Oxyalkylation of Condensed Tannin with Propylene Carbonate as an

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Oxyalkylation of condensed tannin with propylene carbonate as alternative to propylene oxide Antoine Duval, and Luc Averous ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00081 • Publication Date (Web): 21 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016

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ACS Sustainable Chemistry & Engineering

Oxyalkylation of condensed tannin with propylene carbonate as alternative to propylene oxide

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 Propylene carbonate (PC) was used as an alternative to the hazardous propylene oxide (PO) for the oxypropylation of condensed tannins from Acacia mearnsii. The influence of the catalyst, temperature and stoichiometry was evaluated in details. All the synthesized polymers were characterized by 1H and 31P NMR, FTIR and size exclusion chromatography. Polyether chains were successfully grafted on all hydroxyl groups. In addition to the ether linkages, some carbonate linkages were also formed. This can cause some chain coupling reactions, through condensation and/or transesterification reactions, which ultimately increase the molar mass and reduce the OH content of the final products. The use of K2CO3 as catalyst rather than hydroxides (KOH or NaOH) and a high temperature of reaction (170 °C) were found to strongly reduce this phenomenon. Using 10 to 40 equivalents of PC per OH in tannins, polyether/polycarbonate chains of 2 to 6 units in average were grafted, containing less than 25% of carbonate units. The synthesized polyols have similar OH content than those obtained by the standard method using PO, fitting the requirements of, e.g., polyurethane production. The developed method thus appears as a viable, green and non-toxic alternative to the use of PO for the preparation of aliphatic polyols from condensed tannins. Keywords: polyphenols, tannins, oxypropylation, propylene oxide, propylene carbonate, NMR, 1H NMR


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P

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Introduction Studies on aromatic renewable resources for the elaboration of biobased chemicals and polymers are in the core of current academic and industrial researches. After lignin, tannins are the second most abundant source of renewable aromatic structures. They can be directly and easily extracted from different plants, without specific and long fragmentation steps. They are mainly classified in two major groups, the hydrolysable and the condensed tannins. Condensed tannins are secondary plant metabolites, found in all vascular plants. Acacia mearnsii, commonly known as mimosa or black wattle, is the most important source of commercially available condensed tannins, which are extracted from its bark. Although native to Australia, this fast-growing tree is mostly cultivated in Brazil and South Africa, which together gather about 280 000 ha of planted area, and produce almost 90% of the 75 – 85 000 t tannins commercialized each year.1,2 The chemical structure of tannins from Acacia mearnsii has been well characterized, using mass spectroscopy3,4 or 13C NMR.5,6 It is mainly composed of prorobinetinidin, with the other monomers as minor components (Scheme 1), assembled in branched structures of average degree of polymerization (DP) around 5.3–6

Scheme 1. Structure of condensed tannins monomers. Dashed lines represent the possible inter-unit linkages. The interest for the use of tannin as bio-based building block for the formulation of polymer materials is growing.7 The multiple phenolic OH groups can be used as a platform for chemical derivatization. However, their reactivity and accessibility is rather low, thus strongly



