The ring-opening polymerization of 3-methyl-1,4-dioxan-2-one (MDO) mediated by a catalytic system composed of Y[N(TMS)(2)](3) and benzyl alcohol (BnOH) led to a new polymer (PMDO) comprised of perfectly alternating lactic acid and ethylene oxide repeat units. Samples of PMDO were characterized by NMR spectroscopy, size exclusion chromatography (SEC), and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). In a series of MDO polymerizations ([MDO](0) = 3.0 M in toluene, between -30 and 60 degreesC), the equilibrium monomer concentrations were measured. The thermodynamic parameters for the polymerization reaction were determined: DeltaH(p)(o) = -12.1 +/- 0.5 kJ mol(-1) and DeltaS(p)(o) = -42 +/- 2 J mol(-1) K-1. The glass transition temperature (T-g) of PMDO was determined to be approximate to -24 degreesC by differential scanning calorimetry and found to vary little over. the molecular weight range studied (3-21 kg/mol). Mixtures of low-molecular-weight PMDO and atactic polylactide (PLA) were prepared by solution casting and subsequent annealing significantly above the T-g's of the individual components. Miscibility of PLA and PMDO was evinced by single T-g's that were well-described by the Fox relationship for miscible blends. Because of its miscibility with PLA and low T-g, PMDO has potential as a macromolecular plasticizing agent for commercially relevant PLA.