Cyclic Carbonates as Safe and Versatile Etherifying Reagents for the

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Cyclic carbonates as safe and versatile etherifying reagents for the functionalization of lignins and tannins Antoine Duval, and Luc Averous ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01502 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 18, 2017

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ACS Sustainable Chemistry & Engineering

Cyclic carbonates as safe and versatile etherifying reagents for the functionalization of lignins and tannins

Antoine Duval, Luc Avérous* BioTeam/ICPEES-ECPM, UMR CNRS 7515, Université de Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France * Corresponding author: Prof. Luc Avérous, Phone: + 333 68852784, Fax: + 333 68852716, E-mail: [email protected]

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Abstract The potential of cyclic carbonates for the functionalization of lignins and condensed tannins was studied in detail. Four different cyclic carbonates, ethylene, propylene, vinyl ethylene and glycerol carbonates, were evaluated. Full conversion of the phenolic hydroxyl groups is observed within very short reaction times (less than 2 h and as low as 15 min with ethylene carbonate). The comparison between the different cyclic carbonates shows that the substituent influences the reactivity as follow: -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, vinyl groups, 1,2- and 1,3diols or 5-membered cyclic carbonates. All the derivatives have an enhanced thermal stability and a lowered Tg with respect 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 represent 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 Lignins and tannins are the two most important and widely available renewable sources of aromatic structures. These biobased polyphenols continue to generate a growing interest to develop aromatic macromolecular architectures, or additives for polymer formulations. Many chemical modifications of lignins and tannins have been described for these purposes, and recently reviewed.1–6 Among them, aryl and/or alkyl etherifications are particularly large 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 matrices.8–11 Classical etherification reagents are diazomethane12,13 and dimethyl7–11,14 or 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 bring new properties or allow further chemical modifications and polymerizations. Three main types of reactions have been reported for this purpose: (i) the Williamson-type ether synthesis, (ii) the ring-opening reaction of epoxides, and more recently (iii) the reaction with cyclic carbonates. Organohalides are used in the Williamson-type ether synthesis to react with OH groups under basecatalyzed conditions. Lignin or lignin-derived monomers have for example been modified with allyl18– 21

or propargyl halides22–24 to introduce reactive double or triple bonds, 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 ethanol20 or DMF21,24), and the necessity to add stoichiometric bases to catalyze the reaction and entrap the released gaseous toxic by-products (HCl or HBr).



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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 (EO)27 or propylene oxide (PO).28–38 It has also been used to introduce various functional groups on lignins or tannins, such as benzene39 or furan rings.40,41 The reactions can be run in aqueous environment, catalyzed by stoichiometric bases, leading to nearquantitative derivatization of phenolic OH for a slight excess in epoxide (around 1.5 eq).39,40 They can also be run in 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 bars). Besides, it leads 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 they have been known as phenol alkylating agents for decades,42 cyclic carbonates were only recently used for the etherification of lignins and tannins.38,43,44 They lead to products structurally similar to those obtained by reactions with epoxides (Scheme 1). However, they are high boiling point liquids of low toxicity, with high flash points and low evaporation rates, 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, since the cyclic carbonates can act as both reagent and solvent.


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Scheme 1. Reaction of cyclic carbonates with phenolic OH (a), aliphatic OH (b) and COOH groups (c). R = H (EC), -CH3 (PC), -CH=CH2 (VEC) or –CH2-OH (GC). R1, R2 and R3 depend on the type of polyphenol considered.

The reactivity of cyclic carbonates with amines has been widely studied,46–48 but only scarce information is available regarding their reactivity towards phenols, which involves a different mechanism (Scheme 1).4242 In this study, we compared the reactivity of 4 different cyclic carbonates with soda lignin (SL) and condensed tannins (CT): ethylene carbonate (EC), propylene carbonate (PC), vinyl ethylene carbonate (VEC) and glycerol carbonates (GC) (Scheme 2). All these reactions were conducted in solvent-free conditions, with K2CO3 as economic and benign catalyst. The reaction kinetics were first evaluated, and revealed the differences in reactivity based on the nature of the substituent. The obtained lignin and tannin derivatives were then fully characterized with 31P and 1H NMR, FTIR, SEC, DSC and TGA. Finally, the possibility to graft oligomeric chains was also examined.



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Scheme 2. Structure of the cyclic carbonates used in this study: ethylene carbonate (EC), propylene carbonate (PC), vinyl ethylene carbonate (VEC) and glycerol carbonate (GC), and nomenclature of the carbonate ring.

