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 DeltaH(p)degrees = 0.2 +/- 0.7 kJ mol(-1) and DeltaS(p)degrees = +16 +/- 2 J mol(-1) K-1 (standard state [S,S-3] = 1.0 M). Theoretical calculations of the standard state enthalpy (DeltaH(p)degrees) 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 degreesC) 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 degreesC 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.