Homogeneous Esterification of Xylan-Rich Hemicelluloses with Maleic

Nov 5, 2010 - The biopolymers with degrees of substitution (DS) between 0.095 and 0.75 were accessible in a completely homogeneous system by changing ...
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Biomacromolecules 2010, 11, 3519–3524

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Homogeneous Esterification of Xylan-Rich Hemicelluloses with Maleic Anhydride in Ionic Liquid Xin-wen Peng,† Jun-li Ren,*,† and Run-cang Sun*,†,‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China, and Institute of Biomass Chemistry and Technology, Beijing Forest University, Beijing 100083, China Received August 27, 2010; Revised Manuscript Received October 24, 2010

Generation of bioenergy, new functional polymers, or chemicals and biomaterials from hemicelluloses are important uses for biomass. In this paper, a novel functional biopolymer with carbon-carbon double bond and carboxyl groups was prepared by a homogeneous esterification of xylan-rich hemicelluloses (XH) with maleic anhydride in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) ionic liquid using LiOH as catalyst. The biopolymers with degrees of substitution (DS) between 0.095 and 0.75 were accessible in a completely homogeneous system by changing reaction temperature, reaction time, the dosage of catalyst, and the molar ratio of maleic anhydride to anhydroxylose unit in XH. Results obtained from FT-IR and 13C NMR spectroscopies confirmed the structure of hemicellulosic derivatives with carbon-carbon double bond and carboxyl groups, implying an efficient method to prepare a novel and important functional biopolymer for biomaterials.

Introduction Hemicelluloses, which are generally defined as the noncellulose polysaccharides in plant cell wall and represent about 20-40% of the biomass, are a renewable resource for obtaining bioenergy, biopolymers, or chemicals and biomaterials to reduce global dependence on fossil fuels.1-6 There has been interest in converting hemicelluloses to chemicals such as furfural, xylitol, ethanol, or lactic acid.7-9 Glucuronoxylan and arabinoxylan films are found to have a low oxygen permeability and can thus be used in packaging for oxygen-sensitive products.8,10-12 Development of new bioactive and biocompatible polymers capable of exerting a temporary therapeutic function from hemicelluloses is another strategic area. Hemicelluloses have been proposed to be suitable polymers for preparation of biomedical materials such as hydrogels and films and, thus, will have key applications in drug release systems and tissue engineering.4,8,13,14 There has been increasing interest in chemically modified hemicelluloses for producing more biomaterials and functional polymers from hemicelluloses, for example, cationic hemicelluloses,15-17 carboxymethyl hemicelluloses,18-20 lauroylated hemicelluloses,21,22 acylated hemicelluloses,23,24 and oleoylated hemicelluloses.25 These resulting polymers may find a variety of potential applications in functional biomaterials. Derivation reactions will be greatly facilitated if the chemical modification can be carried out in a homogeneous phase. Recently, public attention has focused on the use of ionic liquids (ILs) because of their various advantages such as eco-friendliness,26,27 lower hydrophobicity and viscosity, enhanced electrochemical and thermal stability, and higher reaction rates.28-32 Until now, however, the information available in the literature about the chemical modification of hemicelluloses in ionic liquids is very limited. IL was first reported to be used in the acetylation of wheat straw hemicelluloses in 2007.24 Results indicated * To whom correspondence should be addressed. Phone/Fax: +86-2087111861. E-mail: [email protected]; [email protected]. † South China University of Technology. ‡ Beijing Forest University.

that IL could be used as the satisfactorily homogeneous media, and no derivation occurs on hemicelluloses during the dissolving of hemicelluloses in IL. Maleic anhydride modified polymers that contain carboxyl groups and unsaturated double bonds are very important macromolecules. These carboxyl groups can be used as functional groups in superabsorbents or for binding of heavy metals,33,34 drug delivery,35,36 electrostatic adsorption self-assembly,37 immobilization of bioactive molecules and enzyme,38-40 genetic attachment,41,42 and permselectivity for water.43 The unsaturated double bonds can act as active and polymerizable sites for cross-linking.44,45 The modified polymers can also be used as a versatile platform for macromolecules modification and biosurface engineering.4,8,39,46 Therefore, maleic anhydride modified hemicelluloses like other maleic anhydride modified polymers may find their potential applications in preparation of functional biomaterials. However, only a few studies have been reported on the esterification of hemicelluloses with maleic anhydride. The objective of the current work was to perform a fundamental exploration of the homogeneous modification of hemicelluloses in ILs and prepare a novel maleic anhydride modified biopolymer. This study offers an efficient method to prepare a novel and important functional biopolymer with unsaturated double bonds and carboxyl groups.

