Cellulose derivatives are widely used in biomedical and pharmaceutical applications. Most significant from a commercial and technical point of view are cellulose ether and cellulose ether.
The common structural element of all cellulose derivatives is the polymeric backbone of cellulose, which contains as the basic repeating structure a β-d-anhydroglucose unit with three hydroxyl groups at the C–2, C–3, and C–6 positions available for substitution.
The nature of the substituent and the number of substituted hydroxyl groups are mainly responsible for the individual properties of the different cellulose derivatives like solubility, gelation, or water retention.
Important substituents are low molecular mass alkyl (ethers) and aliphatic or aromatic carboxylate groups (esters), which are listed in Table 27.1.
The amount of substitution groups on the anhydroglucose unit is usually designated by the degree of substitution (DS), the average number of substituted hydroxyl groups with a maximum value of three or by weight percent.
Hydroxyalkyl substituents are characterized by weight percent or by the molar degree of substitution (MS), which is the average number of alkylene oxide reacted with each anhydroglucose unit. The MS includes also side chain formation due to further etherification of hydroxyl groups of the hydroxyalkyl substituent and may take on any value.
Other characteristics of cellulose derivatives that influence its properties are molecular weight (MW), MW distribution, and substituent distribution in and over the polymer chains.The cellulose ether production process is heterogeneous in nature and consists of two major steps: an initial mercerization step and the final etherification step. Both steps may be conducted simultaneously or subsequently. In the mercerization step, the cellulose is being suspended and pre-activated in aqeous caustic soda and then reacted with alkyl chlorides (Williamson etheri cation) and/or epoxides (alkali-catalyzed oxalkylation) as etherifying reagents (etheri cation step).Purification is performed by hot water washing or by treatment with mixtures of water and organic solvents for cellulose ethers without occulation point.
For the synthesis of cellulose ether at industrial scale, purified cellulose ether is reacted with carboxylic anhydrides in acetic acid as solvent system and sodium acetate as catalyst. The dissolved cellulose ether is being obtained then by precipitation after addition of water. Purification is performed by washing with water.