Distribution Of Substituents Research Articles

Homogeneous hydroxyethylation of cellulose in NaOH/urea aqueous solution

The 6 wt.% NaOH/4 wt.% urea aqueous solution was proved to be an aqueous non-derivatizing solvent for cellulose by 13C NMR. O-(2-hydroxyethyl)cellulose (HEC) was prepared by a totally homogeneous hydroxyethylation of cellulose using this new solvent for the first time, and the distribution of substituents within anhydroglucose units (AGU) was examined by 13C NMR. It was found that the relative reactivity of the hydroxyl groups within AGU and the new hydroxyl group was in the order C-x > C-6 > C-2 > C-3, an analogous functionalization pattern as HEC obtained by the heterogeneous slurry process. The ethylene oxide efficiency in this homogeneous etherification reaction was 20 – 30%.

AFM and STM characterization of thiol and thiophene functionalized SWNTs: pitfalls in the use of chemical markers to determine the extent of sidewall functionalization in SWNTs

Thiol- and thiophene-functionalized SWNTs prepared via the reaction of a substituted amine with fluoronanotubes show similar levels of sidewall functionalization, however, the use of Au nanoparticles as chemical markers for AFM gives misleading results for substituent distribution since STM shows the thiol substituents grouped in bands while the thiophene substituents uniformly distributed along the SWNTs.

Direct evidence of substituent clustering in lower valent K+ modified PbZrO3 by high-resolution Z-contrast imaging

The distribution of substituents in lead zirconate (PZ) modified by K+ (PKZ) has been studied by high-resolution Z-contrast imaging. Pronounced intensity variations were observed between various Pb columns, giving direct evidence that a number of columns had higher concentrations of (K+) substituents. High-resolution Z-contrast imaging was also performed in La+3-modified lead zirconate titanate (PLZT) for comparison.

Reaction pathways during the thermal conversion of polysiloxane precursors into oxycarbide ceramics

A detailed evaluation of the events that occur during the heating of organic–inorganic hybrid solids obtained by the sol–gel method has been established. The correlation of many characterization techniques, in particular TG/MS analysis associated with specific alkoxide structural units, has provided some explanations in terms of a set of mechanisms. It will be shown that the residual SiOH and SiOEt, arising from incompletely hydrolyzed and condensed alkoxides, play an important part in the degradation of gel precursors. They undergo reactions with SiH in the temperature range 200–700 °C and with SiMe starting around 450 °C. The thermal stability of SiMe is strongly linked to the nature and distribution of the other functional substituents (SiOH, SiH) present in the gels. The cleavage of SiMe may thus take place within a large temperature range varying from 450 °C to 1000 °C.

Analysis of the substituent distribution along the chain of water-soluble methyl cellulose by combination of enzymatic and chemical methods

Abstract Four methyl cellulose samples in the degree of substitution range from 0.5 to 2.0 were characterised by combination of different analytical methods. Samples were analysed regarding their partial degree of substitution by hydrolysis and anion exchange chromatography with pulsed amperometric detection. For calibration of the chromatographic system, standard substances were isolated by preparative HPLC and their structure was confirmed by 13C-NMR spectroscopy. For two methyl cellulose samples per-acetylation and 13C-NMR with inverse gated decoupling was carried out for comparison with the chromatographic analysis. Endoglucanase fragmentation of methyl celluloses was performed and water-soluble and insoluble fractions were analysed separately. A preparative size exclusion chromatography system for enzymatic-degraded water-soluble methyl cellulose was developed and the molar masses of the individual fractions were examined by analytical size exclusion chromatography. By combination of endoglucanase fragmentation, preparative chromatography, hydrolysis and anion exchange chromatography an approach for the analysis of the substitutent distribution along the polymeric chain of water-soluble methyl cellulose could be established.

Enzyme-aided investigation of the substituent distribution in cationic potato amylopectin starch.

