Chemical Modification of Polysaccharides

Simple chemical reactions on natural polymers are well known to produce polymers such as hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, cellulose acetates and propionates, and many others that have been in commerce for many years. Their biodegradability is often taken for granted, but, in many cases, is not at all well established. Carboxymethyl cellulose, for example, has been claimed as biodegradable below a degree of substitution of about 2, which is similar to the claim for cellulose acetate. More recently, there has been attempts to more rigorously quantify the biodegradation of the cellulose acetates [174,175] and to establish a property-biodegradation relationship.

Rhone-Poulenc indicates that cellulose acetate with a degree of substitution of about 2 is biodegradable, in agreement with its earlier reference [176]. Cellulose has been discussed as a renewable resource [177]. A recent publication [178] on chitosan reacted with citric acid indicates that the ampholytic product is biodegradable. Chitosan acetate liquid crystals [179], hydrophobic amide derivatives [180], and crosslinked chitosan [181] are also claimed to be biodegradable.

Carboxylated natural polymers have been known for many years with the introduction of carboxymethyl cellulose, as noted above. This product has wide use in detergents and household cleaning formulations, even though of questionable biodegradability at the level of substitution required for performance. Nevertheless, carboxylated polysaccharides are a desirable goal for many applications, and the balance of biodegradation with performance has been recognized as an attractive target with a high probability of success by many people. Three approaches have been employed: esterification, oxidation, or Michael addition of the hydroxyl groups with a suitable vinyl receptor. Attempts have also been made to react specifically at the primary hydroxyl, the 6 position, or secondary sites at the 2,3 positions of polysaccharides.

Esterification with poly(carboxylic anhydrides) can be controlled to minimize diesterification and crosslinking to produce carboxylated cellulosic esters. Eastman Kodak in a recent patent claimed the succinylation of cellulose to different degrees: 1 per 3 anhydroglucose rings [182] and 1 per 2 rings [183]. Henkel [184] also has a patent for a surfactant by the esterification of cellulose with alkenylsuccinic anhydride, presumably substitution degree governs the hydrophile-hydrophobe balance of the product and its surfactant properties.

Oxidation of polysaccharides is a far more attractive route to polycarboxylates and potentially cheaper and cleaner than esterification. Selectivity at the 2,3-secondary hydroxyls and the 6-primary is claimed possible. Total biodegradation with acceptable property balance has not yet been achieved, though. For the most part, oxidations have been with hypochlorite/periodate under alkaline conditions, but more recently catalytic oxidation has appeared as a possibility, and chemical oxidations have also been developed that are specific for the 6-hydroxyl group.

Matsumura [185-188] has oxidized a wide range of polysaccharides, starch, xyloses, amyloses, pectins, and the like with hypochlorite/periodate. The products are either biodegradable at low oxidation levels or functional at high oxidation levels; the balance has not yet been established for commercial success. Other than Matsumura, van Bekkum and co-workers, at Delft University, has been the major player in the search to control the hypochlorite/periodate liquid-phase oxidations of starches [189-191]. He has been searching for catalytic processes to speed up the oxidation with hypochlorite. Hypobromite is a more active oxidant than hypochlorite but more expensive, however, it may be generated in situ from the cheap hypochlorite and bromide ion in one solution [191, 192]. This is shown in Scheme 16.

deNooy et al. [193] has also published a method for oxidizing specifically the 6-hydroxyl group (primary) of starch by using TEMPO and bromide/hypochlorite, as shown below in Scheme 17.

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Scheme 17. Catalytic oxidation of C-6 hydroxyl of polysaccharides.

Chemical oxidation of polysaccharides with strong acid is reportedly selective at the 6-hydroxyl, with a mixture of nitric acid/sulfuric acid/vanadium salts [194], which is claimed as specific for up to 40% conversion. Alternatively, dinitro-gen tetroxide in carbon tetrachloride has similar specificity up to 25% conversion [195].

Catalytic oxidation in the presence of metals and oxygen is claimed as both nonspecific and specific for the 6-hydroxyl oxidation depending on the metals used and the conditions employed for the oxidation. Nonspecific oxidation is achieved with silver or copper and oxygen [196] and with noble metals with bismuth and oxygen [197]. Specificity results with platinum catalyst at pH 6-10 in water in the presence of oxygen [198]. A related patent to produce a water-soluble carboxylated derivative of starch is Hoechst's on the oxidation of ethoxylated starch and another on the oxidation of sucrose to a tricarboxylic acid; all the oxidations are specific for primary hydroxyls and use a platinum catalyst at a pH near neutrality in the presence of oxygen [199, 200].

For further reading on polysaccharides as raw materials in the detergent industry may be found in articles by Swift et al. [201, 202].

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