The proposed mechanism of initiation and course of ring-opening polymerization of cyclic trimethylene carbonate (TMC) involving zinc(II) acetylacetonate is in accordance with the mechanism of monomer activation. At the first stage of the process, coordination of carbonate to Zn(Acac)(2)center dot H2O complex occurs with the release of weakly coordinated water molecules. This free water molecule reacts with active TMC-Zn(Acac)(2) complex. The reaction results in the formation of propanediol and CO2 emission. During further stages of the investigated process, the formed propanediols, or later the oligomeric diols produced with polymerization, are cocatalysts of the chain propagation reaction. The chain propagation occurs because of repeating activation of the TMC monomer through the creation of an active structure resulting in the exchange/transfer reaction between the zinc complex and the monomer, with its following attachment to the hydroxyl groups, carbonate ring opening, and formation of the carbonic unit of polymer chain. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 2504-2512, 2011