The distribution of substituents along the polymer chain in cationic potato amylopectin starch, modified in solution, granular slurry, or dry state, was investigated. The starch derivatives were successively hydrolyzed by different enzymes, followed by characterization of the hydrolysis products obtained by means of electrospray mass spectrometry (ESI-MS) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). ESI-MS and MALDI-MS were proved to be appropriate techniques for identification of the substituted hydrolysis products, for which there are no standard compounds available. No highly substituted oligomers were found in the hydrolysates, which was taken as an indication of a more or less homogeneous distribution of cationic groups in the amylopectin molecules. Furthermore, from the results obtained it was suggested that the enzymes cleave glucosidic linkages only between unsubstituted glucose units and, preferentially, linkages in sequences containing more than two adjacent unsubstituted units. The determination of the amount of unsubstituted glucose produced from every successive hydrolysis step revealed slight differences between the different starch samples with respect to the homogeneity of the substitution pattern. Among the three samples under investigation, starch cationized in solution was found to have the most and dry-cationized starch the least homogeneous distribution of substituents.

Determination of the substituent distribution along cellulose acetate chains as revealed by enzymatic and chemical methods

Eight cellulose acetate samples with degree of substitution (DS) in the range from 0.4 to 2.7 were characterised regarding their DS, substituent distribution in the anhydroglucose units, molar mass distribution and intrinsic viscosity. Samples were intensively hydrolyzed with a purified endoglucanase and the degradation monitored by analytical size exclusion chromatography (SEC). A preparative SEC system using pyridine:water was developed, suitable for the analysis of samples over a wide range of DS. Two methods were established to determine the DS in milligram scale for further analysis of isolated fragments. For cellulose acetate samples with DS 0.4, 0.7 and 1.2 endoglucanase fragmentation followed by preparative chromatography and analysis of isolated fragments was performed. The procedure was suitable to determine differences in the substituent distribution along the polymeric chain of cellulose acetates susceptible to endoglucanase hydrolysis.

Determination of the DS and substituent distribution of cationic alkyl polyglycosides and cationic starch ethers by GLC after dealkylation with morpholine

The total DS and substituent distribution of starch and alkyl polyglycosides functionalised as O-(2-hydroxy-3-trimethylammonium)propyl ethers were determined by GLC. To achieve volatile analytes, the samples were submitted to methanolysis, N-demethylation and O-trimethylsilylation. Alternatively hydrolysis, reduction with NaBH 4 and subsequent O-acetylation were performed, but suffered from intramolecular acetal formation of 2- O-substituted residues, preventing reduction. Morpholine as nucleophile was superior to thiophenolate with regard to quantitative dealkylation and side product formation. The ratio of un-, mono-, di-, tri-, and tetrasubstituted compounds was determined. The total DS values calculated from these mole fractions were in good agreement with those obtained from elemental analysis or NMR from standards. Regioselectivity of the cationisation reaction was determined after methanolysis, permethylation and Hofmann elimination by GLC.

Optimization of a process for carboxymethyl cellulose (CMC) preparation in mixed solvents

A benzene-ethanol-water mixture was investigated as the solvent medium in synthesizing carboxymethyl cellulose (CMC). The process of carboxymethylation was optimized to produce CMC with DS > I.0, solution viscosity about 1500 mpa·s, and with uniform distribution of substituents. It is possible to scale up this process by using a kneading machine.

Enzyme Aided Analysis of the Substituent Distribution along the Chain of Cellulose Acetates Regioselectively Modified by the Action of an Aspergillus niger Acetylesterase

Two cellulose acetates (CA) were regioselectively deacetylated by action of a pure Aspergillus niger acetylesterase from the carbohydrate esterase family 1. The action of acetyl esterase along the polymeric chain was monitored by a new enzyme-aided method. CA with and without esterase modification was hydrolysed with a pure endoglucanase. The fragments were deutero-acetylated and separated by preparative SEC in CHCl3. The partial degree of substitution of the individual fragments was determined by 1H-NMR spectroscopy. The investigation confirmed the uniform regioselective and regular deacetylation along the polysaccharide chain. The partial substitution in C2 seemed to be of major importance for the enzyme's mode of action.

