Starch-lanthanum complexes

Starch-lanthanum complexes were produced by the reaction of soluble starch:lanthanum triisopropoxide [La(OPr-i)(3)] using the proportions 1:1, 1:2, and 1:3. The reactions were performed by either of the two methods: (i) 48 h of reflux in dry toluene under argon and (ii) by heating for 20-30 min in a microwave oven at high energy. Starch dextrinization resulted from the microwave-induced reaction of starch with lanthanum chloride. Depending on the reaction method and reagent proportions, different starch-lanthanum complexes were formed according to thermogravimetric analysis, differential thermal analysis, and X-ray photoelectron spectroscopy. The latter method proved the chemical bonding of lanthanum to starch. A product with intermolecular O-La(OPr-i)-O bridges was synthesized by reacting toluene with a 1:1 reagent blend. A starch-lanthanum complex with intramolecular O-La(OPr-i)-O bridges formed upon microwave heating the same blend. Increased concentrations of the lanthanum reagent resulted in products simultaneously having intra- and inter-molecular oxygen-lanthanum bridges. Excess, unreacted lanthanum reagent remained in the final product in the case of the 1:3 starch:lanthanum reagent blends. Starch-lanthanum complexes readily hydrolyzed in open air. (C) 2000 Elsevier Science Ltd. All rights reserved.