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limiting the possibility to synthesize, e.g., polyesters8 or polyurethanes (PU).9 A common strategy to enhance the reactivity is to derivatize the phenolic OH, to expose aliphatic OH instead. The oxypropylation reaction, which involves the ring opening of propylene oxide (PO) onto the phenolic OH (Scheme 2a), has been widely used for this purpose on lignin,10,11 and more recently on tannins.12,13 Depending on the reaction conditions, it is possible to graft oligomeric chains, with an average DP in the range 1 – 7,10 leading to a liquid polyol containing only aliphatic OH groups. The corresponding oxypropylated lignins or tannins have successfully been used in PU synthesis.10,14 However, PO is a highly toxic liquid, with low boiling point, high vapor pressure and explosive vapors, making its handling hazardous. In addition, since the reaction has to be performed above its boiling point, the use of a pressurized reactor is mandatory.13 The search for alternative reagents able to perform a similar reaction thus appears worthwhile. Cyclic carbonates, such as propylene carbonate (PC) and ethylene carbonate (EC), are well-known as O-alkylating agents of phenols.15 PC presents many attractive features, such as low toxicity, high boiling point and low vapor pressure, making it an interesting solvent16 or reagent in organic chemistry.17,18 Besides, new synthetic routes using CO2 as building block are highly promising in terms of green chemistry perspectives.19,20 Several mechanisms are involved in the reaction of PC with alcohols (Scheme 2b). With phenols, only the alkylene attack mechanism takes place. This reaction proceeds in two steps: the nucleophilic attack can happen at any of the two alkylene carbons, and is followed by the loss of CO2. The structure thus obtained is the same as during the reaction with PO. Many kinds of catalysts have been described in the patent literature for this reaction, including alkali metals and their alkoxides,21 halides,22 hydroxides,23 or carbonates. However, in most cases, only the monoalkylated product was sought. In this work, we focused on the grafting of longer polyether chains, to mimic the structures obtained by the oxypropylation with PO 4 ACS Paragon Plus Environment

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(Scheme 2a), because it allows to tailor the properties of the final products, by modulating the final OH content, or reducing the viscosity and the Tg.



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Scheme 2. (a) Oxypropylation of tannin with PO; (b) Mechanism of the reaction with PC; (c) Oxypropylation of tannin with PC. The propagation of the reaction can involve two mechanisms. In addition to the alkylene attack, carbonyl attack can occur, leading to the formation of carbonate linkages (Scheme 2b). At the end, the structure of the tannin reacted with PC should be as shown on Scheme 2c, with polyether-polycarbonate grafted chains terminated with aliphatic OH groups. If one can limit the formation of carbonate linkages, similar structures as those formed during oxypropylation with PO could be obtained. The reaction of PC with lignin was recently reported, but only a single unit was grafted.24 The formation of longer chains still remains a challenge. Several studies reported on the ring opening polymerization of PC or EC, with various initiators and catalysts.25–29 When phenolic initiators were used, only basic catalysts allowed the oligomerization (potassium or lithium carbonates KHCO3, K2CO3, Li2CO3, or potassium hydroxide KOH).30 This work is focused on the oxypropylation of Acacia mearnsii tannins (MT) using both chemistries, for comparison. A first set of reference samples was prepared by the standard method using PO, on the basis of previous work performed in our team (Scheme 2a),13 to obtain oxypropylated tannins with grafted chains of various lengths. An alternative protocol was then developed using PC, with the purpose to obtain equivalent polymers. Different parameters, such as the influence of the nature of the catalyst, the temperature and the stoichiometry were analyzed. All the synthesized polymers were characterized with 1H and exclusion chromatography (SEC).


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P NMR, FTIR, and size

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Experimental section Materials Condensed tannins from Acacia mearnsii, Fintan OP®, were kindly supplied by Silva Chimica (St. Michele Mondovi, Italy). Prior to use, they were dried overnight at 70 °C and then stored in a desiccator under vacuum. Pyridine (sequencing grade, ≥ 99.5%) was purchased from Fisher Scientific, acetic anhydride (ACS reagent, ≥ 97%) and (+)-propylene oxide (PO, 99%) from Acros Organics, propylene carbonate (PC, anhydrous, 99.7%), methanol (laboratory reagent grade, ≥ 99.6%), dichloromethane (DCM, puriss., ≥ 99%), cyclohexane (laboratory reagent grade, ≥ 99.8%), 2,3,4,5,6-pentafluorobenzaldehyde

