A new isotactic, perfectly alternating polymer of (S)-lactic acid and oxetane was synthesized by the entropically driven ring-opening polymerization of a 14-membered cyclic diester (S,S-3) mediated by a zinc alkoxide catalyst. The polymer (S,S-PMOD) was characterized by NMR spectroscopy, size exclusion chromatography, and matrix-assisted laser desorption ionization mass spectrometry. Under polymerization conditions the equilibrium concentration (0.17 ± 0.05 M) of the monomer in toluene was essentially independent of temperature, and the average thermodynamic parameters for the ring-opening polymerization were found to be ΔH° p = 0.2 ± 0.7 kJ mol -1 and ΔS° p = +16 ± 2 J mol -1 K -1 (standard state [S,S-3] = 1.0 M). Theoretical calculations of the standard state enthalpy (ΔH° p) for ring-opening polymerization of various oxocyclics, including S,S-3, were well-correlated with experimental values. Kinetic studies showed that the polymerization of S.S-3 was slower (k p = 0.82 ± 0.04 M -1 s -1 and k dp = 0.12 ± 0.04 s -1 at 25°C) than that of lactide (k p = 2.2 M -1 s -1) using the same zinc alkoxide catalyst. Differential scanning calorimetry of S,S-PMOD showed a glass transition temperature of -30°C for samples with molecular weights between 5 and 72 kg mol -1. In support of the potential utility of S,S-PMOD as a PLA plasticizer, the complete miscibility of the polymeric components was demonstrated by the observation of single T g values for a series of blends of S,S-PMOD and atactic PLA.