Dissolution, derivatization, and functionalization of chitin in ionic liquid

Accepted Manuscript Dissolution, derivatization, and functionalization of chitin in ionic liquid

Jun-ichi Kadokawa PII: DOI: Reference:

S0141-8130(18)35409-6 https://doi.org/10.1016/j.ijbiomac.2018.11.165 BIOMAC 11042

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

9 October 2018 14 November 2018 17 November 2018

Please cite this article as: Jun-ichi Kadokawa , Dissolution, derivatization, and functionalization of chitin in ionic liquid. Biomac (2018), https://doi.org/10.1016/ j.ijbiomac.2018.11.165

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ACCEPTED MANUSCRIPT

Dissolution, derivatization, and functionalization of

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chitin in ionic liquid

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Jun-ichi Kadokawa*

Department of Chemistry, Biotechnology, and Chemical Engineering, Graduate School of

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Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065,

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Japan

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∗Corresponding author. E-mail address: [email protected] (J. Kadokawa).

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Keywords: acylation; graft polymerization; ion gel ABSTRACT

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In this article, a review of the researches, which endeavor concerning ionic liquids, used as

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media for dissolution, derivatization, and functionalization of chitin is presented. Although chitin has been noted to show poor solubility and processability, leading to mostly an unutilized biomass resource, some ionic liquids have been found to dissolve chitin. For example, an ionic liquid, 1-allyl-3-methylimidazolium bromide (AMIMBr), was reported to dissolve chitin in concentrations up to 4.8 wt% and to form ion gels at higher contents of chitin. A cellulose/chitin binary ion gel was fabricated from the individually prepared cellulose/chitin solutions with ionic liquids, which was further converted into a binary film by regeneration. The binary ion gel was

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ACCEPTED MANUSCRIPT applied as a novel electrolyte for an electric double layer capacitor. Acetylation of chitin using acetic anhydride in AMIMBr gave chitin acetates with high degrees of substitution. The acetylation method has been extended to synthesize several chitin acylates using acyl chlorides after optimization of the conditions. The derivatization technique of chitin in AMIMBr was

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applied to the synthesis of a chitin macroinitiator for atom transfer radical polymerization

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(ATRP). Grafting of styrene by ATRP from the resulting macroinitiator was thus conducted to

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give chitin-graft-polystyrene.

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1. Introduction

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Chitin is a natural aminopolysaccharide composed of (14)-linked N-acetyl-D-glucosamine

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repeating units (Fig. 1(a)) [1-3]. Although it is a very important biomass resource because of its huge annual production in nature, chitin still remains an unutilized biomass resource, primarily

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due to its intractable bulk structure and insolubility in water and common organic solvents.

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Accordingly, the research on its processing to new chitin-based functional materials through

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proper dissolution processes has attracted much attention even in recent years [4]. In the limited solvents systems for chitin, 5-7% LiCl/N,N-dimethylacetamide (DMAc) [5] and CaCl2.2H2O-

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saturated methanol [6-9] have been extensively used for the dissolution of chitin. Although acyl derivatives of polysaccharides have been identified one of the useful functional materials, which exhibit high performance properties and are used in practical applications, as representatively seen in cellulose acylates [10], chitin acylates have hardly been employed in practical application. This is principally because efficient acylation methods for chitin from native sources to be substantially employed in practical fields have hardly been developed in previous investigations

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ACCEPTED MANUSCRIPT [1, 11-14]. In the two representative crystalline structures of chitin, that is, - and -chitins with stable antiparallel and metastable parallel chain alignments (Fig. 1(b)), furthermore, the former material shows lower solubility than the latter one [2, 3]. Owing to such strict poor solubility of -chitin in organic solvents, its acylation has been performed principally under heterogeneous

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conditions or in strong acidic solvents, resulting in low yields, low degrees of substitution (DSs),

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and lowering molecular weights of chitin chains [13]. Besides a chitin acetate, the most well-

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known acyl derivative [1, 13, 15], particularly, the limited studies on the synthesis of chitin

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acylates with different substituents from -chitin have been reported so far [16-21]. Because of a much weaker intermolecular forces of -chitin, it has shown a higher reactivity than -chitin.