(98%),

2-chloro-4,4,5,5-tetramethyl-1,3,2-

dioxaphospholane (95%), DMSO-d6 (99.9% D atoms) and CDCl3 (99.8% D atoms) from Sigma-Aldrich. All chemicals were used as received without further purification. Oxypropylation with propylene oxide (PO) Oxypropylation with propylene oxide was performed following the protocol described by Arbenz and Avérous on gambier tannins,13 using 5 wt% KOH as catalyst. The quantities of PO and MT were adjusted to obtain 2 to 10 equivalents PO per OH groups in MT. The poly(propylene oxide) homopolymer fraction was removed by reflux extraction with cyclohexane.13,31 Oxypropylation with propylene carbonate (PC) A three-necked round bottom flask was charged with 1 g MT (corresponding to 12.6 mmol OH groups), 1.26 mmol catalyst (K2CO3, KOH or NaOH) and various amounts of PC, to obtain 10 to 40 PC equivalents to OH groups in MT. The mixture was then stirred and heated under argon up to 130, 150 or 170 °C. After 24 h reaction, the mixture was cooled to room 7 ACS Paragon Plus Environment


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temperature and precipitated in a 10-fold volume of acidified water (pH2). The precipitate was recovered by centrifugation, washed 3 times with acidified water (pH2) and freeze-dried. Acetylation Acetylation was performed in a pyridine/acetic anhydride mixture (1:1 v/v) for 24 h at room temperature, as previously reported on tannin samples.13,32 Size Exclusion Chromatography (SEC) Size exclusion chromatography (SEC) measurements were performed in chloroform (HPLC grade) in a Shimadzu liquid chromatograph equipped with a LC-10AD isocratic pump, a DGU-14A degasser, a SIL-10AD automated injector, a CTO-10A thermostated oven with a 5µ PLGel Guard column, two PL-gel 5µ MIXED-C and a 5µ 100 Å 300mm-columns, and 2 online detectors, a Shimadzu RID-10A refractive index detector and a Shimadzu SPD-M10A diode array (UV) detector, respectively. Molar masses and dispersity were calculated from a calibration with polystyrene standards (580 to 1 650 000 g mol-1). All the oxypropylated samples were fully soluble in chloroform, and analyzed as such. The initial tannin MT could only be analyzed after acetylation, because it was initially insoluble in chloroform. The samples were dissolved in chloroform and filtered through a 0.2 µm PTFE membrane. For all analyses the injection volume was 50 µL, the flow rate 0,8 mL min-1 and the oven temperature set at 25°C. NMR spectroscopy Quantitative 1H NMR spectra were measured on the acetylated samples dissolved in DMSOd6, in the presence of 2,3,4,5,6-pentafluorobenzaldehyde as standard, as previously reported.33,34 The spectra were acquired on a Bruker 400 MHz spectrometer (32 scans at 25 °C). 8 ACS Paragon Plus Environment

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P NMR was performed after phosphitylation of the samples with 2-chloro-4,4,5,5-

tetramethyl-1,3,2-dioxaphospholane, according to standard protocols.35 The spectra were measured on a Bruker 400 MHz spectrophotometer (128 scans at 25 °C). Peaks assignations and quantitative analysis were performed based on previous reports.36,37 Fourier Transform Infrared Spectroscopy (FTIR) FTIR spectra were recorded in the attenuated total reflectance (ATR) mode on a Nicolet 380 FTIR spectrometer, in the range 400 – 4000 cm-1, as the average of 32 scans with 4 cm-1 resolution. The samples were directly deposited on the ATR crystal and carefully pressed to ensure a good contact. The background was recorded with the empty ATR crystal in air.



