tically suppress the transesterification reactions by screening linear polymer chains from the active center during the polymerization of є-caprolactone (CL) with rare earth alkoxide. Therefore, isopropoxy rare earth diethyl acetoacetate ((EA)2LnOiPr Ln=Nd, Y) and Nd(OiPr)3-donor adducts (1,10-phenanthroline, 2,2¢-bipyridyl, 18-crown-6 ether) are excellent catalysts for the living polymerization of є-caprolactone, giving poly(є-caprolactone) (PCL) with a narrow molecular weight distribution. Block copolymerizations of є-caprolactone with trimethylene carbonate (TMC) and D,L-lactide (LA) have been attained successfully using (EA)2LnOiPr as catalyst. The characterizations by GPC, NMR, DSC, and polarizing microscope showed that the block copolymers, P(CL-b-TMC) and P(CL-b-D,L-lactide), have well-controlled sequences without random placement.