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For example, in acetylation of -chitin, a fully acetylated chitin was readily obtained when -

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chitin was treated with acetic anhydride in pyridine in the presence of N,N-dimethyl-4aminopyridine (DMAP) [22]. Modification of polysaccharides such as cellulose by graft

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polymerization via atom transfer radical polymerization (ATRP) provides a significant route to

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combine both the advantageous properties of natural and synthetic polymers to produce functional composite materials in a wide range of potential applications [23, 24]. ATRP is a

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robust and versatile technique to accurately control chain length and polydispersity of the

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resulting polymers, and thus has been used to synthesize a wide range of copolymers with the controlled structure, unit length, and block sequence [25, 26]. However, graft ATRP on chitin chains has hardly been reported so far owing to its poor solubility. Ionic liquids, which are low melting-point molten salts, are identified as good solvents for natural polysaccharides [27-36], since Rogers et al. have reported the dissolution of cellulose in an ionic liquid, 1-butyl-3-methylimidazolium chloride (BMIMCl) [37]. Furthermore, various types of chemical derivatizations and modifications of cellulose in ionic liquids have also been

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ACCEPTED MANUSCRIPT studied such as acetylation, carbanilation, carboxymethylation, and so on [28, 29, 32, 38]. However, only the limited investigations have been reported regarding the dissolution of chitin with ionic liquids even in recent years [33-36, 39, 40]. Accordingly, researches concerning the dissolution of chitin with proper ionic liquids have been being increasingly much attention to

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fabricate new chitin-based functional materials.

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In 2009, the author fortunately found that an ionic liquid, 1-allyl-3-methylimidazolium

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bromide (AMIMBr), dissolved -chitin in concentrations up to 4.8 wt% by a simple operation

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(Fig. 2) [41]. Since this finding, derivatization and functionalization of -chitin have been investigated through dissolution and gelation with this ionic liquid [42]. In this review article, the

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author presents the use of AMIMBr as a powerful media for materialization of -chitin such as

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derivatization and functionalization.

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2. Dissolution and gelation of chitin with ionic liquids

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Different from cellulose, ionic liquids, which dissolve chitin, have hardly been found until

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almost ten years ago. For example, when the dissolution behavior of chitin in a series of alkylimidazolium chloride and dimethyl phosphate and 1-allyl-3-methylimidazolium acetate has

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been investigated, the former two series of the ionic liquids did not dissolve the certain amounts of chitin (less than 1.5 wt%), but the latter ionic liquid was found to dissolve chitin in 5 wt% (Fig. 2) [34]. Furthermore, the dissolution behavior was affected by the degree of deacetylation, the degree of crystallinity, and the molecular weight. Besides, 1-butyl- and 1-ethyl-3methylimidazolium acetates (BMIMOAc and EMIMOAc, respectively), have also been found to dissolve chitin in certain concentrations (Fig. 2) [34, 43, 44]. BMIMOAc was reported to dissolve both - and -chitins with different molecular weights at relatively lower temperature.

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ACCEPTED MANUSCRIPT A cooling process of the chitin/BMIMOAc solutions to ambient temperature resulted in corresponding chitin/BMIMOAc gels, which further formed chitin sponge and film materials by regeneration using water or methanol coagulant. Because it was also found that EMIMOAc dissolved chitin, extraction of chitin from raw crustacean shells, such as shrimp shell, was

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demonstrated using EMIMOAc. EMIMOAc has also been used for the formation of gels and

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films from chitin [45-47].

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Recently, some tetrabutylphosphonium amino acid salts and 1-ethyl-3-methylimidazolium alkanoates were found to dissolve chitin (Fig. 2) [48]. The dissolution of chitin with deep

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eutectic solvents (DESs), that is, ionic liquid analogs, composed of mixtures of choline halide-

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urea, chlorocholine chloride-urea, and choline chloride-thiourea were also investigated [49]. Consequently, maximum dissolution of chitin (9% w/w) was obtained in the choline chloride-

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thiourea system.

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The author has reported that another ionic liquid, AMIMBr, dissolves chitin and the maximum concentration for the dissolution is 4.8 wt% by heating at 100 oC for 48 h (Fig. 3) [41].

