Esterification, Etherification, and Hydrolysis of Polymers

The chemical modifications discussed in this section are historically and scientifically so closely linked to one polymer, cellulose, that although the latter occurs primarily as a fiber and not an elastomer, a discussion of this group of cellulose modifications seems appropriate. Apart from the fact that some cellulose derivatives, like ethyl cellulose, when plasticized, can be quite elastomeric, the effects of modification of a basic polymer are particularly well demonstrable on a substance as stiff and highly crystalline as cellulose. Moreover, in view of the expected hydrocarbon shortage, cellulose may soon gain a new role as a polymeric starting commodity. 

Cellulose, identified chemically as j8-l,4-glucan, is the most widely found natural polymer, constituting the permanent structure of plant cell walls. For the general properties and chemistry of cellulose itself the reader is referred to standard textbooks and recent reviews. 

The complete solubility of cellulose in cuprammonium solutions, discovered in 1857 by Schweizer, led to the development of the rayon industry, but, as in the case of alkali cellulose, the regenerated polymer is chemically the same as the precursor. Regeneration via cellulose xanthate solutions, invented by Cross 

Since this early work, a very large range of organic acids has been used to prepare cellulose esters, mixed esters, and ethw esters (Rouse, 1965). A typical example of considerable commercial importance is the acetylation of cellulose. As in aU esterifications of macromolecular materials, the accessibility of the hydroxyl groups to the esterilying acid is of prime importance. Reaction (11.1) represents complete esterification, a process that is probably never fuUy achieved. The identification of the esterified products is, therefore, dependent not only on the content of acetyl groups but also on the location of these groups on the macromolecular backbone. Both factors are affected by the method of preparation and the esterification conditions. 

Although many esterification reactions (Bruxelles and Grassie, 1965) are based on inorganic acids, for insoluble hydroxyl compounds like cellulose, xanthation is more important. Sodium hydroxide normally is used to produce the swollen alkali cellulose, which (after aging) is reacted with carbon disulfide to form the sodium salt of cellulose xanthate  

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