Experimental Section Materials. Xylan-rich hemicelluloses (XH) were isolated from Dendrocalamus membranaceus Munro (DmM) according to the procedure in a previous paper.47 XH were isolated using 10% KOH at 23 °C for 10 h with a solid to liquid ratio of 1:20 (g/mL) from holocellulose, which was obtained by delignification of the extractivefree DmM (40-60 mesh) with sodium chlorite in an acidic solution (pH 3.7-4.0, adjusted by 10% acetic acid) at 75 °C for 2 h. The sugar analysis showed the following sugar composition (relative percent): 89.38% xylose, 5.75% arabinose, 1.87% glucose, 0.66% galactose, 1.78% glucuronic acid, and 0.55% galactose acid. 1-Butyl-3-methylimidazolium chloride ([BMIM]Cl) ionic liquid was purchased from

10.1021/bm1010118  2010 American Chemical Society Published on Web 11/05/2010

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Scheme 1. Proposed Dissolution Mechanism of XH in [BMIM]Cl Ionic Liquid

Peng et al. B)

DS )

Scheme 2. Reaction Scheme of Homogeneous Esterification of XH with MA in [BMIM]Cl Ionic Liquid

Shanghai Cheng Jie Chemical Co. Ltd. Ethanol, LiOH, and maleic anhydride (A. P.) were purchased from Guangzhou Chemical Reagent Factory, China. Sugar Composition. The sugar composition in the DmM was determined by high-performance anion exchange chromatography (HPAEC). The neutral sugars in XH were liberated by hydrolysis with 6% H2SO4 at 105 °C for 2.5 h. Afterward, the samples were filtered and injected into the HPAEC system (Dionex ISC 3000) with an amperometric detector, an AS50 autosampler and a Carbopa PA1 column (4 × 250 mm, Dionex). Samples were separated in 18 mM NaOH (carbonate free and purged with nitrogen) with postcolumn addition of 0.3 M NaOH at a rate of 0.5 mL/min. Running time was 45 min followed by a 10 min elution with 0.2 M NaOH to wash the column and then a 15 min elution with 0.018 M NaOH to re-equilibrate the column. The uronic acid was eluted with 0.4 M NaOH for 20 min at a rate of 1 mL/min with postcolumn addition of 0.3 M NaOH at a rate of 0.5 mL/min. Calibration was performed with standard solutions L-arabinose, D-glucose, D-xylose, D-glucose, D-mannose, D-galactose, glucuronic acid, and galacturonic acid. Homogeneous Esterification of XH in ([BMIM]Cl) Ionic Liquid. Homogeneous esterification of XH with maleic anhydride (MA) was carried out in [BMIM]Cl ionic liquid (Schemes 1 and 2). Dry XH (0.33 g) were added into 13.0 g ionic liquid (2.5%, w/w) in a threenecked flask with a magnetic stirrer, and the mixture was stirred at 90 °C up to 1.5 h to guarantee complete dissolution of XH. The flask was continuously purged with gaseous N2. The solution was checked by placing a drop between two glass slides and observing the solution microscopically between crossed polars (×40). Then the required quantities of LiOH (0.005, 0.01, 0.02, and 0.025 g) and MA (the molar ratios of MA to XH were 1:1, 2:1, 4:1, and 8:1) were added at 40, 60, 80, and 100 °C, and the reaction ran for 40, 60, 80, and 100 min, respectively. After the required time, the mixture, which was cooled to room temperature, was precipitated with 150 mL of 95% (w/w) ethanol under stirring for 60 min and then centrifuged at 4000 rpm for 30 min. The precipitate was washed with 150 mL of 95% (w/w) ethanol twice. Finally, the product was dried at 45 °C in a vacuum oven for 16 h. Determination of Degree of Substitution. The average values of degree of substitution (DS) of MA-modified XH (MXH) were determined by acidometric titration.20 Exactly 0.05 g MXH was dissolved in 50 mL of distilled water with a magnetic stirrer. The pH of the solution was adjusted to 8 with NaOH. And then the solution was titrated with 0.05 M H2SO4 to pH 3.74. The DS was calculated based on the equations shown below.

