Use of monocarboxylic iron derivatives in the ring-opening polymerization of L-lactide

L-Lactide was ring-opening polymerized in the melt by using different organ ic monocarboxylic iron complexes. The complexes were those of iron and acet ic acid, butyric acid, isobutyric acid, dichloroacetic acid, and trifluoroa cetic acid. The polymerization temperature was in the range 170-210 degrees C, polymerization time between 0.5 and 25 h, and amount of catalyst added varied between 0.12 and 1.20 wt %. Iron butyrate and iron dichloroacetate c omplexes were low efficient catalysts in the ring-opening polymerization du e to hydrolysis during the preparation. Iron acetate, iron trifluoroacetate , and iron isobutyrate complexes were efficient catalysts yielding a high m olar mass poly(L-lactide) with a high monomer conversion. Under optimum con ditions a poly(L-lactide) with a molar mass (M-w) of ca. 150 000 g/mol coul d be prepared. Monomer conversions over 85% were obtained in many experimen ts. High polymerization temperatures are required though with these kinds o f iron catalysts, and some racemization of the polymerization products is e vident. The polymerization experiments indicate that the oxidation state of the iron has an influence on the efficiency of the catalysts and that the iron is chemically bound to the polymer.