Cellulose reactions sites

Processes need to be developed to expose LHC for effective penetration of chemicals that promote separation of LHC or which render cellulose accessible by swelling. These processes can have many objectives (1) to get at cellulose (2) to remove hemicellulose from the reaction site before cellulose hydrolysis (3) to remove lignin from the site and (4) to bring penetrating or hydrolysis agents into effective contact with cellulose. 

The results imply a competition for the reaction sites between the lignin and the cellulose, and therefore a low specificity of the reaction between the reactive species and the surrounding pulp suspension has been observed. The intensity of the variation in the measured properties (die increase in ISO brightness or the decrease in physical properties) is proportional to the dose rate used, and therefore to the concentration of... 

Limitations in possibility of chemical modifications of starch result from steric hindrance of reaction sites, solubility, viscosity of reaction medium, and susceptibility to side reactions among them, depolymerization almost always accompanies intended modification. As a rule, polysaccharides are soluble, although frequently only sparingly, in water and dimethyl sulfoxide. Polysaccharides solubilize on xanthation, i.e., on reaction with CS2 in alkaline medium, to form syrups of xanthates. On acidification polysaccharides could be recovered. Such procedure was utilized for several decades for production of artificial silk from cellulose. 

Work needs to be done to find the specific activity of purified enzymes on the cellulose structure, especially the synergism between different cellulolytic enzymes. With this understood, one can then undertake the task of studying the endoglucanase-cellobiohydrolase system. This requires the identification of binding sites, reaction sites, and changes in cellulose structure as a result of the enzyme action. 

Mechanochemically Initiated Graft Copolymerization. According to the ESR study, it is clear that mechanoradicals were generated in cellulose either by means of mechanical cutting or ball milling. These mechanoradicals may be utilized as reaction sites for the initiation of vinyl polymerization which would result in graft copolymerization of cellulose. Based on this principle, the ability of cellulose mechanoradicals to initiate copol3nnerization was pursued. 

These steric and ionization effects seriously limit the usefulness of substituted celluloses in site-of-action studies. Further uncertainties are introduced by the fact that the substitution reactions used to solubilize cellulose distribute the substituents randomly on the polysaccharide. As a consequence the reaction products become a very large family of variously substituted fragments rather than a single product or a few identifiable oligosaccharides. 

In the commercial process, it is believed that the carbon disulfide, which is added to the reaction mass as a liquid, must first dissolve in the aqueous alkali adsorbed on the alkali cellulose (AC) so that it can be transferred to the reaction sites on the cellulose molecules. There the hydrated CS2 adds onto the alcoholate ion. The basic reaction is summarized in the following equations ... 

Polymers containing unsaturated rubber and semicrystalline polymers are often effectively stained using osmium tetroxide. What about materials that do not show such differential staining Two examples will be described where reactive (unsaturated) materials are included into the polymer to provide reaction sites. Inclusion of a stainable unsaturated polymer was shown for cellulosics [101] and synthetic fibers [127]. The initial work was focused on improvement of the properties of cellulosics by inclusion of an... 

Cellulose is also the raw material for many modified polymers. Ott was an expert on cellulose reactions. Because of the multiple sites on cellobiose for... 

Willard and co-workers [21] have shown that the preferred reaction site on cellulose is the primary (Ce) hydroxyl. A/-Methylol compounds can also react with other AT-methylol compounds and form resins by self-condensation. The reaction with cellulose is, therefore, accompanied by the formation of resinous products in varying amounts. Because A/-methylol compounds can react with compounds that have an active hydrogen atom (e.g., alcohols, amines, and carboxylic acids), they are very useful for introducing hydrophobes into the repellent molecule. Because acids catalyze the self-condensation to resins, N-methy-lol compounds are converted with an alcohol, usually methyl alcohol, to ethers, which are more stable and can be reacted with a long-chain fatty acid to form the repellent molecule. 

Before going into detailed descriptions about the CSAS application to explore the cellulose self-assembly, let us first briefly describe some common features of the two synthetic routes. The artificial synthesis of cellulose to be discussed here, which was first reported by Kobayashi et involves the enzymatic polymerization of the special substrate monomer at a specific reaction site of the so-called cleft in cellulase used as a catalytic enzyme that will be detailed in Section 2.13.3. The biosynthesis of cellulose involves also the enzymatic polymerization of uridine diphosphate glucose (UDP-glucose) at the specific reaction sites of the so-called terminal complex (TC) on the outer membrane of cytoplasm of bacteria genera Acetobacter xylinum (AX)In both systems, the synthesized cellulose molecules are self-assembled in situ in the reaction medium. 

