Lactide polymerization using zirconium(IV) acetylacetonate [Zr(acac)(2)] as an initiator was investigated. In the reaction between Zr(acac), and the monomer molecule, lactide deprotonation and the release of acetylacetone occurred. The structures of the obtained complexes were analyzed with high-resolution NMR spectroscopy. A computational method was used to calculate the hypothetical structures. The role of the obtained complexes in the initiation of polymerization and the reaction of chain growth was proposed. The influence of the reaction temperature on the structures of the complexes was investigated. Polylactide chain growth proceeded by an insertion-coordination mechanism. The polymer chain grew on one ligand, which was formed in advance from a deprotonated lactide. The molecular masses of the obtained polymers were the same as the theoretical masses and were directly proportional to the reaction conversion. (C) 2004 Wiley Periodicals, Inc. J Polym. Sci Part A: Polym Chem 42: 1886-1900, 2004.