Renewable ThiolâEne Thermosets Based on Refined and Selectively

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Renewable thiol-ene thermosets based on refined and selectively allylated industrial lignin Marcus Jawerth, Mats Johansson, Stefan Lundmark, Claudio Gioia, and Martin Lawoko ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02822 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Sustainable Chemistry & Engineering

Renewable thiol-ene thermosets based on refined and selectively allylated industrial lignin Marcus Jawertha, Mats Johanssona, Stefan Lundmarka,b, Claudio Gioia*a, and Martin Lawoko*a * Corresponding author s: [email protected] and [email protected] a

Wallenberg Wood Science Center, WWSC, Department of Fibre and Polymer Technology,

KTH Royal Institute of Technology, Teknikringen 56-58, 100 44 Stockholm, Sweden b

Perstorp AB, Innovation, Perstorp Industrial Park, 284 80 Perstorp, Sweden.

Abstract

Aromatic material constituents derived from renewable resources are attractive for new biobased polymer systems. Lignin, derived from lignocellulosic biomass, is the most abundant natural source of such structures. Technical lignins are, however, heterogeneous in both structure and polydispersity and require a refining to obtain a more reproducible material. In this paper the ethanol soluble fraction of Lignoboost Kraft lignin is selectively allylated using allyl chloride by means of a mild and industrially scalable procedure. Analysis using 1H-,

31

P-, and 2D HSQC-

NMR give a detailed structural description of lignin, provide evidence of its functionalization and that the suggested procedure is selective towards phenols with a conversion of at least 95%. The selectively modified lignin is subsequently crosslinked using thermally induced thiol-ene

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chemistry. FT-IR is utilized to confirm the crosslinking reaction and DSC measurements determined the Tg of the thermosets to be 45-65 °C depending on reactive group stoichiometry. The potential of lignin as a constituent in thermoset application is demonstrated and discussed.

KEYWORDS: Lignoboost Kraft lignin, Controlled refinery, Ethanol, Selective Allylation, Scalable procedure, Thiol-ene thermoset Introduction Renewable resources based polymers are being introduced to the market at an increasing rate, both due to sustainability reasons as well as an increased cost competiveness of bio-based feedstocks. It can be foreseen that this trend to replace fossil based alternatives will continue in the strive towards a more sustainable society.1-2 Most of these new materials are derived from sources such as fatty acids, terpenes, and carbohydrates.1-2 The vast majority is aliphatic in character, which to some extent limits the application areas when it comes to material performance. Material properties such as modulus (stiffness) on a molecular level relates to the polymer chain rigidity and this is in many cases only possible to achieve by introducing aromatic building blocks into the polymer structure.3-4 One area where this is evident is in high performance thermoset polymers used in applications such as composites or organic coatings.5-6 The most promising aromatic monomer that so far has reached furthest in this respect is furan dicarboxylic acid (FDCA) derived from fructose or glucose as a replacement of terephthalic acid in PET plastics.2 FDCA is suitable for terephthalic acid replacement in many cases but there is still a need for other biobased aromatics, especially in the area of thermoset polymers. The most abundant source of aromatic structures in nature is lignin and there are extensive research


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activities aiming to develop new routes to utilize this source towards novel polymer materials.5, 79

Lignin is a major constituent in lignocellulosic biomass providing a promising resource for many future applications.8-10 The structure of lignin is constituted by three main aromatic monolignols called p-coumaryl, coniferyl, and sinapyl alcohol differentiated by the substitution of methoxy groups in the meta positions, Figure 1.8-10

Figure 1: Monolignols, labelled according to established literature. When incorporated in lignin the monolignols form substructures called p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units respectively.8, 10 The ratio between these units in lignin composition is greatly influenced by its plant origin. Softwood lignins are comprised of mostly G- with traces of H-Units, hardwood lignins of S- and G-units while lignins from annual plants are comprised of all three types.8-10 In lignin biosynthesis, two main chemical reactions occur. The first is initiated by a mild oxidation of the phenolic hydroxyl, forming a phenoxy-radical whose resonance forms couple to form lignin dimers. The second involves a nucleophilic addition reaction to an electrophilic site on a quinone methide intermediate formed subsequent to dimerization. The radical couplings and nucleophilic addition reactions yield a polymer with


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different types of inter-unit linkages, with the most prominent being the aryl ether (β-O-4), phenylcoumaran (β-5), resinol (β-β), Spirodienone (β-1), dibenzodioxin (5-5-O-4) and diaryl ether (4-O-5),10 structures seen in Figure 2.

