The ring-opening polymerization of trimethylene carbonate (TMC) using homoleptic lanthanide amidinate complexes [CyNC(R)NCy](3)Ln as single component initiators has been fully investigated for the first time. The substituents on amidinate ligands and center metals show great effect on the catalytic activities of these complexes, that is, Me > Ph, and La > Nd > Sm > Yb. Among them, [CyNC(Me)NCy](3)La shows the highest catalytic activity. Some features of the TMC polymerization initiated by [CyNC(Me)NCy](3)La were studied in detail. A mechanism that the polymerization occurs via acyl-oxygen bond cleavage rather than alkyl-oxygen bond cleavage was proposed. The copolymerization of TMC with epsilon-caprolactone initiated by [CyNC(Me)NCy](3)La was also tested. (c) 2006 Wiley Periodicals, Inc.