Nitroxide-mediated oxidation of cellulose using TEMPO derivatives: HPSEC and NMR analyses of the oxidized products

Regenerated cellulose (viscose rayon) was oxidized using NaBr, NaClO and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) or one of ten related nitroxyl radicals in water at pH 10–11. The C6 primary hydroxyl groups in rayon were oxidized to carboxyl groups in most cases, thus giving water-soluble products. However, the oxidation times required for complete dissolution of the products varied substantially, depending on the nitroxyl radical used. Weight average degrees of polymerization (DPw) of the oxidized products were determined by means of high performance size exclusion chromatography (HPSEC) using pullulan standards. All the products had bimodal HPSEC distribution patterns, probably reflected by the solid-state structure of viscose rayon. When 4-acetamido-TEMPO and 4-carboxy-TEMPO were used, cellouronic acids having almost homogeneous chemical structures with higher DPw than for TEMPO were obtained quantitatively within 30 min. The oxidations using 4-amino-TEMPO, 4-carboxy-PROXYL and 4-carbamoyl-PROXYL gave cellouronic acids having the highest DPw, although reaction times of more than 4 h were required, and some side reactions occurred on the products.     

TEMPO-oxidation of cellulose: Synthesis and characterisation of polyglucuronans

A series of pseudo amorphous cellulose samples were reacted with catalytic amounts of 2,2,6,6-tetramethyl-1-piperidine oxoammonium salt (TEMPO), sodium hypochlorite and sodium bromide in water. In all samples the primary alcohol groups were selectively oxidised into carboxyl groups, and several water-soluble polyglucuronic acid sodium salts were obtained with different molecular weights. With this reaction system, the degradation of the amorphous cellulose samples may be minimized, provided the oxidation is performed at 4°C and at constant pH 10, with controlled amounts of TEMPO and sodium hypochlorite.     

Water dispersion of cellulose II nanocrystals prepared by TEMPO-mediated oxidation of mercerized cellulose at pH 4.8

Mercerized wood cellulose was oxidized by 4-acetamide-TEMPO/NaClO/NaClO₂ system at 60 °C and pH 4.8 for 1–5 days. Mostly individual nanocrystals 4–7 nm in width and 100–200 nm in length were obtained by ultrasonication of the oxidized product in water. The nanocrystals had the cellulose II structure, and carboxylate contents of 2.0–2.4 mmol/g, indicating that these carboxylate groups were selectively formed on the cellulose II crystallite surfaces in mercerized cellulose. Moreover, the original wood cellulose and mercerized cellulose were acid-hydrolyzed, and then subjected to the TEMPO-mediated oxidation under the same conditions at pH 4.8 to prepare reference samples. TEM images, light transmittance and rheological properties of water dispersions showed that the nanocrystals prepared from mercerized cellulose by the TEMPO oxidation and sonication in water had the highest dispersibility of individual nanocrystals with less amounts of bundles in water, resulting from the highest carboxylate contents.     

Depolymerization of cellouronic acid during TEMPO-mediated oxidation

The mechanism of partial depolymerization of cellouronic acid (-1,4-linked polyglucuronic acid sodium salt) during 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation of cellulose was studied by treating cellouronic acid with one reagent or a combination of TEMPO-NaBr-NaClO under various conditions. Although NaClO, NaBrO and an alkali at pH 11 brought about partial depolymerization of cellouronic acid, the use of these reagents themselves did not seem to be the primary reason for depolymerization. On the other hand, when all the reagents, i.e. TEMPO-NaBr-NaClO, were applied to cellouronic acid at pH 11, a remarkable decrease in weight-average degree of polymerization (DPw) from 430 to ca. 20 was observed within the initial 10 min. Probably hydroxyl radicals formed from NaBrO and TEMPO at pH 10–11 cause the depolymerization during the oxidation. Some radical scavengers were then used for the TEMPO-mediated oxidation of cellulose in order to suppress the depolymerization. Although the addition of crotonic acid under certain conditions gave cellouronic acid having higher DPw, none of the radical scavengers examined so far could completely prevent the depolymerization. When regenerated celluloses having higher DP were used as the starting materials, cellouronic acids having DPw of more than 1000 were obtained, although they still had large low-molecular-weight fractions.     