a)

m′ m

2MV am

0.132B 1 - 0.1B

where a is the purity of the products. m and m′ (g) are weights of products purified after and before, 0.132 (g mmol-1) is the molar mass of an xylose unit, 0.1 (g mmol-1) is the net increase in the mass of an xylose unit for each MA substituted, M ) normality of H2SO4 used, V ) mL of H2SO4 used to titrate sample, and B ) mmol/g of H2SO4 consumed per gram of the products.48,49 This procedure makes the assumption that the untreated XH do not have any carboxyl content. All the titrations were carried out in triplicates and standard deviations were less than 4.0%. Characterization of MXH. FT-IR transmission spectra of XH and MXH were measured by using a Nicolet 750 spectrophotometer within the wavenumber range 400-4000 cm-1, and the 1% finely ground samples were mixed with KBr to press a plate for measurement. The solution-state 13C NMR spectra were obtained on a Bruker MSL300 spectrometer operating in the FT mode at 74.5 MHz. The sample (80 mg) was dissolved 1 mL D2O. The 13C NMR spectra were recorded at 25 °C after 30000 scans. Chemical shifts (δ in ppm) are expressed relative to the resonance of Me4Si (δ ) 0). A 60° pulse flipping angle, a 3.9 µs pulse width, and a 0.85 s delay time between scans were used. The molecular weights of XH and MXH were determined by gel permeation chromatography (GPC) on a PL aquagel-OH 50 column (300 × 7.7 mm, Polymer Laboratories Ltd.) and calibrated with PL pullulan polysaccharide standard (average peak molecular weights of 783, 12200, 100000, 1600000). A flow rate of 0.5 mL/min was maintained. The eluent was 0.02 N NaCl in 0.005 M sodium phosphate buffer (pH 7.5). Detection was achieved with a Knauer differential refractometer. The column oven was kept at 30 °C. XH and MXH were dissolved with 0.2 N NaCl in 0.005 M sodium phosphate buffer, pH 7.5, at a concentration of 0.1%. Thermal analysis of XH and MXH were performed using thermogravimetric analysis (TGA) and differential thermal analysis (DTA) on a simultaneous thermal analyzer (Pyris Diamond TG/DTA, PE Instrument). The apparatus was continually flushed with nitrogen. The sample weighed between 9 and 11 mg and was heated from room temperature to 600 °C at a heating rate of 10 °C per minute.

Results and Discussion Proposed Mechanism of the Dissolution of XH in Ionic Liquid. It is presumed that both anions and cations are involved in the dissolution process of cellulose. The oxygen atoms in the OH of cellulose serve as electron donors and hydrogen atoms act as electron acceptors. In corresponding fashion, the cations and anions in ionic liquid solvents act as the electron acceptor centers and electron donor centers, respectively. For their interaction, the oxygen and hydrogen atoms in the OH of cellulose form electron donor-electron acceptor (EDA) complexes with the charged species of the IL. This interaction results in the separation of the hydroxyl groups of different cellulose chains, leading to the dissolution of cellulose in ionic liquid.50,51 XH have two hydroxyl groups in their sugar units and can form the electron donor-electron acceptor (EDA) complexes with the ionic liquid, thus, resulting in dissolution of XH in the ionic liquid.52,53 Scheme 1 shows the proposed dissolution mechanism of XH in ionic liquid. Therefore, the interaction between the hydroxyl group of polysaccharides and ILs is crucial for dissolution of XH. Complete homogeneous xylan solutions with concentrations of 3-20 wt % could be obtained at dissolution temperatures of 45-95 °C.53 These solutions showed no phase separation upon cooling to room temperature or even after

Esterification of Xylan-Rich Hemicelluloses

Figure 1. Dissolution process of XH in [BMIM]Cl at 90 °C with a concentration of 2.6%: 0 (a), 40 (b), and 75 min (c).