Preparations of regioselectively methylated cellulose acetates and their 1H and 13C NMR spectroscopic analyses

AbstractSeven possible regioselectively methylated cellulose acetates (RS‐MCAs)—2,3,6‐tri‐O‐methyl cellulose acetate, 3,6‐di‐O‐methyl cellulose acetate, 2,6‐di‐O‐methyl cellulose acetate, 2,3‐di‐O‐methyl cellulose acetate, 6‐O‐methyl cellulose acetate, 3‐O‐methyl cellulose acetate, and 2‐O‐methyl cellulose acetate—were prepared for the first time from chemically synthesized cellulose derivatives obtained by cationic ring‐opening polymerization and then were analyzed by 1H and 13C NMR spectroscopy. The chemical shifts of ring protons and carbons were influenced by substituent groups (methyl or acetyl) and clearly reflected the pattern of substituent distribution in anhydroglucose units. These data may conveniently be used for the determination of the substituent distribution of methyl cellulose. The synthesized RS‐MCAs also may be used for the elucidation of the structure–property relationship. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4167–4179, 2002

Initial characterization of ethyl(hydroxyethyl) cellulose using enzymic degradation and chromatographic methods.

Two different ethyl(hydroxyethyl) cellulose (EHEC) samples were characterized by size-exclusion chromatography (SEC) with multiangle light scattering (MALS) detection and high-performance anion-exchange chromatography (HPAEC) with pulsed amperometric detection (PAD). The aim of the study was to investigate the molar mass distribution and the heterogeneity of the substituent distribution, factors that are thought to affect the functional properties of EHEC. The presence of blocks of unsubstituted glucose units was studied by enzymic degradation of EHEC by two different endoglucanases from Trichoderma reesei. The SEC-MALS analysis of the hydrolysis products showed that both enzymes were strongly inhibited by the large number of substituents along the cellulose chain. However, as the weight-average molar mass was reduced from approximately 360,000 to 80,000 g/mol in one of the polymers and from 770,000 to 60,000 g/mol in the other polymer, it was suggested that both samples were composed of some unsubstituted regions where the enzymes got access to the glucosidic bonds. The amount of glucose released upon endoglucanase hydrolysis was determined by HPAEC-PAD, which gave information on the homogeneity of the substituent distribution. The production of unsubstituted glucose units indicated that one of the polymers had a more uneven distribution compared with the other. It was demonstrated that chemical characterization of EHEC is a complex task, which requires an analytical approach involving numerous different methods and techniques.

Methylation with sodium naphthalenide for the analysis of substituent distribution of trimethylsilyl celluloses and their evaluation

The substitution pattern of trimethylsilyl celluloses silylated withhexamethyldisilazane and N,N-dibutylamino­trimethylsilane was determined bymethylation analysis. Methylation with sodium hydride/methyl iodide wasaccompanied by significant desilylation because sodium hydride is alwayscontaminated with sodium hydroxide. Sodium naphthalenide prepared in tetrahydrofuran under anhydrous reaction conditions alloweda reduction of the extent of desilylation to less than 10 percent. In allcases preferred silylation of the 6-position was found. Celluloses silylatedwith hexamethyldisilazane and N,N-dibutylaminotrimethylsilane had almostidentical substituent distributions. Lower reaction temperature resulted inhigher regioselectivity.