Figure 2: Common lignin inter-unit linkages. The dotted bond line indicates a proton, methoxy group, or other linkage. The latter two have recently been suggested to exist as phenolic end groups, implying that native lignins may be more linear11-12 than previously thought13. Other common end groups include cinnamyl alcohol and aldehydes.10 The main functionalities in native lignin are phenolic and aliphatic hydroxyls, however, other functionalities can form as the biomass is processed. Structural changes that occur during the treatment mainly depend on the conditions and how the lignin stream is retrieved.9 Technical lignins are mainly by-products in the production of cellulosic pulps.14 The main pulping methods are the Kraft (sulphate)-, Sulphite-, Soda and Organosolv processes.9 The Kraft process is by far the most utilized producing roughly 70 Mt of lignin annually.15 The main chemistry leading to lignin dissolution in Kraft pulping is the hydrogen sulfide anion-catalyzed cleavage of phenolic aryl ether linkages (β-O-4) leading to molar mass decrease and formation of phenolate anions, both of which enhance the solubility of lignin in the alkaline media.16 Lignin


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condensation and side chain cleavage reactions leading to un-saturation, e.g., formation of stilbene, also occur and these form stable structures.17 The resulting dissolved lignin, also referred to as Kraft lignin (KL), has thus increased functionality, i.e., phenolic and aliphatic hydroxyls and a small amount of formed carboxylic functionality, a decreased amounts of etherbonds, and an increased number of carbon-carbon bonds.9 Technically feasible methods to recover lignin from the process liquor have seen progress in recent years due to the potential of use in different applications. One such method is the Lignoboost technology for Kraft lignin recovery, which was developed in Sweden14 and recently commercialized by Stora Enso. In this process the pH of the black liquor is lowered causing lignin to precipitate.14, 18 The recovered lignins are however heterogeneous with respect to both structure and dispersity. Processes to obtain more homogeneous fractions then becomes important when a high degree of control over material properties is desired. Sequential solvent extraction and ultrafiltration are two common examples to fractionate lignin.19-23 A sequential solvent fractionation method applying solvents that fall within recommended (green) solvents24 was, for example, developed by Duval et al in 201525. The use of sequential fractionation and subsequent use of the soluble fractions furthermore ensure that the synthesis is performed under homogeneous reaction conditions i.e. any effects of heterogeneity is avoided. Chemical modification of lignin has been increasingly studied with a large variety of different functionalization to explore its potential field of application. Acetylation, silylation , and epoxidation are a few examples of modifications evaluated on model compounds, lignin or lignin products.26-28 This work, however, is focused on allylation as a derivatization route for lignin phenols. The phenolic groups in technical lignins are a suitable target for modification due to several reasons. While the intrinsic reactivity of phenols allows it to be modified to high


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conversions it also can lead to problems in thermoset formulations if still present. The phenolic groups, for example, negatively affect miscibility with less polar substances and can also negatively interfere with conventional crosslinking chemistries such as free radical reactions and base catalysed reactions. The allyl group provides a versatile chemical handle with many promising chemical pathways and has been examined as a possible derivatisation route in different variations before. Dournel et al29, Zoia et al27, and Over et al30, to mention a few, have elaborated on this chemical handle before in different systems. A route to selectively modify lignin phenols to the corresponding allyl ether leaving aliphatic hydroxyls unreacted was recently described as yet another tool for precise tailoring of lignin derived materials.31 One way of utilizing the allylated lignin is through thiol-ene reactions. Known for its efficiency32-33, it is a popular procedure to covalently couple two molecular entities to each other. In many cases it is described to have “Click” characteristics with little to no side reactions.32-34 This make the thiol-ene reaction a versatile tool for modification and crosslinking of ene containing moieties. One general drawback of thiol-ene based materials is, however, the flexible thio-ether bonds resulting in materials with relatively low glass transition temperatures, Tg:s, often below room temperature.34-35 One way to provide a larger degree of rigidity to these kinds of system could be to incorporate rigid aromatic constituents, such as lignin. In the present work, the phenols of well characterized ethanol soluble Lignoboost Kraft lignin (L-KLEtOH) are selectively allylated and crosslinked using thermally induced thiol-ene chemistry. It is done to explore a viable synthetic pathway for this notorious raw material towards thermoset material applications. The steps of the process are performed in a scalable and viable manner