storing them at 5 °C for several months. Therefore, ILs are ideal reaction media for preparation of novel biopolymer and biomaterials from XH. Optical microscopy investigation clearly demonstrates that the solution was not homogeneous after 50 min dissolution (Figure 1a and b), indicating an incomplete dissolution in [BMIM]Cl, whereas, a period of 75 min resulted in an almost homogeneous solution (Figure 1c), which indicated most of XH had been dissolved in IL. In this case, a clear dissolution could be obtained. Therefore, a prolonging dissolution time of 15 min (the total dissolving time was 90 min) was adopted to guarantee a complete dissolution of XH in IL, and this fully homogeneous phase was prepared for the sequent chemical modification. Effects of Reaction Conditions on the DS of the MXH. Because of various advantages, such as environmental compatibility and chemical and thermal stability, ionic liquids show their potential in replacing most conventional organic solvents for organic and polymerization reactions.27,54,55 The melting point of [BMIM]Cl is about 75 °C and XH can be completely soluble in [BMIM]Cl IL at 90 °C for 1.5 h up to 2.6% by weight in this work. The XH modification was performed by esterification of hydroxyl groups on the backbone of XH with MA using [BMIM]Cl IL as a homogeneous reaction medium in the presence of LiOH as catalyst, which is shown in Scheme 2. Table 1 (Supporting Information) shows the extent of the chemical modification expressed by DS when the reaction was carried out under different conditions. There was an increase in the DS with the molar ratio of MA/anhydroxylose unit in XH from 1:1 to 4:1. A maximum DS of 0.26 was obtained with a molar ratio of MA/anhydroxylose unit in XH of 4:1, indicating that MA should be employed in large excess to give MXH with a relatively high DS. On the other hand, further increase in the molar ratio to 8:1 resulted in a slight decrease in DS (0.23). The increase in the DS of the MXH is probably due to the greater availability of MA at a higher concentration to XH. At a molar ratio higher than 4:1, the decrease in reaction efficiency may suggest that a sufficient time may not be given for large extents of esterification at the higher MA concentration. Similar results were also reported in the literature.24 An increase in the reaction temperature from 60 to 80 °C led to an increment in the DS of the products from 0.26 to 0.45 (Table 1, Supporting Information). A maximum DS of 0.45 was obtained at 80 °C; thereafter, DS decreased to 0.31 when the reaction temperature increased to 120 °C. The reason for this increase is probably that a higher reaction temperature favors the compatibility of the reaction ingredients, swellability of XH, diffusion of the etherifying reagent, and mobility of the reactant molecules. However, the decrease in the DS beyond 80 °C may be due to the XH degradation and an adverse condition for the side reaction. Reaction time also shows a significant influence on the reaction efficiency of esterification with MA. As shown in Table 1 (Supporting Information), the DS increased from 0.095 to 0.52 when the reaction time increased from 40 to 80 min; thereafter, DS decreased to 0.25. This observation indicates that sufficient