Experimental section Materials Condensed tannins (CT) from Acacia catechu were obtained from Silvateam (Italy). Soda lignin (SL) from a mixture of wheat straw and Sarkanda grass (Protobind 1000) was obtained from Green Value SA (Switzerland). EC (1,3-dioxolan-2-one, ≥99%) was purchased from Acros Organics, PC (4-methyl1,3-dioxolan-2-one, 99.7%) from Sigma-Aldrich, VEC (4-vinyl-1,3-dioxolan-2-one, 99%) from Alfa Aesar, and GC (4-hydroxymethyl-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 eq. with respect to reactive groups, i.e. the sum of phenolic and aliphatic OH, and COOH groups) and cyclic carbonate (10 eq.) were added in a round-bottom flask. It 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 mixture was cooled down to room temperature in a water bath, and precipitated in water acidified to pH 2 by addition of a 2M 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.


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Acetylation About 50 mg of sample was dissolved in 1 mL pyridine / acetic anhydride (1:1, v/v) and stirred at room temperature for 24 h. It was then precipitated in about 50 mL of acidic water (pH 2, adjusted by the addition of a 2 M HCl solution). The acetylated sample was recovered by filtration on a 0.45 µm PVDF membrane (Durapore, Millipore), further washed on the filter, and dried overnight at 40 °C in a vacuum oven. Characterizations NMR spectra were recorded on a Bruker 400 MHz spectrometer. 31P NMR was measured after derivatization of the samples with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane in pyridine/CDCl3 (1.6:1 v/v), in the presence of cholesterol as internal standard.49 128 scans were recorded at 25 °C with 15 s delay. Peak assignation was performed based on literature reports.49,50 For 1H NMR, the samples were dissolved in 550 µL DMSO-d6 and 100 µL of a standard solution of 2,3,4,5,6-pentafluorobenzaldehyde was added. 32 scans were recorded at 25 °C. Fourier Transform Infrared Spectrometry (FTIR) was performed on a Nicolet 380 spectrometer, in the attenuated total reflectance (ATR) mode. 32 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 up 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 down to 0 °C at 10 °C min-1, and heated up 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.



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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, a 200 Å, 2.5 µm and a 450 Å, 2.5 µm) thermostatted at 40 °C. Tetrahydrofuran (THF, HPLC grade, Fisher Scientific) was used as eluent at a flow rate of 0.6 mL min-1. The detection was performed by an Acquity refractive index (RI) detector and an Acquity TUV detector operating at 280 nm. Acetylated samples were dissolved in THF at 5 mg mL-1 and filtered through 0.2 µm PTFE syringe filters prior to injection. The average molar masses and dispersity were calculated based on a calibration with polystyrene standards.

Results and Discussion Comparison of the reactivity of CT and SL with various cyclic carbonates Four different cyclic carbonates were evaluated for their ability to etherify CT and SL. All the reactions were performed in solvent-free conditions at 150 °C. The conversion of the phenol groups, χPh-OH, was calculated from the results of 31P NMR. Figure 1 shows the evolution of the conversion of phenols with the reaction time for both substrates.


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Figure 1. Evolution of the conversion of phenol groups (χPh-OH) with the reaction time for CT (a) and SL (b) All the reactions are quantitative for reaction times shorter than 2 h, but strong differences are observed depending on the side chain of the cyclic carbonate. The simplest cyclic carbonate, EC, is also the most reactive. It leads to a full conversion of phenols in only 15 min. All the other cyclic carbonates possess a substituent in position 4 (Scheme 2), which leads to a reduced reactivity. With both substrates, the influence of the substituent on the reaction rate is similar, and gives the corresponding 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 ESI (Figure S1). In any case, CT reacts faster than SL, and shorter times are required to reach the full conversion with both PC and GC. pKa of lignins and tannins are similar (7.4 – 11.3 and 8.7 – 11.6 for lignin51 and tannin model compounds,52 respectively), and thus cannot 9 ACS Paragon Plus Environment


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explain the observed differences. However, since all the reactions were performed with 10 eq of cyclic carbonates, the concentration of the solutions were higher for SL than for CT (180 – 260 g L-1 for SL against 90 – 130 for CT), as a result of its lower content in reactive groups (5.8 mmol g-1 against 11.6 for CT). This can result in an 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 when increasing the reaction time (ESI, Figure S1). The reaction with cyclic carbonates is not limited to the phenolic OH groups. As shown on 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 can be calculated from the 31P NMR spectra, because the newly formed OH groups do not overlap with the aliphatic OH 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 of CT is always between 70 – 80%, irrespective of the reaction time and cyclic carbonate (ESI, Figure S2). The COOH groups of SL are converted into the corresponding esters, according to Scheme 1c.42 Their pKa being lower than the phenolic OH (3.43 – 4.46 in lignin model compounds),51 they react even faster, and are quantitatively converted in less than 1 h with all the cyclic carbonates (ESI, 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 the sample preparation (Table 1). The yields were calculated on a mass basis, taking into account the increase in mass caused by the grafting, using Equation (1):