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using ethanol as a benign solvent for the synthesis. An overview of the procedure can be seen in Scheme 1.

Scheme 1: Selective allylation of the ethanol soluble fraction of Lignoboost Kraft lignin with subsequent thiol-ene crosslinking. Experimental Section Materials All Chemicals were of analytic grade and used as received unless stated otherwise. Allyl chloride (98%), sodium hydroxide (NaOH, ≥98%) and Trimethylolpropane tris(3mercaptopropionate) (TMP 3MP3, 95%) were obtained through Sigma Aldrich. Ethanol (EtOH, 96%), ethyl acetate (EtOAc) and heptane were purchased from VWR chemicals. Hydrochloric acid

(HCl,

37%)

was

bought

from

Fischer

Scientific.

Lignoboost Kraft Lignin (L-KL) was provided by colleagues at Chalmers University of Technology, Gothenburg, Sweden. Methods


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Nuclear Magnetic Resonance (NMR) (1H-,

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31

P-, and 2D hetero nuclear single quantum

coherence (HSQC)) was recorded at room temperature on a Bruker Avance III HD 400 MHz instrument with a BBFO probe equipped with a Z-gradient coil for structural analysis. Data were processed with MestreNova (Mestrelab Research) using 90° shifted square sine-bell apodization window; baseline and phase correction was applied in both directions. 31

P-NMR samples were prepared and analysed

according the procedure reported by

Argyropolous in 1994.36 Size Exclusion Chromatography (SEC), using a SEC 1260 infinity (Polymer standard service, Germany) equipped with a PSS precolumn, PSS column 100 Å and PSS GRAM 10000 Å analytical columns thermostated at 60°C, was performed to determine the molecular weight and dispersity of the different lignin samples. The detection system included a UV detector in series with a refractive index detector. DMSO + 0.5% LiBr was used as eluent with a constant flow rate of 0.5 ml/min. A calibration plot was constructed with pullulan standards. All samples were fully soluble in DMSO. Fourier Transform Infrared Spectroscopy (FT-IR) was performed using a Perkin-Elmer Spectrum 2000 FT-IR equipped with a MKII Golden Gate single reflection ATR system of Specec LTD. All spectra were recorded in the range of 600 to 4000 cm-1 with 32 scans averaged at 4.0 cm-1 resolution at room temperature. All data were analysed using PerkinElmer Spectrum software V10.5.1. Spectra were normalized over the carbonyl peak at 1732 cm-1 to be able to clearly visualise the difference between samples. Differential scanning calorimetry (DSC) was conducted using a Mettler-Toledo DSC equipped with a sample robot and a cryo-cooler and evaluated with Mettler Toledo STARe


