Ring-opening polymerization of epsilon -caprolactone (CL) and delta -valerolactone (VL) was carried out using the initiator system [Sm2I(NPPh3)(5)(DME)] in the presence of toluene. Experimental molecular weights were in good agreement with the theoretical molecular weights for [CL](0)/[1](0) up to 350. Polydispersities are low at 0-20 degreesC but increase at higher temperatures. Addition of small amounts of coordinating solvents such as THF and DME decreased the rate of polymerization and thereby reduced the polydispersity. The active polymer chain ends have been shown by sequential copolymerization of CL and VL, resulting in block copolymers.