% = 100 ×

1 + ∆ × +


(1)

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where mi and mf represent the initial and final mass, 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 in 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 lignin, and are thus harder to precipitate during the workup at acidic pH, which ultimately reduces the recovered yield. Table 1. Characterization of the CT and SL derivatives obtained with the various cyclic carbonates

Substrate

Cyclic carbonate

t (h) a

Yield (%) b

[OH] (mmol g-1) c

CT

2.25 EC 0.25 33 6.31 PC 1.5 46 5.55 GC 1 51 3.51 VEC 1 59 5.23 SL 1.76 EC 0.25 83 4.10 PC 2 73 2.98 GC 1 79 2.94 VEC 1 78 3.56 a Reaction time to reach the full conversion of phenolic OH b Yield on a mass basis, calculated according to Equation (1) c Content in aliphatic OH, determined by 31P NMR d Determined by SEC in THF with PS calibration

Mn (g mol-1) d

Ðd

820 2 040 2 310 1 250 1 470 770 1 580 1 690 1 300 1 250

1.43 2.00 1.94 1.66 2.14 5.90 3.85 2.99 3.29 3.83

SEC measurements were performed for all the lignin and tannin derivatives on an ultra-performance liquid chromatography (UPLC) system. Compared to conventional SEC, the use of UPLC allows a drastic reduction in the time of analysis and solvent consumption (only 6 mL per analysis with the system used, against about 40 – 50 mL with conventional SEC). The chromatograms are available in the ESI (Figures S4 and S5), and the average molar masses and polydispersity are reported in Table 1. All the derivatives have an increased molar mass as a result of grafting. The tannin derivatives



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obtained with EC and PC have the highest molar mass, even though the grafted groups are the smallest. Since they also present the lowest yields, it seems that the losses during the workup concern mainly the low molar mass derivatives. The lignin derivatives present very similar molar mass distributions (ESI, Figure S5), confirming a more homogeneous recovery during the workup. The chemical structure of the derivatives was assessed by 31P and 1H NMR as well as FTIR spectroscopy. In addition to the conversion of OH groups, 31P NMR spectra also give important information on the structure 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 a secondary (structures A) or primary alcohol (structures B). The detail of the aliphatic OH region of the 31P NMR spectra of CT derivatives is shown on Figure 2. The main peak of the neat CT aliphatic OH groups at 145.3 ppm doesn’t overlap with the newly formed aliphatic OH. With EC, a single band appears in the primary OH region, between 146.5 – 148 ppm. With PC, two distinct bands appear. The main one between 145 – 146.5 ppm corresponds to secondary OH (structure A) and the second one between 147 – 148 ppm to primary OH (structure B). Similarly, two distinct regions are visible with GC, corresponding to primary (147 – 148.5 ppm) and secondary OH (146 – 147 ppm). With VEC, a single large peak is visible between 146.5 – 148.5 ppm, but cannot be assigned unequivocally to primary or secondary OH. Similar observation can be done on the 31P NMR spectra of SL derivatives (ESI, Figure S6), but the aliphatic OH of the neat SL overlap with the newly formed OH groups, precluding the quantification.


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Figure 2. Detail of the aliphatic OH region of the 31P NMR spectra of CT derivatives obtained with the different cyclic carbonates. IS = internal standard (cholesterol). 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. 15 and 35% of structures of type B are formed, respectively. The effect of the substituents on the isomers can be explained by two distinct phenomena, steric hindrance and electronic effects, which will favor the reactivity at 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 or –CH=CH2, reduce the electrophilicity of C4, further leading preferentially to structures of type A.53 On the opposite, electron withdrawing substituents, such as –CH2-OH, will favor structures of type B by increasing the electrophilicity of C4. It explains the higher proportion of structures of type B measured with GC, even though the substituent is bulkier than in PC. FTIR spectra of CT derivatized with the various cyclic carbonates are shown on Figure 3a. Some features are common to all the derivatives. The OH band is shifted towards 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 the grafted groups contain at least 2 CH2 or CH groups, and a new peak of C-O stretch appears (1257 – 1275 cm-1) as a result of the formation of ether bonds (Scheme 1). In addition, all the derivatives show a new peak in the C=O stretch region (1720 – 1800 13 ACS Paragon Plus Environment


