Heterometal alkoxide clusters of lanthanide and sodium, LnNa(8)[OC(CH3)(3)](10)(OH) [where Ln = Nd (1) or Yb (2)], which were synthesized by the metathesis reaction of LnCl(3) with NaOC(CH3)(3) and NaOH in a 1 : 10 : 1 molar ratio in high yields and fully characterized including X-ray analysis for 2, were found to be highly active catalysts in the ring-opening polymerization of epsilon-caprolactone and trimethylene carbonate and their copolymerization. All of the polymers obtained showed a unimodal molecular weight distribution, indicating that I and 2 could really be used as single-component catalysts. The dependence of catalytic activity on the lanthanide metals was observed: Yb < Nd. A coordination-insertion mechanism for the ring-opening polymerization is proposed. (c) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 454-460, 2009