Khaled Beyaz

Michael Charton

Antoine Rouilly

Emeline Vedrenne

Carlos Vaca-Garcia
email: carlos.vacagarcia@ensiacet.fr

Ahmed Benaboura

Sophie Thiebaud-Roux

Synthesis of graft-copolymers from palm cellulose and solketal acrylate and their characterization

Keywords:

The use of partially biobased monomers to functionalize palm cellulose via graft-copolymerization was explored. The aim of this work is to recycle a waste, the leaves of palm trees, which are abundant in North African countries in order to obtain novel cellulose derivatives having interesting physico-chemical properties. Furthermore, the monomer that was used for the graft-copolymerization was synthesized from a biobased synthon, glycerol. The synthesis of graft-copolymers from cellulose and (2,2-dimethyl-1,3-dioxolan-4-yl)methyl acrylate (solketal acrylate, DMDMA) was studied to determine the optimal conditions for grafting. The maximum grafting weight gain was 27% obtained after 72 min of reaction at 65 • C with 6.4 mmol KPS/eq OH and 1.51 mol DMDMA/eq OH. THF, used as dispersion solvent, hinders the homopolymerization side-reaction by creation of terminating radicals. FTIR spectroscopy confirmed the grafting of monomers to cellulose and X-ray diffraction revealed an organized structure of the side-chains. Thermogravimetry showed that the grafting could not confer a higher thermal stability to cellulose (loss of 35 • C in the decomposition temperature). Even though partial melting was observed when thermopressed, DSC analysis could not show a neat glass transition temperature but rather multiple exothermal peaks attributed to side-chains reorganization. Thermopressed grafted samples showed improved mechanical properties compared to palm cellulose.

Introduction

Interest in the chemical modification of cellulose has never decreased, with a view to enlarging the field of applications of this inexpensive and abundant biopolymer. The graft-copolymerization of monomers onto cellulose is one way to link two entities with different physicochemical properties within the same macromolecule. The result is a branched polymer in which the principal chain consists of cellulose and the lateral chains are formed from another oligomer or polymer, the length of the latter depends on the reaction conditions.

Abbreviations: DMDMA, 2,2-dimethyl-1,3-dioxolan-4-yl methyl acrylate; GP, grafting percentage; KPS, potassium persulfate; THF, tetrahydrofuran; TEA, triethylamine; NMR, nuclear magnetic resonance; FTIR, Fourier transform infrared; XRD, X-ray diffraction; TGA, thermogravimetric analysis; SEM, scanning electron microscopy.

Grafting can be used to improve certain physicochemical properties of cellulose [START_REF] Roy | Cellulose modification by polymer grafting: a review[END_REF][START_REF] Kang | Cellulose derivatives and graft copolymers as blocks for functional materials[END_REF][START_REF] Kang | Graft modification of cellulose: methods, properties and applications[END_REF]. Thermoplasticity or solubility is expected to be attained due to the diminution of hydrogen bonding during grafting, but at the same time, these properties largely depend on the nature and size of the side-chains and their statistical distribution along the backbone chain. To the best of our knowledge, there is no model to predict the resulting properties of the graft-polymer.

In this context, graft polymerization has been envisaged to create new materials with improved chemical and physical properties and to match various specifications. For example, self-assembled micelles can be obtained from cellulose graft copolymers and be used as carriers for drug and gene controllable delivery [START_REF] Xu | Comb-shaped conjugates comprising hydroxypropyl cellulose backbones and low-molecular-weight poly(N-isopropylacryamide) side chains for smart hydrogels: synthesis, characterization, and biomedical applications[END_REF]. The surface graft modification of cellulosic materials has also been envisaged using various monomers to produce adsorbents for removal of toxic ions [START_REF] O'connell | Heavy metal adsorbents prepared from the modification of cellulose: a review[END_REF][START_REF] Wojnárovits | Radiation-induced grafting of cellulose for adsorption of hazardous water pollutants: a review[END_REF][START_REF] Tian | Modified native cellulose fibers-a novel efficient adsorbent for both fluoride and arsenic[END_REF], antibacterial surfaces (Roy et al., 2008) and additives to reinforce composites [START_REF] Cao | One-pot polymerization, surface grafting, and processing of waterborne polyurethane-cellulose nanocrystal nanocomposites[END_REF].

In this work we selected a partially-biobased vinyl monomer to react with cellulose: (2,2-dimethyl-1,3-dioxolan-4-yl)methyl [START_REF] Tosh | Grafting of cellulose based materials[END_REF]. These monomers include acrylic acid [START_REF] Hajeeth | Removal of Cu(II) and Ni(II) using cellulose extracted from sisal fiber and cellulose-g-acrylic acid copolymer[END_REF][START_REF] Halib | Topological characterization of a bacterial cellulose-acrylic acid polymeric matrix[END_REF], ethyl acrylate (Thakur et al., 2013d;[START_REF] Zampano | Poly(ethyl acrylate) surface-initiated ATRP grafting from wood pulp cellulose fibers[END_REF], methyl methacrylate [START_REF] Wan | Graft copolymerization of methyl methacrylate onto bamboo cellulose under microwave irradiation[END_REF], methyl acrylate (Thakur et al., 2013b), glycidyl methacrylate [START_REF] Anirudhan | Improved performance of a cellulose-based anion exchanger with tertiary amine functionality for the adsorption of chromium(VI) fromaqueous solutions[END_REF], butyl acrylate (Thakur et al., 2013a,c), acrylonitrile mixed to styrene and to butanediene [START_REF] Ma | Preparing cellulose nanocrystal/acrylonitrile-butadiene-styrene nanocomposites using the master-batch method[END_REF], acrylonitrile [START_REF] Lv | Studies on graft copolymerization of chitosan with acrylonitrile by the redox system[END_REF], acrylamide [START_REF] Ye | Ion-responsive liquid crystals of cellulose nanowhiskers grafted with acrylamide[END_REF], 4vinylpyridine [START_REF] Dhiman | Synthesis of a cellulose-grafted polymeric support and its application in the reductions of some carbonyl compounds[END_REF] and N-vinylpyrrolidone [START_REF] Gupta | Co(III) Acetylacetonate-complex-initiated grafting of N-vinyl pyrrolidone on cellulose in aqueous media[END_REF][START_REF] Ibrahim | Cellulose membranes grafted with vinyl monomers in homogeneous system[END_REF].