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Furthermore, we investigated the molecular dynamics (MD) simulation to evaluate the

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dissolution behavior of chitin crystal in AMIMBr [50]. It was observed by the MD simulation result in AMIMBr that the chitin chains were peeled from the crystal surface, accompanied with

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cleavage of hydrogen bonds. The MD result also revealed the following dissolution process, in which Br- contributed to cleavage of intermolecular hydrogen bonds, whereas AMIM+ prevented to return to the crystalline phase after the peeling. On the other hand, other imidazolium bromides, such as 1-methyl-3-propylimidazolium and 1-butyl-3-methylimidazolium bromides, did not dissolve chitin at all in dissolution experiments under the same conditions. The above dissolution results have suggested that the allyl substituent in AMIMBr strongly affects the

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ACCEPTED MANUSCRIPT dissolution ability, although the reason, why AMIMBr specifically dissolves chitin, has not been yet clear. When the larger amounts of chitin (6.5-10.7 wt%) were successively immersed in AMIMBr at room temperature, heated at 100 oC, and cooled to room temperature, gel-like materials (ion gels) with higher viscosity were totally formed (Fig. 3). The dynamic rheological

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measurement showed that both the 4.8 wt% and 6.5 wt% liquids of chitin with AMIMBr

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behaved as the weak gels.

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An attempt was made to fabricate a cellulose/chitin binary ion gel with the two ionic liquids, BMIMCl and AMIMBr [51]. A 9.1 wt% cellulose solution with BMIMCl and a 4.8 wt% chitin

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solution with AMIMBr were first mixed at 100 oC to obtain a homogeneous solution. The

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resulting solution was left standing at room temperature for 4 days, followed by washing with ethanol, to form the cellulose/chitin binary ion gel with excluding the excess ionic liquids (Fig.

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4). A cellulose/chitin binary film was fabricated by regeneration from the thinly prepared binary

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ion gel (Fig. 4) [52]. The cellulose/chitin binary ion gel was employed as a novel electrolyte for an electric double layer capacitor (EDLC). The binary ion gel was first treated with 2.0 mol/L

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H2SO4 aqueous solution for 3 h to give an acidic binary gel [53-55]. Electrochemical

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characteristics of the resulting acidic gel electrolyte were evaluated by galvanostatic chargedischarge measurements. The test cell with the acidic binary gel electrolyte exhibited a specific

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capacitance of 162 F/g at room temperature, which was higher than that for a cell with an ordinal H2SO4 electrolyte (155 F/g). The acidic binary gel electrolyte showed the excellent high-rate discharge capability in a wide range of current densities as well as H2SO4 aqueous solution. In addition, the discharge capacitance of the test cell retained over 80 % of its initial value in 10 5 cycles even at a high current density of 5000 mA/g.

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ACCEPTED MANUSCRIPT 3. Acylation of chitin in AMIMBr Acetylation of -chitin using acetic anhydride in AMIMBr has been investigated to obtain a chitin acetate (Fig. 5, m = 0) [56]. The reactions were carried out using acetic anhydride (5-20 equiv. with a repeating unit) in 2 wt% AMIMBr solution at 60-100 oC for 24 h. The IR spectra of

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the most products, isolated as fractions insoluble in methanol, observed carbonyl absorptions

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assignable to ester linkage, indicating the occurrence of acetylation. The DS values, which were determined by the IR spectra, increased with increasing the amounts of acetic anhydride used for

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the reaction. The highest DS value was 1.86, which was obtained by using 20 equiv. of acetic anhydride under the conditions at 100 oC (entry 1). As the product with such a high DS value

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was soluble in DMSO, the structure was further confirmed by the 1H NMR measurement in

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DMSO-d6. Consequently, the NMR result fully supported the structure of the chitin acetate. The DS value estimated by the integrated ratio of the signals due to acetyl protons to the signal due to

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anomeric protons to be 1.9, which was in good agreement with that determined from the IR

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spectrum.