Synthesis and characterization of oxidized cellulose

Samples of oxidized cellulose (OC) with various carboxyl contents and degrees of crystallinity were obtained by the oxidation of native and mercerized cellulose with a solution of nitrogen(IV) oxide in CCl₄. A detailed characterization of these OC samples was performed. The effect of oxidation conditions (concentration of N₂O₄ in the solution and oxidation time) and starting cellulose material on OC characteristics (carboxyl, carbonyl and nitrogen content, degree of crystallinity and polymerization, surface area and swelling, and acidic properties) was investigated. Reactivity in the oxidation process was higher in mercerized cellulose than in native cellulose. The action of dilute solutions (10–15%) of N₂O₄ did not affect the degree of crystallinity of cellulose samples. Under these conditions, the oxidation took place mainly in amorphous regions and on the surface of crystallites. Oxidation in a concentrated (40%) N₂O₄ solution led to the destruction of crystallites, which increased the surface area and swelling of cellulose in water. The surface area and the swelling of OC samples increased with a decrease in the index of crystallinity. The acidic properties of OC were shown to increase with an increase of swelling in water. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4785–4791, 2004     

Oxidation of wood cellulose using 2-azaadamantane N-oxyl (AZADO) or 1-methyl-AZADO catalyst in NaBr/NaClO system

A wood cellulose was oxidized with catalytic amounts of 2-azaadamantane N-oxyl (AZADO) or 1-methyl-AZADO, in an NaBr/NaClO system, in water at pH 10. The oxidation efficiency, carboxylate/aldehyde contents, and degree of polymerization (DP_v) of the oxidized celluloses thus obtained were evaluated in terms of the amount of AZADO or 1-methyl-AZADO catalyst added, in comparison with those prepared using the TEMPO/NaBr/NaClO system. When the AZADO/NaBr/NaClO and 1-methyl-AZADO/NaBr/NaClO oxidation systems were applied to wood cellulose using the same molar amount of TEMPO, the oxidation time needed for the preparation of oxidized celluloses with carboxylate contents of 1.2–1.3 mmol/g was reduced from ≈80 to 10–15 min. Moreover, the molar amounts of AZADO and 1-methyl-AZADO that had to be added for the preparation of oxidized celluloses with carboxylate contents of 1.2–1.3 mmol/g were reduced to 1/32 and 1/16 of the amount of TEMPO added, respectively. The DP_v values for the AZADO- and 1-methyl-AZADO-oxidized celluloses after NaBH₄ treatment were in the range of 600–800. This indicated that not only C6-carboxylate groups but also C2/C3 ketones were formed to some extent on the crystalline cellulose microfibril surfaces during the AZADO- and 1-methyl-AZADO-mediated oxidation. When the AZADO-oxidized wood cellulose, which had a carboxylate content of 1.2 mmol/g, was mechanically disintegrated in water, an almost transparent dispersion consisting of individually nano-dispersed oxidized cellulose nanofibrils was obtained, with a nanofibrillation yield of 89 %.     

SEC-MALLS analysis of TEMPO-oxidized celluloses using methylation of carboxyl groups

Two cellouronic acids [sodium (1 → 4)-β-polyglucuronates, CUAs] and one 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized wood cellulose (TOC) became soluble in 8 % lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) after the methylation of C6 carboxyl groups in these samples using trimethylsilyldiazomethane (TMSD). The obtained solutions were diluted to 1 % LiCl/DMAc and subjected to size-exclusion chromatography combined with multi-angle laser-light scattering (SEC-MALLS). Neither depolymerization nor side reactions took place during methylation; this was confirmed by SEC-MALLS and nuclear magnetic resonance analyses, using CUAs as models. The SEC-MALLS analysis of the original wood cellulose and the carboxyl-methylated TOC prepared from it, using 1 % LiCl/N,N-dimethyl-2-imidazolidinone and 1 % LiCl/DMAc, respectively, as eluents, showed that the weight-average degree of polymerization of the original wood cellulose decreased from 3,100 to 2,210 through TEMPO-mediated oxidation. The molecular-mass distributions of the original wood cellulose and the TOC both consisted of one large peak with a small shoulder, indicating that some of the oxidized hemicelluloses remained in the TOC. The combination of methylation of carboxyl groups in polysaccharides using TMSD and subsequent SEC-MALLS analysis using 1 % LiCl/DMAc as an eluent may be applicable not only to TOCs, but also to other polysaccharides with carboxyl groups, for evaluation of their molecular-mass parameters.     