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reaction time facilitates the homogeneous esterification of XH with MA in ionic liquid, whereas excessive time may lead to degradation of XH and their derivatives over a longer reaction period. Hence, attempts were made to carry out the chemical reaction for 80 min so as to prepare a product with a high DS. Table 1 (Supporting Information) also shows that the dosage of catalyst (LiOH) plays an important role in the homogeneous esterification of XH in [BMIM]Cl ionic liquid. An increase in the dosage of catalyst (LiOH) from 0.005 to 0.02 g gave rise to a significant increment from 0.14 to 0.72, which suggests that LiOH exhibits a more remarkable influence on the rate of esterification of XH with MA. Excessive LiOH could not improve the reaction efficiency much more; as a result, the DS showed a slight increase (0.75) when 0.025 g LiOH was applied. Molecular Weight Distribution. The average molecular weights of MXH were determined by gel permeation chromatography (GPC). The weight-average (Mw) and number-average (Mn) molecular weights as well as polydispersity (Mw/Mn) are listed in Table 2 (Supporting Information). The Mw of XH were 42169 g · mmol-1, which were higher than that of DXH (38264 g · mmol-1), indicating that some degradation occurred to XH during dissolving process. The depolymerization by dissolving in ionic liquid was also reported for cellulose.56,57 This may be due to the fact that the dissociated Cl- and [BMIM]+ in [BMIM]Cl weakens the glycosidic linkages in the macromolecules. All of the Mw and Mn of MXH were lower than that of DXH, which may be due to the degradation of DXH during etherification reaction in elevating temperature (60-120 °C). In addition, the MXH has a relatively low index of polydispersity (1.53-1.83) than the DXH (1.89), which indicates that DXH has a more uniform molecular weight distribution. Table 2 (Supporting Information) also indicates the important effect of reaction temperature on the molecular weight of MXH. The Mw of MXH decreased from 32852 to 24330 g · mmol-1 when the temperature increased from 60 to 120 °C, suggesting a more significant degradation of the macromolecules at a higher reaction temperature. Prolonging reaction time also facilitated the degradation of XH, as indicated by a relatively low Mw (26787 g · mmol-1) over a period of 100 min. The dosages of MA and LiOH show less remarkable impact on the molecular weights of MXH. FT-IR Spectra Analyses. The effect of chemical modification on the structure of XH was demonstrated by FT-IR in the region of 400-4000 cm-1. Figure 2 illustrates the FT-IR spectra of XH (spectrum 1) isolated from Dendrocalamus membranaceus Munro and MXH (spectrum 2, sample 15) prepared in [BMIM]Cl ionic liquid. For the unmodified XH, the absorptions at 3430, 2933, 1466, 1400, 1248, 1163, 1044, 987, and 897 cm-1 seen in spectrum 1 are indicative of XH.58 A sharp band at 897 cm-1 is assigned to β-glucosidic linkages between the sugar units, indicating that the xylose residues forming the backbone of the macromolecule are linked by β-form bonds.59 The low intensity of the band at 987 cm-1 suggests the presence of arabinosyl units, which are attached only at position 3 of the xylopyranosyl constituents.60 The region between 1466 and 1044 cm-1 relates to the C-H and C-O bond stretching frequencies. A strong broadband belonging to hydrogen-bonded hydroxyls occurs at 3430 cm-1, and a symmetric C-H vibration band is at 2933 cm-1.61 In spectrum 2, the presence of a few new bands occurs as compared with the spectrum 1 in Figure 2. Two peaks at 1727 and 1164 cm-1 are observed due to carbonyl stretching vibrations of CdO groups present in anhydride groups and the C-O vibration in MXH,62 which indicates the esterification reaction between hydroxyl groups of XH and anhydride groups

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Figure 2. FT-IR spectra of XH (spectrum 1) and MXH (spectrum 2, sample 15, Table 1, Supporting Information).

Figure 3. FT-IR spectra of MXH sample 2 (spectrum 1, Table 1, Supporting Information), sample 10 (spectrum 2, Table 1, Supporting Information), and sample 15 (spectrum 3, Table 1, Supporting Information).

of MA.63 The significant decrease in the intensity of the peak at 3430 cm-1 (O-H) indicates that the esterification occurs between the hydroxyl groups of XH and the anhydride groups of the MA, as shown in Scheme 2. The absence of absorption region between 1850-1780 cm-1 demonstrates that the product does not contain the unreacted MA, and the lack of a peak at 1700 cm-1 for carboxylic groups indicates that the product is also free of the byproduct (acetic acid).64 Figure 3 illustrates the FT-IR spectra of MXH: samples 2 (spectrum 1), 10 (spectrum 2), and 15 (spectrum 3). The similarity of spectral profiles indicates the structural similarity of the derivatives. However, on close examination of the spectra, some small differences can be identified. Comparison of the relative intensity at 1727 cm-1 showed that it increases from spectrum 1 (sample 2, DS ) 0.14) to spectrum 3 (sample 15, DS ) 0.72), and the absorbance for hydroxyl band at 3430 cm-1 decreases from spectrum 1 to spectrum 3 as the DS increased from 0.14 to 0.72. 13 C NMR Spectra. Figure 4 shows the 13C NMR spectra of XH (a) and MXH (b). In Figure 4a, five major signals at 102.4, 76.0, 75.2, 73.6, and 63.2 ppm are attributed to the C-1, C-4, C-3, C-2, and C-5 of the (1f4)-linked β-D-Xyl units, respectively.65,66 The signals at 109.8, 86.4, 80.3, 78.4, and 61.7 ppm are attributed to the C-1, C-4, C-3, C-2, and C-5 of R-Larabinofuranosyl residues, respectively. The 13C NMR spectrum