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Results and discussion Characterization of Acacia mearnsii tannins (MT) Acacia mearnsii tannins (MT) have already been widely studied, because they are the most important source of commercially available condensed tannins. The combined use of MALDI-TOF-MS and 13C NMR revealed an oligomeric structure, with average DP around 5, and prorobinetinidin as principal monomeric unit (50 – 70% of total),3 with prodelphinidin and profisetinidin as minor components (Scheme 1).3–6 For the purpose of this study, a critical parameter is the initial OH content, which will control the quantity of initiating points for the ring opening oligomerization of PO or PC. It was measured by 31P NMR, which has been shown to precisely identify and quantify the different kinds of OH groups in tannins.36–38 The spectrum and the detailed peak assignation are given in Figure 1a. The wide peak centered at 142.5 ppm is representative of ortho-disubstituted phenols, and confirms the presence in large amounts of pyrogallol B rings, i.e. the predominance of prorobinetinidin units. Two peaks located at 138.4 and 138.9 ppm are representative of catechol-like phenols, which are present in all the monomers reported in MT. Two other peaks at 137.9 and 138.1 ppm are in the range of ortho-substituted phenols, and correspond either to the phenol in position C5 in prodelphinidin units, or to the phenol in position C7 in units linked at C6 or C8 positions. Finally, the peak at 137.5 ppm is characteristic of the phenol in position C7 in terminal units. The integration of the relevant regions gives a total phenol content in MT of 7.7 mmol g-1 (details of the integrations are given as SI, Table S1). In addition, several peaks are measured in the aliphatic OH region. Some of them are representative of the aliphatic OH groups in position C3 in the flavanol units, but it also indicates that some impurities, most likely carbohydrates, are present in the sample. The 10 ACS Paragon Plus Environment

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content in aliphatic OH groups is 4.9 mmol g-1, giving a total OH content of 12.6 mmol g-1. A similar quantification performed using 1H NMR measured on acetylated sample gives 4.2 and 6.9 mmol g-1 of aliphatic and phenolic OH groups, respectively (spectrum is available as SI, Figure S1).

Figure 1. (a) 31P NMR spectrum of MT, with the detailed assignation of the phenolic region, showing the most important monomers. Peak assignation based on previous reports,36,37 IS = internal standard (cholesterol). (b) SEC of acetylated MT, measured in chloroform with RID and UV detections. SEC of MT was measured after acetylation to ensure full dissolution, using RID and UV detection (Figure 1b). Both detections show very similar profiles, the only difference being observed in the low molar mass region, where a small peak is observed at 31 min with RID detector, but is not visible in UV. It can correspond to a small fraction of free carbohydrates, which can’t be detected by UV. However, most of the carbohydrate fraction detected by 31P NMR should be bound to the tannin, since no distinction can be found between RID and UV signals. Based on a polystyrene calibration, average molar masses of 1560 (Mn) and 2310 g mol-1 (Mw) were obtained. Oxypropylation with PO



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The oxypropylation of MT with PO has been performed with increasing amounts of PO, from 2 to 10 equivalents, in order to obtain different grafted chains lengths. In addition to the grafting onto tannins OH, homopolymerization of PO also occurred, as evidenced by SEC, where two distinct elution peaks are visible (Figure 2, top curves). At high molar mass, a peak is observed with both RID and UV detector. Since UV only detects the aromatic parts, this peak corresponds to the derivatized tannins. At low molar mass, a peak is observed with the RID detector, but do not appear in UV: it is related to the homopolymer fraction, which does not absorb the UV light. To obtain neat oxypropylated tannins, cyclohexane extraction of the homopolymer fraction has been performed.13,31 This step is fully controlled by SEC, and only the high molar mass peak remains after the extraction (Figure 2, middle curves). The extracted homopolymer fraction contains only very small amounts of tannins (Figure 2, bottom curves). Similar data for all the other samples are available as SI (Figures S2 to S5). The amount of homopolymer formed increases with the PO content, to reach up to 45 wt% for 8 – 10 equivalents (SI, Figure S6). It thus constitutes a clear drawback to the obtaining of oxypropylated tannins.


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Figure 2. SEC of MT oxypropylated with 10 equivalents of PO, measured by RID (continuous line) and UV detection (dashed line): before purification (top curves), after purification (middle curves) and homopolymer fraction (bottom curves). The purified oxypropylated tannins were then analyzed by FTIR, corresponding spectra are available as SI (Figures S7 to S9).