Mechanism and kinetics studies of carboxyl group formation on the surface of cellulose fiber in a TEMPO-mediated system

2,2,6,6-Tetramethylpiperidine-1-oxyl radical (TEMPO) can selectively oxidize primary hydroxyl groups of cellulose to carboxyl groups. However, the depolymerization also occurs during the process. The kinetics and mechanism of carboxyl group formation on the surface of cellulose fiber oxidized by TEMPO/NaClO₂/NaClO were discussed. The oxidization and depolymerization of cellulose occurred simultaneously, according to analysis of FTIR and ¹³C CP/MAS NMR. The glucuronic acid and some small molecular fragments, formed by hydrolysis or β-elimination during the oxidation, are also discussed. The crystallization index increased and crystal size decreased, as shown by X-ray analysis. The degradation steps in the TEMPO/NaClO₂/NaClO system was discussed, according to carbon conversion analyzed by ¹³C CP/MAS NMR. The oxidation of cellulose can be described well by the kinetics model established based on the degradation of cellulose. It was found that temperature is one of the key parameters for controlling the oxdation and degradation level. The possible mechanism for oxidation of cellulose was composed.     

Oxidation of Primary Alcohol Groups of Naturally Occurring Polysaccharides with 2,2,6,6-Tetramethyl-1-Piperidine Oxoammonium Ion

ABSTRACT

The primary alcohol groups of ten polysaccharides, with widely different structures and water solubilities, were oxidized to carboxyl groups using 2,2,6,6-tetramethyl-1-piperidine oxoammonium ion (TEMPO) at pH 10.8 and 0°C. The yield and selectivity for the primary alcohol group were high for all ten of the polysaccharides. The oxidation greatly increased the water-solubility of the polysaccharides. Water-insoluble polysaccharides such as amylose, cellulose, and chitin became water-soluble to the extent of approximately 10% (w/v). The water-soluble polysaccharides had their degree of solubility doubled or tripled. The specific optical rotation, viscosity, and gelling properties with calcium ion were determined. The oxidized polysaccharides are new anionic polymers with unique structures that could have application as gums, gels, and films.     

Catalytic oxidation of cellulose with a novel amphiphilic nitroxide block copolymer as a recoverable catalyst

A novel amphiphilic block copolymer of poly(ethylene glycol)-b-poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl-methacrylate) was prepared through activators regenerated by electron transfer atom transfer radical polymerization of 2,2,6,6-tetramethylpiperidine methacrylate monomer, followed by oxidizing it with 3-chloroperoxybenzoic acid. This nitroxide block copolymer was used as a recoverable catalyst instead of free 2,2,6,6-tetramethylpiperidine-1-oxyl for selective catalytic oxidation of cellulose. According to its amphiphilic property, a mixture of acetonitrile and water was used as the reaction medium. The resulting carboxyl content of oxidized cellulose reached 1.07 mmol/g, equivalent to 73.2% of free TEMPO, was satisfactory. Furthermore, the block copolymer was easy to recycle and the activity did not decrease to a noticeable level after 4 cycles.     

TEMPO-mediated Oxidation of Native Cellulose: SEC–MALLS Analysis of Water-soluble and -Insoluble Fractions in the Oxidized Products

