Cellulose esters esterification

G. Celluloseester R esters cellulosiques Reaction of - cellulose with inorganic or organic acids yields cellulose esters (->esterification) ... 

Cellulosics. CeUulosic adhesives are obtained by modification of cellulose [9004-34-6] (qv) which comes from cotton linters and wood pulp. Cellulose can be nitrated to provide cellulose nitrate [9004-70-0] which is soluble in organic solvents. When cellulose nitrate is dissolved in amyl acetate [628-63-7] for example, a general purpose solvent-based adhesive which is both waterproof and flexible is formed. Cellulose esterification leads to materials such as cellulose acetate [9004-35-7], which has been used as a pressure-sensitive adhesive tape backing. Cellulose can also be ethoxylated, providing hydroxyethylceUulose which is useful as a thickening agent for poly(vinyl acetate) emulsion adhesives. Etherification leads to materials such as methylceUulose [9004-67-5] which are soluble in water and can be modified with glyceral [56-81-5] to produce adhesives used as wallpaper paste (see Cellulose esters Cellulose ethers). 

Cellulose triacetate is obtained by the esterification of cellulose (qv) with acetic anhydride (see Cellulose esters). Commercial triacetate is not quite the precise chemical entity depicted as (1) because acetylation does not quite reach the maximum 3.0 acetyl groups per glucose unit. Secondary cellulose acetate is obtained by hydrolysis of the triacetate to an average degree of substitution (DS) of 2.4 acetyl groups per glucose unit. There is no satisfactory commercial means to acetylate direcdy to the 2.4 acetyl level and obtain a secondary acetate that has the desired solubiUty needed for fiber preparation. 

Determining the degree of substitution using standard proton nmr refles on the integral ratio between the ceUulosic ring protons ( i 5.0-2.96) and the ester alkyl protons ( i 1.26 for butyryl and propionyl and i 2.06 for acetyl methyl groups). This simple procedure is used extensively to determine the extent of esterification and is currently the fastest, easiest way for determining the DS of mixed cellulose esters. 

However, this method is appHed only when esterification cannot be effected by the usual acid—alcohol reaction because of the higher cost of the anhydrides. The production of cellulose acetate (see Fibers, cellulose esters), phenyl acetate (used in acetaminophen production), and aspirin (acetylsahcyhc acid) (see Salicylic acid) are examples of the large-scale use of acetic anhydride. The speed of acylation is greatiy increased by the use of catalysts (68) such as sulfuric acid, perchloric acid, trifluoroacetic acid, phosphoms pentoxide, 2inc chloride, ferric chloride, sodium acetate, and tertiary amines, eg, 4-dimethylaminopyridine. 

The main focus of this account is to review some aspects of the chemistry of cellulose esters. Emphasis is placed on the esterification reaction, carried out under the homogenous reaction conditions (HRC) scheme. Unconventional methods for the synthesis of cellulose derivatives, e.g., esters and ethers... 

The discussion is organized in the following order First the advantages of HRC scheme, relative to the industrial (i.e., heterogenous) process are briefly commented on second, the relevance of celMose activation and the physical state of its solution to optimization of esterification are discussed. Finally, the use of recently introduced solvent systems and synthetic schemes, designed in order to obtain new, potentially useful cellulose esters with controlled, reproducible properties is reviewed. A comment on the conformity of these methods with the concepts of green chemistry is also included. 

The primary OH group can be selectively blocked by the bulky triphenyl-methyl (trityl) moiety, followed by esterification at the secondary OH groups and removal of the protecting trityl group. Thus 2,3-di-O-acetyl cellulose has been obtained by this procedure. Moreover, regioselectively substituted mixed cellulose esters, acetate/propionate, were prepared by subsequent acy-... 

Two methods of prepn are listed in Ref 3 a)By esterification of lactic acid with ethanol and b)By combining acetaldehyde with hydrocyanic. acid to form acetaldehyde cyanohydrin, and this is treated wi th ethanol HC1 to ethyl lactate. Used as a solvent for cellulose acetate and nitrate, other cellulose esters, resins, lacquers, paints and enamels Refs l)Beil 3, 264, 267, 280,(102,109) ... 

CCOOC3Ha)2 mw 146.14, col, unstable aromatic liquid combustible but not flammable sp gr 1-079 at 20/4° (Lange), 1.09 at 20/20° (Ref 3), fr P -40.6°, bp 185-4-186° fl p 168°F v si isol in w with gradual deoompn miscible with ale, eth, eth acetate and other common org solvents. Can be prepd by standard esterification procedure using ethanol oxalic acid. The final purification, however calls for specific technique and equipment (Ref 3). Used as solvent for cellulose esters ethers and for synthetic resins also for radio tube cathode fixing lacquers, pharmaceuticals, etc... 

Polyesters from propylene glycol and dicarboxylic acids, especially adipic and sebacic acid, are commercial products suggested for PVC as well as for cellulose esters. The well known Paraplex resins of Rohm Haas, which are compatible with nitrile and GRS rubber, belong to this group. Other products are the Ultramolls of Farbenfabriken Bayer. Some polyesters of this type have a tendency to exude on storage, especially if esterification is not complete. 

Most commercial preparations of cellulose esters still follow, basically, the methods described by Franchimom and Miles—esterification with sulfuric acid catalyst followed by hydrolysis. The principal steps in this process are shown in Fig. I. 

Cellulose esters (e.g., cellulose triacetate, cellulose diacetate, cellulose propionate, and cellulose butyrate) are prepared by initially treating cellulose with glacial acetic acid (or propionic acid and butyric acid) followed by the corresponding acid anhydride with a trace of strong acid as a catalyst in chlorinated hydrocarbon. Complete esterification reactions result in the formation of a triester, which undergoes water hydrolysis to form a diester. Cellulose acetate alone or in combination with cellulose triacetate or cellulose butyrate is used as a semipermeable membrane for osmotic pumping tablets, primarily in controlled release systems. The permeability of the membrane can be further modulated by adding water-soluble excipients to the cellulose esters. 

Many cellulose esters, such as cellulose nitrate, cellulose acetate, and mixed esters of cellulose acetate butyrate, have found popularity in commercial scale production. Many new esters continue to appear in the market. Traditionally, esterification is conducted on a heterogeneous system (topo-chemical reaction) however, homogeneous systems employing mixed organic solvents have recently been developed. For example, Ikeda et al. [17] demonstrated that homogeneous esterification and acetalization of cellulose in LiCF DM AC can be achieved. 

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. 

No systematic studies of the principles governing the synthesis of cellulose esters by the trans-esterification reaction had been available in the literature until the present authom published work permitting one to ascertain the relation between the reactivity cf low-mol ular-weight esters and their structure, and the direction trf the reactions involved. 

Much helpful information on starch esterification methods and on the probable properties of starch esters may be obtained by examining similar reactions and esters in the cellulose field. This is because both starch and cellulose consist of polymerized glucose units capable of esterification to the triester stage, and because the superior ability of the cellulose esters to form plastics, films, fibers, and lacquers has promoted an accumulation of information on cellulose esters which far exceeds the present knowledge of starch esters. 

It is natural that with the growing interest in cellulose acetate, chemists have investigated other organic cellulose esters. Cellulose formate, the ester of the lowest member of the fatty acid series, has been studied. sufficiently to show that it is unlikely to be of commercial im-portance. It is difficult to produce a high degree of esterification of cellulose with this acid, and the ester which is obtained is very limited in solubility, and is highly unstable toward moisture and elevated temperatures. 

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. 

VL Discussion of the Principles Governing the Synthesis of Cellulose Esters by the Trans-esterification Reaction.124... 

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