Abstract
In adiabatic vacuum and dynamic calorimeters the temperature dependence of the heat capacity C~p~^0^ of 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 Δ__U__~comb~ of the monomer and the polymer were measured. From the experimental data the thermodynamic functions C~p~^0^, H^0^(T) – H^0^(O), S^0^(T), G^0^(T) – H^0^(0) were calculated in the range of 0 to 450 K, and enthalpies of combustion Δ__H__~comb~^0^ and thermochemical parameters of formation Δ__H__~f~^0^, Δ__S__~f~^0^, Δ__G__~f~^0^ of the compounds studied were estimated at T = 298.15 K and standard pressure. The results obtained were used to estimated the thermodynamic characteristics of the equilibrium of THO polymerization in bulk (Δ__H__~pol~^0^, Δ__S__~pol~^0^, Δ__G__~pol~^0^), 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 Δ__G__~pol~^0^ values are always negative and change from −18 kJ · mol^−1^ at 0 K to −14 kJ · mol^−1^ at 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 is T~ceil~ = 750 K, that is, considerably higher than the temperature of the onset of the thermal polymer decomposition (450 K).