Linter cellulose was suspended in water and oxidized by the NaClO/NaBr/2,2,6,6-tetramehylpiperidine-1-oxy radical (TEMPO) system at pH 10.5 (TEMPO-mediated oxidation), and the oxidized products were separated into several fractions by filtration and centrifugation, depending on their particle sizes and apparent water-solubility. The major fraction (>ca. 80 mass % of the original linter cellulose) is the filter paper-trapped fibers, which can form inter-fiber hemiacetal linkages when handsheets are prepared thereof. Size-exclusion chromatographic analysis with multi-angle laser light scattering detection (SEC–MALLS) of these fibrous fractions dissolved in 0.5% LiCl/N,N-dimethylacetamide (DMAc) showed that some depolymerization occurred on cellulose chains during the TEMPO-mediated oxidation. On the other hand, the apparently water-soluble fractions (<ca. 20 mass % of the original linter cellulose) in the TEMPO-oxidized linter cellulose consisted of small amounts of colloidal particles having the cellulose I crystal structure, which came off from linter cellulose by the TEMPO-mediated oxidation and were mixed in the apparently water-soluble fraction even after filtration using 0.45 μm membrane. The presence of such colloidal cellulose crystals in the water-soluble fractions of the TEMPO-oxidized linter cellulose brings about anomalous bimodal SEC-elution patterns and extremely large molecular-mass values calculated from the SEC–MALLS data. Truly water-soluble cellouronic acid and/or over-oxidized compounds having glucuronic acid and hexeneuronic acid units are also present in the water-soluble fractions.     

Optimization of cellouronic acid synthesis by TEMPO-mediated oxidation of cellulose III from sugar beet pulp

Microfibrillated cellulose from purified sugar beet pulp was converted into cellulose III by immersion in liquid ammonia. When freed from ammonia, this product was oxidized in water at pH-10 using NaBr, NaOCl and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) under various conditions and concentrations. The resulting water-soluble cellouronic acid—i.e. cellulose oxidized at the C6 position- was analyzed by high performance size exclusion chromatography (HPSEC) together with ¹³C NMR spectroscopy. The oxidation parameters, namely reaction time, temperature, NaBr and TEMPO concentrations were varied to determine the optimum reaction conditions. A low TEMPO concentration, a rather fast reaction time and the conducting of the oxidation at 0 °C were critical to obtain pure cellouronic acid in high yield, high purity and high DP.     

TEMPO/NaBr/NaClO and NaBr/NaClO oxidations of cotton linters and ramie cellulose samples

Dried cotton linters and ramie cellulose samples were oxidized with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)/NaBr/NaClO and NaBr/NaClO (i.e., TEMPO-free) in water at pH 10. The carboxy contents, degrees of polymerization (DPs), X-ray diffraction patterns, and solid-state ¹³C NMR spectra were measured or obtained for the oxidized products with and without subsequent NaBH₄ reduction. Cellulose nanofibrils were prepared from the oxidized cellulose samples by sonication in water and observed by atomic force microscopy and transmission electron microcopy. Because the cellulose molecules were depolymerized with NaBr/NaClO, the depolymerization behavior of the cellulose samples with TEMPO/NaBr/NaClO can be mainly explained by depolymerization with NaBr/NaClO (i.e., not TEMPO-related compounds or reactions). However, because C6-aldehydes formed in the disordered regions periodically present along the longitudinal direction of each cellulose microfibril, the viscosity-average DP values of the TEMPO/NaBr/NaClO-oxidized cellulose samples decreased to 200–300, while those with subsequent NaBH₄ reduction exhibited much higher DP values. The nanofibrils prepared from the TEMPO/NaBr/NaClO-oxidized cellulose samples had smallest fibril heights or widths of 5–6 nm. However, significant amounts of unfibrillated bundles with heights of 10–40 mm were present in the nanofibril/water dispersions. The high carboxy contents of the TEMPO/NaBr/NaClO-oxidized cellulose samples (1.62–1.63 mmol/g) indicated that significant amounts of carboxy groups were likely present in the disordered regions, probably forming tail-like polyglucuronate chains. Solid-state ¹³C NMR analysis revealed that some of the glucosyl units originally with the tg C6–OH conformation were transformed to other conformations by TEMPO/NaBr/NaClO oxidation, while the crystalline C4 signal areas remained constant.

