New Sm(III) mu-halo-bis(trimethylsilyl)amido complexes [Sm(mu-X) {N(SiMe3)(2)}(2)(THF)](2) (X = Cl, Br) were synthesized and their crystal structures were analyzed. These complexes were used as catalysts for the homopolymerization of epsilon-caprolactone and delta-valerolactone and found to initiate ring-opening polymerization (ROP) of these monomers in high conversion over a short period, giving homopolymers with a moderate polydispersity index (similar to 2.0). The effect of temperature and the catalyst amount on the molecular weight distribution was also studied. Mechanistic studies were conducted using 1 : 1 and 1 : 6 molar ratios of mu-halo-bis-(trimethylsilyl)amido complexes and epsilon-caprolactone by H-1-NMR and C-13-NMR spectroscopic techniques and revealed the presence of more than one initiating species.