Another important choice in this work was the cellulose source. Graft-polymerization studies have been carried out on cellulose of different origins such as bacterial [START_REF] Mohd Cairul | Synthesis and characterization of thermo-and pH-responsive bacterial cellulose/acrylic acid hydrogels for drug delivery[END_REF], nanocrystalline [START_REF] Ma | Preparing cellulose nanocrystal/acrylonitrile-butadiene-styrene nanocomposites using the master-batch method[END_REF] or microcrystalline cellulose [START_REF] Tao | Graft copolymers prepared by atom transfer radical polymerization (ATRP) from cellulose[END_REF] as well as cellulose fibers from wood [START_REF] Zampano | Poly(ethyl acrylate) surface-initiated ATRP grafting from wood pulp cellulose fibers[END_REF] or cotton [START_REF] Amatjan | Photopolymerization and characterization of maleylated cellulose-g-poly(acrylic acid) superabsorbent polymer[END_REF]. In the present work we chose cellulose extracted from a natural resource cultivated abundantly in North Africa: palm tree. [START_REF] Chehma | Valorisation des sous-produits du palmier dattier en vue de leur utilisation en alimentation du bétail[END_REF] estimated the number of date palm trees to 10 millions in Algeria. [START_REF] Boudjelal | Production d'acide lactique par Lactobacillus rhamnosus sur milieu à base de jus de dattes[END_REF] reported that 60 thousand tons per season of date palm fruits waste are available. This gives an idea of the potential availability of palm tree leaves rich in cellulose (almost 50%).

The accessibility of hydroxyl groups is one of the main concerns when a cellulose source is to be selected. In some cases, the cellulose is subjected to pretreatment before the reaction such as solventexchange [START_REF] Mccormick | Preparation of cellulose derivatives via ring-opening reactions with cyclic reagents in lithium chloride/N,N-dimethylacetamide[END_REF]. Another approach is starting from a functional derivative such as cellulose acetate or carboxymethylcellulose, in which the accessibility of the hydroxyl groups is higher [START_REF] Toledano-Thompson | Characterization of henequen cellulose microfibers treated with an epoxide and grafted with poly acrylic acid[END_REF].

Summing up, the grafting and its distribution depend on the type of cellulosic raw material used, the nature of the monomer and the initiator used. For the grafting of DMDMA onto palm cellulose, we selected potassium persulfate (KPS) as initiator, as it has been reported to be efficient [START_REF] Liu | Radical graft functional modification of cellulose with allyl monomers: chemistry and structure characterization[END_REF], soluble in water [START_REF] Ghosh | Modification of cotton by acrylic acid (AA) in the presence of NaH2PO4 and K2S2O8 as catalysts under thermal treatment[END_REF], readily available and inexpensive. No transition metal catalysts were deliberately used.

The monomer was thus reacted onto cellulose in a heterogeneous environment, as in most previous reports. The monomers were expected to be preferentially attached to amorphous regions of cellulose, whereas the crystalline regions should be less modified due to the hydrogen bonds between the hydroxyl groups.

The effects of reaction time and reactant amounts on grafting rates were studied. The synthesized molecules were analyzed: the structure of the monomer was determined by NMR and FTIR spectroscopy. The extracted cellulose and its grafted copolymers were characterized by FTIR spectroscopy, X-Ray Diffraction, SEM Microscopy and thermal analyses.

Experimental

Materials and methods

1,2-O-Isopropylidene glycerol (solketal, 98% [100-79-8]), acryloyl chloride, potassium persulfate (99%), magnesium sulfate (97%), dichloromethane (99.8%), tetrahydrofurane (inhibitor-free) (99.9%) and hydrogen peroxide (30%) were supplied by Sigma-Aldrich. Triethylamine (99%) and sodium bicarbonate (99%) were supplied by Acros Organics. Sodium hydroxide (97%) was supplied by Carlo Erba. Ethanol (99.9%) was supplied by VWR. Toluene (99.9%) was supplied by Fluka. All reagents were used directly, without prior purification.

Cellulose extraction

The cellulose was obtained by chemical treatment of palm leaves collected from Washingtonia filifera palm trees cultivated in Algeria using different organic solvents. These treatments are intended to remove any other components present in this plant.

Thus at first, the palm leaves were ground and washed several times with distilled water and dried at 80 • C for 12 h, thereafter fat was removed by Soxhlet extraction with a mixture of ethanol and toluene (1/2, v/v), [START_REF] Moran | Extraction of cellulose and preparation of nanocellulose from sisal fibers[END_REF]. Palm leaves were treated subsequently with an aqueous sodium hydroxide solution at different concentrations (18%, 10%, and 2%). Finally, the cellulose fibers were obtained after a bleaching treatment using a hydrogen peroxide solution (5%) at 60 • C for 4 h [START_REF] Jiang | Isolation of cellulose with ionic liquid from steam exploded rice straw[END_REF] then washed with distilled water and dried at 40 • C for 12 h. The Van Soest and Wine (1967)'s ADF-NDF analysis of the raw palm leaves and palm extracted cellulose revealed the compositions shown in Table 1.

2.3. Synthesis of (2,2-dimethyl-1,3-dioxolan-4-yl)methyl acrylate (DMDMA, [13188-82-4])
A 250 mL round-bottom flask equipped with a magnetic stirrer and a dropping funnel was charged with 5.0 g (37.9 mmol) of solketal (racemic mixture), 4.6 g (45.5 mmol) of triethylamine and 80 mL of CH 2 Cl 2 . The reaction mixture was cooled to 0 • C. A solution of 4.1 g (45.5 mmol) of acryloyl chloride in 20 mL of CH 2 Cl 2 was added dropwise, via the dropping funnel, over a period of 2 h. The reaction mixture was stirred continuously, with the magnetic stirrer, for 16 h, at room temperature. Triethylamine chlorhydrate salts were eliminated from the solution by filtration and the filtrate was washed with a 10% (w/w) aqueous solution of NaHCO 3 . The organic phase was then dried with magnesium sulfate and filtered. The solvent was eliminated by rotary evaporation. The DMDMA was then purified by vacuum distillation (at 12 mbar). DMDMA was obtained with a yield of 90%, as a colorless oil. NMR spectra were acquired on a Fourier 300 (300 MHz) spectrometer from Bruker Corporation (Karlsruhe, Germany): 1 H NMR (300 MHz, CDCl 3 ) ı 6.45 (dd, J = 17.3, 1.5 Hz, 1H, H-1), 6.16 (dd, J = 17.3, 10.4 Hz, 1H, H-2), 5.86 (dd,

O O O O 1 2 3 4 5 6 7 O O OH Cl O + TEA
Scheme 1. Synthesis of the DMDMA monomer [START_REF] Sousa | Microwave-promoted Morita-Baylis-Hillman reactions: efficient synthesis of new monoacylglycerols (MAGs) as potential anti-parasitic compounds[END_REF][START_REF] Moity | Glycerol acetals and ketals as bio-based solvents: positioning in Hansen and COSMO-RS spaces, volatility and stability towards hydrolysis and autoxidation[END_REF]. J = 10.4, 1.5 Hz, 1H, H-1), 4.40-4.32 (m, 1H, H-5), 4.26 (dd, J = 11.5, 4.6 Hz, 1H, H-4 or H-6), 4.18 (dd, J = 11.5, 5.9 Hz, 1H, H-4 or H-6), 4.10 (dd, J = 8.5, 6.4 Hz, 1H, H-4 or H-6), 3.78 (dd, J = 8.5, 6.1 Hz, 1H, H-4 or H-6), 1.44 (s, 3H, CH 3 ), 1.38 (s, 3H, CH 3 ) (Scheme 1). 13 C NMR (75 MHz, CDCl 3 ) ı 166.1 (C-3), 131.5 (C-1), 128.1 (C-2), 110.0 (C-7), 73.8 (C-5), 66.5 (C-4/6), 64.9 (C-4/6), 26.8 (CH 3 ), 25.6 (CH 3 ).