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software V15.00a. All measurements had a heating and cooling rate of 10 K/min and were performed under N2-atmosphere. The thermosets were analysed by heating from 25 °C to 100 °C and 100 °C to -40 °C to remove thermal history of each sample before being cycled between -40 °C to 120 °C 2 times to find the Tg of the thermosets. Dynamical Mechanical Analysis (DMA) was performed on a TA instrument DMA Q800 in tensile mode. The geometry of the sample was retained from the mould (6.5 x 6.9 x 1 mm). The starting temperature was set to 0°C where it was held isothermally for 10 min before heating with a rate of 3°Cmin-1 up to 90°C. The frequency was set to 1 Hz with a strain amplitude of 6.5 µm. Procedures Fractionation of Lignoboost Kraft lignin The fractionation procedure was based on the previous work of Duval et al.25 Lignoboost Kraft lignin (L-KL), 20 g, was dispersed in 200 ml of EtOAc in a round bottom flask equipped with magnetic stirring. The suspension was stirred at room temperature for 2 hours before the lignin fraction that had not dissolved was recovered from the mixture by filtration. The filtrate was washed with additional EtOAc. The EtOAc insoluble lignin fraction was furthermore introduced to a round bottom flask and dispersed in 200 ml of EtOH. After 2 hours of stirring the EtOH soluble fraction was obtained by filtration of the dispersion and the ethanol was removed under reduced pressure at 40 °C. The sticky residue was solubilized in 20 ml of acetone before being precipitated in deionized water to obtain a homogeneous water dispersion. The dispersion was finally freeze dried and a fine brown powder was obtained with an overall yield of 37.5 wt% of


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ethanol soluble L-KL (L-KLEtOH). This last step being performed for practical handling in lab scale. Selective allylation of the ethanol soluble fraction The allylation procedure was based on the previous model recipe developed in our lab.31 L-KLEtOH, 1g (4.98 mmol of phenols), was added to a round bottom flask equipped with a magnetic stirrer and dissolved in 33 ml of ethanol and 21.5 ml of a NaOH (7 mmol) water solution. When the solution appeared homogeneous 1.2g of allyl chloride (14.94 mmol) was added and the round bottom flask was sealed. The mixture was left to react under stirring at 55 °C for 30 h before 50 ml of deionized water was poured into the mixture and the pH lowered to 3.5 with a few drops of HCl, 0.1 M. The resulting precipitated product was recovered by filtration. The filtrate was washed 2 times with roughly 50 ml of deionized water to remove traces of acid. The filtrate was dissolved in about 1-2 ml of acetone and precipitated in deionized water to obtain a homogeneous water dispersion. The mixture was finally freeze dried to obtain the allylated product (Allyl-L-KLEtOH) as a fine light brown powder with a 95 wt% yield, this last step being performed for practical handling in lab scale. The quantification of the allyl functionalities was obtained by 1H-NMR, employing p-nitrobenzaldehyde as internal standard. The signal comprised between 6.33 and 5.5 ppm, related to the allyl ether double bond, was therefore correlated with the aldehydic proton at 10.1 ppm of the internal standard. Thermal, initiator free, thiol-ene crosslinking


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The lignin–thiol resins were prepared by adding TMP 3MP3 and Allyl-L-KLEtOH to a vial and using EtOAc to aid the mixing of the two components. Resins with different stoichiometric ratios, with respect to thiol vs allyl ether functionalities, were prepared. A typical recipe includes roughly 60 mg of TMP 3MP3 and 113 mg of Allyl-L-KLEtOH to give a 1:1 relationship between the reactive groups and mixed using roughly 150 mg of EtOAc. The mixtures were then spread onto microscopy glass substrates and the solvent was let to evaporate before curing at 120 °C for 24 hours. Five different ratios were prepared: 0.8:1, 0.9:1, 1:1, 1.1:1 and a 1.2:1 with respect to thiol/allyl ether groups. The curing was analysed by FT-IR where the reactive groups, thiol and ene, can be found at 2568 cm-1 and 1647 cm-1 respectively. In order to obtain self-standing samples, suitable for DMA analysis the mixture of allylated lignin and TMP 3MP3 were poured into a silicon mould and cured at 120°C for 24 hours. Results and Discussion In this work, Lignoboost Kraft lignin (L-KL) was used as the starting raw material for a thermoset resin system. The L-KL was refined by solvent fractionation to obtain a more homogeneous, ethanol soluble, fraction (L-KLEtOH). The L-KLEtOH was subsequently selectively allylated using allyl chloride to form a thermoset resin constituent (Allyl-L-KLEtOH). In all procedures only solvents that are considered green and benign24 have been used. The use of allyl chloride will be subsequently discussed and justified by its specific reactivity. Lignin fractionation and characterization


