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Hydrophobic Modification of Starch by Alkali‐Catalyzed Addition of 1,2‐Epoxyalkanes
Authors:Frank Bien  Berthold Wiege  Siegfried Warwel
Abstract:An efficient method of preparing hydrophobic α‐hydroxy starch ethers using aqueous alkaline conditions is described. α‐Hydroxy starch ethers were synthesized by addition of 1,2‐epoxyalkanes to an aqueous alkaline starch gel in the presence of sodium sulfate as a co‐catalyst. The reaction, carried out in a stirred autoclave at 140 °C and 3.9 bar, was optimized with respect to the concentrations of sodium hydroxide and 1,2‐epoxyalkane. Optimum yields and molar substitutions (MS) were obtained at molar ratios of sodium hydroxide to anhydroglucose unit (AGU) of 0.5 to 1.0. The amount of molar substitution could be controlled by 1,2‐epoxyalkane concentration. Thus, a series of α‐hydroxyoctyl starch ethers with MS from 0.7 to 2.4 were synthesized in yields up to 90% by using these conditions. Starches with different amylose contents were also converted to the corresponding ethers using a threefold excess of 1,2‐epoxyoctane and an equimolar ratio sodium hydroxide : AGU. The reaction is hardly effected by the origin of the starch and its amylose content. The influence of the 1,2‐epoxyalkane chain length was investigated by performing the conversion with a series of terminal epoxyalkanes from 1,2‐epoxyhexane to 1,2‐epoxydodecane. The results indicated that the hydrophobic character of the starch ethers increased by increasing the molar substitution and alkyl chain length. All products were insoluble in water, but soluble in mixtures of methanol and methylene chloride. Furthermore the starch ethers can be converted into shaped articles by extrusion technology without the addition of plasticizers.
Keywords:Hydrophobic modification  1  2‐Epoxyalkanes  Etherification
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