Synthesis and characterization of cellulose derivatives prepared in NaOH/urea aqueous solutions

We successfully synthesized hydroxypropylcellulose (HPC) and methylcellulose (MC) in high yields from cellulose in 6 wt % NaOH/4 wt % urea aqueous solutions at 25 °C. The cellulose derivatives were characterized with NMR, size exclusion chromatography/laser light scattering, gas chromatography (GC), ultraviolet, and solubility measurements in different solvents. According to the results of solution ¹³C NMR and GC, the individual degree of substitution (DS; i.e., the average number of substituted hydroxyl groups in the monomer unit) at C‐2 hydroxyl groups was slightly higher than the DS values at C‐3 and C‐6 hydroxyl groups for HPC and MC. In comparison with traditional systems, NaOH/urea aqueous solutions were proved to be a stable and more homogeneous reaction medium for preparing cellulose ether with a more uniform microstructure. The low limits for the average number of moles of the substituent groups per monomer unit and the DS value of water‐soluble HPC were 1.03 and 0.85, respectively. MC (DS = 1.48) had good solubility in both water and organic solvents, and the precipitation point occurred at about 67 °C for a 2% (w/v) aqueous solution. In this way, we could provide a simple, pollution‐free, and homogeneous aqueous solution system for synthesizing cellulose ethers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5911–5920, 2004     

Relationship between distribution of substituents and water solubility of O-methyl cellulose

The solubility behavior of O-methyl cellulose (MC) in water was investigated in terms of the distribution of substituents along the cellulose chain as well as in the anhydroglucose (AHG) units. For this purpose, three different types of MC samples were prepared by respective homogeneous reaction, i.e.. (i) methylation of cellulose acetate (CA) prepared from cellulose triacetate (CTA), followed by deacetylation, (ii) methylation of CA prepared by direct acetylation of cellulose in a 10% LiCl–dimethylacetamide (DMAc) solution, followed by deacetylation, and (iii) methylation of cellulose with dimethyl sulfate in a 10% LiCl–DMAc solution. Their water solubility was compared with that of MC samples prepared by the alkali cellulose process, i.e., by the heterogeneous reaction, including commercial products. It was found that water-soluble MC samples prepared by the alkali cellulose process exhibit a thermally-reversible sol-gel transition in aqueous solution, but all of the MC samples preapred homogeneous reactions show a normal phase separation in aqueous solution. This result gives a direct support for the consideration that the highly substituted glucose sequences present in the commercial MC act as “crosslinking loci” on warming. The distribution of substituents in the AHG units was estimated by ¹³C-NMR method. The results on the water solubility of MC were also discussed in terms of the distribution of substituents in the AHG units.     

二乙烯基砜交联的可德胶化学水凝胶的制备与表征

以二乙烯基砜(DVS)作为交联剂通过亲电加成反应制备了可德胶化学水凝胶。采用傅里叶变换红外光谱(FT-IR)、扫描电子显微镜(SEM)和质构分析手段表征了所得凝胶的结构与性能,并对其形貌、溶胀率以及水凝胶的力学性质与制备条件的关系进行了研究。结果表明:所得凝胶网络结构较为致密均匀,溶胀率依赖于交联度。可德胶化学水凝胶的强度和韧性可以通过DVS的用量、可德胶的浓度和碱溶液的浓度进行调控。     

Neutron Total Scattering Investigation of the Dissolution Mechanism of Trehalose in Alkali/Urea Aqueous Solution

The atomic picture of cellulose dissolution in alkali/urea aqueous solution is still not clear. To reveal it, we use trehalose as the model molecule and total scattering as the main tool. Three kinds of alkali solution, i.e., LiOH, NaOH and KOH are compared. The most probable all-atom structures of the solution are thus obtained. The hydration shell of trehalose has a layered structure. The smaller alkali ions can penetrate into the glucose rings around oxygen atoms to form the first hydration layer. The larger urea molecules interact with hydroxide groups to form complexations. Then, the electronegative complexation can form the second hydration layer around alkali ions via electrostatic interaction. Therefore, the solubility of alkali aqueous solution for cellulose decreases with the alkali cation radius, i.e., LiOH > NaOH > KOH. Our findings are helpful for designing better green solvents for cellulose.     