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Analysis of the raw material revealed that the parent L-KL had an average Mw of 6820 g/mol and a dispersity (ÐN) of around 6.1, as determined by SEC. The high dispersity and partial solubility could make it difficult to provide predictable and reproducible results in material applications. A recently published sequential solvent extraction25 to obtain a fraction presenting lower dispersity and defined solubility was therefore applied to achieve fractions with a lower heterogeneity and desirable solubility in benign solvents. The process initially involves a first extraction with EtOAc. The EtOAc soluble fraction (L-KLEtOAC) is composed of low molecular weight oligomers with an average Mw of about 850 g/mol. The solid insoluble fraction, recovered from EtOAc after filtration, was subjected to a second extraction using EtOH as the solvent. The L-KLEtOH was obtained in a 37.5 wt% yield, presenting an average Mw of 2120 g/mol with ÐN≈2.5. The molecular weight and dispersity of the fractions a represented in Figure 3. Both ethyl acetate and ethanol are classified as a green solvents.24

Figure 3: SEC traces of L-KL, L-KLEtOAc, and L-KLEtOH with relevant data. The solvent fractionation yields lignin fractions with considerably lower ÐN, soluble in benign solvents.


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A detailed analysis of the hydroxyl functionalities of L-KL and L-KLEtOH was achieved by 31PNMR, Table 1. L-KLEtOH shows a higher amount of phenolic groups and carboxylic acids compared to the L-KL, while the aliphatic alcohol content is significantly lower. The higher amounts of phenolics indicate, in agreement with the SEC results, that L-KLEtOH consists of oligomers rather than longer polymer chains. Table 1: The difference in hydroxyl functionality between the original Lignoboost Kraft lignin and the ethanol soluble fraction as determined by 31P NMR. Phenolics [mmol/g]

Aliphatics [mmol/g]

Carboxylic acids [mmol/g]

L-KL

4.4

1.4

0.4

L-KLEtOH

5.0

1.0

0.6

More information about the molecular structure of the different fractions can be achieved by 2D HSQC NMR. The common lignin inter-unit linkages were semi-quantified by using the aromatic H-C2 cross peak as internal reference,37 spectra found in ESI (Figure S3) and the results are shown in Table 2.


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Table 2: Occurrence of common linkages and end groups in L-KLEtOH as determined by 2D HSQC and 31P-NMR*. Molecular structures are presented in the ESI Figure S1. Bond type

Number/100 units [%]

β-O-4

6

β-51

2

β-β1

3

β-β2

3-4

Stilbene

4-6

*5-5, 4-O-5 and β-52

35

Cinnamyl alcohol

1

Dehydro cinnamyl alcohol

5

Conj. carbonyl/acid

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Some condensed linkages such as 5-5 and the stable diaryl ether linkage (4-O-5) could not be quantified by this method, and may in part explain the generally low percentages of lignin interunits accounted for by the 2D HSQC NMR. Quantification of some of these so called C5condensed phenolics, which include 5-5 and 4-O-5, can however be achieved by 31P-NMR, since they presumably involve a phenolic moiety.11 These will appear between 143-139 ppm in the spectra (Figure 4). Allylation of ethanol soluble fraction In recent work31 it was demonstrated that allylation of coniferyl alcohol, performed in ethanol, using allyl chloride, gives an outstanding selectivity towards phenols in the presence of aliphatic hydroxyls. The chloride presents a suitable leaving group in terms of reactivity and allyl chloride


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is already a commercially available chemical used in industry.38 This introduces the possibility to obtain selectively allylated lignin under mild reaction conditions since L-KLEtOH make up the largest portion25, well above a third, of the total amount of L-KL and allyl chloride is highly soluble in ethanol in accordance to the reaction protocol that was previously established31 making this fraction a promising candidate for selective allylation. Achieving a high level of selectivity when modifying lignin, in this case selectivity towards phenols, provide a useful tool to actively design the chemical panorama of new materials with tailor-made properties. The reaction is performed in ethanol, being an environmentally benign (green) solvent, which is promising for future processing of this raw material. SEC measurements show that the average molecular weight (Mw) is slightly increased after the allylation procedure, from 2120 to 2300 g/mol, due to the additional allyl ether groups introduced in the reaction. The occurrence of degradation or condensation reactions under these mild reaction conditions can therefore be excluded. As can be seen in Figure 4, the 31P-NMR analysis of L-KLEtOH and Allyl-L-KLEtOH shows that the allylation occurs to high conversion selectively on the phenolic hydroxyls. While 95% of the aromatic hydroxyl region, found between 143 and 137 ppm, disappear after allylation, both the aliphatic alcohols, between 148 and 145 ppm, and carboxylic acids, between 135 and 133 ppm, remain unreacted.


