MODIFICATION OF NATURAL POLYMERS BY THERMO-MECHANICAL PROCESSING
Location: Plant Polymer Research
Title: Iodine-Catalyzed Polysaccharide Esterification
Submitted to: Chemistry Today/Chimica Oggi
Publication Type: Review Article
Publication Acceptance Date: May 29, 2009
Publication Date: August 5, 2009
Citation: Biswas, A., Selling, G.W., Shogren, R.L., Willett, J.L., Buchanan, C.M., Cheng, H.C. 2009. Iodine-Catalyzed Polysaccharide Esterification. Chemistry Today/Chimica Oggi. 27(4):33-35.
Interpretive Summary: Starch and cellulose, earth's most abundant biopolymers, are of tremendous economic importance. Over 90% of cotton and 50% of wood are made of cellulose. Wood and cotton are the major resources for all cellulose products such as paper, textiles, construction materials, cardboard, as well as such cellulose derivatives as cellophane, rayon, and cellulose acetate. Similarly, acetylated starches have been known for over 100 years and have found applications. However, cellulose acetates are industrially more important and it is estimated that annually 1.5 billion pounds of cellulose acetates are manufactured globally. Cellulose acetate is widely used in textiles because of its low cost, toughness, gloss, high transparency, natural feel, and other favorable aesthetic properties. Cellulose acetate fibers in cigarette filters are designed to absorb vapors and accumulate particulate smoke components. Cellulose acetate is also used as a carrier for photographic negatives, motion picture film (celluloid), microfilm, microfiche and audio tape. We would like to report a simple solvent-less rapid method to acetylate polysaccharide (starch and cellulose). We have found that acetic anhydride in presence of catalytic amount of iodine is an excellent acylating reagent for cellulose. It is believed that iodine activates the carbonyl group of acetic anhydride, which renders it more reactive. Thus, iodine-activated acetic anhydride is a strong enough acylating agent to react with polysaccharide directly. This reagent works only in the absence of a solvent. Furthermore, when cellulose was heated with acetic acid and iodine, there was no reaction and cellulose remained unreacted. This solvent-free method that we discovered would help the cellulose acetate manufacturers to prepare cellulose acetate in an environment friendly way. This method reduces the use of solvents and acids.
A review is provided of the recent reports to use iodine-catalyzed esterification reaction to produce esters from polysaccharides. The process entails reaction of the polysaccharide with an acid anhydride in the presence of a catalytic level of iodine, and in the absence of additional solvents. The reactant stoichiometry, reaction temperature, and reaction time all have effects on the degree of substitution of the ester and the reaction yield. Included in the review are the proposed modes of action for iodine, the mechanism, and assessment of the utility of this approach.