Cyclic Carbonates as Safe and Versatile Etherifying Reagents for the

Research Article pubs.acs.org/journal/ascecg

Cyclic Carbonates as Safe and Versatile Etherifying Reagents for the Functionalization of Lignins and Tannins Antoine Duval and Luc Avérous* BioTeam/ICPEES-ECPM, UMR CNRS 7515, Université de Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France S Supporting Information *

ABSTRACT: The potential of cyclic carbonates for the functionalization of lignins and condensed tannins was studied in detail. Four different cyclic carbonates, namely, ethylene, propylene, vinyl ethylene, and glycerol carbonates, were evaluated. Full conversion of the phenolic hydroxyl groups was observed within very short reaction times (less than 2 h and as low as 15 min with ethylene carbonate). Comparison among the different cyclic carbonates shows that the substituent influences the reactivity as follows: CH3 < CHCH2 < CH2OH < H. The developed method is a safe alternative to the use of organohalides or epoxides for the introduction of functional groups of interest onto lignins and tannins. The prepared derivatives expose primary or secondary hydroxyl groups, vinyl groups, 1,2- and 1,3-diols, or five-membered cyclic carbonates. All of the derivatives have an enhanced thermal stability and a lowered glass transition temperature (Tg) compared to the neat lignins and condensed tannins and thus present a high potential for the preparation of various types of biobased and aromatic polymers. The proposed method also allows the grafting of oligomeric chains when the reaction time is extended and thus represents an alternative to the use of the toxic and hazardous ethylene or propylene oxides for the oxyalkylation of polyphenols. KEYWORDS: Lignin, Tannins, Cyclic carbonates, Alkylation, Polyphenols, Ether linkage, Ring-opening polymerization

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INTRODUCTION

Organohalides are used in the Williamson-type ether synthesis to react with OH groups under base-catalyzed conditions. Lignin or lignin-derived monomers have, for example, been modified with allyl halides18−21 or propargyl halides22−24 to introduce reactive double or triple bonds, respectively, or with epichlorhydrin to introduce oxirane groups.25,26 The main drawbacks of these reactions are the high toxicity of the organohalides, the use of organic solvents [acetone,19 ethanol,20 or dimethylformamide (DMF)21,24], and the necessity to add stoichiometric bases to catalyze the reaction and entrap the released gaseous toxic byproducts (HCl or HBr). The ring-opening reactions of epoxides also allow the formation of ether linkages. Unlike Williamson ether synthesis, the opening of the epoxy ring leads to the formation of an aliphatic OH group. It is thus commonly used to derivatize the phenolic OH groups and expose a more reactive aliphatic OH groups by reaction with ethylene oxide (EO)27 or propylene oxide (PO).28−38 It has also been used to introduce various functional groups onto lignins and tannins, such as benzene39 and furan rings.40,41 The reactions can be run in aqueous environments, catalyzed by stoichiometric bases, leading to

Lignins and tannins are the two most important and widely available renewable sources of aromatic structures. These biobased polyphenols continue to generate growing interest for the development of aromatic macromolecular architectures or additives for polymer formulations. Many chemical modifications of lignins and tannins have been described for these purposes and were recently reviewed.1−6 Among them, aryl and/or alkyl etherifications are particularly broad in scope and have thus been studied in great detail. Etherifications are commonly used to mask the phenolic OH groups of lignin by methylation or ethylation. This leads to enhanced thermal properties7 and increased compatibility between lignin and apolar polymer matrixes.8−11 Classical etherification reagents are diazomethane,12,13 dimethyl sulfate,7−11,14 and diethyl sulfate.10,11,15 Recently, more benign dimethyl carbonate was successfully used as an alternative to these highly toxic and hazardous reagents.16,17 Etherification reactions can also be used to introduce new functional groups of interest onto the polyphenol, to furnish new properties or allow further chemical modifications and polymerizations. Three main types of reactions have been reported for this purpose: (i) Williamson-type ether synthesis, (ii) ring-opening of epoxides, and (iii) reaction with cyclic carbonates. © 2017 American Chemical Society

