The paper presents the mechanism of the polymerization of epsilon-caprolactone with the use of Zr(Acac)(4). At the first stage of the initiation of the polymerization, a reaction consisting of proton transfer from caprolactone to acetylacetonate ligand and then ligand exchange reaction with release of free acetylacetone (HAcac) occur. Complex 1, arisen as a product of this reaction, is the actual initiator of the observed polymerization. During the conducted polymerization the molar mass increase is directly proportional to monomer conversion degree. On the basis of observed phenomenon, the polymerization of cyclic trimethylene carbonate (TMC) applying epsilon-caprolactone as a co-initiator is possible. Complex 1 arose in the initial phase of this reaction as well, being an equally efficient initiator for TMC polymerization. Using Zr(Acac)(4) as an initiator for an epsilon-caprolactone/TMC copolymerization resulted in obtaining a series of copolymers with varied, predicted composition and high molar mass values. (c) 2007 Elsevier Ltd. All rights reserved.