Calorimetric study of tetramethylene urethane and of poly(tetramethylene urethane) between 0 and 300 K

摘要

In an adiabatic vacuum calorimeter the temperature dependence of the heat capacity C_p~0 of tetramethylene urethane (TMU) (2-oxo-hexahydro-1,3-oxazepine) and poly(tetramethylene urethane) (PTMU) was studied between 4.3 and 300 K with an uncertainty of about 0.2. In a calorimeter with a static bomb and an isothermal shield the energies of combustion #DELTA#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(0), S~0(T), G~0(T)-H~0(0) were calculated in the range of 0 to 300 K, and enthalpies of combustion #DELTA#H_(comb)~0 and thermochemical parameters of formation #DELTA#H_f~0, #DELTA#S_f~0, #DELTA#G_f~0 of TMU and PTMU were estimated at T=298.15 K and standard pressure. The results obtained were used to calculate the thermodynamic characteristics of TMU polymerization in bulk (#DELTA#H_(pol)~0, #DELTA#S_(pol)~0, #DELTA#G_(pol)~0) with opening of the seven-membered ring and formation of the linear polymer PTMU, in the range of 0 to 300 K. It was found that the values of the standard Gibbs function of the process (#DELTA#G_(pol)~0=(-41)-(-42) kJ (centre dot) mol~(-1)) are always negative and do not substantially depend upon temperature in the whole temperature interval studied. This implies that the equilibrium TMU (->implied by) PTMU is far on the side of the polymer. The upper (ceiling) limiting temperature of TMU polymerization is T_(ceil)~0>1000 K.