Graphic abstract

     

Biodegradation of β-1,4-linked polyglucuronic acid (cellouronic acid)

Biodegradability of -1,4-linked polyglucuronic acid (cellouronicacid), which was prepared from regenerated cellulose by2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation underaqueous conditions, was examined by enzymatic treatments and incubationtreatments with microorganisms collected from some soil samples. Degradation ofcellouronic acid was traced by size exclusion chromatography (HPSEC) or totalorganic carbon (TOC) of the treated products or solutions, respectively.Cellouronic acid was depolymerized by a commercial crude cellulase anddecreasedin its weight average degree of polymerization from about 1600 to 40 by thecellulase treatment at 20 °C for 40 days. ¹³C-NMRanalysis and liquid chromatography of the treated products showed thathydrolysis-type enzymes present in the crude cellulase as contaminantsprimarilydepolymerized cellouronic acid to give glucuronic acid. When aqueous solutionscontaining cellouronic acid were incubated with soil microorganisms for morethan 3 days, the TOC values decreased to less than 20% of the initial value,depending on molecular weight of the cellouronic acid used. The decreasing rateof TOC for cellouronic acid was clearly higher than that ofcarboxymethylcellulose, which is one of the cellulose derivatives havingcarboxymethyl substituents. These results imply that cellouronic acid has bothbiodegradability and metabolizability in the natural environment.     

Selective oxidation of cellulose catalyzed by NHPI/Co(OAc)2 using air as oxidant

With NHPI/Co(OAc)₂ as catalyst and air as oxidant, carboxylic group functionalized cellulose was prepared by oxidation of cellulose in acetic acid. Fourier transform infrared spectroscopy was utilized to detect the generation of carboxylic group and the acid amount was determined by acid–base titration method. The present results revealed that C6 primary hydroxyl groups on glucose units were partly converted to carboxylic groups during the catalytic oxidation process. The degree of polymerization of oxidized cellulose, which was determined by viscosity measurement, decreased slightly as compared with its parent. The structure of cellulose was characterized by X-ray diffraction and scanning electron microscopy, and it was almost unchanged.     

Characterization of cellulose nanofibrils prepared by direct TEMPO-mediated oxidation of hemp bast

Hemp bast (α-cellulose 79.4%, Klason lignin 4.9%) was directly oxidized by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation in water at pH 10 and room temperature for 2 h. The level of added NaClO in oxidation varied from 5 to 30 mmol/g (based on dry weight of hemp bast). Weight recovery ratios of the TEMPO-oxidized hemp bast celluloses were in the range of 81–91%, and their carboxylate contents increased up to 1.2 mmol/g with the increased NaClO addition level. The lignin contents decreased to 0.5–0.9% after oxidation, and the viscosity-average degrees of polymerization decreased from 1100 to 560 because of depolymerization during oxidation. Thus, direct TEMPO-mediated oxidation of hemp bast introduced a significant number of carboxylate groups and simultaneously achieved sufficient delignification. Small amounts of xylose, mannose, and rhamnose originating from hemicelluloses remained in the TEMPO-oxidized hemp bast samples prepared by oxidation with 5–20 mmol/g NaClO. However, oxidation with 30 mmol/g NaClO completely removed these hemicellulose-originating sugars, and produced almost pure TEMPO-oxidized cellulose. When TEMPO-oxidized hemp bast samples were mechanically disintegrated in water, their nanofibrillation yields were 58–65%. After removal of unfibrillated fractions by centrifugation, transparent dispersions showed birefringence when observed between cross-polarizers, while atomic force microscopy images showed near-individually dispersed nanofibril elements with widths of ~2 nm.     

Dissolution of Cellulose in Aqueous NaOH Solutions

Dissolution of a number of cellulose samples in aqueous NaOH was investigated with respect to the influence of molecular weight, crystalline form and the degree of crystallinity of the source samples. A procedure for dissolving microcrystalline cellulose was developed and optimized, and then applied to other cellulose samples of different crystalline forms, crystallinity indices and molecular weights. The optimum conditions involved swelling cellulose in 8–9 wt % NaOH and then freezing it into a solid mass by holding it at –20°C. This was followed by thawing the frozen mass at room temperature and diluting with water to 5% NaOH. All samples prepared from microcrystalline cellulose were completely dissolved in the NaOH solution by this procedure. All regenerated celluloses having either cellulose II or an amorphous structure prepared from linter cellulose and kraft pulps were also essentially dissolved in the aqueous NaOH by this process. The original linter cellulose, its mercerized form and cellulose III samples prepared from it had limited solubility values of only 26–37%, when the same procedure was applied. The differences in the solubility of the celluloses investigated have been interpreted in terms of the degrees to which some long-range orders present in solid cellulose samples have been disrupted in the course of pre- treatments.     