Etherification. The accessible, available hydroxyl groups on the 2, 3, and 6 positions of the anhydroglucose residue are quite reactive (40) and provide sites for much of the current modification of cotton ceUulose to impart special or value-added properties. The two most common classes into which modifications fall include etherification and esterification of the cotton ceUulose hydroxyls as weU as addition reactions with certain unsaturated compounds to produce ceUulose ethers (see Cellulose, ethers). One large class of ceUulose-reactive dyestuffs in commercial use attaches to the ceUulose through an alkaH-catalyzed etherification by nucleophilic attack of the chlorotriazine moiety of the dyestuff ... 

Following the findings of Mino and Kaizerman [51] that ceric ion can form a redox system with cellulose, grafting onto various natural polymers has been carried out by the ceric ion method. In the case of cellulose, the reaction between ceric ion and cellulose occurs to produce active sites on cellulose in the following manner ... 

An effective method of NVF chemical modification is graft copolymerization [34,35]. This reaction is initiated by free radicals of the cellulose molecule. The cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. Then, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with a suitable solution (compatible with the polymer matrix), for example vinyl monomer [35] acrylonitrile [34], methyl methacrylate [47], polystyrene [41]. The resulting copolymer possesses properties characteristic of both fibrous cellulose and grafted polymer. 

Copper phthalocyanine derivatives are well established as turquoise blue direct and reactive dyes for cellulosic fibres. Chlorosulphonation at the 3-position, followed by hydrolysis, yields sulphonated direct dyes such as Cl Direct Blue 86 (5.32 X = H) and Blue 87 (5.32 X = S03Na). Solubility and dyeing properties can be varied by introducing four chlorosulphonyl groups, some of which are hydrolysed and some converted to sulphonamide by reaction with ammonia or alkylamines. This approach is also the main route to reactive dyes of the copper phthalocyanine type. The reactive system Z is linked to a 3-sulphonyl site... 

The kinetics of homogeneous reaction of several reactive dyes of the vinylsulphone type with methyl-a-D-glucoside (7.9), selected as a soluble model for cellulose, were studied in aqueous dioxan solution. The relative reactivities of the various hydroxy groups in the model compound were compared by n.m.r. spectroscopy and the reaction products were separated by a t.l.c. double-scanning method [38]. The only sites of reaction with the vinylsulphone system were the hydroxy groups located at the C4 and C6 positions [39,40]. 

The kinetics of alkaline hydrolysis of a series of eleven vinylsulphone reactive dyes fixed on cellulose have been investigated at 50 °C and pH 11. Bimodal hydrolytic behaviour was observed under these conditions, the reaction rates being rapid at first but becoming slower as the concentration of fixed dye remaining gradually decreased. These results were attributed to differences in the degree of accessibility of the sites of reaction of the dyes within the fibre structure [87]. 

The hydroxyl radicals formed may abstract hydrogen from the cellulose fiber substrate which gives grafting sites and subsequently grafted polymer with monomer present. The HO- radicals may also initiate homopolymerization. This means that reaction (17) is not specific for initiation of grafting. Another disadvantage is that the Fe + ions formed - if not carefully removed -may cause discoloration of the resulting product. 

Proceeding on the same line, Hagerdal et al. reported that perfluorinated resin supported sulfonic sites (NATION 501) can hydrolyze disaccharides [25]. In particular, these authors studied the effect of the addition of sodium chloride in the hydrolysis of cellobiose, a subunit of cellulose much more resistant to hydrolysis than sucrose. They observed that the presence of sodium chloride in water dramatically increased the conversion of cellobiose. Indeed, in the presence of 10 wt% of sodium chloride, 80% of cellobiose was converted at 95°C after 6 h. For comparison, when 1% of sodium chloride was added, only 50% of cellobiose was hydrolyzed. It should be noted that without addition of sodium chloride only 15% conversion was achieved, thus pushing forward the key role of sodium chloride on the reaction rate. Effect of salt on the reaction rate was attributed to a change of the pH caused by the release of proton in the reaction medium (due to an exchange of the supported proton by sodium). 

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