Synthesis of poly-DMDMA (PDMDMA homopolymer)

A 100 mL triple-neck round-bottom flask equipped with a nitrogen line, a magnetic stirrer, a cooler and a dropping flask was charged with 6.5 g (35 mmol) of DMDMA. The flask was then immersed in a water bath maintained at 65 • C with a thermostat. 4 mL of an aqueous solution (0.4% w/w) of KPS were added dropwise whilst bubbling nitrogen through the mixture. After 2 h at 65 • C, the reaction mixture was cooled to 0 • C. The purified homopolymer was then obtained after several washes with THF and oven-drying at 50 • C for 14 h. Comparison of the FTIR spectra of the DMDMA monomer and the prepared homopolymer clearly showed a disappearance of the bands corresponding to the elongation vibration and deformation of the carbon-carbon double bond (1643 cm -1 and 1417 cm -1 , respectively) indicating lack of unreacted monomer (Fig. 1).

The homopolymer, PDMDMA, was then characterized by differential scanning calorimetry (DSC). The thermogram showed a glass-transition temperature of 11 • C. The flexible nature of the polymer observed at room temperature comforted this measurement. Surprisingly, the obtained homopolymer was not soluble in any of the common organic solvents, indicating probably a very high molecular weight, which could not be quantified due to insolubility.

Graft-copolymerization of DMDMA onto cellulose

A 50 mL triple-neck round-bottom flask equipped with a nitrogen line, a cooler and a dropping flask was charged with 500 mg (9.3 meq OH) of extracted cellulose in 20 mL of THF. The resulting mixture was stirred at room temperature for 3 h.

The flask was then immersed in a water bath maintained at 65 • C with a thermostat. 4 mL of an aqueous solution of KPS and then the DMDMA were added to the reaction mixture. In our experiments, the added amount of KPS was varied from 0.8 to 12.8 mmol/eq OH, and the amount of monomer, from 1 to 5 mL (0.75-3.77 mol/eq OH). Nitrogen was bubbled through the resulting reaction mixture for 2 h. The reaction mixture was then cooled on ice. The graft-copolymer (Scheme 2) was recovered by filtration followed by several washes with THF. It was then dried at 50 • C for 14 h.

The grafting percentage (GP) was calculated as [START_REF] Dhiman | Synthesis of a cellulose-grafted polymeric support and its application in the reductions of some carbonyl compounds[END_REF] and [START_REF] Lin | Cellulose functionalization via ATRP grafting of glycidyl methacrylate for Cr(VI) adsorption[END_REF] suggest:

GP (%) = W 1 -W 0 W 0 × 100
where W 0 and W 1 are the weights of the cellulosic polymers before and after graft-copolymerization, respectively.

2.6. Characterization of the monomer, the homopolymer and the graft-cellulose 2.6.1. Nuclear magnetic resonance (NMR) spectroscopy NMR spectra were acquired on a Fourier 300 (300 MHz) spectrometer from Bruker Corporation (Karlsruhe, Germany). Samples were dissolved in CDCl 3 . Chemical shifts were relative to tetramethylsilane at 0 ppm. Standard pulse sequences were used for 1 H and 13 C NMR. Signals were assigned on the basis of DEPT135, 2D COSY and HSQC data.

FTIR spectroscopy

The FTIR spectra were recorded in the 500-4000 cm -1 range, with a Perkin Elmer Spectrum 65 device. The dry powdered samples were mixed with KBr at a concentration of 7%.

Thermogravimetric analysis

Thermogravimetric analyses of the initial cellulose derivatives and graft copolymers were performed with a Mettler Toledo TGA (Stare system) between 30 and 600 • C, at a heating rate of 10 • C/min under a nitrogen atmosphere.

Differential scanning calorimetry

Around 10 mg of sample for analysis was weighed and placed in an aluminum capsule, which was then sealed. Analyses were carried out on a Jade Perkin Elmer machine, between -40 and 300 • C, with a temperature ramp of 10 • C/min under a nitrogen atmosphere.

X-ray diffraction studies

The X-ray diffraction (XRD) curves of the initial cellulose and graft copolymers were recorded on a Rigakuminiflex II (Japan) Xray diffractometer, with a Cu K target, at 30 kV and 15 mA. The diffraction angle ranged from 5 • to 40 • (0.05 • /min).

Scanning electron microscopy

The samples were deposited on metal grids and covered with araldite epoxy resin. The product was well insulated, so the surfaces were rendered conductive by the deposition of a layer of silver several nanometers thick with an Edwards Scancoat SIX metallizing device. A morphological analysis of the samples was carried out under a LEO 435VP scanning electron microscope.

Mechanical properties

Palm cellulose and corresponding graft copolymers were thermopressed at 180 • C under 100 bar for 10 min. The thermopressed sheets were used to get six specimens sized 50 × 15 mm. A metal spacer was used to produce sheets of about 1 mm (Mangavel et al., Micro Systems, London, UK) was used to assess their mechanical properties. Stress-strain measurements were carried out to determine tensile strength ( max ) and elongation ( max ) at break value.

A speed of 1 mm s -1 was used, with an initial grip separation of 55 mm, as described in standards.

Results and discussion

The DMDMA was first prepared by reacting solketal with acryloyl chloride in the presence of triethylamine. This vinyl monomer was obtained in 90% yield after purification by vacuum distillation.

The PDMDMA homopolymer was then obtained by radical polymerization reaction with the use of KPS as initiator and in the absence of solvent. The obtained solid product (Fig. 2 left) was supple and elastic when hand-pulled.

