AbstractIn adiabatic vacuum and dynamic calorimeters the temperature dependence of the heat capacityCp0of tetrahydro‐1,3‐oxazin‐2‐one (THO) and poly(oxy‐1,3‐propanediyliminocarbonyl) (PTHO) was studied between 5 and 500 K. The melting temperature of the monomer and the polymer, the enthalpies of melting of THO and the glass transition temperature of PTHO were determined. In a calorimeter with a static bomb and an isothermal shield the energies of combustion ΔUcombof the monomer and the polymer were measured. From the experimental data the thermodynamic functionsCp0,H0(T) –H0(O),S0(T),G0(T) –H0(0) were calculated in the range of 0 to 450 K, and enthalpies of combustion ΔHcomb0and thermochemical parameters of formation ΔHf0, ΔSf0, ΔGf0of the compounds studied were estimated atT= 298.15 K and standard pressure. The results obtained were used to estimated the thermodynamic characteristics of the equilibrium of THO polymerization in bulk (ΔHpol0, ΔSpol0, ΔGpol0), with opening of the six‐membered ring and formation of the linear polymer PTHO, in the range 0 to 450 K. It was found that the ΔGpol0values are always negative and change from −18 kJ · mol−1at 0 K to −14 kJ · mol−1at 450 K. Thus, the process equilibrium is everywhere shifted towards the formation of the polymer. The upper (ceiling) limiting temperature of THO polymerization in bulk at standard pressure isTceil= 750 K, that is, considerably higher than the temperature of the onset of the t