Effect of pulp composition on the characteristics of residuals in CMC made from such pulps

Four different spruce sulphite pulp samples were used for the preparation of carboxymethylcellulose (CMC). The characteristics of the unreacted fibre and particle residuals obtained in the CMC-preparation were used to establish a correlation between the hemicellulose in the pulp and the intrinsic viscosity, i.e.,␣cellulose chain length and the occurence of unreacted residuals. It was shown that the residual particles in the CMC consisted of fibres, fibre fractions and gel particles of different degrees of substitution. The results suggested that pulps with long cellulose chains, i.e., pulps with high intrinsic viscosities, resulted in particles that were more substituted and more swollen. These pulps also resulted in more substituted hemicelluloses in the CMC and more substituted residuals. It was also suggested that galactoglucomannan in the cellulose pulps is favourable for the swelling which results in more substituted hemicelluloses in the CMC and more swollen residuals. The amount of residuals was influenced mainly by the characteristics of the cellulose in the pulp. It is therefore believed that a combination of high viscosity and a suitable combination of hemicelluloses is the most favourable way of eliminating the occurrence of undissolved residuals in CMC.     

Quantitative analysis of cellulose-reducing ends

Methods for the quantification of total and accessible reducing ends on traditional cellulose substrates have been evaluated because of their relevance to enzyme-catalyzed cellulose saccharification. For example, quantification of accessible reducing ends is likely to be the most direct measure of substrate concentration for the exo-acting, reducing end-preferring cellobiohydrolases. Two colorimetric assays (dinitrosalicylic acid [DNS] and bicinchoninic acid [BCA] assay) and a radioisotope approach (NaB³H₄ labeling) were evaluated for this application. Cellulose substrates included microcrystalline celluloses, bacterial celluloses, and filter paper. Estimates of the number of reducing ends per unit mass cellulose were found to be dependent on the assay system (i.e. the DNS and BCA assays gave strikingly different results). DNS-based values were several-fold higher than those obtained using the BCA assay, with fold-differences being substrate specific. Sodium borohydride reduction of celluloses, using cold or radiolabeled reagent under relatively mild conditions, was used to assess the number of surface (solvent-accessible) reducing ends. The results indicate that 30–40% of the reducing ends on traditional cellulose substrates are not solvent accessible; that is, they are buried in the interior of cellulose structures and thus not available to exo-acting enzymes.     

Chemically and physically crosslinked porous gels

Chemically crosslinked gels made by polymerization and copolymerization of trimethylolpropane trimethacrylate have been characterized. Bimodal pore size distributions were obtained. The radii of the large pores (100 Å to 1 μm) varied with concentration while the small ones (20 to 30 Å) were largely independent of the concentration of TRIM. A mechanism of formation of the pores is outlined. Physically crosslinked gels of polyacrylonitrile with highly porous structures were made by cooling polymer solutions in DMF containing varying amounts of water. By controlling the thermal treatment the pore sizes could be varied from 0.1 to 1 μm. Finally the surface modification of cellulose fibers with derivatives of dichlorotriazine is described. It is shown that derivatives containing double bonds in the side chain can form covalent bonds across the interface between cellulose and an unsaturated polyester.     

Polymers as reagents and catalysts. part x. Halogenations of acetophenone and 1,3-diketones with polymer-supported reagents

A crosslinked copolymer of styrene and 4-vinylpyridine (40-43% of monomer units) was reacted with hydrogen iodide to give a polymer containing pyridinium iodide residues. Reaction of this with chlorine in chloroform at 0° gave a polymer containing pyridinium tetrachloroiodate residues. In a similar manner but using methyl iodide in place of hydrogen iodide, crosslinked polymers containing N-methylpyridinium iodide and N-methylpyridinium tetrachloroiodate residues were prepared. The latter contained up to three chlorine molecules per iodine atom. Both reagents reacted with acetophenone, thus forming iodomethyl-phenyl ketone ($\text{4}$) and chloromethyl-phenyl ketone ($\text{5}$), the ratios depending on the reagent used and the reaction time. Chlorinations of 5,5-dimethyl cyclohexane-l,3-dione and indane-1,3-dione with polymer-supported reagent ($\text{2}$) resulted in the formation of geminal dichlorides in high yields.     