Since graft-polymerization of cellulose uses THF as dispersion solvent, we attempted the synthesis of homopolymer under the same operation conditions (see Section 2.4) in the presence of THF (20 mL). After 2 h of reaction at 65 • C, and contrarily to the previous experiment, no solid product was obtained. The removal of THF with a rotary evaporator yielded a viscous liquid (Fig. 2

right).

According to FTIR data shown in Fig. 1 (3), the liquid consisted of some oligomers (significant diminution of the C C 1643 cm -1 band) solubilized in monomer.

DMDMA monomer polymerizes by a radical reaction by the association of radicals created by the initiator KPS (Scheme 2). However, in the presence of an important number of THF molecules, a competing reaction with a stable radical THF• comes into play to inhibit the homopolymerization reaction (Scheme 3), giving statistical priority to the formation of oligomers with a low molecular weight.

The question now is: how will the presence of THF affect the graft-polymerization? Grafting only a few monomers on the side-chains can be enough to modify the properties of the cellulose polymer. Thus, the DMDMA monomer was used for a graft-copolymerization reaction on cellulose, a reaction that has never, to the best of our knowledge, been studied before. A suspension of extracted cellulose in THF was stirred for 3 h at 65 • C. An aqueous solution of KPS as a radical initiator recently used for the graft-copolymerization of cellulose with acrylic acid [START_REF] Toledano-Thompson | Characterization of henequen cellulose microfibers treated with an epoxide and grafted with poly acrylic acid[END_REF], followed by 3 mL of the DMDMA monomer, were added dropwise to the flask, at 65 • C. The molar ratio of KPS/eq OH was varied to investigate the effect of this experimental parameter on the reaction and, in particular, on the rate of grafting of the DMDMA polymer onto cellulose (Fig. 3).

The experiments were duplicated and quite reproducible. The grafting rate appeared to increase with the amount of KPS, eventually reaching 27% for a ratio of 6.4 mmol KPS/eq OH. Beyond this value, the grafting rate decreased significantly, possibly due to the tendency of DMDMA to undergo preferential homooligomerization when monomer radicals become abundant as KPS renders them readily available.

In another series of reactions, we set the number of moles of KPS to 6.4 mmol/eq OH, which led to the highest rate of grafting and we varied the number of moles of DMDMA monomer from 0.77 to 3.77 mol/eq OH in the reaction medium. The curve illustrating the variation of grafting according to the amount of monomer is shown in Fig. 4. A gradual increase of grafting percentage was obtained, until arriving at the monomer amount of 1.51 mol/eq OH in the reaction medium, reaching 27% in GP. Beyond this value, the amount of monomer had no effect, and the grafting rate remained constant.

We analyzed the effect of reaction time on copolymerization, by fixing the amounts of KPS (6.4 mmol/eq OH) and monomer (1.51 mol/eq OH) at values favoring grafting onto cellulose and varying the reaction time from 24 to 120 min.

The effect of reaction time on grafting rate is clearly illustrated in Fig. 5. There was a progressive increase in yield for the first 72 min. Thereafter, the weight gain remained practically constant (27%).

(

(3) We can therefore conclude that each of the three factors studied the molar quantity of KPS, the molar quantity of monomer and the reaction time -have a direct effect on the grafting percentage. The tendencies for these parameters are difficult to predict.

The best grafting rates were obtained after a reaction time of 72 min in the presence of 6.4 mmol/eq (OH) of KPS and 1.51 mol/eq (OH) of monomer.

A comparison of the FTIR spectra of the palm cellulose, used as raw material, and the grafted cellulose (Fig. 6) revealed a new band at 1735 cm -1 in the grafted cellulose spectrum corresponding to the elongation vibration of the C O bond of the ester group present in the moieties grafted onto the cellulose backbone. This band corresponded to that observed on the spectrum of the synthesized PDMDMA homopolymer. But as we showed above (Fig. 2), there is no significant homopolymer formation when reaction takes place in the presence of THF. The ester band corresponds therefore to the grafted side-chains.

Morphological features of the various obtained samples were then analyzed by scanning electron microscopy (SEM). The SEM images (Fig. 7) revealed a major difference between cellulose and grafted cellulose. The surface of the cellulose was smooth, ordered and appeared to be homogeneous. By contrast, the surface of the grafted cellulose appeared to be more heterogeneous and covered with clusters resulting from the grafts attached during the polymerization reaction.

The palm cellulose used as the starting material and the grafted cellulose were then characterized by X-ray diffraction XRD (Fig. 8).

In these two cases, we obtained diffraction lines for 2 of about 16 • and 22.4 • . These lines correspond to the diffraction of cellulose designating planes (1 01) and (002) of the cellulose [START_REF] Kaushik | Isolation and characterization of cellulose nanofibrils from wheat straw using steam explosion coupled with high shear homogenization[END_REF]. New diffraction lines of low intensity appeared at around 30 • on the spectrum for grafted cellulose, suggesting a new organized structure attributed to the grafting of DMDMA side-chains to the cellulose backbone. Nevertheless, the grafted cellulose had a lower thermal stability than cellulose (Fig. 9). It began to break down at a lower temperature (220 • C, Table 2) than cellulose (360 • C). These results were compared with those obtained for PDMDMA. The thermal decomposition of PDMDMA occurs at 325 • C, but when DMDMA is grafted to cellulose, the thermal resistance is not increased. It becomes evident that the grafted side-chains are too short to improve the thermal resistance of cellulose and consequently, the organized segments observed by X-ray spectroscopy can correspond to the stacking of the pending groups (solketal ester moiety) in two or more neighboring DMDMA grafted monomers (Scheme 2). Differential scanning calorimetry was conducted with rising temperature from 0 to 100 • C, then cooled and heated again from 0 to 300 • C at 10 • C/min. In the second run we could not observe a distinguishable glass transition for the graft-cellulose (Fig. 10). Instead, a first endothermal peak at around 130 • C and two exothermal peaks between 210 and 230 • C were observed, that is, quite below its decomposition temperature. These three peaks were absent in the homopolymer DSC diagram (not shown), which showed only a glass transition at 11 • C. The 130 • C peak could be attributed to melting. However, the complex statistical structure of the graftcopolymer can be subjected to internal rearrangements of the lateral side-chains revealed by the exothermal peaks.

When the sample of grafted palm cellulose was subjected to thermopressing, partial melting, observable by the naked eye, occurred (Fig. 11). The mechanical properties of the obtained thermopressed sheets are shown in Table 3.

Even though the mechanical resistance is roughly twenty times lower than other cellulose derivatives such as cellulose acetate [START_REF] Gedon | Cellulose esters, organic esters[END_REF], the grafting of side-chains led to a significant increase in the elastic modulus accompanied by a slight increase of the tensile strength. In turn, there was also a decrease of the elongation at break, but this is a common phenomenon with the increase of the other two properties. Such low properties can be 3.1 ± 0.9 1.7 ± 0.6 explained by the partial melting leading to heterogeneous samples and a big standard deviation on six specimens.