of XH confirms that the monomeric side chains of the Larabinofuranosyl residues are linked to C-3 of the backbone β-D-xylans. The signals at 97.5, 72.4, 71.7, 82.9, and 177.0 ppm originate from the C-1, C-3, C-2, C-4, and C-6 of 4-O-methylD-glucuronic acid in the xylan, and a 59.4 ppm signal is characteristic of the methoxyl group of a 4-O-methyl-Dglucuronic acid residue in the xylan.67 Figure 4b shows the 13C NMR spectrum of MXH in sample 15. Five major signals at 99.8, 75.1, 72.1, 70.8, and 62.6 ppm are attributed to the C-1, C-4, C-3, C-2, and C-5 of the (1f4)linked β-D-Xyl units, respectively. The chemical shift at 35.6 ppm is assigned to the carbon atoms of C-H. Signals between 165.3 and 171.6 ppm originate from the carbon atom of carbonyl groups, and signals between 122.3 ppm and 136.5 ppm are attributed to carbon-carbon double bonds of the anhydride in MXH. These observations confirm the occurrence of homogeneous esterification of XH with MA in ionic liquid. Thermal Analysis. Thermal degradability is affected by the chemical composition of the material. Figure 5 shows typical TGA/DTA curves of XH and MXH. The thermal stability of MXH was lower than that of XH. The first decomposition can be observed at about 200 °C for XH and 185 °C for MXH, which is probably due to generation of noncombustible gases such as CO2, CO, formic acid, and acetic acid. Tmax (the decomposition temperature corresponding to the maximum rate

Esterification of Xylan-Rich Hemicelluloses

Figure 4.

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C NMR spectrum of XH (spectrum a) and MXH (spectrum b, sample 15, Table 1, Supporting Information).

Figure 5. TGA/DTA curves of XH (curve 1) and MXH sample 15 (curve 2, Table 1, Supporting Information).

of weight loss) of XH and MXH are 278 and 238 °C, respectively. The 50% weight losses were observed at 287 and 274 °C for XH and MXH, respectively. These results indicated that the thermal stability of MXH is lower than that of XH, which can be explained that more hydrogen bonds in macromolecules were destroyed during dissolution and chemical modification and anhydride groups are not stable.49,50,68

Conclusions The homogeneous esterification of xylan-rich hemicelluloses with maleic anhydride can be carried out in ionic liquid by using LiOH as catalyst. The novel hemicellulosic derivatives with a DS range between 0.095 and 0.75 have been prepared, offering a novel biopolymer for biomaterials. The molar ratio of maleic anhydride/anhydroxylose unit in xylanrich hemicelluloses, reaction temperature, time, and dosage

of catalyst (LiOH) affected the reaction efficiency, especially the DS of the hemicellulosic derivative increased significantly to 0.72 when 0.02 g LiOH was applied (molar ratio of maleic anhydride/anhydroxylose unit in xylan-rich hemicelluloses of 4:1, 80 °C, and 80 min). Results obtained from FT-IR and 13C NMR analysis confirmed the structure of hemicellulosic derivatives with carbon-carbon double bond and carboxyl groups. Thermal analysis showed that the thermal stability of the hemicellulosic derivative was lower than that of the native xylan-rich hemicelluloses. The novel biopolymers with carboxyl groups and carbon-carbon double bond can be used as materials for further copolymerization with monomers at unsaturated double bonds, such as acrylamide, acrylonitrile, crylic acid, and acrylic ester to form a grafted copolymer applied in pharmaceutical, agriculture, food, and wastewater treatment.

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Acknowledgment. This work was supported by the grants from Ministry of Science and Technology (973-2010CB732201/ 4), National Natural Science Foundation of China (No. 30930073 and 31070530), Guangdong Natural Science Foundation (No. 07118057), the subject building funds from GuangDong Province Education Office (x2qsN9100230), and the Fundamental Research Funds for the Central Universities (2009ZM0153 and 2009220043), SCUT. Supporting Information Available. Degree of substitution (DS) of the novel MXH obtained by the reaction of XH with MA in various reaction conditions (Table 1) and average molecular weight of XH and MXH (Table 2), as mentioned in the manuscript. This material is available free of charge via the Internet at http://pubs.acs.org.

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