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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 wavenumbers (1786 cm-1), in the region of cyclic carbonates. The peak doesn’t correspond to residual GC, since 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 first be formed, according to the pathway described on Scheme 1. Then, it seems that the 1,2-diol can react with another GC molecule via transesterification, to produce a 5membered cyclic carbonate group and a glycerol molecule (Scheme 3). However, this reaction is not complete, since many OH groups are detected by 31P NMR and FTIR. Nevertheless, it might have a great potential for the preparation of lignins and tannins carrying cyclic carbonate groups,26 which are a key intermediate for the preparation of non-isocyanate polyurethanes.47

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

H NMR spectra (Figure 3b) further confirm the successful grafting, with the disappearance of the

signal of phenol groups between 8 – 10 ppm. With PC, the methyl group is clearly visible as a large peak around 0.5 – 1.5 ppm. With VEC, the vinyl group gives rise to signals at 5.1, 5.3 (-CH=CH2) and 14 ACS Paragon Plus Environment

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5.9 ppm (-CH=CH2).44 The quantifications of these groups are given in Table 2. With GC, a peak at 5.15 ppm confirms the formation of cyclic carbonate structures. All 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 for SL are given in the ESI (Figures S7 and S8), and lead to comparable conclusions. To gain further insights into the formation of cyclic carbonate structures during the reaction with GC, the derivatives were hydrolyzed (reaction scheme shown in the ESI, Scheme S1) and further analyzed by FTIR and 31P NMR. After hydrolysis, the FTIR spectra reveal the disappearance of the peak at 1786 cm-1, indicating that all cyclic carbonate structures have been removed (ESI, Figures S9 and S10). In addition, an increase in the content in OH groups is visible. The increase was further confirmed by 31P NMR (ESI, Figures S11 and S12). It reveals that the content in 1,2-diols (i.e., structures of type A on Scheme 1) is substantially higher after hydrolysis. On the other hand, the content in 1,3-diols (i.e., structures of type B on Scheme 1) is unchanged. This confirms that some of the corresponding 1,2diols have been converted into 5-membered cyclic carbonate structures (Scheme 3). It represents 1.23 and 0.61 mmol g-1 for the CT and SL derivatives, respectively.

Scheme 3. 2-steps reaction leading to the formation of cyclic carbonates during the reaction with GC: the etherification first forms a derivative carrying a 1,2-diol (a), which upon transesterification with GC produces a derivative carrying a cyclic carbonate and glycerol (b).



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Table 2 summarizes the new functional groups that have been introduced onto CT and SL derivatives. It includes primary and secondary OH, 1,2- and 1,3-diols, 5-membered cyclic carbonates and vinyl groups, and thus clearly shows the versatility of the developed method for the functionalization of lignins and tannins. Table 2. Type and quantification of the functional groups introduced by the reaction with the different cyclic carbonates

Substrate CT

Cyclic carbonate EC PC GC

VEC SL

EC PC GC

VEC

Type of functional group primary alcohol a primary alcohol a secondary alcohol a 1,2-diol a 1,3-diol a 5-membered cyclic carbonate b aliphatic alcohol a vinyl group c primary alcohol a primary alcohol a secondary alcohol a 1,2-diol a 1,3-diol a 5-membered cyclic carbonate b aliphatic alcohol a vinyl group c

Content (mmol g-1) 6.31 0.86 4.69 1.15 0.46 1.23 5.23 7.14 nd d nd 1.06 nd nd 0.61 nd 4.36

a

Determined by 31P NMR Determined by the comparison of 31P NMR spectra before and after hydrolysis c Determined by 1H NMR d nd = not determined, because of overlap in the 31P NMR spectra with the aliphatic OH of the neat lignin b

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 the degradation curves are available in the ESI (Figures S13 and S14). In any case, the modification of tannin and lignin leads to derivatives of enhanced thermal stability, with a T95% substantially higher than 200 °C. Indeed, masking the phenolic OH drastically limits the


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possibilities for radical reactions that are known to affect the underivatized lignins.7 This enhanced thermal stability is important to increase the usage and the processability of these derivatives under thermo-mechanical input. All the derivatives present a main degradation between 365 and 395 °C. Table 3. Thermogravimetric analyses of tannin and lignin derivatives

Substrate

Cyclic T (°C) carbonate 95%

CT

EC PC GC VEC SL EC PC GC VEC a Main degradation peak

212 231 295 229 209 165 233 227 213 231

Td (°C) a 291 365 369 373 393 319 377 374 376 386

The glass transition temperatures (Tg) of all the derivatives are shown on Figure 4. All the derivatives present a Tg inferior to those of the neat lignin and tannin. This arises from a reduced H bonding capacity caused by the masking of the phenol groups, as well as from an increased free volume due to the grafting.7 The bulky vinyl group of VEC leads to the lowest Tg, since it induces more free volume. On the other hand, the grafted groups contain OH, 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 2 OH groups per graft, present the highest Tg of the derivatives.



