Syntheses and comparison of 2,6-di-O-methyl celluloses from natural and synthetic celluloses

2,6-Di-O-methylcellulose was prepared from natural and synthetic celluloses. Natural cellulose was converted to 2,6-di-O-thexyldimethylsilylcellulose, then to 3-mono-O-allyl-2,6-di-O-methylcellulose, and finally into 2,6-di-O-methylcellulose. Alternatively, 2,6 di-O-methylcellulose was synthesized from the synthetic cellulose derivative 3-mono-O-benzyl-2,6-di-O-pivaloylcellulose by depivaloylation and methylation to give 3-mono-O-benzyl-2,6-di-O-methylcellulose, which was debenzylated to yield the dimethyl ether. Both types of 2,6-di-O-methylcellulose are insoluble in water and common organic solvents. The structures of all cellulose derivatives were determined by NMR.     

Three-dimensional snow images by MR microscopy

MR microscopy technique was introduced to visualize and quantify the three-dimensional structure of snowpack. Since the NMR signal from the ice was week, we looked at the air space instead filling with dodecane or aniline doped with iron acetylacetonate. Four types of snow were tested: ice spheres, large rounded poly crystals, small rounded mono-crystals and depth hoar crystals. A specific specimen-cooling system was developed to keep the temperature below 0 degrees C. In the experiments 0.5 to 2 h were necessary to accumulate the signals enough to obtain a 3D micro-image; the image matrix 128(3), voxel size (200 microm)3 or 256(3) (120 microm)3. Comparison with the 2D data using the conventional section plane method was also carried out and MR microscopy is proved to be a very useful method to visualize the microstructure of snowpack.     

Solid-state NMR studies of methyl celluloses. Part 1: regioselectively substituted celluloses as standards for establishing an NMR data basis

Three methyl celluloses with completely uniform substitution pattern, 2-O-methyl cellulose (1), 3-O-methyl cellulose (2) and 6-O-methyl cellulose (3), were prepared according to the cationic ring opening polymerization approaches starting from substituted 1,2,4-orthopivalate derivatives of d-glucose. These samples allowed for the first time to sort out the methyl substitution effects on solid-state NMR chemical shifts and relaxation. Dipolar dephasing experiments allowed the detection and assignment (¹H, ¹³C) of the methyl groups. In 1 and 2, these resonances overlapped with those of C-6, whereas in 3, the methyl signal experienced a low-field shift into the region of C-2,3,5. ¹³C T₁ experiments were used to verify different relaxation behavior of the carbon sites, particularly the short relaxation time of at the carbon substitution site next to the methyl groups. This effect was used to unambiguously identify the ¹³C chemical shifts of the carbons carrying the methoxyl substituent, although they overlap with all resonances in the C-2,3,5 region. The data obtained for the standard samples with uniform substitution will now be used as the basis for determining methylation patterns and substitution degree in commercial methyl celluloses.     

Preparation and characterization of redox cellulose Langmuir–Blodgett films containing a ferrocene derivative

The 3‐ferrocenoylpropanoyl group, one of the redox species, was introduced at C‐2 and/or C‐3 positions of 6‐O‐(4‐stearyloxytrityl)cellulose. The spreading behavior of the cellulose derivatives on the water surface and the properties of Langmuir–Blodgett (LB) films were investigated. The surface pressure–area isotherm of the cellulose monolayer was changed by the subphase temperature. Uniform monolayers of 6‐O‐(4‐stearyloxytrityl)cellulose 3‐ferrocene propionate (STCFc) could be deposited successively onto several substrates by the horizontal lifting method at 10 mN m^?1, and this produced X‐type LB films. The successive uniform depositions of STCFc were confirmed by ultraviolet–visible absorption spectra. X‐ray diffraction measurements indicated that the thickness of the STCFc molecules in the LB films was 1.99 nm. Fourier transform infrared spectroscopy measurements supported the idea that hydrocarbon chains in the LB films were highly ordered (trans‐zigzag) and oriented considerably perpendicular to the surface of the substrate. Moreover, the C?O group of the ferrocenoyl groups was perpendicular to the surface of the substrate, and the ferrocene group was occupied in the water phase. Cyclic voltammograms for the STCFc monolayer on a gold electrode exhibited surface waves. The interfacial electron‐transfer process between the redox site incorporated into the cellulose LB monolayer and the electrode surface was fast enough at a scanning rate lower than 100 mV s^?1. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5023–5031, 2005     

First in vitro norlignan formation with Asparagus officinalis enzyme preparation

We report for the first time that an enzyme preparation from fungal-elicited Asparagus officinalis cultured cells catalyses the formation of a norlignan, (Z)-hinokiresinol, from two non-identical phenylpropanoid monomers, 4-coumaryl alcohol and 4-coumaroyl CoA, and from a dimer, 4-coumaryl 4-coumarate, without any additional cofactors.     