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P and 1H NMR. All the

P NMR shows the complete

disappearance of the phenolic OH, indicating their full derivatization for all PO to MT ratios. In addition, new aliphatic OH groups are formed. They give rise to a new sharp peak located at 145.6 ppm (Figure S7), corresponding to secondary OH groups,33 and confirming the chemical structure given in Scheme 2a. The initial aliphatic OH groups are also consumed, indicating that the aliphatic OH of tannin as well as the carbohydrate impurities are also oxypropylated. Indeed, under these reaction conditions, the oxypropylation of various carbohydrates39,40 or carbohydrate-polyphenol mixtures41–43 has already been reported. 1

H NMR spectra were measured after acetylation of the oxypropylated tannins (SI, Figure

S8). The oxypropylation creates a cluster around 0.5 – 1.5 ppm, related to the methyl group of 13 ACS Paragon Plus Environment


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the PO units. After acetylation, the OH groups are derivatized as acetyls, which give rise to signals at 1.7 – 2.1 ppm (aliphatic acetyls) or 2.1 – 2.5 ppm (phenolic acetyls). The oxypropylated tannins only show acetyls signal in the 1.7 – 2.1 ppm region, confirming the full consumption of phenolic OH groups. The aliphatic acetyl signal thus gives the quantity of OH chain ends, whereas the methyl signal gives the total quantity of PO units. By comparing the integrals, the average number of units per polyether chain can be obtained,44 applying Equation 1:

.. 1 ..

where I0.5-1.5 ppm and I1.7-2.1 ppm represent the integrals of the corresponding regions. Figure 3 shows the average number of units per polyether chains, depending on the amount of PO used for the reaction. The chain length increases linearly with the amount of PO, to culminate at 4 units. It is clearly limited by the concomitant formation of homopolymer that consumes part of the PO. These results are in good agreement with previous results reported on other tannins.13

Figure 3. Average number of units per grafted chains (measured by 1H NMR), depending on the amount of PO. 14 ACS Paragon Plus Environment

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FTIR spectra further confirm the grafting of polyether chains onto tannins (SI, Figure S9). New peaks related to the polyether chains appear, and become more intense as the chain length increases. A strong peak for the C-O stretch of the ether linkages at 1090 cm-1 is observed, and two for the C-H stretches of the methyl and methylene groups at 2690 and 2790 cm-1. The latter is relatively free from overlap with tannin-related signal. Since all phenolic OH are derivatized, the O-H stretch signal at 3450 cm-1 is only related to the aliphatic OH, representing the chain ends. Thus, the ratio of the C-H to O-H stretch bands can give information on the length of the grafted chains. When the IC-H / IO-H ratio is plotted against the average polyether chain length measured by 1H NMR (SI, Figure S10), a linear correlation is observed, showing that FTIR data can give semi-quantitative information on the length of the grafted polyether chains. Screening of reaction conditions for the oxypropylation with PC: catalysts and temperature Three different basic catalysts were evaluated, K2CO3, KOH and NaOH, because they have already been shown to allow the oligomerization of cyclic carbonates onto phenolic initiators.30 The reactions were carried out using 10 equivalents PC. The progress of the reaction was followed by

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P NMR (spectra available as SI, Figure S11). With K2CO3 as

catalyst, all phenolic OH groups were consumed after 1 h, whereas 2 and 3 h were necessary with KOH and NaOH, respectively. The progress of the reaction was also monitored by FTIR, taking the C-H to O-H stretch bands intensity ratio as representative of the grafted chain length, as described above (all FTIR spectra are given as SI, Figure S12). Figure 4a shows its evolution with the reaction time for all catalysts.



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Figure 4. (a) Evolution of the ratio of the intensities of FTIR peaks (C-H / O-H stretches) with time depending on the catalyst (10 eq PC, 170 °C); (b) SEC of tannins oxypropylated with PC (10 eq, 24 h reaction at 170 °C) and different catalysts. SEC of the initial tannin, given for comparison, was measured on the acetylated sample. The average lengths of the grafted chains were measured by 1H NMR after acetylation, as described previously (spectra available as SI, Figure S13). In average, chains of 2.4 ± 0.3 units were grafted after 24 h of reaction, independently of the catalyst used (within the experimental error of the NMR measurement). SEC was also performed to evaluate the increase in molar mass with the grafting. The chromatograms of the final products (24 h reaction) obtained with all the catalysts are depicted on Figure 4b. When hydroxides are used, especially NaOH, signal is observed in the