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Figure 4:

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P-NMR Spectra of L-KLEtOH before and after the allylation procedure. It can be seen

that the peaks corresponding to the phenolic hydroxyls have disappeared while the aliphatic hydroxyls as well as the carboxylic acids remains unreacted.

2D HSQC NMR was used for structural analysis of Allyl-L-KLEtOH to confirm the introduction of the allyl ether functionality, as well as to monitor if any structural modifications had occurred during the reaction. The spectrum, Figure 5, shows similar spectral features as the L-KLEtOH, Figure S3 in the ESI, however, when quantified, some decrease in the gamma-hydroxylated β-O4, β-β and β-5 are observed. This could be simply due to shifts of some of the signals due to the allylation. Thus, no major changes seem to have occurred to the lignin structure apart from the intended capping of phenols. Signals from the added allyl group are clearly seen in the spectra. The first and second carbons of the allyl ether appear to give rise to more than one signal. These shifts can tentatively be explained by the different condensation structures of the aromatic ring.


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Figure 5: 2D HSQC-NMR of Allyl-L-KLEtOH, the allyl ether functionality can clearly be found and identified in this spectrum. Full spectra can be found in the ESI, Figure S3. The 2D HSQC was also used for spectral quality control when choosing signals for quantification of allyl functionalities by 1H-NMR, Figure S2 of the ESI, which is a fast and quantitative method. The signal at 6.33-5.5 ppm was not overlapped and could therefore be used for quantitation of the allyl group. Thermosets using thiol-ene chemistry The Allyl-L-KLEtOH has, thanks to the modification, an increased double bond functionality that can readily be used as a chemical handle for further derivatisation. In the present study the Allyl-L-KLEtOH has been employed in a thermoset resin system as a viable and promising example of how these structures can be utilized in material applications. A three-functional thiol (TMP 3MP3) was used as a cross linker since it is a standard crosslinking agent already extensively used in industry with suppliers such as Bruno Bock Gmbh in Germany.


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Thiol-ene chemistry is usually promoted by UV-light34, however, due to strong UV absorbance of the lignin structure, the activation of the initiator is extensively prevented making the UVcuring a poor process choice for this system. It is well established that one of the main drawbacks with conventional thiol-ene systems is a poor pot-life, i.e., these systems tend to spontaneously polymerize without activation.34 This feature is used as an advantage in the present study since it allows for an initiator free thermally induced crosslinking. The resins were cured in an oven at 120 °C and under air atmosphere, without initiator. The strong absorbance of lignin is also the reason to why FT-IR was performed as the curing analysis method rather than FT-Raman that would give significantly more pronounced signals for the thiol S-H as well as ene C=C vibrations. FT-IR analysis of the system before and after curing, Figure 6, could nevertheless clearly show a decrease of thiol (2568 cm-1) as well as double bond (1647 cm-1) peaks providing evidence of consumption of the respective groups during curing, however, due to high crosslinking density, full conversion of the reactive groups is not achieved.


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Figure 6: FT-IR spectra over the resin and the cured resin of the 1:1 ratio. The decrease of the thiol (2568 cm-1) and ene (1647 cm-1) peaks indicate that the thiol-ene coupling occurs. Spectra of the different resins are provided in the ESI, Figure S4. To examine the effect of the reactive group stoichiometry, five different resins with different thiol-ene ratios were prepared. The thermosets are rigid when cured and DSC measurements reveal very broad transitions showing Tg:s between 45 and 65 °C. These broad transitions indicate that the network is tightly crosslinked. Even though the Tg:s of the samples are difficult to determine precisely it can clearly be stated that they are significantly higher than many thiolene systems due to the incorporation of rigid aromatic lignin structures. Variation in the stoichiometry render thermosets either having residual amounts of thiols or allyl ether groups depending on which one is in excess, FT-IR spectra of all different resins are presented in Figure S4. Only minor differences in the Tg is found when varying the stoichiometry as seen in Table 3.


