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 organic monocarboxylic iron complexes. The complexes were those of iron and acetic acid, butyric acid, isobutyric acid, dichloroacetic acid, and trifluoroacetic 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 complexes were low efficient catalysts in the ring-opening polymerization due to hydrolysis during the preparation. Iron acetate, iron trifluoroacetate, and iron isobutyrate complexes were efficient catalysts yielding a high molar mass poly(L-lactide) with a high monomer conversion. Under optimum conditions a poly(L-lactide) with a molar mass (M(w)) of ca. 150 000 g/mol could be prepared. Monomer conversions over 85% were obtained in many experiments. High polymerization temperatures are required though with these kinds of iron catalysts, and some racemization of the polymerization products is evident. 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.