Solvents, for acetylation of cellulose

Halohydrocarbons, as solvents for acetylation of cellulose, I, 313 Haptenes, dextrans as, II, 215 pneumococcus polysaccharide as, II, 221... 

Recently, use of LiCl/DMAc and LiCl/l,3-dimethyl-2-imidazolidinone as solvent systems for acetylation of cellulose by acetic anhydride/pyridine has been compared. A DS of 1.4 was obtained the substituent distribution in the products synthesized in both solvents was found to be the same, with reactivity order Ce > C2 > C3. Therefore, the latter solvent system does not appear to be better than the much less expensive LiCl/DMAc, at least for this reaction. It appears, however, to be especially efficient for etherification reactions [178]. It is possible, however, that the effect of cellulose aggregation is more important for its reaction with the (less reactive) halides than with acid anhydrides this being the reason for the better performance of the latter solvent system in ether formation, since it is more efficient in cellulose dissolution. 

Recent Developments. A considerable amount of cellulose acetate is manufactured by the batch process, as described previously. In order to reduce production costs, efforts have been made to develop a continuous process that includes continuous activation, acetylation, hydrolysis, and precipitation. In this process, the reaction mixture, ie, cellulose, anhydride, catalyst, and solvent, pass continuously through a number of successive reaction zones, each of which is agitated (92,93). In a similar process, the reaction mass is passed through tubular zones in which the mixture is forced through screens of successively small openings to homogenize the mixture effectively (94). Other similar methods for continuous acetylation of cellulose have been described (95,96). 

Miyamoto et al. [ 164,165] conducted homogeneous acetylation of cellulose in a 10% LiCl-dimethylacetamide solvent. The 6-OH group was about twice as reactive as the 2-OH group for DS up to 1.2, and the factor was reduced to 1.4 for higher DS samples. 

Cellulose acetate was produced by the room temperature homogeneous acetylation of cellulose with acetic anhydride in the presence of amines in DMSO-H2CO systems Cellusose acetate was also prepared by adding 7ml acetic anhydride to 100ml of a solution of 5 percent cellulose in DMAc-LiCl to which had been added 0.5ml of 71 percent perchloric acid solution. The solution was allowed to stand for one day at room temperature and was then heated at 40°C for 2 hrs. before the ester was precipitated from the solvent by the addition of methanol. Cellulose and chitin have also been esterified and carbanylated at room temperature by the addition of phenyl isocyanate and pyridine to a solution in DMAc-LiCl systems. 

After the acetylation of cellulose, effective recycling of the solvent ILs is absolutely required. For example, at the end of the acetylation comhusk cellulose in the presence of the IL AmimCl, the comhusk cellulose acetates (CCA) product was precipitated with a large excess of water. The polymer was filtered off, and the residual IL in the filtrate was recovered by a simple evaporation, giving dean AmimCl. The purity of the recovered AmimCl was above 99%, confirmed by IH NMR sp>ectroscopy [Cao et al., 2007). 

Cellulose Acetates (CA). Acetylation of cellulose is obtained by reaction of the natural polymer with acetic anhydride. The reaction is catalyzed by sulfuric acid. However, to obtain derivatives of high D.S. (>92%), it is advisable to operate in the presence of a diluent. When the diluent is a solvent of cellulose acetate—for instance, acetic acid—the cellulose is gradually swollen by the solvent as substitution proceeds, the latter being catalyzed by mineral acids (Lewis or Brpnsted acid). This process is called acetylation in the homogeneous phase. 

The solution process consists of four steps preparation of cellulose for acetylation, acetylation, hydrolysis, and recovery of cellulose acetate polymer and solvents. A schematic of the total acetate process is shown in Figure 9. 

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. 

---