Products of low-temperature pyrolysis of nanocellulose esters and implications for the mechanism of thermal stabilization

Esterification was used to improve the thermal stability of nanocellulose to extend its application as reinforcing filler to polymer matrices with high melting point. The effect of the structure of ester groups on thermal stability was studied in detail. Various types of nanocellulose esters (straight-chain, C2–C14; cyclic adamantoyl, ADM; aromatic benzoyl, BNZ; and branched pivaloyl, PIV) with degree of substitution values in the range of 0.40–0.47 were prepared from bacterial cellulose nanofibers and nanocrystals. The reaction conditions used to prepare the esters maintained the viscosity-average degree of polymerization (DPv) and crystallinity of the starting materials. Thermogravimetric analysis showed that the temperature at maximum weight loss rate (T_max) increased after esterification. The structure of the ester groups and the DPv, however, showed no varying effect on T_max. The 5 % weight loss temperature (WLT) which was used to assess the thermal stability at the onset of thermal degradation varied with the type of ester. Lower 5 % WLT was observed in straight-chain esters than those of the bulky esters of ADM, BNZ and PIV; which also showed high resistance to weight loss when subjected to isothermal heating. To understand the event at the onset of thermal degradation, low temperature pyrolysis was conducted. The evolved gases were separated and identified by gas chromatography–mass spectrometry technique. Results showed that at the onset of thermal degradation, levoglucosan (LG) is produced from the untreated BC nanocrystals. After esterification, LG formation was inhibited. The removal of the ester groups or deprotection is the main event at the onset of thermal degradation of nanocellulose esters. From the structure of the pyrolysis products, the mechanism of thermal deprotection of nanocellulose esters is proposed for the first time.     

ABA- and BAB-triblock cooligomers of tri-<Emphasis Type="Italic">O</Emphasis>-methylated and unmodified cello-oligosaccharides: syntheses and structure-solubility relationship

Triblock cooligomers consisting of tri-O-methyl-glucopyranosyl and unmodified glucopyranosyl residues, methyl 2,3,4,6-tetra-O-methyl-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-methyl-β-d-glucopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-methyl-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-methyl-α-d-glucopyranoside (1: ABA triblock cooligomer; DS = 2.1) and β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-methyl-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-methyl-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-methyl-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-methyl-β-d-glucopyranosyl-(1 → 4)-d-glucopyranose (2: BAB triblock cooligomer; DS = 1.8) were prepared. Compound 1 dissolved both in distilled water and chloroform but compound 2 dissolved in distilled water not in chloroform, though compounds 1 and 2 consist of 4 tri-O-methyl-glucopyranosyl and 2 unmodified anhydro glucopyranosyl units.     

Synthesis and Structure/Property Relationships of Regioselective 2‐O‐, 3‐O‐ and 6‐O‐Ethyl Celluloses

Regioselectively ethylated celluloses, 2‐O‐ ( 1 ), 3‐O‐ ( 2 ), and 6‐O‐ethyl‐ ( 3 ) celluloses were synthesized via ring‐opening polymerization of glucopyranose orthopivalate derivatives. The number‐average degrees of polymerization (DP_ns) of compounds 1 and 2 were calculated to be 10.6 and 49.4, respectively. Three kinds of compound 3 with different DP_ns were prepared: DP_ns = 12.9 ( 3‐1 ), 60.3 ( 3‐2 ), and 36.1 ( 3‐3 ). The 2‐O‐, 3‐O‐, and 6‐O‐ethylcelluloses were soluble in water, confirmed by NMR analysis. Furthermore, the 3‐O‐ ( 2 ), and 6‐O‐ethyl‐ ( 3‐2 ) celluloses showed thermo‐responsive aggregation behavior and had a lower critical solution temperature (LCST) at about 40 °C and 70 °C, respectively, based on the results from turbidity tests and DSC measurements. The 6‐O‐ethyl‐cellulose ( 3‐3 ) with DP_n = 36.1 and DP_w = 54.6 showed gelation behavior over approx 70 °C, whereas the 6‐O‐ethyl‐celluloses 3‐1 and 3‐2 with lower and higher molecular weight, such as DP_ns 12.9 and 60.3, did not show gelation behavior at this temperature. It was revealed that the position of ethyl group affected the phase transition temperature. According to our experiments, the 3‐O‐ethyl and 6‐O‐ethyl groups along the cellulose chains caused the thermo‐responsive property of their aqueous solutions. The appropriate DP of the regioselective 6‐O‐ethyl‐cellulose existed for gelation of the aqueous solution.