Synthesis and NMR characterization of regiocontrolled starch alkyl ethers

Selective polysaccharide functionalization is of importance in molecular and supramolecular structure design and the development of new products and properties. As a typical partially branched polysaccharide, starch (72% amylopectin) was selectively and completely alkylated in the secondary positions. Starting from 6-O-thexyldimethylsilyl starch (6-O-TDS starch) prepared regio-controlled, an effective alkylation takes place by using methyl iodide and benzyl bromide, respectively, and sodium hydride as a base in THF solution. Under these conditions, the silyl ether linkages are stable. The subsequent desilylation proceeded with tetra-n-butylammonium fluoride (TBAF) under mild conditions to form the alkyl starches without cleavage of the alkyl ether linkage. The pattern of functionalization could be analyzed by using the silyl alkyl ethers or subsequently 6-O-acetylated derivatives and multidimensional NMR techniques. In the case of the methyl ethers, the substituent distribution could be detected not only in the anhydroglucose units (AGUs), but also in the non-reducing end-groups (NEGs). As a result, the starch ethers are alkylated regio-controlled in 2,3-di-O (AGU) and 2,3,4-tri-O (NEG) positions.

Electrosorption of sodium cellulose sulfates with different substitution patterns

Various types of sodium cellulose sulfate (SCS), dissolved in a 1M LiCl solution, were investigated by alternating current (AC)polarography. The SCS samples differed in the degree of substitution (DS), thedistribution of substituents within the anhydroglucose unit (AGU), and alongthechain, due to the method of synthesis. The goal was to study theelectrosorptionbehavior, characterized by the shape of the desorption wave in the polarogram,as a function of the chemical structure of the SCS samples. The shape of thedesorption wave reflects domains of different substitution. A superimpositionofparameters like DS and pattern of substitution on the electrosorption behaviorwas observed. The AC polarography method described can be used as a tool todistinguish between an even or uneven distribution of substituents along thecellulose chain.

Endoglucanase fragmentation of cellulose sulfates derived from different synthesis concepts

Nine cellulose sulfates, derived from different synthesis concepts, were characterised regarding the degree of substitution, substituent distribution in the anhydroglucose unit, molar mass distribution, intrinsic viscosity and solubility. The degree of substitution ranged from 0.37 to 1.46, and the degree of polymerisation amounted from 160 to 350. All samples were intensively incubated with an endoglucanase, and the degradation was monitored using multi-detected size exclusion chromatography. For samples with a degree of substitution up to 0.8 a severe fragmentation occurred, while for samples with higher substitution at least a significant modification of the derivatives could be achieved. The enzyme altered not only the molar mass distribution and viscosity but also its water solubility.The investigations on cellulose sulfates with different substitution pattern indicated that a homogeneous substituent distribution resulted in the best solubility of the derivatives. Investigation of samples after enzymatic fragmentation suggested that the C6 position inhibited the enzyme most effectively.

Influence of the p-toluenesulphonylation of cellulose on the polymer dynamics investigated by dielectric spectroscopy

Cellulose p-toluenesulphonates (tosyl cellulose) were studied by dielectric spectroscopy. Two relaxation processes were observed for samples of low degrees of substitution (DS) in the low temperature range (−130 to 0 °C). These are related to the local chain and the tosyl substituent motion, β- and γ-relaxations, respectively. For DS values higher than 1.4, the dielectric spectra no longer could be separated into the individual processes, because the dielectric response became reduced in intensity and its distribution in the frequency scale was broadened by cross-correlation of different motions between the modes of the substituents and the segmental motions of the chain. Significantly, different local chain (β-relaxation) behaviour was found for derivatives of DS≈0.9 with block-like and random distribution of substituents along the backbone. The dynamic behaviour of the tosylated polymers is compared with other cellulose derivatives and revealed similar qualitative behaviour, but the type of the substituents at the anhydroglucose unit has a characteristic influence on the dynamic parameters of the relaxation time, the activation energy related and the shape parameter of the relaxation processes.

Rheological evaluation of the interactions between cationic celluloses and Carbopol 974P in water.