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high molar mass region, leading to unexpectedly high values of Mw. The formation of such high molar mass compounds is most likely to result from chain coupling reactions. They could occur via the reaction of a terminal carbonate anion with an OH-terminated chain,45 or via transesterification reactions involving a carbonate linkage in the middle of the chain (Scheme 3).28,46 Since the aim of the reaction is to obtain a star-like polymer architecture, with a tannin core grafted with polymer chains terminated by aliphatic OH, such chain coupling reactions are undesirable, and K2CO3 was thus chosen as the best catalyst for the rest of the study.

Scheme

3.

Potential

chain

coupling

reactions:

(a)

condensation

reaction;

(b)

transesterification reaction The O-alkylation of phenols with PC generally requires rather high temperatures, in the range of 150 – 200 °C.15 Similar temperatures are reported concerning the ROP of PC or EC initiated on aliphatic25–28 or phenolic OH groups.30 However, CO2 bubbling in the reaction mixture has been observed from 110 °C during the reaction of PC with BPA,30 suggesting that the reaction can also proceed at lower temperature. Considering the thermal stability of the initial MT, 170 °C was considered as the maximum acceptable reaction temperature, and temperatures from 130 to 170 °C were thus evaluated. Figure 5a shows the average number of units per grafted chain, depending on the reaction temperature. The corresponding 1H NMR spectra are given as SI (Figure S14). It clearly appears that increasing the temperature facilitates the chain extension process. 17 ACS Paragon Plus Environment


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Figure 5. Influence of the reaction temperature (K2CO3 as catalyst, 10 eq PC, 24 h reaction): (a) average number of units per chain (1H NMR), (b) SEC. SEC of the initial tannin, given for comparison, was measured on the acetylated sample. Size exclusion chromatograms are presented on Figure 5b. Increasing the grafted chain length causes a global shift of the curves to higher molar masses. When the reaction is performed at 150 °C, more signal is detected in the high molar mass region. As discussed above, this is likely to originate from chain coupling reactions (Scheme 3).45 Based on these observations, an optimum reaction temperature of 170 °C has thus been chosen for the rest of the study. Oxypropylation with PC The previously determined conditions (K2CO3 as catalyst, 170 °C) were later applied with increasing amounts of PC, from 10 to 40 equivalents. In the reaction, PC plays both the roles 18 ACS Paragon Plus Environment

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of reagent and solvent, which precludes reducing its amount below 10 equivalents, to ensure full dissolution of the tannins. 31

P NMR spectra of the modified tannins are given as SI (Figure S15). They show the

complete disappearance of phenol-related signals, confirming their full consumption during the reaction (Scheme 2c). The signals from the initial aliphatic OH groups also disappeared, meaning that aliphatic OH group on the tannin as well as carbohydrate impurities also reacted. It should be noted that the purification procedure, employing successive water wash, could also remove eventual unreacted carbohydrate impurities. A new peak appears in the aliphatic secondary OH region, at 145.6 ppm. It is similar to the one observed after oxypropylation with PO, and corresponds to secondary alcohols at the end of the grafted polyether/polycarbonate chains. As expressed before, the reaction with PC can proceed via two distinct mechanisms (Scheme 2b). The grafted chain can thus be composed of ether units (linear propylene oxide units, LPO) or carbonate units (linear propylene carbonate units, LPC) (Scheme 2c). FTIR spectra, given as SI (Figure S16), reveal the occurrence of carbonate linkages with a strong peak at 1740 cm-1.



