Phosphido-diphosphine Group 3 metal complexes 1-4 [(o-C6H4PR2)(2)P-M(CH2SiMe3)(2); R = Ph, 1: M = Y, 2: M = Sc; R = iPr, 3: M = Y, 4: M = Sc] are very efficient catalysts for the ring-opening polymerization (ROP) of cyclic esters such as epsilon-caprolactone (epsilon-CL), L-Iactide, and delta-valerolactone under mild polymerization conditions. In the ROP of epsilon-CL, complexes 1-4 promote quantitative conversion of high amount of monomer (up to 3000 equiv) with very high turnover frequencies (TOF) (similar to 4 x 10(4) mol(CL)/mol(I) h) showing a catalytic activity among the highest reported in the literature. The immortal and living ROP of epsilon-CL and L-lactide is feasible by combining complexes 1-4 with 5 equiv of 2-propanol. Polymers with controlled molecular parameters (M-n, end groups) and low polydispersities (M-w/M-n = 1.05-1.09) are formed as a result of fast alkoxide/alcohol exchange. In the ROP of delta-valerolactone, complexes 1-4 showed the same activity observed for lactide (L- and D,L-lactide) producing high molecular weight polymers with narrow distribution of molar masses. Complexes 1-4 also promote the ROP of rac-beta butyrolactone affording atactic low molecular weight poly(hydroxybutyrate) bearing unsaturated end groups probably generated by elimination reactions. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 403-413, 2011