The anionic lanthanide-sodium-2,6-di-tert-butyl-phenoxide complexes [Ln (OAr)(4)][Na(DME)(3)](DME)-D-. (Ln = Nd 1 (neodymium), Sm 2 (samarium), or Gd 3 (gadolium); DME = dimethoxyethane) were synthesized by the reaction of anhydrous LnCl(3) with 4 equiv of sodium-2,6-di-tert-butyl-phenoxide NaOAr in high yields and structurally characterized. These complexes showed high catalytic activity in the ring-opening polymerizations of F-caprolactone (E-CL) and trimethylene carbonate (TMC). The catalytic activity profoundly depended on the lanthanide metals. The active order of Gd < Sm < Nd for the polymerization of F-CL and TMC was observed. The polymers obtained with these initiators all showed a unimodal molecular weight distribution, indicating that the [Ln(OAr)(4)][Na(DME)(3)](DME)-D-. anionic complexes could be used as single-component initiators. The anionic complex was more efficient than the corresponding neutral complex, Ln(OAr)(3)(THF)(2). (c) 2007 Wiley Periodicals, Inc.