The samarium(II) aryloxide complexes Sm(OAr)(2)(THF)(3) and [(C5Me5)Sm(mu-OAr)](2) (Ar = (C6H2Bu2)-Bu-t-2,6-Me-4) showed an extremely high activity for the ring-opening polymerization of epsilon-caprolactone (CL) and delta-valerolactone (VL). By using Sm(OAr)(2)(THF)(3) as an initiator, polyesters with very high molecular weight (M-n up to 6 x 10(5)) and relatively narrow molecular weight distributions (M-w/M-n < 1.65) could be quantitatively obtained within a few minutes at room temperature, gamma-Butyrolactone (BL) did not polymerize under the similar conditions. However, the copolymerization of BL with CL took place under the coexistence of both monomers, which gave CL-BL copolymers in which the BL units all exist in an isolated form, and the CL units all in blocks, as confirmed by the two-dimensional H-1-C-13 HMQC and HMBC NMR studies. Part of the previously reported C-13 NMR spectrum of poly(epsilon-caprolactone) was reassigned on the basis of the two-dimensional H-1-C-13 HMQC NMR Study In all these polymerization reactions, incorporation of the CH3O group of quenching methanol into the end of polymer chains was observed.