The ring-opening polymerization of trimethylene carbonate (TMC) using homoleptic lanthanide guanidinate complexes [RNC(NR'(2))NR](3)Ln as single component initiators has been fully investigated for the first time. The substituents on guanidinate ligands and center metals show great effect on the catalytic activities of these complexes, that is, -N(CH2)(5) > -(NPr2)-Pr-i > -NPh2 (for R'), -Cy > -Pr-i (for R), and Yb > Sm > Nd. Among them, [Ph2NC(NCy)(2)](3)Yb shows the highest catalytic activity. Some features and kinetic behaviors of the TMC polymerization initiated by [Ph2NC(NCY)(2)](3)Yb were studied in detail. The polymerization rate is first order, with the monomer concentration and M-n of the polymer increasing with the polymer yield increasing linearly. The results indicated the present system having "living character." 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 (epsilon-CL) initiated by [Ph2NC(NCy)(2)](3)Yb was also tested. (c) 2005 Wiley Periodicals, Inc.