Degradation of TEMPO-oxidized cellulose fibers and nanofibrils by crude cellulase

The biodegradation behavior of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose fibers (TOCs) suspended in water and TEMPO-oxidized cellulose nanofibrils (TOCNs) dispersed in water by a commercial crude cellulase was studied. Products crude cellulase-treated for 0–7 days were separated into water/ethanol-insoluble and -soluble fractions. Weight recovery ratios and viscosity-average degrees of polymerization of the water/ethanol-insoluble fractions clearly decreased with crude cellulase-treatment time, showing that both TOCs and TOCNs have biodegradability. Water/ethanol-soluble fractions were subjected to size-exclusion chromatography (SEC) with photodiode array (PDA) detection to obtain SEC elution patterns detected by reflective index and UV spectra of each SEC pattern elution slice. SEC–PDA and ¹³C-NMR analyses showed that glucuronosyl unit-containing molecules present on microfibril surfaces in TOCs and TOCNs were primarily cleaved by hydrolyzing enzymes present as contaminants in the crude cellulase to form glucuronic acid as one of the major water-soluble degradation compounds. After the glucuronosyl units in TOCs and TOCNs were degraded and removed from microfibril surfaces by the hydrolyzing enzymes, cellulose chains newly exposed on the microfibril surfaces were rapidly hydrolyzed by cellulases predominantly present in the crude cellulase to form cellobiose. Both TOCs and TOCNs having sodium carboxyl groups are thus biodegradable, but TOCN having free carboxyl groups had clearly low biodegradability by the crude cellulase. Thus, biodegradation behavior may be controllable by controlling the structure of carboxyl group counter ions in TOCs and TOCNs.     

Wood cellulose nanofibrils prepared by TEMPO electro-mediated oxidation

A softwood bleached kraft pulp (SBKP) was subjected to electro-mediated oxidation in water with TEMPO or 4-acetamido-TEMPO without any chlorine-containing oxidant. Solid recovery ratios of water-insoluble fractions of the oxidized SBKPs were more than 80%, and C6-carboxylate contents increased up to approximately 1 mmol g⁻¹ after oxidation for 48 h. Significant amounts of C6-aldehyde groups (0.17–0.38 mmol g⁻¹) were also formed in the oxidized SBKPs. The degree of polymerization decreased from 2,200 to 520 and 1,400 by the oxidation for 48 h with TEMPO at pH 10 and 4-acetamido-TEMPO at pH 6.8, respectively. The original cellulose I crystal structure and crystallinity of SBKP were maintained after the oxidation, indicating that all C6-oxidized groups were selectively formed on crystalline cellulose microfibril surfaces. The oxidized SBKPs with carboxylate contents of more than 0.9 mmol g⁻¹ were convertible to individual cellulose nanofibrils in yields of more than 80% by disintegration in water.     

Degrees of polymerization (DP) and DP distribution of cellouronic acids prepared from alkali-treated celluloses and ball-milled native celluloses by TEMPO-mediated oxidation

Various cellulose II samples, ball-milled native celluloses and ball-milled wood saw dust were subjected to 2,2,6,6-tetramethypyperidine-1-oxyl radical (TEMPO)-mediated oxidation to prepare cellouronic acid Na salts (CUAs). The TEMPO-oxidized products obtained were analyzed by ¹³C-NMR and size-exclusion chromatography (SEC). When the cellulose II samples with degrees of polymerization (DP) of 220–680 were used as the starting materials, the CUAs obtained had weight-average DP (DPw) values of only 38–79. Thus, significant depolymerization occurs on cellulose chains during the TEMPO-mediated oxidation. These DP values of CUAs correspond to the cellulose II crystal sizes along the chain direction in the original cellulose II samples, but not necessarily to their leveling-off DP values. CUAs can be obtained also from ball-milled native celluloses in good yields by TEMPO-mediated oxidation, although their DPw values are lower than about 80. On the other hand, CUA with DPw of about 170 was obtained from ball-milled wood saw dust.