To clarify the equilibrium polymerization behavior of 1,4-dioxan-2-one, PDO, bulk polymerization was investigated with Sn(II) ethylhexanoate or triethylaluminum over a range of 60-180 degrees C. The thermodynamic parameters Delta H-1s, Delta S-1s, and ceiling temperature for the polymerization were -14.1 kJ mol(-1), -26.15 mol(-1) K-1, and 265 degrees C, respectively. The parameters support the thermal instability of PPDO. The calculated thermal parameters of activation were E-a = 71.8 kJ mol(-1), Delta G(p)(double dagger) = 106.2 kJ mol(-1), Delta H-p(double dagger) = 68.5 kJ mol(-1), and Delta S-p(double dagger) = -95.7 J mol(-1) K-1 at 120 degrees C. These parameters of activation are nearly equal to those of L-lactide. PDO and PPDO are easily polymerizable and depolymerizable materials, respectively.