Received: May 22, 2017 Revised: July 8, 2017 Published: July 18, 2017 7334

DOI: 10.1021/acssuschemeng.7b01502 ACS Sustainable Chem. Eng. 2017, 5, 7334−7343

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Reaction of Cyclic Carbonates with (a) Phenolic OH, (b) Aliphatic OH, and (c) COOH Groupsa

a

R = H (EC), CH3 (PC), CHCH2 (VEC), or CH2OH (GC). R1, R2, and R3 depend on the type of polyphenol considered.

near-quantitative derivatization of phenolic OH for a slight excess in epoxide (about 1.5 equiv).39,40 They can also be run under solvent-free conditions, in the presence of an excess of epoxide, as commonly reported for the oxypropylation with PO.28,34,35,37,38 However, in that case, the reactions have to be performed in pressurized reactors at high temperature and high pressure (usually 150−180 °C and 10−20 bar). Moreover, the reactions lead to the grafting of oligomeric chains and to the concomitant formation of homopolymer that has to be removed by solvent extraction with n-hexane28−31,34,35 or cyclohexane.37,38 Although cyclic carbonates have been known as phenol alkylating agents for decades,42 they were only recently used for the etherification of lignins and tannins.38,43,44 They lead to products that are structurally similar to those obtained by reactions with epoxides (Scheme 1). However, they are highboiling-point liquids of low toxicity, with high flash points and low evaporation rates, that are much safer to handle than epoxides.42,45 The reactions can be conducted at high temperature under atmospheric pressure, without the use of pressurized reactors, and in the absence of additional solvent, because the cyclic carbonates can act as both reagent and solvent. The reactivity of cyclic carbonates with amines has been widely studied,46−48 but only scarce information is available regarding their reactivity toward phenols, which involves a different mechanism (Scheme 1).42,42 In this study, we compared the reactivities of four different cyclic carbonates with soda lignin (SL) and condensed tannins (CT): ethylene carbonate (EC), propylene carbonate (PC), vinyl ethylene carbonate (VEC), and glycerol carbonate (GC) (Scheme 2). All of these reactions were conducted under solvent-free conditions, with K2CO3 as an economical and benign catalyst. The reaction kinetics were first evaluated and revealed

Scheme 2. Structures of the Cyclic Carbonates Used in This Study and Nomenclature of the Carbonate Ring: Ethylene Carbonate (EC), Propylene Carbonate (PC), Vinyl Ethylene Carbonate (VEC), and Glycerol Carbonate (GC)

differences in reactivity based on the nature of the substituent. The obtained lignin and tannin derivatives were then fully characterized by 31P and 1H nuclear magnetic resonance (NMR) spectroscopies, Fourier transform infrared (FTIR) spectrometry, size-exclusion chromatography (SEC), dynamic scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Finally, the possibility of grafting oligomeric chains was also examined.

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EXPERIMENTAL SECTION

Materials. Condensed tannins (CT) from Acacia catechu were obtained from Silvateam (San Michele Mondovi, Italy). Soda lignin (SL) from a mixture of wheat straw and Sarkanda grass (Protobind 1000) was obtained from GreenValue SA (Orbe, Switzerland). EC (1,3-dioxolan-2-one, ≥99%) was purchased from Acros Organics, PC (4-methyl-1,3-dioxolan-2-one, 99.7%) from Sigma-Aldrich, VEC (4vinyl-1,3-dioxolan-2-one, 99%) from Alfa Aesar, and GC (4hydroxymethyl-1,3-dioxolan-2-one, Jeffsol Glycerine Carbonate) from Huntsmann. 2-Chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (95%) was purchased from Sigma-Aldrich. Reaction between Polyphenols and Cyclic Carbonates. CT or SL (0.25−10 g), K2CO3 (0.1 equiv with respect to reactive groups; i.e., the sum of phenolic and aliphatic OH and COOH groups), and cyclic carbonate (10 equiv) were added to a round-bottom flask. The flask was then flushed with argon and immersed in an oil bath regulated at 150 °C for various reaction times (0.25−17 h). Then, the 7335


