Catalytic behavior of tetrahydroborate complexes of rare earth metals, Ln(BH4)(3)(THF)(x) (1: Ln = La, x = 3; 2: Ln = Pr, x = 2; 3: Ln = Nd, x = 3; 4: Ln = Sm, x = 3; 5: Ln = Y, x = 2.5; 6: Ln = Yb, x = 3), for ring-opening polymerization (ROP) of six-membered cyclic esters, delta-valerolactone (VL) and D,L-lactide (D,L-LA), was studied. The controlled polymerization of VL with 1-6 proceeded in THF at 60 degrees C. The catalytic activities of these complexes for the ROP of VL were observed to be in order of the ionic radii of the metals: 1(La)>= 2(Pr)>= 3(Nd)>4(Sm)>5(Y)>6(Yb). The obtained polymers were demonstrated to be hydroxy-telechelic by H-1 NMR and MALDI-TOF MS spectroscopy. The controlled ROP Of D,L-LA also proceeded by these complexes. The activities of these complexes for the D,L-LA ROP were also in order of the ionic radii of the metals. (C) 2009 Elsevier Ltd. All rights reserved.