Cellulose esters hydroxyl

The acetyl content of cellulose acetate may be calculated by difference from the hydroxyl content, which is usually determined by carbanilation of the ester hydroxy groups in pyridine solvent with phenyl isocyanate [103-71-9J, followed by measurement of uv absorption of the combined carbanilate. Methods for determining cellulose ester hydroxyl content by near-infrared spectroscopy (111) and acid content by nmr spectroscopy (112) and pyrolysis gas chromatography (113) have been reported. 

In unmodified cellulose the hydroxyl groups give a large amount of hydrogen bonding which leads to insolubility in most solvents. On the other hand if these arc changed by chemical reactions to ether or ester groups a much more tractable material results. Cellulose acetate, butyrate and nitrate methyl and ethyl ether and carboxy methyl ether are widely used modified celluloses. Starches also are modified, but much less commercial success has been had with them. 

Cellulose, the most abundant renewable agricultural raw material, is transformed into multifarious products affecting every phase of our daily life. The presence of active hydroxyl groups in cellulose has been utilized in a variety of chemical reactions to produce commercially important cellulose derivatives, such as cellulose ethers and cellulose esters. Although the practical purpose of cellulose derivatization is by and large to improve various properties of the original cellulose, these cellulose derivatives are often not competitive with most of the petrochemically derived synthetic polymers. In order to provide a better market position for cellulose derivatives, there is little doubt that further chemical modification is required. Grafting of vinyl monomers onto cellulose and... 

Cellulose esters are usually classified in organic and inorganic depending on the acid that is used to esterify the hydroxyl groups of cellulose. 

The cyanoethylation of polysaccharides has been studied extensively in starch and cellulose. The hydroxyl groups (typically the C-4) on the molecule react with acrylonitrile in the presence of alkali to form a cyanoethyl ester. O-Cyanoethylated cellulose is used in the paper industry to enhance the mechanical strength, heat resistance, and microbiological resistance of the paper. Cyanoethylated starch is used in the textile industry. 

Cellulose Esters. Cellulose contains primary and secondary hydroxyl groups. Hence, cellulose esters can be made with all inorganic and organic acids. Traditionally, cellulose esters are made by a controlled acid-catalyzed reaction between an acid or acid anhydride and the hydroxyl groups of cellulose. The reaction requires the absence of water for completion because it is a reversible reaction. The general reaction scheme can be illustrated as shown in Scheme 1. 

Cellulose esters and cellulose ethers are prepared based on the substitution of cellulose hydroxyl groups with short chain regents. Cellulose can also be modified by introduction of long chain polymer(s) onto its main chain. The products are mostly grafted copolymers, and in some cases, block copolymers can also be made. 

A relatively novel class of derivatives is obtained by the covalent incorporation of organometallic moieties into cellulose. For example, cellulose ferro-cenyl derivatives have been prepared by esterification of cellulose with an intermediate derived from ferrocene carboxylic acid and triphenyl phosphite in the presence of pyridine [84]. An enzymatically cleavable cellulose ester has been developed [85], and prodrugs have been coupled to the hydroxyl or carboxyl functions of C-terminal aromatic amino acids of cellulose peptide derivatives for controlled release applications [86]. 

Among the various types of cellulose derivatives, it is the cellulose esters that find the widest practical application and complete succesfully in a number of branches of the industry with synthetic polymers. The complete or partial esterification of the hydroxyl groups of cdlulose can yield cellulosic materials having such technically valuable properties as thermoplasticity, hydrophobicity, resistance to heat and light, stability to the action of putrefying microorganisms, bactericidal action, etc. 

The solubility restrictions that apply to the manufacture of the mixed esters are the same as those for the cellulose acetate, in that no soluble products are obtained by partial esterification. Hydrolysis of the esters in acid solution, however, yields uniform products showing gradually changing physical properties with increasing free hydroxyl content. The exact ratio of hydrolysis of acetyl to hydrolysis of propionyl or butyryl groups depends upon the composition of the hydrolysis solution. Thus, a cellulose acetate propionate hydrolyzed in acetic acid solution will retain a higher proportion of acetyl groups than would the same cellulose ester hydrolyzed in propionic acid. 

Of the methods of synthesis of cellulose esters, the one that has been most thoroughly studied is the reaction of trans-esterification, and this method is widely used for the synthesis of low-molecular-weight esters. The alcoholysis of a low-molecular-weight ester (methyl- and n -propyl-borate) with hydroxyl groups of cellulose was first used (37) for die preparation of cellulose borate. This was followed by the trans-esterification, with cellulose, of the esters of phosphorous acids (see above), i.e. mono-, di- and trimethylphosphites (71, 72, 75), esters of phosphonic acids (76), and also phenyl-/ -chloroethyl- and / -fluoroethylphosphites (77, 78). Of considerable interest is the reaction of alcoholysis, with cellulose, of the esters of aryl- and naphthalenesulphonic add, which results in the formation of cellulose ethers, rather than esters (79-81). 

Polymer chemists have found the fully acetylated product, called a trlester, to be of limited value in the plastics and coatings industries. Some free hydroxyl groups along the polymer chain are necessary to effect solubility, flexibility, compatibility, toughness, etc. Acylation to a predetermined degree less than the triester is not feasible if a uniform, soluble product is desired. Therefore, the cellulose is fully acylated and then hydrolyzed back to the desired hydroxyl level. The hydrolysis reaction is relatively slow and may be controlled and terminated as required. Commercial cellulose esters may contain up to 5% by weight of hydroxyl obtained in this manner. 

Importance of Hydroxyl Functionality. The hydroxyl content of each specific cellulose ester type is a very Important chemical variable. Relatively small changes may dramatically affect such properties as solubility or compatibility of the ester with other resins. Thermoplastic coatings formulated with cellulose esters with varying hydroxyl content exhibit varying degrees of compatibility with... 

Cellulose esters and ethers of various types and viscosities are characterized by their degree of substitution (DS), i.e. the average number of substituted hydroxyl groups (the maximum value is 3, see also Table 3). Furthermore, ethers or ether esters with hydroxyalkyl groups attached are characterized by the degree of reaction (DR), also frequently named molar substitution (MS), i.e. the average number of molecules of reagent (alkylene oxide) reacted with each... 

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