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ACS Sustainable Chemistry & Engineering mixture was cooled to room temperature in a water bath and precipitated in water acidified to pH 2 by addition of a 2 M HCl solution. The derivatized polyphenols were recovered by filtration or centrifugation, further washed with acidic water, and dried overnight in a vacuum oven at 40 °C. Acetylation. About 50 mg of sample was dissolved in 1 mL of pyridine/acetic anhydride (1:1, v/v) and stirred at room temperature for 24 h. It was then precipitated in about 50 mL of water acidified to pH 2 by addition of a 2 M HCl solution. The acetylated sample was recovered by filtration through a 0.45-μm poly(vinylidene fluoride) (PVDF) membrane (Durapore, Millipore), further washed on the filter, and dried overnight at 40 °C in a vacuum oven. Characterization. NMR spectra were recorded on a Bruker 400 MHz spectrometer. 31 P NMR spectra were measured after derivatization of the samples with 2-chloro-4,4,5,5-tetramethyl-1,3,2dioxaphospholane in pyridine/CDCl3 (1.6:1 v/v), in the presence of cholesterol as an internal standard.49 One hundred twenty-eight scans were recorded at 25 °C with a 15-s delay. Peak assignment was performed based on literature reports.49,50 For 1H NMR spectroscopy, the samples were dissolved in 550 μL of deuterated dimethyl sulfoxide (DMSO-d6), and 100 μL of a standard solution of 2,3,4,5,6pentafluorobenzaldehyde was added. Thirty-two scans were recorded at 25 °C. Fourier transform infrared (FTIR) spectrometry was performed on a Nicolet 380 spectrometer in attenuated total reflectance (ATR) mode. Thirty-two scans were collected between 500 and 4000 cm−1 at 4 cm−1 resolution. Dynamic scanning calorimetry (DSC) was performed on a TA Q200 calorimeter. The samples were first heated at 10 °C min−1 to 105 °C and maintained at this temperature for 15 min to erase the thermal history. They were then cooled to 0 °C at 10 °C min−1 and heated to 200 °C at 10 °C min−1. The glass transition temperature (Tg) was taken as the midpoint of the change in slope during the second heating run. Thermogravimetric analysis (TGA) was performed on a TA Q5000 instrument. The temperature was ramped at 20 °C min−1 to 700 °C under helium. Size-exclusion chromatography (SEC) was performed on a Waters Acquity Advanced Polymer Chromatography (APC) system, equipped with three 150-mm APC XT columns (a 45 Å, 1.7 μm column; a 200 Å, 2.5 μm column; and a 450 Å, 2.5 μm column) thermostated at 40 °C. Tetrahydrofuran (THF, HPLC grade, Fisher Scientific) was used as the eluent at a flow rate of 0.6 mL min−1. Detection was performed with an Acquity refractive index (RI) detector and an Acquity tunable UV (TUV) detector operating at 280 nm. Acetylated samples were dissolved in THF at 5 mg mL−1 and filtered through 0.2‑μm polytetrafluoroethylene (PTFE) syringe filters prior to injection. The average molar masses and dispersities were calculated based on a calibration with polystyrene standards.

Figure 1. Evolution of the conversion of phenol groups (χPh−OH) with the reaction time for (a) CT and (b) SL.

were similar and gave the following ranking: H > CH2OH > CHCH2 > CH3. Comparative plots showing the differences in reactivity between SL and CT with each cyclic carbonate are shown in the Supporting Information (Figure S1). In all cases, CT reacts faster than SL, and shorter times are required to reach full conversion with both PC and GC. The pKa values of lignins and tannins are similar (7.4−11.3 and 8.7−11.6 for lignin51 and tannin52 model compounds, respectively) and thus cannot explain the observed differences. However, because all of the reactions were performed with 10 equiv of cyclic carbonate, the concentrations of the solutions were higher for SL than for CT (180−260 g L−1 for SL versus 90−130 g L−1 for CT), as a result of SL’s lower content of reactive groups (5.8 mmol g−1 versus 11.6 mmol g−1 for CT). This can result in the easier dissolution of CT in the cyclic carbonate during the early stages of the reaction, leading to a better reactivity for short reaction times. This feature becomes less important as the reaction progresses, and the differences in reactivity between CT and SL thus tend to disappear as the reaction time is increased (Supporting Information, Figure S1). The reaction with cyclic carbonates is not limited to the phenolic OH groups. As shown in Scheme 1, both aliphatic OH and COOH groups also react with cyclic carbonates.42 For the reactions of CT with EC, GC, and VEC, the conversion of aliphatic OH groups can be calculated from the 31P NMR spectra, because the newly formed OH groups do not overlap with the aliphatic OH groups of the neat CT. This is unfortunately not the case for the other reactions (CT with PC and all reactions with SL). The conversion of the aliphatic OH

