A series of magnesium complexes have been prepared and characterized, in which [(EDBP-Me)Mg(μ-OBn)]₂ (4) has shown high activity toward the ring-opening polymerization (ROP) of ε-caprolactone and L-lactide. 2,2′-Ethylidene-bis(4,6-di-tert-butylphenol)-monomethyl ether ([EDBP-(Me)H]) is prepared by the reaction of 2,2′-ethylidene-bis(4,6-di-tert-butylphenol) (EDBP-H₂) with dimethyl sulfate. The reaction of [EDBP-(Me)H] with ⁿBu₂Mg yields [(EDBP-Me)MgⁿBu]₂, which further reacts with benzyl alcohol and N,N-dimethylethylenediamine giving complexes [(EDBP-Me)Mg(μ-OBn)]₂ (4) and {[EDBP(Me)]Mg(μ-Me₂NCH₂CH₂NH)}₂ (5), respectively. Experimental results indicate that the activity of complex 4 toward cyclic esters is higher than that of [(μ-EDBP)Mg]₂/(BnOH).