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Figure 4. Glass transition temperature (Tg) of CT and SL derivatives obtained with the different cyclic carbonates. 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 on 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, it is an alternative to the classic oxyethylation or oxypropylation reactions performed with ethylene or propylene oxide, respectively.38,43 To evaluate these possibilities, several 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 became insoluble for reaction times superior to 2 – 3 h.44

Scheme 4. Grafting of oligomeric chains based on ether and carbonate units for extended reaction times.


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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 on Figure 5a. Similar data recorded for SL as well as the FTIR spectra are available in the ESI (Figures S15 – 17). 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 confirm the chain extension, which result in an increase in the molar mass 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 occurrence of important crosslinking reactions. Since FTIR spectra show the presence of a large quantity of carbonate linkages, it is most likely that the crosslinking reactions happen by chain coupling through transesterification reaction (reaction scheme shown in the ESI, Scheme S2).62



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Figure 5. (a) Evolution of the intensities of the FTIR peaks of C-H (2872 cm-1), C=O (1741 cm-1) and CO (1257 cm-1) stretches with the reaction time during the reaction between CT and EC. (b) SEC traces of the CT derivatives obtained after various reaction times with EC. 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 measured on acetylated samples, as previously described.38 The results are presented on Figure 6. The average length of the grafted chain is equal to one unit after 2 h of reaction with CT. With 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 ESI 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 the control of the final chain length easy to achieve. However, the studied conditions do not allow the grafting of more than 1.5 units per chains on average, which doesn’t cause significant changes in the SEC traces (ESI, Figures S18 and S19). To achieve the grafting of longer chains, higher amounts of PC are necessary.38

Figure 6. Average length of the grafted chains during the reaction with PC depending on the reaction time.


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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 5-membered cyclic carbonate groups, as depicted on Scheme 3.42 Then, the OH groups might also react with GC according to the general pathway shown on Scheme 4, leading to the grafting of hyperbranched polyglycerol chains on the tannin or lignin core.63–65 SEC results reported on Figure 7 shows that the molar mass of the derivatives increases with the reaction time (the corresponding chromatograms are reported in the ESI, Figures S20 and S21). In addition, FTIR spectra do not show any increase in the carbonate band at 1780 cm-1 (ESI, Figures S22 and S23). This shows that the second pathway is preferred, and that hyperbranched polyglycerol chains are successfully grafted onto the lignin or tannin core.

Figure 7. Evolution of the number-average molar mass of the CT and SL derivatives prepared with GC depending on the reaction time.



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Conclusion This study explores the advantages of cyclic carbonates for the modification of lignins and tannins. Cyclic carbonates are liquids of high boiling points and low volatility that are safe to handle. Their reaction with lignins and tannins can be performed in solvent-free conditions, with low amounts of an inexpensive and benign catalyst (K2CO3). The phenolic OH 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 reactivity of 4 different cyclic carbonates with condensed tannins and lignins shows that the reactivity is affected by the substituent on the cyclic carbonate as follow: -CH3 < -CH=CH2 < -CH2-OH < H. The prepared lignin and tannin derivatives expose interesting functional groups, such as primary or secondary aliphatic OH, vinyl groups, 1,2 and 1,3-diols or 5-membered cyclic carbonate groups, and present a 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 reveals the possibility to graft oligomeric chains if the reactions are conducted over longer periods. With EC and PC, it is of particular interest since 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, since the lengths of the grafted chains increase linearly with the reaction time. With GC, it 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 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


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other functional groups than those reported in this study, such as aromatic rings, double or triple bonds or protected amines.

Supporting information All figures, tables and schemes mentioned in the text, including additional NMR and FTIR spectra, SEC traces and TGA curves.



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For Table of Contents use only

Biobased aromatics (lignins and tannins) were quantitatively derivatized with various cyclic carbonates in a solvent-free process to yield versatile aromatic building blocks for polymer synthesis.


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Cyclic Carbonates as Safe and Versatile Etherifying Reagents for the

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