Polymers anhydroglucose

Cellulosic binders - Cellulose is a natural anhydroglucose polymer that is insoluble in water and all organic solvents. The cellulose esters, nitrocellulose (actually cellulose nitrate) and cellulose acetobutyrate are solvent-soluble derivatives used in coatings. 

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. 

An important characterization parameter for ceUulose ethers, in addition to the chemical nature of the substituent, is the extent of substitution. As the Haworth representation of the ceUulose polymer shows, it is a linear, unbranched polysaccharide composed of glucopyranose (anhydroglucose) monosaccharide units linked through thek 1,4 positions by the P anomeric configuration. 

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. 

Other methods that are used to characterize maltodextrins and corn syrup solids include liquid chromatography which can be used to quantify the relative amounts of shorter chain polymers found in a particular DE product. Maltodextrins and corn syrup solids are made up of polymers of anhydroglucose units having varied chain lengths rather than one particular polymer size (Table I). 

Due to the 3 hydroxyl groups available for oxidation within one anhydroglucose unit and due to the polymeric character of the cellulose a great variety of structural modifications and combinations is possible. As with other types of chemical changes at the cellulose molecule also in this case the oxidation can affect different structural levels differently. Depending on the oxidative stress imposed on the cellulose, the individual hydroxyls within the AGU and within the polymer chain are involved to varying extent and may respond to further treatment and reactions in a specific way. Despite their low concentration in the imol/g range, oxidative functionalities are one of the prime factors to determine macroscopic properties and chemical behavior of cellulosic materials (Fig. 1). 

The application of fluorescence labels in combination with GPC can be considered a step forward in the analysis of oxidized functionalities in cellulosics. However, a large number of questions still remain to be addressed in the future. If oxidized functionalities are considered as substituents along the polymer chain of cellulose, then a thorough analysis of the substituent distribution within the cellulose chains and per anhydroglucose unit should provide many new insights. The differentiation of aldehyde and keto functions will be a next step. Also the exact position of carbonyls (keto or aldehyde) within the AGU needs to be resolved, and differences in their reactivity determined. Furthermore, it is an open question whether oxidation occurs statistically within cellulose chains or forms clusters of highly oxidized areas. 

Starch molecules are polymers of anhydroglucose and occur in both linear and branched form. The degree of polymerization and, accordingly, the molecular weight of the naturally occurring starch molecules vary radically. Furthermore, they vary in the ratio of branched-chain polymers (amylopectin) to linear-chain polymers (amylose), both within a given type of starch and from one type to another. These factors, in addition to any type of chemical modification used, affect the viscosity, texture, and stability of the starch sols significantly. 

FIGURE 6.1 Ordinary reactions of chemical agents with cellulose are almost exclusively with the 2,3 and 6 hydroxyl groups that are not involved in formation of the linear polymer consisting of D-anhydroglucose units joined via (3-1,4-linkages. 

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