Conclusion

This work was focused on the radical polymerization reaction between palm cellulose and 2,2-dimethyl-1,3-dioxolan-4-yl methyl acrylate (DMDMA) in the presence of potassium persulfate (KPS).

The maximum grafting weight gain was 27% obtained after 72 min of reaction at 65 • C with 6.4 mmol KPS/eq OH and 1.51 mol DMDMA/eq OH. If THF is used as dispersion solvent, the homopolymerization side-reactions are reduced. FTIR and TGA analyses excluded the hypothesis of a final product consisting of a mixture of homopolymer and cellulose. Organized sub-structures of the sidechains were revealed by a new peak in XRD spectroscopy and by two new exothermal peaks in DSC. The obtained adduct is partially thermoplastic and shows improved mechanical properties (modulus 5 times higher and strength 2 times higher compared to palm cellulose, but around twenty times lower than cellulose acetate). It does not possess a well-defined glass transition and its decomposition temperature is 35 • C lower than that of cellulose.

Fig.

  Fig. FTIR spectra of DMDMA (1, monomer), PDMDMA (2, homopolymer) and a mixture of oligomer and DMDMA (3).

Scheme 2 .

 2 Scheme 2. Simplified reaction mechanism for the synthesis of cellulose-g-PDMDMA.

Fig. 2 .Fig. 3 .Fig. 4 .

 234 Fig. 2. (left) PDMDMA homopolymer obtained in solvent-free conditions, (right) viscous liquid obtained in the flask after synthesis in THF (magnetic stirring) and evaporation of the solvent.

Fig. 5 .

 5 Fig. 5. Variation of grafting percentage with reaction time.

Fig. 6 .

 6 Fig. 6. FTIR spectra of palm cellulose before and after grafting of DMDMA.

Fig. 7 .

 7 Fig. 7. SEM images of palm cellulose (left) and PALM cellulose-g-PDMDMA (right). Upper row: magnification ×1000, lower row: ×5000.

Fig. 8 .

 8 Fig.8. X-ray diffraction spectra of palm cellulose before and after grafting.

Fig. 9 .

 9 Fig. 9. TGA and DTG thermograms of samples of PDMDMA, cellulose and cellulose-g-PDMDMA.

Fig. 10 .

 10 Fig. 10. DSC thermograms (2 nd run) of samples of cellulose and cellulose-g-PDMDMA.

Fig. 11 .

 11 Fig. 11. Thermopressed sheet of cellulose-g-PDMDMA.

  Table 3Mechanical properties (tensile) of palm cellulose and grafted palm cellulose (average and standard deviation on six specimens).

Table 1

 1 ADF-NDF composition of cellulosic raw materials.

	Raw palm leaves (ground)	Extracted cellulose from palm leaves

acrylate (solketal acrylate, DMDMA), a derivative of glycerol and acrylic acid that never has been used, to our knowledge, for graft-polymerization on cellulose. Recent studies have reported the use of other types of vinyl monomers for radical polymerization processes for cellulose (or other polysaccharides) grafting

Table 2

 2 Decomposition temperature of cellulose, grafted cellulose, and PMDMA.


Cédric Goncalves

Camille Favre

Pierre Feuardant

Sarah Klein

Carlos Vaca-Garcia

Marie-Christine Cecutti

Sophie Thiebaud-Roux

Emeline Vedrenne
email: emeline.vedrenne@ensiacet.fr

Christine Cecutti

Sophie Thiébaud-Roux

Synthesis of new cellulose ethers using Suzuki-Miyaura reactions

Keywords: Cellulose ethers Biopolymer Functionalization of polymers Suzuki-Miyaura reaction DS, degree of substitution, DG, degree of grafting, DMA, N,N-dimethylacetamide, GY, grafting yield, THF, tetrahydrofuran

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Cellulose ethers are polymers derived from cellulose that display very interesting properties. Indeed, they can be used as thickeners, binders, films formers, water-retention agents, emulsifiers [START_REF] Fukasawa | Meat additive, pickling liquid and processed meat product[END_REF][START_REF] Thielking | Cellulose Ethers[END_REF][START_REF] Porter | Controlled-release film coatings based on ethylcellulose[END_REF][START_REF] Wang | Blends and composites based on cellulose and natural polymers[END_REF], etc. Most of these derivatives are water-soluble and low concentrations are required to greatly modify the formulation rheology. The most commercially important cellulose ethers are carboxymethylcellulose (CMC; [START_REF] Thielking | Cellulose Ethers[END_REF] and alkyl cellulose such as methyl cellulose (MC; [START_REF] Majewicz | Cellulose ethers[END_REF] and ethyl cellulose (EC; [START_REF] Koch | Properties and uses of ethylcellulose[END_REF] but some derivatives bearing hydroxyl (hydroxyethylcellulose, HEC; [START_REF] Bert | Substitution patterns of cellulose ethers -influence of the synthetic pathway[END_REF], carboxyl [START_REF] Liebert | Synthesis path versus distribution of functional groups in cellulose ethers[END_REF] or amino [START_REF] Zhang | Antifungal activity and humidity sensitivity of quaternized cellulose synthesized in NaOH/urea aqueous solution[END_REF] moieties have also been developed. These compounds are generally synthesized in two steps. The first step is a mercerization process, which results in a reduction in crys-tallinity of cellulose. It is followed by a functionalization that often requires the use of gaseous substances such as methyl chloride, ethyl chloride, and ethylene oxide [START_REF] Thielking | Cellulose Ethers[END_REF]. This step is generally realized in continuously stirred tank reactors, under pressure (around 3 MPa) at 50-120 • C or in the presence of solvents that solubilize gases. Faced to the small number of available cellulose ethers, their great potential and the drastic conditions used for their preparation, it is thus necessary to develop new processes that could easily provide a wide range of new ethers. These processes should use mild conditions and respect to the maximum the Twelve Principles of Green Chemistry [START_REF] Anastas | Green chemistry: Theory and practice[END_REF].

