Cellulose cuprammonium complexes

Dissolution of the cellulose in cuprammonium solution followed by acid coagulation of extruded fibre ( cuprammonium rayon —no longer of commercial importance). In this case the acid converts the cuprammonium complex back into cellulose. 

Taking into consideration the nature of the complex compounds between nitrocotton and metals or metal oxides, Miles [15] suggested that they approach in chemical character the cuprammonium complexes of cellulose. 

In another oudine, cellulose was complexed with cuprammonium ions (Nicoll and Conaway, 1943). Lately, laboratory-scale isolation has relied on polar aprotic solvents and solvent systems, e.g., dimethylsulfoxide, pyridine, Af,7V-dimethylacetamide-lithium chloride, and l-methyl-2-pyrrolidinone-lithium chloride (Baker et al., 1978 McCormick and Shen, 1982 Seymour et al., 1982 Arnold et al., 1994). These solvents have enabled such homogeneous17 reactions as O- and N-derivatization of cellulose and chitin (Williamson and McCormick, 1994) and selective site chlorination (Ball et al., 1994). Dimethylsulfoxide was the solvent in a homogeneous reaction of cellulose and paraformaldehyde, prior to isolation of purified cellulose (Johnson et al., 1975). In yet another outline, paraformaldehyde enabled superior quality extracts when the parent tissues were presoaked in this solution (Fasihuddin et al., 1988). 

An alternate procedure used in a few specialty applications is the cuprammonium process. This involves stabilization of cellulose in an ammonia solution of cupric oxide. Solubilization occurs by complex formation of cupric ion with ammonia and the hydroxyl groups of cellulose. Regeneration of cellulose, after formation of the desired products, is accomplished by treatment with acid. The main application of the cuprammonium process is for the synthesis of films and hollow fibers for use in artificial kidney dialysis machines. The cuprammonium process yields products with superior permeability and biocompatibility properties compared to the xanthation process. Less than 1% of all regenerated cellulose is produced by the cuprammonium process. 

Today rayon is made by either the viscose or the cuprammonium process. The latter process is based on Schweitzer s discovery in 1857 that it is possible to dissolve cellulose in cuprammonium hydroxide, the soln being due to the formation of a Cu cellulose complex. The mfg procedure involves processing the cuprammonium soln by filtration and deaeration prior to pumping it thru holes in a spinneret into si alkaline w which coagulates the Cu-cellulose soln into rayon filaments. The filaments are then stretched to the desired fineness (Ref 11). The viscose process is the most widely used because of its great versatility and low cost operation. 

The literature contains numerous observations on the properties of polysaccharides in cuprammonium solutions the work on cellulose is especially voluminous. Viscometric measurements in cuprammonium solution are regularly employed to determine the size of cellulosic molecules. However, before the spatial requirements for complexing with cuprammonium became known the properties of the complexes of polysaccharides could not be interpreted in terms of the structure of their monosaccharide units. With the present understanding of cupram-monium-glycol complexing, some of the earlier observations will be reexamined. 

The chemical reactions and mechanism of fixation of the am-moniacal preservatives such as ACA have not been studied extensively. The main mechanism of fixation is believed to be the formation of insoluble copper arsenate upon evaporation of the ammonia. However, the overall mechanism is undoubtedly more complex because cuprammonium ions react by ion exchange with functional groups, such as carboxyl, in wood (52). In addition, copper complexes can be formed with cellulose (52). 

Among the best-known nonderivatizing solvent systems is a combination between copper, alkali, and ammonia termed Schweizer s reagent. Solutions of cuprammonium hydroxide have been used for both analytical and industrial cellulose dissolution. Regenerated fibers with silk-like appearance and dialysis membrane have been (and partially continue to be) industrial products on the basis of cellulose dissolution in cuprammonium hydroxide. The success of this solvent is based on the ability of copper and ammonia to complex with the glycol functionality of cellulose as shown inO Fig. 11. Because of the potential side reactions (oxidation and crosslinking, Norman compound formation), alternatives to both ammonia as well as copper have been developed. Cuen and cadoxen are related formulations based on the use of ethylene diamine and cadmium, respectively. The various combinations of alkali, ammonia. 

Complexatlon principle of cellulose with derivatizing solvent molecules illustrated for the case of cuprammonium hydroxide. Solvent molecules replace the existing hydrogen bonds with solvating Cu-complexes. (After Burchardt et al. [47])... 

Several known systems dissolve cellulose (126-129). These systems range from solutions in protonic acids (e.g., 78% phosphoric acid) to metallic complexes (e.g., cuprammonium). All known methods for dissolving cellulose can be fit into four main categories (128) cellulose acting as abase, cellulose acting as an acid, cellulose complexes, and cellulose derivatives. The cellulose derivatives are distinguished from those discussed previously in that dissolution occurs simultaneously with derivative formation and the derivative produced can easily be regenerated (129). 

Regenerated cellulose films and hollow fibres used in haemodialysers have been prepared by a method known as the cuprammonium process. Cellulose is dissolved in a solution of ammonia and cupric oxide. The complex cupric salts are water-soluble and cellulose is regenerated by treatment with acid. Cuprophan is prepared by this process. 

The Optical Rotation of Cellulose and Glycosides in Cuprammonium Hydroxide Solution. Science 99, 148 (1944) Reeves, R. E. The Optical Activity of the Copper Complexes of Polysaccharides and Substituted Methyl Glucosides. J. Biol. Chem. 154, 49 (1944). 

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