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RESULTS AND DISCUSSION Comparison of the Reactivities of CT and SL with Various Cyclic Carbonates. Four different cyclic carbonates were evaluated for their abilities to etherify CT and SL. All of the reactions were performed under solvent-free conditions at 150 °C. The conversion of the phenol groups, χPh−OH, was calculated from the results of 31P NMR spectroscopy. Figure 1 shows the evolution of the conversion of phenols with the reaction time for both substrates. All of the reactions were quantitative for reaction times shorter than 2 h, but strong differences were observed depending on the side chain of the cyclic carbonate. The simplest cyclic carbonate, EC, is also the most reactive. It leads to the full conversion of phenols in only 15 min. All of the other cyclic carbonates have a substituent in position 4 (Scheme 2), which leads to a reduced reactivity. With the two substrates, the effects of the substituent on the reaction rate 7336


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ACS Sustainable Chemistry & Engineering

increased molar masses as a result of grafting. The tannin derivatives obtained with EC and PC had the highest molar masses, even though the grafted groups were the smallest. Because they also presented the lowest yields, it seems that the losses during workup involved mainly the low-molar-mass derivatives. The lignin derivatives presented very similar molar mass distributions (Supporting Information, Figure S5), confirming a more homogeneous recovery during the workup. The chemical structures of the derivatives were assessed by 31 P and 1H NMR spectroscopies as well as FTIR spectrometry. In addition to the conversion of OH groups, 31P NMR spectra also provide important information on the structures of the derivatives. With asymmetrical cyclic carbonates, such as PC, GC, and VEC, two different isomers can be obtained, as depicted in Scheme 1 (structures A and B). They lead to the creation of either secondary (structures A) or primary (structures B) alcohols. Details of the aliphatic OH region of the 31P NMR spectra of the CT derivatives are shown in Figure 2. The main peak of the

groups of CT is always between 70−80%, irrespective of the reaction time and cyclic carbonate (Supporting Information, Figure S2). The COOH groups of SL are converted into the corresponding esters, according to Scheme 1c.42 Because their pKa values are lower than those of the phenolic OH groups (3.43−4.46 in lignin model compounds),51 they react even faster and are quantitatively converted in less than 1 h with all of the cyclic carbonates (Supporting Information, Figure S3). Structural Characterization of the Derivatized Lignins and Tannins. The fully derivatized lignins and tannins were then characterized in more detail. The shortest reaction time leading to full conversion was used for sample preparation (Table 1). The yields were calculated on a mass basis, taking Table 1. Characterization of the CT and SL Derivatives Obtained upon Reaction with Various Cyclic Carbonates substrate CT

SL

cyclic carbonate

t (h)a

yield (%)b

[OH]c (mmol g−1)

− EC PC GC VEC − EC PC GC VEC

− 0.25 1.5 1 1 − 0.25 2 1 1

− 33 46 51 59 − 83 73 79 78

2.25 6.31 5.55 3.51 5.23 1.76 4.10 2.98 2.94 3.56

Mnd (g mol−1) 2 2 1 1 1 1 1 1

820 040 310 250 470 770 580 690 300 250

Đd 1.43 2.00 1.94 1.66 2.14 5.90 3.85 2.99 3.29 3.83

a Reaction time to reach the full conversion of phenolic OH. bYield on a mass basis, calculated according to eq 1. cContent in aliphatic OH, determined by 31P NMR spectroscopy. dDetermined by SEC in THF with PS calibration.

into account the increase in mass caused by the grafting, using the equation mf yield (%) = 100 × m i (1 + ΔMgraft[OH + COOH])

