Lanthanide(II) complexes supported by amido ligands, [(C6H5)(Me3Si)N](2)Ln(DME)(2) [Ln = Sm (1) or Yb (2); DME = 1,2-dimethoxyethane] and [(C6H3-Pr-i(2)-2,6)(Me3Si)N](2)Ln(THF)(2) [Ln = Sm (3) or Yb (4); THF = tetrahydrofuran], were found to initiate the polymerization of methyl methacrylate (MMA) as efficient single-component initiators (in toluene for 3 and 4 and in toluene with a small amount of THF for 1 and 2) to produce syndiotactic polymers. The catalytic behavior was highly dependent on both the amido ligand and the polymerization temperature. Initiators 3 and 4 initiated MMA polymerization over a wide range of temperatures (20 degrees C to -40 degrees C), whereas the polymerization with 1 and 2 proceeded smoothly only at low temperatures (<= 0 degrees C). The kinetic behavior and some features of the polymerizations of MMA initiated by 3 and 4 were Studied at -40 degrees C. The polymerization rate was first-order with the monomer concentration. The molar masses of the polymers increased linearly with the increase in the polymer yields, whereas the molar mass distributions remained narrow and unchanged throughout the polymerization; this indicated that these systems had living character. A polymerization mechanism initiated by bimetallic bisenolate formed in situ was proposed. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2403-2409, 2009