This report describes the synthesis of a series of sodium complexes bearing cavity-like conformations formed with the aid of sterically hindered phenoxide and 15-crown-5 ether ligands. All the complexes are well-characterized via NMR spectroscopy analysis, and through single crystal X-ray studies and DFT calculations it is found that the size of the cavity is very similar to that of macrolactone monomers. During subsequent studies of the ring-opening polymerization (ROP) of macrolactones, the present sodium complexes demonstrate high catalytic efficiencies in the ROP of pentadecalactone and ethylene brassylate, affording the corresponding polyester products in high yields. Moreover, due to chain propagation being confined within the cavity, transesterification can be suppressed because of mutual steric repulsion between the ligands and polymer chains, which further allows polymerization to proceed in a well-controlled manner. Owing to such uniqueness, block copolymerization between macrolactones and other cyclic esters can be also successfully achieved.