Cellulose, alkali etherification

A purified form of cellulose is reacted with sodium hydroxide to produce a swollen alkali cellulose that is chemically more reactive than untreated cellulose. The alkali cellulose is then reacted with propylene oxide at elevated temperature and pressure. The propylene oxide can be substituted on the cellulose through an ether linkage at the three reactive hydroxyls present on each anhydroglucose monomer unit of the cellulose chain. Etherification takes place in such a way that hydroxypropyl substituent groups contain almost entirely secondary hydroxyls. The secondary hydroxyl present in the side chain is available for further reaction with the propylene oxide, and chaining-out may take place. This results in the... 

The molecular weight may be regulated by controlled degradation of the alkali cellulose in the presence of air. This can be done either before or during etherification. The molecular weight of commercial grades is usually expressed indirectly as viscosity of a 5% solution in an 80 20 toluene-ethanol mixture. 

The hydroxyl groups of the cellulose appear to be somewhat acidic. While studies of the composition of alkali cellulose and adsorption of sodium hydroxide have not clearly proved the presence of any sodium compound in alkali cellulose, the reactions of alkali cellulose with carbon disulfide and with etherifying agents would seem to justify the assumption that such an intermediate exists or that the hydroxyl hydrogen at least ionizes. This view is strengthened by the fact that the rate of etherification is proportional to a high power of the concentration of alkali.19... 

Cellulose Ethers. Cellulose ethers are formed when cellulose, in the presence of alkali or as alkali cellulose, is treated with alkyl or arylalkyl halides. Two types of reaction are employed in the preparation of cellulose ethers. The most common is nucleophilic substitution. Methylation of alkali cellulose with a methyl halide is an example of this type. The other type of etherification reaction is Michael addition. This reaction proceeds by way of an alkali-catalyzed addition of an activated vinyl group to the cellulose. The reaction of acrylonitrile with alkali cellulose is a typical example. The general reaction is outlined in Scheme 4. 

Treatments based on condensation reactions (such as in the classical Williamson synthesis) produce the most stable cotton derivatives. On the other hand, treatments based on addition reactions (such as in the Michael reaction) yield cellulose ethers that are somewhat less stable. This lower level of stability is because of the equilibrium nature of the addition reaction. Typical examples of these two types of cellulose etherification are carboxymethyla-tion [9,329,330] and cyanoethylation [9,329,330,332], respectively, both of which proceed in the presence of alkali. 

Chemical modification reactions continue to play a dominant role in improving the overall utilization of lignocellulosic materials [1,2]. The nature of modification may vary from mild pretreatment of wood with alkali or sulfite as used in the production of mechanical pulp fibers [3] to a variety of etherification, esterification, or copolymerization processes applied in the preparation of wood- [4], cellulose- [5] or lignin- [6] based materials. Since the modification of wood polymers is generally conducted in a heterogeneous system, the apparent reactivity would be influenced by both the chemical and the physical nature of the substrate as well as of the reactant molecules involved. 

The discrepancies among reported data, besides possibly being caused by different analytic techniques employed, may be partly attributed to variations in the alkali concentration used as shown in Fig. 1. Rammas and Samuelson [197] also demonstrated that the reactivity of the 2-OH and 6-OH with ethylene oxide was quite comparable in dilute alkali and that the C6 hydroxyethylation was preferentially promoted by an increase in the alkali concentration. In etherification of cotton cellulose with sodium 2-aminoethyl sulfate [192], sodium allyl sulfate [193], or acrylamide [194], the 6-OH group was generally found to be more reactive than the 2-OH group. 

Alkali cellulose is prepared by steeping cellulose obtained from wood pulp or cotton fibers in sodium hydroxide solution. The alkaline cellulose is then reacted with sodium monochloro-acetate to produce carboxymethylcellulose sodium. Sodium chloride and sodium glycolate are obtained as by-products of this etherification. 

OH. These results suggest that some of the intramolecular hydrogen bonds between the 3-OH groups and 0-5 in the adjacent anhydro-glucose residues, known to occur in native celluloses, are retained even in swollen alkali cellulose, and thus have an effect on the etherification process. 

The cellulose ethers constitute another important group of cellulose derivatives prepared from alkali cellulose by standard etherification reactions between the hydroxyl groups and an alkyl halide. The properties of the ethers depend on the extent of the reaction that is, the degree of etherification. In general, the ethyl celluloses are water-insoluble thermoplastic materials, whereas methyl ether, ethyl hydroxyethyl cellulose, and carboxymethyl cellulose are soluble in cold water and are used as viscoelastic thickeners and adhesives. 

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