponding to the sharp narrow peak (2 in Fig. 3), is especially noteworthy. In our experiments, the TPD spectra constantly preserved this peak as compared to 1, 3 6 ones. This peak can be assigned to the orthorhombic-tetragonal phase transition (22,25) in Pr2NiO . According to single crystal neutron diffraction investigations performed by M. Ceretti et al. ( 23) on Pr2NiO at 2O2 rock salt layer from an embedding matrix of NiO2 layers takes place in this temperature range. Thus, this peak induced by crystal lattice transformation can be found whatever the initial oxygen stoichiometry. In refs. (16,22) it has been confirmed that the initial non-stoichiometry exceeding 4.20, does not significantly affect the presence of this phase transition.

Th 4, Fig. 3) is supposed to result from oxygen removal from the direct environment of the nickel cations.

Comparison of TPD Spectra

Summarizing the above-mentioned features concerned with non-monotonous change of oxygen desorption when heating at constant rate, one can correlate the nature of released oxygen with titration current peaks (maxima) in TPD spectra: 1) phase transformation in a material, 2) un-equivalent crystallographic positions of oxygen atoms in the crystal lattice of the compounds. In the case of the phase transformations, an abrupt change in bonding energy of oxygen with the lattice takes place and, as a consequence, an anomaly in the oxygen desorption rate can occur. In the second case, with the increase of temperature, distinct consecutive removal and/or migration of oxygen from interstitial and regular positions can proceed in accordance with the increase in bonding energy of oxygen in the crystal lattice (12). The sharp narrow peak matching the phase transition is observed only for PNO and not detected for PLNO phases. A hypothesis is that the amount of oxygen removed during the phase transition for PLNO phases is significantly lower than for PNO (cf. Table 2 and Fig. 2,4) due to their lower ionic mobility in the attendance of lanthanum cations. The second reason for the absence of the narrow peak in TPD spectra for Pr2-xLaxNiO4+ (x 0) could be the vicinity of temperature ranges for weakly-bounded oxygen desorption (first peak) and oxygen removal linked to the phase transition (25). Indeed, from Fig. 2 it can be seen that the location of the first peak shifts towards higher temperature with decreasing x (for lowest Pr contents, which again supposes a lower mobility of such oxygen in PLNO), and can then overlap the narrow peak of oxygen desorption occurring during the phase transition.

To confirm the existence of the TDP peaks and their resolution for Pr2NiO and La2NiO , experiments were conducted As shown in the TPD of PNO (Fig. 5) all oxygen desorption maxima were either present in the temperature range under consideration for intensity and shape slightly changed (Fig. 5). Prolonged thermal treatment (depending on the heating rate) results in both cases in the shift of the peaks location to lower temperatures, as well as in a change of the shape of the last maximum (Fig. 5(a)).

Most likely, the two sharp maxima in TPD for Pr2-xLaxNiO (x 0) might be associated to removal of oxygen from different crystallographic positions in these nickelates. Over-stoichiometric oxygen in La2NiO structure is located in interstitial position with the coordinates ( , , close to layers formed by NiO6 octahedra. These interstitial oxygen ions, apparently, are the most weakly in the lattice (26). Our suggestion is that during the heating of the nickelates interstitial oxygen ions leave the compound lattice at lower temperatures and cause the occurrence of the first oxygen desorption peak in the range 240 stoichiometry content, a further departure of oxygen apparently occurs accounting for the removing of oxygen from its normal (regular) lattice sites. Regular oxygen causes the occurrence as shown in Fig. 2.

