Poly(ethylene glycol) (PEG) has been extensively used in diverse applications. However, it is not biodegradable and shows abnormal immune responses. Herein, a fast, controlled, ring-opening polymerization (ROP) of 2-oxo-15-crown-5 (O-15C5) is reported to prepare well-defined PEG-like polyesters, poly(O-15C5). This approach relies on a coordination between the macrocyclic monomer and Na+ that increases the electrophilicity of the carbonyl group of O-15C5 and leads to a fast controlled ROP (dispersity, ĐM < 1.2). Both computational and mechanistic studies show that the selective Na+ binding to the monomer over poly(O-15C5) allows the ring-opening initiation and propagation to be more energetically favorable than side transesterifications. This is the key to control the challenging entropy-driven ROP of O-15C5. Moreover, with the aid of Na+ and organic base, poly(O-15C5) depolymerized readily into O-15C5 in 2 h. Also, it degraded in a buffer of pH 7.4 by hydrolysis.