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Table 3: The Tg:s of the different ratios as determined by DSC. However, due to a tight network formation the samples exhibits broad transition curves which make precise calculations difficult. Curves are represented in Figure S5. Sample

Tg [°C]

L-KLEtOH

133

Thiol:Ene ratio 0.8:1

62


53

Thiol:Ene ratio 1:1

51


47


51

This can be explained by counteracting factors that reduce the effect of stoichiometry variations i.e. crosslinking density vs weight fractions of the ingoing components. Similar effects have previously been seen on studies of thermosets based on epoxidized vegetable oils.39 Initial DMA characterization of the thermoset, 1:1 ratio, reveals a mechanical performance that corroborate well with the results obtained from the DSC study with a peak in the tan δ, presented in Figure S6. The storage modulus (E´) trace furthermore shows that the thermoset has a modulus around 1 GPa below Tg followed by a rather broad Tg transition indicating that a densely cross-linked thermoset has been formed. Practical issues measuring precise geometries as well as the brittle character of the sample, however, limited the accuracy of the characterization above the Tg. A more extensive study on the mechanical properties of these thermosets will be a subject for a future study.


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The cured material appears homogeneous and transparent, presenting a dark amber color; it has a high affinity to glass and does not have any smell of residual thiol. A photograph of the cured material is shown in Figure 7.

Figure 7: The cured thermoset has a brown-red color; it is transparent with no visual agglomerates or defects. It is glossy and sticks very well to the glass substrates. It can be said that the Allyl-L-KLEtOH can be readily employed as a thermoset resin constituent in thiol-ene systems able to provide materials with relatively high Tg:s, above room temperature. Details of the exact reaction mechanisms of the thermally induced crosslinking is not revealed in the present study but the simultaneous disappearance of the thiol as well as the allyl groups imply that the overall reaction is a thiol-ene coupling reaction. It can also be concluded that conversion to high levels is readily obtained using this process. The potential of this type of material is evident and much can be done to further elaborate on these kinds of polymer systems. Conclusions An ethanol soluble fraction of Lignoboost Kraft lignin has been refined using scalable procedures and thoroughly characterized by 1H-, 31P- and 2D HSQC-NMR. The retrieved


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fraction could readily be selectively modified through allylation of the phenols using allyl chloride under mild conditions to a high conversion (95%). The modified lignin could be mixed in a thiol-ene resin system and crosslinked, initiator free, at 120 °C to form a thermoset. The material was cured to high conversions and determined to have Tg:s between 45 and 65 °C, well above room temperature. Associated content Supporting information Molecular structures of inter unit linkages; 1H-NMR spectrum of Allyl-L-KLEtOH; 2D HSQC NMR spectra of L-KLEtOH and Allyl-L-KLEtOH; FT-IR of thermoset resins and thermosets; DSCcurves of thermosets as well as L-KLEtOH; DMA curves of 1:1 thiol:ene ratio thermoset. Corresponding Authors Claudio Gioia: [email protected] Wallenberg Wood Science Center, WWSC, Department of Fibre and Polymer Technology, KTH Royal Institute of Technology,100 44 Stockholm, Sweden Martin Lawoko: [email protected] Wallenberg Wood Science Center, WWSC, Department of Fibre and Polymer Technology, KTH Royal Institute of Technology,100 44 Stockholm, Sweden Author Contributions The manuscript was written through contribution of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements The authors are grateful to the Knut and Alice Wallenberg Foundation for financial support through the Wallenberg Wood Science Center at KTH Royal Institute of Technology.


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For Table of Content Use Only

In this work Kraft lignin has been refined and selectively modified to be successfully employed in a thiol-ene thermoset synthesis.


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Renewable ThiolâEne Thermosets Based on Refined and Selectively

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