Figure 2. Detail of the aliphatic OH region of the 31P NMR spectra of CT derivatives obtained with different cyclic carbonates. IS = internal standard (cholesterol).

neat CT aliphatic OH groups at 145.3 ppm does not overlap with that of the newly formed aliphatic OH groups. With EC, a single band appears in the primary OH region, between 146.5− 148 ppm. With PC, two distinct bands appear. The main peak between 145 and 146.5 ppm corresponds to secondary OH groups (structure A), and the second peak between 147 and 148 ppm corresponds to primary OH groups (structure B). Similarly, two distinct regions are visible with GC, corresponding to primary (147−148.5 ppm) and secondary (146−147 ppm) OH groups. With VEC, a single large peak is visible between 146.5 and 148.5 ppm, but it cannot be assigned unequivocally to primary or secondary OH groups. Similar observations were made for the 31P NMR spectra of the SL derivatives (Supporting Information, Figure S6), but the aliphatic OH groups of the neat SL overlap with the newly formed OH groups, precluding quantification. For CT derivatives obtained with PC and GC, the percentages of structures of types A and B can be calculated from the 31P NMR spectra. It was found that 15% and 35% of the structures formed were of type B for PC and GC, respectively. The effect of the substituents on the isomers can be explained by two distinct phenomena, namely, steric hindrance and electronic effects, which favor the reactivity at

(1)

where mi and mf represent the initial and final masses, respectively; ΔMgraft is the increase in molar mass caused by the grafted chemical group (44, 58, 70, and 74 g mol−1 for EC, PC, VEC, and GC, respectively); and [OH + COOH] is the total content of reactive groups in CT or SL (11.6 × 10−3 and 5.8 × 10−3 mol g−1 for CT and SL, respectively). The yields are reported in Table 1. They are higher for the lignin derivatives (73−83%) than for the tannin derivatives (33−59%). Tannins are water-soluble over a much wider pH range than lignins and are thus harder to precipitate during workup at acidic pH, which ultimately reduces the recovered yield. SEC measurements were performed for all of the lignin and tannin derivatives on an ultraperformance liquid chromatography (UPLC) system. Compared to conventional SEC, the use of UPLC allows a dramatic reduction in the time of analysis and solvent consumption (only 6 mL per analysis with the system used versus about 40−50 mL with conventional SEC). The chromatograms are available in the Supporting Information (Figures S4 and S5), and the average molar masses and dispersities are reported in Table 1. All of the derivatives had 7337


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Figure 3. (a) FTIR and (b) 1H NMR spectra of CT modified with different cyclic carbonates.

Scheme 3. Two-Step Reaction Leading to the Formation of Cyclic Carbonates during the Reaction with GC: (a) Etherification First Forms a Derivative Carrying a 1,2-Diol, (b) Which, upon Transesterification with GC, Produces a Derivative Carrying a Cyclic Carbonate and Glycerol

wavenumbers (1786 cm−1), in the range of cyclic carbonates. The peak does not correspond to residual GC, because 1H NMR spectra confirmed the purity of the sample. It thus seems that the reaction with GC leads to the formation of cyclic carbonate groups. Derivatives containing a 1,2-diol should be formed first, according to the pathway described in Scheme 1. Then, it seems that the 1,2-diol can react with another GC molecule through transesterification, to produce a fivemembered cyclic carbonate group and a glycerol molecule (Scheme 3). However, this reaction is not complete, because many OH groups were detected by 31P NMR spectroscopy and FTIR spectrometry. Nevertheless, it might have a great potential for the preparation of lignins and tannins carrying cyclic carbonate groups,26 which are key intermediates for the preparation of nonisocyanate polyurethanes.47 1 H NMR spectra (Figure 3b) further confirmed the successful grafting, with the disappearance of the signal of phenol groups between 8 and 10 ppm. With PC, the methyl group is clearly visible as a large peak at about 0.5−1.5 ppm. With VEC, the vinyl group gives rise to signals at 5.1, 5.3 ( CHCH2) and 5.9 ppm (CHCH2).44 The quantifications of these groups are given in Table 2. With GC, the peak at 5.15 ppm confirms the formation of cyclic carbonate structures. All of the other new signals fall in the 3−5 ppm region and overlap with other tannin-related signals, precluding the quantification of the grafting with EC and GC. Similar data