When the second oxygen desorption peak occurs in Pr2-xLaxNiO (x 0), the over-0.090 (Fig. 4, Fig. 6 (a) (c)). This over-stoichiometric content of oxygen is possibly so strongly linked to the nickelate lattice that it does not result in desorption of oxygen at heating the powders up to temperatures 650 1, Fig. 6). In this temperature range, oxygen is more easily removed from the regular lattice sites. predominantly result from weakly bonded over-stoichiometric oxygen, i.e. interstitial oxygen in the crystal lattice. The statement might be supported by the fact that, when increasing the Pr amount, whose cationic radius is smaller than that of La, the oxygen over-stoichiometry of the nickelates (synthesized in the same conditions) increases and the oxygen desorption shifts to lower temperature compared to La2NiO . TPD spectrum of Pr2NiO recorded under the same conditions drastically differs from the spectra of Pr2-xLaxNiO with lower Pr contents: three sharp and two local maxima of titration current are located in range of temperature 210 prominent first maximum in a relatively low temperature range is supposed to have the same nature as for PLNO and might be associated with the removal of interstitial sites with the coordinates , close to layers formed by NiO6 octahedra. The second (narrow) maximum of the TPD at 370 is most probably attributed to the orthorhombic-tetragonal phase transition in Pr2NiO . The peaks observed in the temperature range 400 65 different (nonequilibrium) migration positions of mobile oxygen from crystal lattice under heating.

The maxima of TPD spectra observed at temperature above 650 considered nickelate are apparently connected with the removal of oxygen from NiO6 octahedra building perovskite layers. It has been shown that the removal of oxygen from perovskite layers occurs when the lattice maintains some amount of oxygen over-0.09 for Pr2-xLaxNiO 0.14 for Pr2NiO ).

Figure 1 .

 1 Figure 1..

Figure 2 .

 2 Figure 2. TPD spectra recorded on Pr2-xLaxNiO4+ (x = 0, 0.5, 1.0, 1.5, 2) powders in the temperature range 20 900 (argon flow, pO2 = 50 Pa, heating-6 C /min).

Figure 3 .

 3 Figure 3. TPD spectra (oxygen partial pressure in argon flow p(O2)=50 Pa; heating-cooling of Pr2NiO4.26 after .

Figure 4 .

 4 Figure 4. Oxygen non-stoichiometry value vs. temperature (converted TPD spectra) of Pr2-xLaxNiO4+ nickelates in argon flow (pO2 = 50 Pa) and heating rate 6 /min.

Figure 5 .

 5 TPD spectra for Pr2NiO4+ at T= 20 in argon flow (pO2 = 50 Pa) and /min (b).

Figure 6 .

 6 TPD spectra for Pr2-xLaxNiO4+ (x = 0.5, 1.0, 1.5) nickelates, including in the temperature cycle 20 900 in Ar flow (pO2 = 50 Pa) and heating-/min. Conclusions TPD spectra of Pr2-xLaxNiO (x 0) exhibit two sharp peaks (maxima) of titration current with excellent resolution at p(O2) = 50 Pa that are connected with removal of oxygen from different sites of the crystal lattice. The maxima observed during heating of Pr2-xLaxNiO powders at temperatures 200

Table I .

 I Oxygen over-

	Sample	TGA	2-xLaxNiO	powders at room temperature in air. Iodometry Average
	La2NiO Pr0.5La1.5NiO Pr1.0La1.0NiO Pr1.5La0.5NiO Pr2NiO	0.16 0.17 0.19 0.21 0.27		0.16 0.17 0.19 0.20 0.25	0.16 0.17 0.19 0.205 0.26

Table II .

 II Oxygen

	20 900	over-(argon flow, pO2 = 50 Pa, heating-Pr2-xLaxNiO4+ (x = 0, 0.5, 1.0, 1.5, 2) in heating-cooling cycle /min). Sample name Initial after heating Final after cooling Difference between the initial and final
	La2NiO4.16 Pr0.5La1.5NiO4.17 Pr1.0La1.0NiO4.19 Pr1.5La0.5NiO4.205 Pr2NiO4.26	0.160 0.170 0.190 0.205 0.260	0.052 0.057 0.059 0.044 0.070	0.124 0.135 0.148 0.132 0.181	0.036 0.035 0.042 0.073 0.079

Acknowledgments

This research was supported by the European Community Mobility Programme .