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Synthesis of a new mononuclear magnesium complex with a bulky bis(alkoxide) ligand environment and its reactivity in ring-opening polymerization (ROP) and ring-opening copolymerization (ROCOP) are reported. Reaction of n -butyl- sec -butylmagnesium with two equivalents of HOR (HOR = di- tert -butylphenylmethanol, HOC t Bu 2 Ph) formed Mg(OR) 2 (THF) 2 . The reaction proceeded via the Mg(OR)( sec -Bu)(THF) 2 intermediate that was independently synthesized by treating n -butyl- sec -butylmagnesium with one equivalent of HOR. Mg(OR) 2 (THF) 2 led to active albeit not well-controlled ROP of rac -lactide. In contrast, well-controlled ROCOP of epoxides with cyclic anhydrides was observed, including efficient and alternating copolymerization of phthalic anhydride with cyclohexene oxide as well as rare copolymerization of phthalic anhydride with limonene oxide and terpolymerization of phthalic anhydride with both cyclohexene oxide and limonene oxide. In addition, novel copolymerization of dihydrocoumarin with limonene oxide is described.