This paper reports on the influence of the structural properties of two cationic hydroxyethylecelluloses, polyquaternium-4 (PQ-4; 1.13-1.27% N) and polyquaternium-10 (PQ-10; 1.88-1.95% N), on the rheological behavior of their dispersions and of the dispersions of their blends with Carbopol 974P (60.9% COOH) in water. Dynamic shear oscillation measurements showed that the rheological behavior of 1 and 2% (w/w) cellulose polymers was mainly viscous. Flow curves of PQ-10 dispersions presented a higher consistency and pseudoplasticity than PQ-4 dispersions, in which the thixotropy was higher. Structural features, such as type and distribution of substituents and molecular weight, explain why they behave different. The presence of 0.00125-0.02% Carbopol in diluted cationic cellulose (0.01-0.08%) dispersions produced an important decrease in viscosity, due to a strong associative process via an electrostatic interaction phenomenon. Similar results were obtained when sodium acetate was added instead of Carbopol. In contrast, in the concentrated range (1% cellulose polymer), the viscous (G") and especially the elastic (G') moduli of the dispersions increased monotonically with Carbopol concentration (0.010-0.125%) and became physical gels, except in the proximity of the neutralization of the ammonium groups (0.07% Carbopol for PQ-4, and 0.1% Carbopol for PQ-10) in which G" and G' decreased. The concentrated dispersions showed pH-sensitivity when the Carbopol added was around or above the neutralization point. In dispersions with this composition, consistency increased dramatically when pH changed from 4.5 to 7.4. This property can open a wide range of applications, especially in the pharmaceutical field to prepare gelling in situ systems.

High-performance anion-exchange chromatography–electrospray mass spectrometry for investigation of the substituent distribution in hydroxypropylated potato amylopectin starch

The use of high-performance anion-exchange chromatography (HPAEC) with pulsed amperometric detection (PAD) coupled on-line with electrospray mass spectrometry (ESI-MS) for analysis of the substitution pattern in chemically modified starch, has been investigated. In order to characterise the distribution of substitution groups along the polymer chain, hydroxypropylated potato amylopectin starch (HPPAP) was subjected to enzymic hydrolysis, followed by analysis of the degradation products by HPAEC-PAD-MS. When using conventional chromatographic techniques for characterisation of enzymic hydrolysates, standard compounds are required for identification of the hydrolysis products. However, the on-line coupling with ESI-MS allowed identification of all products obtained, substituted as well as unsubstituted, and also of those compounds that co-eluted, without the need for standards. Further, HPAEC-PAD-MS was shown to be useful for analysis of the substitution pattern in modified starch; from results obtained it was suggested that the hydroxypropyl groups were homogeneously distributed in the amylopectin molecule. It was also shown that the starch hydrolysing enzymes were hindered by the hydroxypropyl groups and preferentially cleaved glucosidic linkages between unsubstituted glucose units.

Specificity of Microbial α-Amylase and Amyloglucosidase Hydrolysis of Methyl Amyloses

Methyl amyloses prepared under various conditions were exhaustively digested by means of α-amylase (B. licheniformis) and amyloglucosidase (A. niger). The amount of glucose that was released by the action of the enzymes was determined. Degradation products were further investigated as their O-methyl-O-deuteromethyl derivatives by mass spectrometry. The substituent distribution in the reduced and O-methylated-O-ethylated oligosaccharides was determined, differentiating between non-reducing terminal glucosyl residues, 1→4-linked inner glucosyl units and reducing glucose end groups. The portion of glucose that could be liberated by the enzymes decreased with increasing degree of substitution (DS) but at the same DS it was considerably higher for heterogeneously prepared amylose ethers than for those prepared under homogeneous conditions. Mass spectrometry (fast atom bombardment, FAB- and matrix-assisted laser desorption ionisation, MALDI-MS) gave evidence of different oligomer patterns and average values of degree of substituent/degree of polymerisation (DS/DP) in dependence on the methylation conditions applied. More detailed analysis of the O-methylated positions of the oligosaccharides showed that the reducing glucose end group was usually unsubstituted, while the non-reducing glucosyl residues could be 2-, 6- or even 2,6-di-O-substituted. In contrast, all 3-O-methyl groups were located in the inner 1,4-linked glucosyl units, indicating inhibition of both enzymes. The results of this selective partial degradation are compared with those obtained after chemical random degradation and mass spectrometry.