Glucose,not cellobiose,is the repeating unit of cellulose and why that is important

Despite nomenclature conventions of the International Union of Pure and Applied Chemistry and the International Union of Biochemistry and Molecular Biology, the repeating unit of cellulose is often said to be cellobiose instead of glucose. This review covers arguments regarding the repeating unit in cellulose molecules and crystals based on biosynthesis, shape, crystallographic symmetry, and linkage position. It is concluded that there is no good reason to disagree with the official nomenclature. Statements that cellobiose is the repeating unit add confusion and limit thinking on the range of possible shapes of cellulose. Other frequent flaws in drawings with cellobiose as the repeating unit include incorporation of O-1 as the linkage oxygen atom instead of O-4 (the O-1 hydroxyl is the leaving group in glycoside synthesis). Also, n often erroneously represents the number of cellobiose units when n should denote the degree of polymerization i.e., the number of glucose residues in the polysaccharide.     

Crosslinking and structural changes of cellulose fibers

The supermolecular structure of various cellulose fibers modified with crosslinking reagents has been investigated by electron microscopy methods. The density, degree of crystallinity (DC), and length changes in alkaline solutions were measured for the modified celluloses. The samples treated with monofunctional analogs of the crosslinking reagents as well as the fiber preparations containing linear and network polymer were also investigated. Three main problems are suggested for the discussion: (1) the general regularities of the structural changes in cellulose in the process of crosslinking; (2) the specific features of the structural changes, as observed in different cellulose samples; (3) the relation between the degree of modification, the type of modifying reagent, and the structure of the crosslinked cellulose. The characteristic structural changes, i.e., the increase in the thickness of fragments, the specific cogged edges, the increase in the lateral dimensions of structural elements all seem to be most representative in native cellulose fibers and are perfectly well distinguished. Similar changes are found in viscose fibers but are less clearly defined. Crosslinking proceeds rather uniformly through the whole of the fiber cross section. It appeared to be most evident when the cross sections are treated with solvents, or when etched in gaseous discharge. Only in cases when the modification is performed in nonaqueous solutions does the reaction proceed mainly in the peripherial regions of the fiber. In fibers subjected to strong swelling, crosslinking results in a real increase in the lateral dimensions of the microfibrils, with the layer thicknesses remaining the same. As a rule, the modification does not imply significant changes in the fiber surface. The crystallite size decreases in the process of crosslinking. This appears to be peculiar to viscose fibers, especially to those subjected to crosslinking in the swollen state. The degree of crystallinity and density of the fibers decrease sharply, which seems to be especially evident in epichlorohydrin-modified samples. Cellulose structure remains unchanged when linear or network polymer forms in the fiber or when the samples are treated with monofunctional reagents. Changes in properties and structure of cellulose caused by crosslinking are most apparent if elongation of the fibers in alkaline solution before and after the modification is compared.     

Click Chemistry with Polysaccharides

Summary: The copper‐catalyzed Huisgen reaction as a typical example of click chemistry was realized with the polysaccharide cellulose for the first time. The generality, selectivity, and the efficiency of click chemistry perfectly fit the requirements of polysaccharide modification, which is demonstrated by the introduction of triazole‐spacer bound functional groups, i.e., carboxylic ester, thiophene, and aniline moieties. Azide moieties introduced into cellulose via the tosyl derivative were simply transferred with ethynyl compounds under Cu(I) catalysis and mild and easily applicable conditions. Hydrolytically stable cellulose derivatives soluble in organic solvents, e.g., DMSO or DMF with DS up to 0.9 are obtained. The triazole substituted cellulose derivatives were characterized by elemental analysis, FTIR, ¹H NMR, and ¹³C NMR spectroscopies and show no impurities or substructures resulting from side reactions.

