Lanthanide metal (11) 2,6-di-tert-butylphen-oxide complexes (ArO)(2)Ln(THF)(3) (Ln = Sm 1, Yb 2) alone have been developed to catalyze the ring-opening polymerization of trimethylenecarbonate (TMC) and random copolymerization of TMC and E-caprolactone (epsilon-CL) for the first time. The influence of reaction conditions, such as initiator, initiator concentration, polymerization temperature, and polymerization time, on monomer conversion, molecular weight, and molecular weight distribution of the resulting PTMC was investigated. It was found that the divalent complex 1 showed higher activity for the polymerization of TMC than complex 2. The random structure and thermal behavior of the copolymers P(TMC-co-CL) have been characterized by H-1 NMR, (13) C NMR, GPC, and DSC analysis. (C) 2007 Wiley Periodicals, Inc.