The acetylation reaction method of -chitin in AMIMBr was extended to synthesize several

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chitin acrylates having different substituents (Fig. 5) [57]. Lauroylation of -chitin using lauric

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anhydride was first attempted according to the reaction manner of the above acetylation in AMIMBr (Fig. 5, m = 10). A mixture of lauric anhydride (20 equiv. with a repeating unit) with 2 wt% chitin/AMIMBr solution was heated at 100 oC for 24 h for the progress of lauroylation. The IR spectrum of the product, which was isolated as a fraction insoluble in water and diethyl ether, exhibited a carbonyl absorption ascribed to ester linkage, suggesting the occurrence of lauroylation. However, the DS value, which was estimated based on the method for calculation of degree of acetylation from the ratio of two carbonyl absorptions due to ester and amido I, was

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ACCEPTED MANUSCRIPT not high (0.54, entry 2). When the reaction was carried out using the other lauroylation reagent, that is, lauroyl chloride (20 equiv. with a repeating unit), the ester carbonyl absorption in the IR spectrum of the product, which was isolated as a fraction insoluble in ethanol, obviously increased, strongly suggesting the production of the higher DS derivative, probably owing to the

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higher reactivity of lauroyl chloride than that of lauric anhydride. Indeed, the DS value of the

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product, estimated by the IR analysis, dramatically increased to be 1.1 (entry 3). When the

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lauroylation of -chitin using lauroyl chloride was further carried out in the presence of pyridine/DMAP as base/catalyst, the DS value of the product increased to be 1.3 (entry 4). As

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the high DS product was soluble in a CHCl3/CF3CO2H mixed solvent (1/1 in volume), the 1H

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NMR measurement of the product was conducted in a CDCl3/CF3CO2H mixed solvent (1/1 in volume). The DS value was calculated from the integrated ratio of the methyl signal of lauryl

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group to the H1-H6 signals of the chitin chain to be 1.9, indicating that hydroxy groups in the

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product were mostly lauroylated under the present reaction conditions. The abovementioned DS value, estimated by the IR analyses, was not in agreement with that calculated by the 1H NMR

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spectrum, and probably underestimated, owing to the fact that the IR method was based on the

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calibration for degree of acetylation.

Acylation of -chitin using various acyl chlorides (hexanoyl, octanoyl, myristoyl, stearoyl (Fig.

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5, m = 4, 6, 12, 16, respectively), and oleoyl chlorides, 20 equiv. with a repeating unit) was then performed under such optimal conditions in the presence of pyridine/DMAP in AMIMBr at 100 o

C for 24 h (entries 5-9). The IR spectra of all the isolated products obviously showed the

detection of C=O absorptions assignable to ester groups, suggesting the efficient occurrence of acylation using all acyl chlorides under the present conditions. As all the products were soluble in CHCl3/CF3CO2H mixed solvents, the 1H NMR analysis was conducted in CDCl3/CF3CO2H

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ACCEPTED MANUSCRIPT mixed solvents to confirm the structures further, which supported the high DS products in all cases (DSs = 1.6 – 2.0). For example, Fig. 6 shows the 1H NMR spectrum of the product (entry 5) by hexanoylation using hexanoyl chloride in a CDCl3/CF3CO2H mixed solvent (1/4 in volume). Besides the signals a-d due to hexanoyl groups, the seven signals ascribable to the H1-

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H6 sugar protons were clearly detected, indicating the production of a chitin hexanoate with the

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signals of chitin chain, the DS value was calculated to be 1.7.

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high DS value. From the integrated ratio of the methyl signal of hexanoyl group to the H1-H6

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4. Graft polymerization from chitin through functionalization of initiating groups in

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AMIMBr

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The above homogeneous derivatization approach of -chitin in AMIMBr as the reaction media was employed to synthesize a chitin macroinitiator for subsequent graft ATRP to produce chitin-

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based composite materials with synthetic polymers [58]. As ATRP is initiated from -

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haloalkylacyl groups, the chitin macroinitiator modified with such initiating groups was synthesized by acylation of hydroxy groups in chitin with 2-bromopropyl bromide as the

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acylation reagent in AMIMBr according to the similar reaction manner of the abovementioned

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acylation (Fig. 7). The DS value of the product, which was synthesized by the reaction using 30 equiv. of 2-bromopropyl bromide with a repeating unit at 100 oC for 24 h, was estimated by the 1

H NMR measurement in DMSO-d6 to be 1.86. Graft polymerization of styrene, a representative

monomer for ATRP, was then carried out by ATRP from the resulting chitin macroinitiator using CuBr and N,N,N’N’,N”,N”-pentamethyldiethylene triamine (PMDETA) as the catalyst system to obtain chitin-graft-polystyrene (Fig. 7). The yields of the products, which were isolated as fractions insoluble in water, increased with increasing monomer/initiating site feed ratios. The

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ACCEPTED MANUSCRIPT graft chains were separated from the products by alkaline hydrolysis of ester linkages at root on the chitin chain to estimate their Mn values by GPC measurement. The GPC results indicated that the Mn values increased with increasing the monomer/initiating site feed ratios, indicating the progress of the graft polymerization in living manner.