In this context, the Suzuki-Miyaura (SM) reaction was chosen as it matches most of these criteria and displays a great functional group tolerance [START_REF] Bej | Palladium nanoparticle catalysis: Borylation of aryl and benzyl halides and one-pot biaryl synthesis via sequential borylation-Suzuki-Miyaura coupling[END_REF][START_REF] Bolliger | Dichloro-bis(aminophosphine) complexes of palladium: Highly convenient, reliable and extremely active Suzuki-Miyaura catalysts with excellent functional group tolerance[END_REF]. This pallado-catalyzed reaction involves the coupling of a boron reagent (boronic acid, esters or organotrifluoroborates) with a halide or a triflate. Most of these compounds do not require high-pressure equipment. In order to engage cellulose in a Suzuki-Miyaura reaction, a first step of functionalization was required to introduce a halide moiety on its scaffold. The SM reaction was then carried out on the halogenated cellulose to yield a series of new cellulose ethers. Noteworthy, to the best of our knowledge, this reaction has never been applied to cellulose derivatives. http://dx.doi.org/10.1016/j.carbpol.2014.03.089

Experimental

Materials and equipment

4-Bromobenzyl bromide, anhydrous dimethylacetamide, lithium chloride, sodium hydroxide, sodium sulphate, potassium carbonate, tetrahydrofuran, isopropanol, toluene were purchased from Aldrich and used without further purification. Palladium-based catalysts were purchased from STREAM and used without further purification. All boron derivatives were purchased from Frontier Scientific and used without further purification.

␣-Cellulose (Aldrich) has been dried at 105 • C for 3 h prior to use.

NMR spectra were acquired with FOURIER300 (300 MHz) and AVANCE (500 MHz with cryoprobe) spectrometers from Bruker Corporation (Karlsruhe, Germany). The temperature probes were set at 300 K. Samples were dissolved in d6-DMSO or CDCl 3 . In that case, chemical shifts were referenced to tetramethylsilane at 0 ppm. Standard pulse sequences were used for 1 H-, 13 C-, DEPT135 and two-dimensional (COSY, HSQC, HMBC) NMR spectra. The DS and the DG were determined from the peak area obtained by integration of the 1 H NMR spectra.

Measurement

Two cellulose ethers 2 and 3 obtained through each step were characterized (Fig. 1).

The first compound (2) is the result of an etherification reaction introducing a bromide functionality on the hydroxyl groups of cellulose (1). The weight gain, the isolated product yield and the degree of substitution of the hydroxyl moieties of 2 (DS) were determined.

The second compound (3), obtained through the Suzuki-Miyaura cross-coupling reaction between 2 and a boron derivative, is characterized by its degree of grafting (DG). It represents the number of hydroxyl groups substituted by O-CH2-Ph-Ph-R per anhydroglucose unit (AGU), after the SM reaction carried out on 2. The grafting yield (GY) represents the yield of the SM reaction (ratio between the DG and the DS).

The weight gain, DS, DG, GY, M (3) and m th were calculated by Eqs. ( 1 (2)

I AGU is the peak area of the protons on the AGU scaffold and CH 2 of the benzyl moiety from 5.25 ppm to 2.88 ppm. I phenyl is the peak area corresponding to the phenyl group of the benzyl moiety from 6.60 ppm to 7.88 ppm.

Degree of grafting (

DG) = 7 + 2 × DS 2 × I CH 2 I AGU (3)
I CH 2 is the peak area of the CH 2 moiety of the ethyl group.

Grafting yield (GY, %) = DG DS × 100 ( 4)

M (3) (g mol -1 ) = M AGU + DS × M CH 2 PhBr + DG × (M PhC 2 H 5 -M Br )
(5)

m th (g) = n starting material × M (3) (6)
M (x) , m x and n x are respectively the molar mass, the experimental weight and the number of moles of compound or group x.

Synthesis of 4-bromobenzyl cellulose 2c

␣-Cellulose (1.0 g, 6.170 mmol) were stirred in 30 mL of DMA at 160 • C for 2 h then cooled down to 100 • C. Lithium chloride (3.4 g, 0.080 mol) was added and the mixture was allowed to cool to room temperature overnight. Mixture was heated to 65 • C and powdered NaOH (9.4 g, 0.235 mol) was added followed by a solution of 4-bromobenzyl bromide (25.4 g, 0.102 mol) in DMA (30 mL) dropwise. The resulting mixture was stirred for 24 h at 65 • C and then cooled to room temperature. 100 mL of distillated water and 100 mL of chloroform were then added. The aqueous layer was extracted with chloroform. The combined organic layers were collected and washed with distillated water and concentrated. Ethanol was added and the resulting mixture was stirred for 24 h and then filtered. The obtained solid was washed with distillated water, ethanol and cyclohexane and then dried under vacuum to give 2.60 g of the desired compound as a white powder (DS = 2.78, 60%).

1 H NMR (500 MHz, CDCl 3 ): ı = 3.13 (H-5), 3.23 (H-2), 3.29 (H-3), 3.38 (H-7 ′ ), 3.60 (H-6), 3.84 (H-4), 4.34 (H-1), 4.14 (H-7), 4.60 (H-7 ′′ a), 4.99 (H-7 ′′ b), 6.78-7.68 (H Ar). 2.4. General procedure for Suzuki reactions: synthesis of compounds 3 4-Bromobenzylcellulose 2c (300 mg, DS = 2.78), the corresponding boronic acid (1.471 mmol), sodium sulfate (15 mg, 0.106 mmol), palladium chloride (7 mg, 0.049 mmol) and potassium carbonate (550 mg, 4.4 mmol) were added to a mixture of 9 mL of THF and 1 mL of water. The resulting mixture was stirred at 45 • C for 8 h. Solvents were evaporated and the resulting grey powder was washed with 3× 100 mL of cold water, 3× 100 mL of ethanol and then dried under vacuum. Compound 3a was obtained as a grey powder (330 mg, DG = 2.18), using (4-ethylphenyl)boronic acid.

1 H NMR (500 MHz, CDCl 3 ): ı = 1.25 ( CH 2 CH 3 ), 2.68 ( CH 2 CH 3 ), 3.15 (H-5), 3.24 (H-2), 3.36 (H-3), 3.63 (H-6), 3.90 (H-4), 4.13, 4.36 (H-7/7 ′ /7 ′′ ), 4.38 (H-1), 4.59, 5.00 (H-7/7 ′ /7 ′′ ), 6.88-7.69 (H Ar).

13 C NMR (125 MHz, CDCl 3 ): ı = 15.6 ( CH 2 CH 3 ), 28.5 ( CH 2 CH 3 ), 68.0 (C-6), 72.1, 72.8, 74.5 (C-7/7 ′ /7 ′′ ), 75.0 (C-5), 77.1 (C-4), 81.6 (C-2), 83.5 (C-3), 102.7 (C-1), 121.5 (C q -Ar), 126.8,126.9,128.2,129.0,129.3,131.1,131.4 (C Ar),136.4,137.7,138.4,140.1,140.6,143.1,143.6 (C q -Ar). 3a is soluble in THF and chloroform. Compound 3b was obtained as a grey powder (326 mg, DG = 1.76), using (4-butylphenyl)boronic acid.