6‐Azido‐6‐deoxy cellulose.     

Carbamoylation Applied for Structure Determination of Cellulose Derivatives

Carbamoylation of cellulose esters (CE) and investigation of the mixed derivatives obtained with NMR spectroscopy represents a useful analytical tool for the determination of the degree of substitution (DS) and analysis of the distribution of substituents on the level of the anhydroglucose unit (AGU). Especially the carbethoxymethylcarbamoylation and the ethylcarbamoylation of CE combined with ¹H NMR spectroscopy are efficient and inexpensive ways to gain information on the over-all DS and partial DS values in position 2, 3, and 6 of the AGU. Complete subsequent phenylcarbamoylation can be achieved even for CE with bulky substituents, e.g., adamantanecarboxylic acid esters. In addition to NMR experiments the carbamoylated CE were studied by HPLC after complete chain degradation. Carbethoxymethylcarbamoylation has turned out to be the most useful tool for this path. Chromatograms comparable to carboxymethylated cellulose (CMC) were obtained, which can be exploited to calculate the mole fractions of the basic building units (un-, mono-, di- and tri-substituted glucoses) of the polymer. Comparison with statistic calculations gave a first hint on the distribution of substituents along the polymer chain. For a commercial cellulose diacetate a statistic pattern of substitution was determined.     

Efficient synthesis of cellulose furoates in 1-<Emphasis Type="Italic">N</Emphasis>-butyl-3-methylimidazolium chloride

The ionic liquid 1-N-butyl-3-methylimidazolium chloride ([C₄mim]⁺Cl⁻) was investigated as reaction media for the homogeneous acylation of cellulose with 2-furoyl chloride in the presence of pyridine. The preparation of cellulose furoate depending on the reaction conditions, the cellulose type and the pyridine content was studied. Cellulose furoates with a degree of substitution in the range from 0.46 to 3.0 were accessible, i.e., under mild conditions, with a low excess of reagent and in a short reaction time. The products were characterized by elemental analysis, perpropionylation, ¹H- and ¹³C NMR spectroscopy and FTIR spectroscopy. Thomas Heinze is the member of the European Polysaccharide Network of Excellence (EPNOE), www.epnoe.eu     

室温下三光气作环合剂合成5-取代-1,3,4-噁二唑-2-酮类化合物

1,3,4-噁二唑具有抗菌,抗肿瘤,抗惊厥等生物活性~([1-6]),在药物化学方面具有重要的理论意义和实际应用价值.1,3,4-噁二唑类材料具有优良的荧光性和闪烁性和优异的电子传输性能,以及特殊的稳定性和良好的耐热性,被认为是下一代显示器件中载流子输送的首选材料~([7-8]).     

Base-free NIS promoted electrophilic cyclization of alkynes: an efficient synthesis of iodo substituted pyrano[4,3-b]quinolines

A simple and mild procedure for the synthesis of iodo substituted 1H-pyrano[4,3-b]quinolines has been achieved using NIS reagent in the absence of base from 2-alkynylquinoline-3-carboxaldehydes via intramolecular electrophilic cyclization onto alkynes in good to excellent yields in a short duration of time. The reactions proceeded smoothly in a normal solvent in aerobic atmosphere at room temperature. The presence of substituent at either quinoline or alkyne moieties did not show effect on reaction rate of cyclization. The palladium-catalyzed transformations of iodo group to C-C bond are also discussed.     

Mineralized Polyvinyl Alcohol/Sodium Alginate Hydrogels Incorporating Cellulose Nanofibrils for Bone and Wound Healing