carbons C4 or C5 (Scheme 2). Steric hindrance reduces the reactivity of the substituted carbon (C4), leading to a greater amount of structures of type A. In addition, electron-donating substituents, such as CH3 and CHCH2, reduce the electrophilicity of C4, further leading preferentially to structures of type A.53 In contrast, electron-withdrawing substituents, such as CH2OH, favor structures of type B by increasing the electrophilicity of C4. This explains the higher proportion of structures of type B measured with GC, even though the substituent was bulkier than for PC. FTIR spectra of CT derivatized with the various cyclic carbonates are shown in Figure 3a. Some features are common to all of the derivatives. The OH band is shifted toward higher wavenumbers as a result of the consumption of phenolic OH and creation of aliphatic OH. The peaks of the CH stretch increased (2800−3000 cm−1) because all of the grafted groups contained at least two CH2 or CH groups, and a new peak for the CO stretch appeared (1257−1275 cm−1) as a result of the formation of ether bonds (Scheme 1). In addition, all of the derivatives show a new peak in the CO stretch region (1720−1800 cm−1) that demonstrates the presence of carbonate linkages. For EC, PC, and VEC, the peak is located between 1728 and 1743 cm−1. It corresponds to linear carbonate linkages, which are known to form during the reaction between cyclic carbonates and aliphatic OH (Scheme 1b). For GC, the peak is surprisingly located at much higher 7338


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ACS Sustainable Chemistry & Engineering Table 2. Type and Quantification of the Functional Groups Introduced by the Reaction with Different Cyclic Carbonates substrate CT

cyclic carbonate EC PC GC

VEC SL

EC PC GC

VEC

type of functional group primary alcohola primary alcohola secondary alcohola 1,2-diola 1,3-diola five-membered cyclic carbonateb aliphatic alcohola vinyl groupc primary alcohola primary alcohola secondary alcohola 1,2-diola 1,3-diola five-membered cyclic carbonateb aliphatic alcohola vinyl groupc

Table 3. Thermogravimetric Analyses of Tannin and Lignin Derivatives

content (mmol g−1) 6.31 0.86 4.69 1.15 0.46 1.23

substrate

cyclic carbonate

T95% (°C)

Tda (°C)

CT

− EC PC GC VEC − EC PC GC VEC

212 231 295 229 209 165 233 227 213 231

291 365 369 373 393 319 377 374 376 386

SL

5.23 7.14 ndd nd 1.06 nd nd 0.61

a

Main degradation peaks.

OH groups dramatically limits the possibilities for radical reactions that are known to affect underivatized lignins.7 This enhanced thermal stability is important in increasing the usage and processability of these derivatives under thermomechanical input. All of the derivatives presented a main degradation peak between 365 and 395 °C. The glass transition temperaturess (Tg) of all of the derivatives are shown in Figure 4. All of the derivatives

nd 4.36

a

Determined by 31P NMR spectroscopy. bDetermined by comparison of 31P NMR spectra before and after hydrolysis. cDetermined by 1H NMR spectroscopy. dnd = not determined, because of overlap in the 31 P NMR spectra with the aliphatic OH of neat lignin.