FT-IR (KBr, cm

1 H NMR (500 MHz, CDCl 3 ): ı = 0.89 ( CH 2 CH 2 CH 2 CH 3 ), 1.41 ( CH 2 CH 2 CH 2 CH 3 ), 1.61 ( CH 2 CH 2 CH 2 CH 3 ), 2.64 ( CH 2 CH 2 CH 2 CH 3 ), 3.11 (H-5), 3.21 (H-2), 3.30 (H-3), 3.60 (H-6), 3.88 (H-4), 4.12 (H-7), 4.33 (H-1), 4.55 (H-7 ′′ a), 4.63 (H-7 ′ ), 4.99 (H-7 ′′ b), 6.77-7.70 (H Ar).

13 C NMR (125 MHz, CDCl 3 ): ı = 14.3 ( CH 2 CH 2 CH 2 CH 3 ), 22.8 ( CH 2 CH 2 CH 2 CH 3 ), 32.0 ( CH 2 CH 2 CH 2 CH 3 ), 37.3 ( CH 2 CH 2 CH 2 CH 3 ), 68.2 (C-6), 72.3 (C-7), 74.3 (C-7 ′ ), 74.7 (C-7 ′′ ), 75.1 (C-5), 76.9 (C-4), 81.8 (C-2), 83.6 (C-3), 102.7 (C-1), 121.2, 121.5 (C q -Ar), 129.1,129.2,129.3,129.4,129.7,131.3,131.4,131.5,131.6,131.7,131.8 (C Ar),136.8,137.0,137.1,138.3 (C q -Ar). 3b is soluble in THF and chloroform. Compound 3c was obtained as a grey powder (330 mg, DG = 1.20), using (4-(ethoxycarbonyl)phenyl)boronic acid.

FT-IR (KBr, cm

1 H NMR (500 MHz, CDCl 3 ): ı = 1.34 ( CH 2 CH 3 ), 3.06 (H-5), 3.17 (H-2), 3.49 (H-3), 3.51 (H-6), 3.77 (H-4), 4.05 (H-7/7 ′ /7 ′′ ), 4.27 (H-1), 4.33 ( CH 2 CH 3 ), 4.43 (H-7/7 ′ /7 ′′ ), 4.64 (H-7/7 ′ /7 ′′ ), 6.71-8.15 (H Ar).

13 C NMR (125 MHz, CDCl 3 ): ı = 14.4 ( CH 2 CH 3 ), 61.1 ( CH CH 3 ), 69.9 (C-6), 72.2, 74.3, 74.6 (C-7/7 ′ /7 ′′ ), 75.2 (C-5), 76.9 (C-4), 81.8 (C-2), 83.6 (C-3), 102.7 (C-1), 121.8 (C q -Ar), 126.9,127.2,128.3,129.0,129.1,129.3,130.0,130.2,131.2,131.4,131.5 (C Ar),143.9,144.3,144.9,166.6 (C q -Ar). 3c is soluble in THF and chloroform.

FT-IR (KBr, cm

Results and discussion

Preparation of the halogenated cellulose derivatives

The first step of this study was the preparation of a halogenated cellulose derivative. Cellulose was thus solubilized in N,N-dimethyl acetamide (DMA)/LiCl in the presence of sodium hydroxide and 1,3dibromopropane was added (Table 1, entry 1) [START_REF] Sachinvala | The physical and NMR characterizations of allyl-and crotylcelluloses[END_REF]. Even after 20 h at 70 • C, we could not observe the formation of the desired halogenated cellulose 2a, but the presence of allyl-cellulose, issued from an elimination reaction of a HBr molecule in this basic medium, was detected. The same behaviour was observed on 1-bromo-4-(2-bromoethyl)benzene, leading to the formation of 1-allyl-4-bromobenzene (Table 1, entry 2). 1-Bromo-4-(bromomethyl)benzene was then chosen (Table 1, entry 3) as no elimination can occur on this reagent. Indeed, the desired halogenated cellulose 2c was obtained but with a DS too low to be soluble in common solvents, such as CDCl 3 (which prevented the DS to be determined by NMR). It seems highly likely that cellulose ethers that have even lower DS are soluble in the aqueous phase and are thus lost during the extraction process, accounting for the negative weight gain.

The brominated reagent was then added in three portions instead of one at the beginning of the reaction, which led to the formation of 2c in 10% yield. Noteworthy, 2c is soluble in THF, DMSO and CDCl 3 . It was thus dissolved in DMSO-d6 in order to be analyzed by NMR spectroscopy (Fig. 2). A DS of 2.76 was determined thanks to the addition of a few drops of d1-TFA in the NMR tube in order to displace downfield the NMR signals of exchangeable protons and water [START_REF] Ross | Downfield displacement of the NMR signal of water in deuterated dimethylsulfoxide by the addition of deuterated trifluoroacetic acid[END_REF]. An excellent yield of 60% was then obtained when 1-bromo-4-(bromomethyl)benzene was added dropwise over the course of 24 h (Table 1, entry 5), which seems to show the poor stability of this reactant in our reaction conditions. Full structure characterization of 2c was carried out at 500 MHz using one-, two-dimensional homo-and 1 H-13 C heterocorrelated techniques. Experiments were run in CDCl 3 as d6-DMSO caused spectral line broadening and required long accumulation time because of the poor solubility of 2c in this solvent. Thanks to the 1 H/ 1 H COSY and 1 H/ 13 C HSQC NMR analyses (Fig. 4), the 1 H and 13 C signals of the modified AGU can be respectively assigned as follows (Fig. 3): 3.13 ppm (H-5), 3.23 ppm (H-2), 3.29 ppm (H-3), 3.60 ppm (H-6), 3.84 ppm (H-4), 4.34 ppm (H-1) and 68.1 ppm (C-6), 75.0 ppm (C-5), 77.0 ppm (C-4), 81.8 ppm (C-2), 83.5 ppm (C-3), 102.7 ppm (C-1). The signals for H-7 and H-7 ′ were respectively found at 4.14 and 3.38 ppm, while the protons in position 7 ′′ are split and give peaks at 4.60 and 4.99 ppm. The corresponding carbons appear at 72.2 ppm (C-7), 74.3 ppm (C-7 ′′ ), 74.7 ppm (C-7 ′ ). Further confirmation of the structure of 2c was then obtained from the HMBC correlation peaks between H-2 and C-7 ′′ , H-3 and C-7 ′ and H-7 and C-6 (Fig. 4).