Bio sustainable hydrogels including tunable morphological and/or chemical cues currently offer a valid strategy of designing innovative systems to enhance healing/regeneration processes of damaged tissue areas. In this work, TEMPO-oxidized cellulose nanofibrils (T-CNFs) were embedded in alginate (Alg) and polyvinyl alcohol (PVA) solution to form a stable mineralized hydrogel. A calcium chloride reaction was optimized to trigger a crosslinking reaction of polymer chains and mutually promote in situ mineralization of calcium phosphates. FTIR, XRD, SEM/EDAX, and TEM were assessed to investigate the morphological, chemical, and physical properties of different mineralized hybrid hydrogels, confirming differences in the deposited crystalline nanostructures, i.e., dicalcium phosphate dehydrate (DCPDH) and hydroxyapatite, respectively, as a function of applied pH conditions (i.e., pH 4 or 8). Moreover, in vitro tests, in the presence of HFB-4 and HSF skin cells, confirmed a low cytotoxicity of the mineralized hybrid hydrogels, and also highlighted a significant increase in cell viability via MTT tests, preferentially, for the low concentration, crosslinked Alg/PVA/calcium phosphate hybrid materials (<1 mg/mL) in the presence of hydroxyapatite. These preliminary results suggest a promising use of mineralized hybrid hydrogels based on Alg/PVA/T-CNFs for bone and wound healing applications.     

Biofunctional paper via the covalent modification of cellulose

Paper-based analytical devices are the subject of growing interest for the development of low-cost point-of-care diagnostics, environmental monitoring technologies, and research tools for limited-resource settings. However, there are limited chemistries available for the conjugation of biomolecules to cellulose for use in biomedical applications. Herein, divinyl sulfone (DVS) chemistry was demonstrated to immobilize small molecules, proteins, and DNA covalently onto the hydroxyl groups of cellulose membranes through nucleophilic addition. Assays on modified cellulose using protein-carbohydrate and protein-glycoprotein interactions as well as oligonucleotide hybridization showed that the membrane's bioactivity was specific, dose-dependent, and stable over a long period of time. The use of an inkjet printer to form patterns of biomolecules on DVS-activated cellulose illustrates the adaptability of the DVS functionalization technique to pattern sophisticated designs, with potential applications in cellulose-based lateral flow devices.     

Reaction Acceleration in Thin Films with Continuous Product Deposition for Organic Synthesis

Thin film formats are used to study the Claisen–Schmidt base‐catalyzed condensation of 6‐hydroxy‐1‐indanone with substituted benzaldehydes and to compare the reaction acceleration relative to the bulk. Relative acceleration factors initially exceeded 10³ and were on the order of 10² at steady state, although the confined volume reaction was not electrostatically driven. Substituent effects were muted compared to those in the corresponding bulk and microdroplet reactions and it is concluded that the rate‐limiting step at steady state is reagent transport to the interface. Conditions were found that allowed product deposition from the thin film to occur continuously as the reaction mixture was added and as the solvent evaporated. Yields of 74 % and production rates of 98 mg h⁻¹ were reached in a very simple experimental system that could be multiplexed to greater scales.     

Convenient Esterification of Carboxylic Acids by SN2 Reaction Promoted by a Protic Ionic-Liquid System Formed in Situ in Solvent-Free Conditions

The reaction of esterification of benzoic acid with benzyl chloride was chosen as a model reaction to study the esterification by S_N2 promoted by tertiary amine as deprotonating agent. The use of ionic liquid (IL) 1,3-dimethylimidazolium methanesulfonate [MMIm][OMs] as reaction medium has proven to give quantitative yield of the ester, but interestingly the reaction does occur even in solvent-free conditions, where the acid + the amine form a liquid system (a protic IL) in situ. This last methodology was extended to several carboxylic acids in conditions of atom economy (i.e., without excess of any reagent), giving moderately good yields of esters (54–78%) recovered by weight in pure form.     

Substituent Distribution in Cellulose Acetates: Its Control and the Effect on Structure Formation in Solution

Architecture-regulated cellulose derivatives were prepared by regioselective substitution of C-2, -3, and -6 position hydroxyls in cellulose by O-acetyls or trityls. Their dynamic structures formed in polar solvents were compared with those by the commercial cellulose acetates where the distribution of hydroxyls and O-acetyls was nearly random in the chain. It was found that the difference in the chain architecture induces a large difference in the chain conformation, the solubility, and the clustering mechanism and structures; i.e., they changed with solvent quality, concentration, temperature, strength of external field, and so on. Copyright 2000 Academic Press.