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Figure 6. 1H NMR spectra of the tannins modified with PC. See the text for details on the assignation. 1

H NMR spectra of the modified tannins are reported on Figure 6. A cluster is observed in the

0.5 – 1.5 ppm region, corresponding to the methyl groups in the either LPO (1.0 ppm) or LPC units (1.2 ppm).47 Unfortunately, the LPC to LPO ratio cannot be quantified by comparing the integrals, because the downfield peak also contains contribution from the methyl group of LPO units close to the aromatic ring,48 as confirmed with COSY measurements (SI, Figure S17). The 3 – 4 ppm region contains numerous contributions, such as CH-O and CH2-O of the LPO units, and tannin-related signals. The bands strongly overlap, making their assignation impossible, and precluding their use for quantification. The H atom in position α to the carbonate linkage (labelled a on Figure 6) shifts at 4.75 ppm. Its assignation was confirmed with COSY measurements (SI, Figure S17). Since it is free from overlap with other signals, it was used to quantify the amount of LPC units within the chains, NLPC, by comparing its integral to the total integral of the -CH3 groups in both LPC and LPO units: 20 ACS Paragon Plus Environment

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3 × $ ..

× %&% 2

where Ia is the integral of H atom a (Figure 6) and I0.5-1.5 ppm is the integral of the –CH3 groups. Factor 3 arises from the multiplicity of –CH3 groups. Ntot is the total number of units per chain. Ntot can be calculated based on the 1H NMR spectra of the acetylated samples, as described previously (Equation 1, corresponding spectra available as SI, Figure S19). Then, the average number of LPO units per chain can be deducted:

(

%&% − ! 3

Ntot, NLPO and NLPC have all been reported on Figure 7 against the amount of PC used for the reaction. The total chain length increases linearly with the amount of PC used for the reaction, to reach about 6 units in average for 40 equivalents PC. As compared to the oxypropylation with PO, larger amounts of PC are required to achieve similar lengths of the grafted chains. However, this is not necessarily an issue, since the unreacted PC can be recovered from low pressure distillation. Up to a total chain length of 2, only LPO units are grafted. This shows that PC can readily be used to mimic the structures obtained during the oxypropylation with PO. Then, for higher chain lengths, the amount of LPC units increases linearly. A maximum of 22% of the units are incorporated in the form of LPC. Figure 6 also shows the presence of a multiplet at 5.9 ppm and two doublets at 5.2 and 5.3 ppm. COSY and HSQC measurements, available as SI (Figures S17 and S18) allowed assigning them as vinyl chain ends, as reported on Figure 6. Such structures can originate from the dehydration of a terminal OH, and are likely to occur because of the high temperature employed, especially for long reaction times.



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Figure 7. Average number of units per chain (measured by 1H NMR) depending on the amount of PC used for the reaction (K2CO3 as catalyst, 170 °C, 24 h reaction). The carbonate linkages that are introduced in the chains when PC is used are sensitive to hydrolysis. The comparison between the polymer structures before and after hydrolysis can give interesting information on the quantity and position of the carbonate linkages.26,29,45 The carbonate linkages were thus hydrolyzed in the presence of KOH, and the resulting samples characterized by 1H, 31P NMR, FTIR and SEC.


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Figure 8. (a) Hydrolysis of the carbonate linkages, releasing free OH groups; (b)

31

P NMR

spectra of the tannins modified with PC before (straight lines) and after hydrolysis (dashed lines); (c) SEC of the tannins modified with PC before (straight lines) and after hydrolysis (dashed lines) 1

H NMR and FTIR confirm the hydrolysis, as shown by the disappearance of the carbonate

related signals (SI, Figures S20 and S21). As shown on Figure 8a, the hydrolysis of the carbonate linkage creates two new OH groups, which can be detected by 31P NMR. Figure 8b shows the

31

P NMR spectra before and after hydrolysis. After hydrolysis, no signal is

observed in the phenolic OH region (136 – 144 ppm). This confirms that carbonate linkages are not formed on tannin phenolic OH, i.e. that the initiation of the ROP of PC on phenolic OH only involves alkylene attack (Scheme 2b). In addition, an increase in the content in