Preparation of new cellulose ethers using Suzuki-Miyaura reactions

The brominated cellulose 2c was then engaged in the key step of this synthesis: the Suzuki-Miyaura reaction. Based on the reaction conditions described by Bora's team [START_REF] Mondal | An efficient protocol for palladium-catalyzed ligandfree Suzuki-Miyaura coupling in water[END_REF], a solution of 2c and 4-ethylphenylboronic acid (1 equiv.) in i-PrOH was stirred in the presence of PdCl 2 (4 mol%), Na 2 SO 3 (8 mol%) and K 2 CO 3 (3 equiv.) at 45 • C for 8 h. These mild, moisture-and aircompatible conditions led to the formation of the cellulose ether 3a (Table 2, entry 1).

A degree of grafting of 1.09 was determined thanks to the 1 H NMR spectra analyses. A typical 1 H NMR of 3a is shown in Fig. 5. Noteworthy, the chemical shift of the ethyl group is not in the region of cellulose scaffold, which allows to obtain reliable integrations of the different signals. The DG determination thus indicated that only a part of the brominated ether moieties have been chemically modified by the SM reaction.

For subsequent assays (Table 2, entries 2-4), i-PrOH was replaced by toluene, water or a mixture of THF/water (9/1). The latest conditions gave the best results with a grafting yield of 79% (Table 2, entry 4). THF/water was thus used for the rest of this work.

The influence of the amount of boronic acid was then investigated. The number of equivalents of 4-ethylphenylboronic acid was doubled (Table 2, entry 5), yielding 3a in 84% yield. However, compared to the cost of boronic acids, this yield improvement was not significant enough and the subsequent assays were carried out with only one equivalent of boron species.

In order to study the influence of the temperature on the grafting yield, the SM reaction was then carried out at room temperature and 78 • C (Table 2, entries 6 and 7). A drop of the grafting yields was observed, indicating that it is better to run this cross-coupling reaction at 45 • C.

We then focused our study on the screening of different palladium-based catalysts. Classical Suzuki-Miyaura catalysts, such as palladium(II) acetate or [1,1 ′ -bis(diphenylphosphino) ferrocene]dichloropalladium(II) were used (Table 2, entries 8 and 9) and palladium chloride appeared to give the best results. The amount of this catalyst was then optimized (Table 2, entries 10-12) with loading varying from 1 to 10 mol%. Similar yields were obtained with 4 and 10 mol% (Table 2, entries 4 and 12). For economical reasons, a catalyst loading of 4 mol% was chosen for the rest of the study.

In order to broaden the scope of the synthesized products, we next proposed to modify the nature of the boronic acid using our optimized conditions. Boronic acids were chosen so that a part of the grafted moiety NMR signals do not superimpose itself on the AGU peaks in order to obtain accurate integrations and so accurate degrees of grafting. Cellulose ether 3b, bearing a longer alkyl chain, was obtained in good yield (Table 2, entry 13) with the use of 4butylphenylboronic acid. Furthermore, electron-poor boronic acids appeared to be compatible with these reaction conditions as 3c was obtained in 43% yield, when 4-(ethoxycarbonyl)phenylboronic acid was engaged in the reaction (Table 2,entry 14). This lower yield can be explained by the low nucleophilicity of electron-deficient aryl boronic acids that decreases the transmetalation rate. These boron species are also prone to react in homo-coupling side reactions [START_REF] Wong | Ligand promoted palladium-catalyzed homocoupling of arylboronic acids[END_REF] and more susceptible to metal-catalyzed protodeboronation [START_REF] Kuivila | Metal ion catalysis in the protodeboronation of areneboronic acids[END_REF].

Noteworthy, in all cases, the experimental and theoretical masses were very close to each other, indicating that the mass balance is in agreement with the NMR analysis and that no degradation occurred during the SM reaction.

Conclusion and perspectives

New cellulose ethers can be easily synthesized through the use of a Suzuki-Miyaura cross-coupling reaction. This new process is carried out in mild, moisture-and air-stable conditions. Thanks to its great functional group tolerance, cellulose ethers bearing various alkyl chains and different functionalities can be obtained in good to excellent yields. Detailed investigation of the physicochemical properties of these new cellulose ethers will be published in due course. This synthetic route might also be interesting for the grafting of sensitive functional groups to the cellulose backbones under mild conditions.

Fig. 1 .

 1 Fig. 1. Definition of the weight gain, DS, DG, GY, M (3) and m th .

  I AGU /I phenyl -1/2)

  13 C NMR (125 MHz, CDCl 3 ): ı = 68.1 (C-6), 72.2 (C-7), 74.3 (C-7 ′′ ), 74.7 (C-7 ′ ), 75.0 (C-5), 77.0 (C-4), 81.8 (C-2), 83.5 (C-3), 102.7 (C-1), 121.1, 121.5, 121.7 (C-11/11 ′ /11 ′′ ), 129.1, 129.2, 129.4 (C-9/9 ′ /9 ′′ ), 131.3, 131.5, 131.6 (C-10/10 ′ /10 ′′ ), 137.0, 137.1, 138.3 (C-8/8 ′ /8 ′′ ). FT-IR (KBr, cm -1 ): 3452 vOH (weak), 2960, 2865 vC H, 1595, 1487 vC C, 1404, 1358, vC H, 1152-1037 vC O C, 802 vC Br. 2c soluble in THF, DMSO and chloroform.

Fig. 2 .

 2 Fig. 2. 1 H NMR spectrum of cellulose ether 2c in DMSO-d6 (with addition of a few drops of d1-TFA).

Fig. 3 .

 3 Fig. 3. 1 H-and 13 C NMR spectra of cellulose ether 2c in CDCl3.

Fig. 4 .

 4 Fig.4. 1 H/ 1 H-COSY NMR, 1 H/ 13 C-HSQC and 1 H/ 13 C-HMBC NMR spectra of cellulose ether 2c in CDCl3.

Fig. 4 .

 4 Fig. 4. (Continued ).

Fig. 5 .

 5 Fig. 5. 1 H NMR spectrum of cellulose ether 3a in CDCl3.

Table 1

 1 Results for the preparation of halogenated cellulose derivatives.

	.

a Reaction time: 24 h.

  -1 ): 3434 vOH (weak), 3024, 2961, 2926, 2868 vC H, 1498 vC C, 1458, 1400 vC H, 1152-1037 vC O C, 810 vC Br.

  -1 ): 3424 vOH (weak), 2924, 2867 vC H, 1488 vC C, 1458, 1358 vC H, 1174-970 vC O C, 802 vC Br.

  -1 ): 3435 vOH (weak), 2867 vC H, 1710 vC O, 1487 vC C, 1397, 1365 vC H, 1183-950 vC O C, 803 vC Br.

Acknowledgement

The authors thank the French Higher Education and Research Minister (MESR) for financial support.

a m th was calculated using the DG that was determined with the NMR analysis.

b Two equivalents of 4-ethylphenylboronic acid were used.