Three novel magnesium aryloxides, [(MCIMP)(2)Mg-2(THF)](2) (1), [(EDBP)Mg(Et2O)](2) (2), and [(EDBP)Mg(THF)](2) (3), have been synthesized by the reaction of 2,2'-methylenebis(4-chloro-6-isopropyl-3-methylphenol) (MCIMP-H-2) or 2,2'-ethylidenebis(4,6-di-tert-butylphenol) (EDBP-H-2) with (MgBu2)-Bu-n in diethyl ether or tetrahydrofuran, respectively. Experimental results show that 1-3 efficiently catalyze the ring-opening polymerization of epsilon-caprolactone and L-lactide in a controlled fashion, yielding polymers with very narrow polydispersity indexes in a wide range of monomer-to-initiator ratios. Compound 1 has paved a way to synthesize as much as 500-fold polymer chains of poly(epsilon-caprolactone) with a very narrow polydispersity index. In addition, block copolymers such as poly(epsilon-caprolactone)-b-poly(L-lactide), poly(ethylene glycol) methyl ether-b-poly(L-lactide), and polystyrene-b-poly(L-lactide) have also been prepared.