Bulk Ring-Opening Polymerization of Lactides Initiated by Ferric Alkoxides

Abstract

Ferric alkoxides (Fe(OR)3), where R = Et, nPr, iPr, and nBu, were used as initiators for bulk ring-opening polymerization of lactides. It was showed that these ferric alkoxides were efficient initiators for the ring-opening polymerization of lactides with higher than 90% of monomer conversions polymerized at 130 °C for 36 h. Maximum molecular weights were achieved at 1/1000 of [I]/[M] mole ratio and then decreased gradually with increasing amount of initiator and prolonging polymerization time. Poly(dl-lactide) and poly(l-lactide) with the maximum viscosity-average molecular weights (Mv) of 7.28 × 104 and 19.00 × 104, respectively, were synthesized by using ferric ethoxide as initiator. The molecular weight decreased and the molecular weight distribution broadened as the polymerization temperature increased. With increasing the bulkiness of the ligands of the ferric alkoxides, the molecular weight decreased and the molecular weight distribution broadened. 1H and 13C NMR analyses indicated that no racemization occurred during the ring-opening polymerization of l-lactide, while isotactic stereoselectivity addition was found in the ring-opening polymerization of dl-lactide. Intermolecular transesterification took place during polymerization of dl-lactide as evidenced by the results of MALDI−TOF MS analysis, and the quantitative evaluation for each initiation system was made by 13C NMR analysis. ICP-AES, 1H NMR, and MALDI-TOF MS analyses indicated that the polymerization of lactides proceeded via a coordination−insertion mechanism involving cleavage of the acyl−oxygen bond of the lactides. All alkoxy groups in Fe(OR)3 were converted into polylactide growing chains.