Anhydroglucose units

The main raw material required for the production of viscose is ceUulose (qv), a natural polymer of D-glucose (Fig. 1). The repeating monomer unit is a pair of anhydroglucose units (AGU). CeUulose and starch (qv) are identical but for the way in which the ring oxygen atoms alternate from side to side of the polymer chain (beta linkages) in ceUulose, but remain on the same side (alpha linkages) in starch. 

In theory, for regular rayon manufacture only one of the hydroxyl groups on each pair of anhydroglucose units needs to be replaced by a xanthate group, ie, the target degree of substitution (DS) is 0.5, which if achievable without waste would need 23% CS2 on ceUulose. 

Fig. 6. Conversion of weight percent to degree of substitution per anhydroglucose unit (max = 3.0), where is butyryl, is propionyl, and 0 is acetyl.

The stmcturaUy similar starch amylose polymer is linked through the a anomeric configuration. The three hydroxyl functions pet anhydroglucose unit ate noteworthy these hydroxyls ate the active sites for ether formation. 

Other examples illustrating the effect of substituent distribution on properties include (/) enzymatic stabiUty of hydroxyethjlceUulose (16,17) (2) salt compatibihty of carboxymethylceUulose (18,19) and (J) thermal gelation properties of methylceUulose (20). The enzymatic stabUity of hydroxyethylceUulose is an example where the actual position of the substituents within the anhydroglucose units is considered important. Increasing substitution at the C2 position promotes better resistance toward enzymatic cleavage of the polymer chain. Positional distribution is also a factor in the other two examples. 

Many cellulose derivatives have been prepared of which the esters and ethers are important. In these materials the hydroxyl groups are replaced by other substituent groups. The degree of substitution is the term given to the average number of hydroxyl groups per anhydroglucose unit that have been replaced. 

The sorption of water by excipients derived from cellulose and starch has been considered by numerous workers, with at least three thermodynamic states having been identified [82]. Water may be directly and tightly bound at a 1 1 stoichiometry per anhydroglucose unit, unrestricted water having properties almost equivalent to bulk water, or water having properties intermediate between these two extremes. The water sorption characteristics of potato starch and microcrystalline cellulose have been determined, and comparison of these is found in Fig. 11. While starch freely adsorbs water at essentially all relative humidity values, microcrystalline cellulose only does so at elevated humidity values. These trends have been interpreted in terms of the degree of available cellulosic hydroxy groups on the surfaces, and as a function of the amount of amorphous material present [83]. 

The oxidation of starch in aqueous suspension with H202 in the presence of iron phthalocyanine gives both carboxylic and carbonyl groups (Table 3.1). The best yields were obtained with a molar ratio 12900/1 (0.0078 mol%), but the oxidation was still quite efficient with 0.0039 mol% of catalyst [25800 per anhydroglucose unit (AGU)/catalyst ratio]. The oxidized starch had almost the same final Fe-content as the initial potato starch. Still, the efficiency of this method in view of scaling up was limited by comparatively low activity and product isolation problems. 

How many hydroxyl groups are present on each anhydroglucose unit in cellulose ... 

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