for SL are provided in the Supporting Information (Figures S7 and S8) and lead to comparable conclusions. To gain further insight into the formation of cyclic carbonate structures during the reaction with GC, the derivatives were hydrolyzed (reaction scheme shown in the Supporting Information, Scheme S1) and further analyzed by FTIR spectrometry and 31P NMR spectroscopy. After hydrolysis, the FTIR spectra revealed the disappearance of the peak at 1786 cm−1, indicating that all of the cyclic carbonate structures had been removed (Supporting Information, Figures S9 and S10). In addition, an increase in the content of OH groups was visible. The increase was further confirmed by 31P NMR spectroscopy (Supporting Information, Figures S11 and S12). This reveals that the content of 1,2-diols (i.e., structures of type A in Scheme 1) was substantially higher after hydrolysis. On the other hand, the content of 1,3-diols (i.e., structures of type B in Scheme 1) was unchanged. This confirms that some of the corresponding 1,2-diols were converted into five-membered cyclic carbonate structures (Scheme 3). Such structures represented 1.23 and 0.61 mmol g−1 for the CT and SL derivatives, respectively. Table 2 summarizes the new functional groups that have been introduced onto CT and SL derivatives. It includes primary and secondary OH groups, 1,2- and 1,3-diols, fivemembered cyclic carbonates, and vinyl groups and thus clearly shows the versatility of the developed method for the functionalization of lignins and tannins. Thermal Properties of the Derivatized Lignins and Tannins. The thermal properties of lignin and tannin derivatives were studied by TGA and DSC. Table 3 gathers the results of TGA, whereas all of the degradation curves are available in the Supporting Information (Figures S13 and S14). In any case, the modification of tannin and lignin leads to derivatives of enhanced thermal stability. The temperature at which 95% of the initial mass remains, T95%, present values substantially higher than 200 °C. Indeed, masking the phenolic

Figure 4. Glass transition temperatures (Tg) of CT and SL derivatives obtained with different cyclic carbonates.

presented Tg values inferior to those of the neat lignin and tannin. This arises from the reduced H-bonding capacity caused by the masking of the phenol groups, as well as from the increased free volume due to the grafting.7 The bulky vinyl group of VEC leads to the lowest Tg value, because VEC induces more free volume. On the other hand, the grafted groups contain OH groups, which are able to induce H bonding and counterbalance the decrease in Tg caused by the increase in free volume.7 This explains why the derivatives obtained with GC, which contain two OH groups per graft, presented the highest Tg values of the derivatives. Increasing the Reaction Time: Toward Chain Extension? After reaching the full conversion of phenol groups, the reaction time can be extended to potentially achieve the grafting of oligomeric chains, as shown in Scheme 4. The chain extension process can include the formation of ether linkages, with the release of CO2, and carbonate linkages.54−61 With EC or PC, this is an alternative to the classic oxyethylation or oxypropylation reactions performed with ethylene or propylene oxide, respectively.38,43 To evaluate these possibilities, several 7339


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ACS Sustainable Chemistry & Engineering Scheme 4. Grafting of Oligomeric Chains Based on Ether and Carbonate Units for Extended Reaction Times

occurrence of significant cross-linking reactions. Because the FTIR spectra showed the presence of a large quantity of carbonate linkages, it is most likely that the cross-linking reactions happen by chain coupling through a transesterification reaction (reaction scheme shown in the Supporting Information, Scheme S2).62 Similarly, reactions between CT or SL and PC were conducted over longer periods, up to 17 h. The average length of the grafted chains was quantified by 1H NMR spectroscopy performed on acetylated samples, as described previously.38 The results are presented in Figure 6. The average length of the

reactions were performed for extended reaction times, up to 17 h, with EC, PC, and GC. VEC was not included in this specific study, because it has been shown that the derivatives with VEC became insoluble for reaction times greater than 2−3 h.44 The FTIR spectra of CT reacted with EC show an increase of the CH, CO, and CO peaks with the reaction time, as reported in Figure 5a. Similar data recorded for SL and the

Figure 6. Average length of the grafted chains during the reaction with PC as a function of the reaction time.

grafted chain is equal to one unit after 2 h of reaction with CT. For SL, it is slightly higher, but signals overlapping in the 1H NMR spectra are probably responsible for the overestimation (SL contains some residual fatty acids that give rise to a signal in the 1.0−1.5 ppm region, and overlap with the methyl group of the grafted PC, see Supporting Information, Figure S6). It is thus very easy to obtain derivatives with a single unit grafted by a solvent-free reaction with PC, whereas reactions with PO under similar conditions lead to the grafting of oligomeric chains.37,38 The length of the grafted chain then increases linearly with the reaction time, making control of the final chain length easy to achieve. However, the studied conditions did not allow for the grafting of more than 1.5 units per chains on average, which does not cause significant changes in the SEC traces (Supporting Information, Figures S18 and S19). To achieve the grafting of longer chains, higher amounts of PC are necessary.38 Finally, further reactions with GC were performed, with extended reaction times up to 17 h. Two distinct features could be expected. First, it is possible that the 1,2-diols react with GC by transesterification to form five-membered cyclic carbonate groups, as depicted in Scheme 3.42 Then, the OH groups might also react with GC according to the general pathway shown in Scheme 4, leading to the grafting of hyperbranched polyglycerol chains on the tannin or lignin core.63−65 The