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aliphatic OH is observed, corresponding to the new terminal OH groups formed during the cleavage of the carbonate linkages (Figure 8a). SEC was measured before and after hydrolysis, to follow the associated decrease in molar mass (Figure 8c). When carbonate linkages are present, i.e. for 20 or more equivalents PC, the hydrolysis is associated to a decrease in molar mass. In particular, the peak at high molar mass is strongly reduced. The fragments that are released are however not detected by the SEC system employed, probably because they are too short. When the sample doesn’t contain carbonate linkages (10 eq PC), the treatment with KOH leads on the other hand to an increase in molar mass, caused by crosslinking reactions. Comparison of the chemical properties of tannins oxypropylated with PO and PC The OH content is a critical parameter, since it controls the properties of the final product.49 The OH content of the tannin-based polyols prepared with either PO or PC is shown on Figure 9a, as measured by

31

P NMR (the same data measured with 1H NMR on acetylated

samples are given as SI, Figure S22). The OH content of the oxypropylated tannins decreases with the length of the grafted chain, but is practically the same when PC or PO is used for the reaction.


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Figure 9. OH content measured by 31P NMR (a) and mass-average molar mass Mw (b) of the tannins oxypropylated with PO and PC, depending on the average length of the grafted chain. Another key parameter is the molar mass of the polyols. SEC of tannins derivatized with PO and PC are given as SI (Figures S23 and S24). The values of the mass-average molar mass Mw have been reported as a function of the average chain length on Figure 9b. The molar masses are always higher when the oxypropylation was carried out with PC instead of PO. This is partly related to the formation of carbonate linkages, because the LPC unit has a molar mass of 102 g mol-1 against 58 g mol-1 for the LPO unit, but mostly to chain coupling reactions in the presence of carbonate linkages, as discussed above (Scheme 3), conducting to a net increase in molar mass.



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Conclusion Propylene carbonate (PC) appears as a viable alternative to propylene oxide (PO) to synthesize liquid polyols from condensed tannins in a green way, without toxic chemicals. Its high boiling point allows using it in a solvent-free process that does not require any specific equipment. It is thus a reagent of great interest, and its ability to similarly modify other types of polyphenols, such as other tannins and lignins, is currently under investigation. The coexistence of carbonyl and ethylene nucleophilic attacks during the ring opening of PC leads to the formation of some carbonate linkages in the chain. Through condensation and/or transesterification reactions, this can lead to chain coupling, thus increasing the molar mass and reducing the OH content. However, different parameters can drastically reduce this issue, such as the use of K2CO3 as catalyst (rather than hydroxides), or the increase in the reaction temperature. The average length of the grafted chains was found to vary linearly with the stoichiometry in PC. Chain lengths of up to 6 units were thus reached, in the range of common oxypropylations with PO. The first two units that are grafted are exclusively propylene oxide. In total, less than 25% of the units are grafted in the form of propylene carbonate. The polyols synthesized with PC have similar OH content than those obtained with PO, but slightly higher molar mass. Oxypropylated aromatic polyols can be useful intermediates for the synthesis of different biobased polymers, e.g., polyethers, polyesters or polyurethanes (PU) for a large range of applications. For instance, the synthesized tannin-based polyols fit the requirement of rigid PU synthesis when the grafted chains length is shorter than 4 units (IOH between 200 and 800,39 corresponding to 3.5 to 14.3 mmol g-1 OH), whereas some others could be useful in the synthesis of soft PU. 26 ACS Paragon Plus Environment

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Acknowledgments Silvateam (Italy) is gratefully acknowledged for kindly supplying Acacia mearnsii tannin samples. Chheng Ngov (ICPEES, Université de Strasbourg) is thanked for performing the SEC measurements.

Supporting information All 31P, 1H, HSQC and COSY NMR spectra, FTIR spectra and SEC traces mentioned in the text.

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For Table of Contents Use Only Oxyalkylation of condensed tannin with propylene carbonate as alternative to propylene oxide Antoine Duval, Luc Avérous

Condensed tannins were modified into aliphatic polyols, using propylene carbonate in replacement of the toxic and hazardous propylene oxide.


Oxyalkylation of Condensed Tannin with Propylene Carbonate as an

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