Carboxylic starch

An interesting possible application in our society is in super absorbers, which are currently made with polyacrylates, but these suffer from poor biodegradability. Since the demand for short-use super absorbers is growing, a product with a short lifetime and therefore a quick and safe disposal is required. Carboxylated starch can be such a product, but first, an efficient method to oxidize starch needs to be developed. Currently used methods produce stoichiometric amounts of waste products, so the search for a catalytic process is ongoing. 

Slow but significant progress is visible in this area. For example, potato starch (containing 27 % amylase and 73 % amylopectin) can be oxidized to superabsorbing biopolymers. The three-component system H202/HBr/CH3Re03 works in the formation of carboxylated starch according to the mechanism proposed in Scheme 10 [104],... 

Catalytic oxidation in the presence of metals and oxygen is claimed as both nonspecific and specific for the 6-hydoxyl oxidation depending on the metals used and the conditions employed for the oxidation. Non-specific oxidation is achieved with silver or copper and oxygen [196] and with noble metals in combination 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 Hoechst on the oxidation of ethoxylated starch produces a water-soluble carboxylated starch and another on the oxidation of sucrose to a tricarboxylic acid are all specific for primary hydroxyls and use a platinum catalyst at a pH near neutrality in the presence of oxygen [199, 200]. 

Carboxymethylcellulose, polyethylene glycol Combination of a cellulose ether with clay Amide-modified carboxyl-containing polysaccharide Sodium aluminate and magnesium oxide Thermally stable hydroxyethylcellulose 30% ammonium or sodium thiosulfate and 20% hydroxyethylcellulose (HEC) Acrylic acid copolymer and oxyalkylene with hydrophobic group Copolymers acrylamide-acrylate and vinyl sulfonate-vinylamide Cationic polygalactomannans and anionic xanthan gum Copolymer from vinyl urethanes and acrylic acid or alkyl acrylates 2-Nitroalkyl ether-modified starch Polymer of glucuronic acid... 

Phosphated, oxidized starch with a molecular weight of 1500 to 40,000 Dalton, with a carboxyl degree of substitution of 0.30 to 0.96, is useful as a dispersant for drilling fluids [926]. 

Figure 30.3 C NMR spectrum of oxidized starch. (Signals at 171.6 and 180.5 ppm are attributed to carboxyl and aldehyde groups, respectively).

We have developed an efficient and practical method for clean oxidation of starch (21-23) resulting in the oxidation of primary alcohol function in Ce position and the cleavage of vicinal diols in C2 and C3 position (Figure 30.2). We used small amounts of cheap iron tetrasulfophthalocyanine catalyst, pure water as reaction medium and H2O2 as clean oxidant to achieve a one-pot conversion of starch resulting in the introduction of aldehyde and carboxyl functions in polymer chains. The iron content... 

Starch esters have been obtained by reactions of starch and carboxylic or sulfonic acid imidazolides in aqueous NaOH or nonaqueous solutions, as described in reference [226]. The esterification of dextran with butyric or palmitic acid using CDI in DMSO or formamide is discussed in reference [174]. 

Poly(starch-g-((l-amidoethylene)-co-(sodium 1-carboxylatoethylene))). Poly(l-amidoethylene) is, however, rarely used as a viscosifier. Instead, the homopolyraer is reacted with base (hydrolyzed with NaOH) to convert some of the amide units of the polymer to carboxylic acid units. The acid units on the hydrolyzed polymer dissociate in water and produce a polyanionic polymer. This polyelectrolyte expands in water because of ion-ion repulsion and, as an enlarged molecule, is a better viscosifier. 

The oxidation of starch in aqueous suspension with H202 in the presence of iron phthalocyanine gives both carboxylic and carbonyl groups (Table 3.1). The best yields were obtained with a molar ratio 12900/1 (0.0078 mol%), but the oxidation was still quite efficient with 0.0039 mol% of catalyst [25800 per anhydroglucose unit (AGU)/catalyst ratio]. The oxidized starch had almost the same final Fe-content as the initial potato starch. Still, the efficiency of this method in view of scaling up was limited by comparatively low activity and product isolation problems. 

This catalytic system was very flexible because by simple modification of the reaction conditions it was possible to prepare oxidized polymers with the desired level of carboxyl and carbonyl functions. No waste was formed because the process did not involve any acids, bases or buffer solutions. The incipient wetness process is very easy to scale up. Hydrophilic starch was prepared in batches of 150 L and incorporated successfully in paint formulations. Good results were also obtained with in vitro and in vivo tests for cosmetic formulation. Interestingly, this is a rather unique example of a heterogeneous catalytic process involving a soluble catalyst and a solid substrate. 

The cleavage of 1,2-diols can be inexpensively achieved at a nickel hydroxide electrode [126], while chemically more expensive reagents such as PbjOAc) or 104 must be used. The latter can be used as mediator for the indirect anodic cleavage of starch [146]. Double bonds can be cleaved at the anode to carboxylic acids by applying the double mediator RuCh, 104 [147]. 

Carbohydrates Ceiiuiose Starch 1 Hemiceiiuiose Lignin J ( monosaccharides "j ( hexoses "j Cx(H20)y < oligosaccharides > pentoses > [ chitin J ( glucosamine J (C2H20)4 unsaturated aromatic alcohols —> polyhydroxy carboxylic aoids HPOy, GO2, CH4, glucose, fructose, galactose, arabinose, ribose, xylose... 

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