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5. Conclusions

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In this review article, an overview of the research endeavors on the derivatization and functionalization of chitin through the dissolution and gelation in AMIMBr is presented. Chitin-

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based materials, e.g., ion gels, acyl derivatives, and graft materials, have efficiently been

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fabricated using the AMIMBr media. For example, the efficient acylation of chitin was achieved in AMIMBr under homogeneous conditions to obtain chitin acylates with high DS values, and

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the acylation method was further applied to the synthesis of the chitin macroinitiator for ATRP.

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The macroinitiator was then used for grafting of styrene on chitin by ATRP. Chitin is one of the most abundant organic resources comparable to cellulose, and accordingly is expected to be

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incorporated in functional bioactive and tissue materials because of its biocompatibility. As the

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fabrication of chitin-based materials using ionic liquids have significantly been developed in recent years as representatively discussed in this review, such materials will be used in the

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application fields related to biomedical and environmental benign industries in the future. References

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Table 1. Acylation of -chitin using acid anhydrides or acyl chlorides in AMIMBra (adapted from Refs. [56] and [57]) Pyridine/DMAPb

DSc

1

Acetic anhydride

-

1.9 (1.86)

2

Lauric anhydride

-

3

Lauroyl chloride

-

4

Lauroyl chloride

+

5

Hexanoyl chloride

6

Octanoyl chloride

7

Myristoyl chloride

8 9

Oleoyl chloride

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Acylation reagent

(1.1)

1.9 (1.3) 1.7

+

2.0

+

1.6

Stearoyl chloride

+

1.6

+

1.8

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+

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(0.54)

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a

Entry

At 100 oC for 24 h in a 2 wt% AMIMBr solution. Acylation reagent; 20 equiv. with a repeating

unit. b

Pyridine; 10 equiv., DMAP; 0.25 equiv. with a repeating unit.

c

DS; degrees of substitution, which were determined by 1H NMR spectra. Values in parentheses

were determined by IR spectra.

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Fig. 1. (a) Chemical structure of chitin and (b) schematic images for representative crystalline

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structures, - and -chitins.

Fig. 2. Representative ionic liquids, which dissolve chitin.

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Fig. 3. Dissolution and gelation of chitin with AMIMBr.

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Fig. 4. Formation of cellulose/chitin binary ion gel, followed by regeneration for fabrication of binary film.

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Fig. 5. Acylation of chitin using acid anhydrides or acyl chlorides in AMIMBr.

Fig. 6. 1H NMR spectrum of chitin hexanoate (entry 5) in CDCl3/CF3CO2H (1/4 in volume).

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CH

H

O

C

Br CH3

Chitin AMIMBr

OR1 O R1O

Styrene PMDETA / CuBr

O NHAc n

O R2O

DMSO

Chitin macroinitiator H C

C

O

H

Br R2 = H,

CH3

C C

N

N

CH3 N CH3

CH2

CH

Br

CH3

p

US

H3C

CR

CH3 H3C

NHAc n

Chitin-graft-polystyrene

O

PMDETA =

O

T

R1 = H,

OR2

IP

Br C

AC

CE

PT

ED

M

AN

Fig. 7. Synthesis of chitin mancroinitiator in AMIMBr and following graft ATRP of styrene.

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ACCEPTED MANUSCRIPT Highlights Dissolution, derivatization, functionalization of chitin in ionic liquid are dealt. 1-Allyl-3-methylimidazolium bromide dissolves chitin and forms ion gels.

IP

T

Cellulose/chitin binary ion gel and film were fabricated using ionic liquids.

CR

Acylation of chitin in ionic liquid was achieved to obtain several chitin acrylates.

AC

CE

PT

ED

M

AN

US

Chitin macroinitiator for grafting was prepared by acylation in ionic liquid.

17