Figure 5. (a) Evolution of the intensities of the FTIR peaks of CH (2872 cm−1), CO (1741 cm−1), and CO (1257 cm−1) stretches with the reaction time during the reaction between CT and EC. (b) SEC traces of the CT derivatives obtained after reaction with EC for various reaction times.

corresponding FTIR spectra are available in the Supporting Information (Figures S15−S17). This shows that the chain extension is successful under these reaction conditions and that it involves the formation of both carbonate and ether linkages (Scheme 3). Unfortunately, overlaps in the 1H NMR spectra preclude the quantification of the chain length. SEC measurements also confirmed the chain extension, which resulted in an increase in the molar masses of the derivatives with the reaction time (Figure 5b). However, the samples recovered after 3 h or more were insoluble and could not be analyzed by SEC. After 5 h, the reaction mixture even gelified, indicating the 7340


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polyphenols. Furthermore, cyclic carbonates carrying many different functional groups have been reported recently,66−68 making this strategy adaptable to the grafting of plenty of other functional groups than those reported in this study, such as aromatic rings, double or triple bonds, and protected amines.

SEC results reported in Figure 7 show that the molar masses of the derivatives increased with the reaction time (the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01502. All supplementary figures and schemes mentioned in the text, including additional NMR and FTIR spectra, SEC traces, and TGA curves (PDF)

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AUTHOR INFORMATION

Corresponding Author

Figure 7. Evolution of the number-average molar masses of the CT and SL derivatives prepared with GC as functions of the reaction time.

*Phone: + 333 68852784. Fax: + 333 68852716. E-mail: luc. [email protected]. ORCID

corresponding chromatograms are reported in the Supporting Information, Figures S20 and S21). In addition, FTIR spectra do not show any increase in the carbonate band at 1780 cm−1 (Supporting Information, Figures S22 and S23). This shows that the second pathway is preferred and that hyperbranched polyglycerol chains can be successfully grafted onto the lignin or tannin core.

Luc Avérous: 0000-0002-2797-226X Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS This study has explored the advantages of cyclic carbonates for the modification of lignins and tannins. Cyclic carbonates are liquids with high boiling points and low volatilities that are safe to handle. Their reactions with lignins and tannins can be performed under solvent-free conditions, with low amounts of an inexpensive and benign catalyst (K2CO3). The phenolic OH groups are quantitatively converted within short reaction times (always less than 2 h and as low as 15 min), yielding derivatives of controlled structure, with a single unit grafted on each OH. The comparative study of the reactivities of four different cyclic carbonates with condensed tannins and lignins showed that the reactivity is affected by the substituent on the cyclic carbonate as follows: CH3 < CHCH2 < CH2OH < H. The prepared lignin and tannin derivatives expose interesting functional groups, such as primary or secondary aliphatic OH groups, vinyl groups, 1,2 and 1,3-diols, and five-membered cyclic carbonate groups, and present great potential for the use in macromolecular architectures or as additives in polymer formulations. Compared to the neat lignins and tannins, the thermal stability is largely increased and the Tg is lowered, thus facilitating the thermal processing. This study also revealed the possibility of grafting oligomeric chains if the reactions are conducted over longer periods. With EC and PC, this is of particular interest because it is a safe way to prepare oxyalkylated derivatives without the use of the toxic, hazardous, and volatile ethylene or propylene oxides. In addition, the control of the grafting is easy to achieve, because the lengths of the grafted chains increase linearly with the reaction time. With GC, this allows the grafting of hyperbranched polyglycerol side chains onto the aromatic core of tannin or lignin. The developed method is very versatile for the controlled introduction of new chemical functions onto biobased 7341


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