Kinetic investigation of thermal decomposition reactions of podophyllotoxin and its derivatives

The thermal behavior and thermal decomposition kinetic parameters of podophyllotoxin (1) and 4 derivatives, picropodophyllin (2), deoxypodophyllotoxin (3), β-apopicropodophyllin (4), podophyllotoxone (5) in a temperature-programmed mode have been investigated by means of DSC and TG-DTG. The kinetic model functions in differential and integral forms of the thermal decomposition reactions mentioned above for first stage were established. The kinetic parameters of the apparent activation energy E _a and per-exponential factor A were obtained from analysis of the TG-DTG curves by integral and differential methods. The most probable kinetic model function of the decomposition reaction in differential form was (1 - a)² for compounds 1 - 3, 2/3·a^-1/2 for compound 4 and 1/2(1 - α)·[ - ln (1 - α)]^-1 for compound 5. The values of E_a indicated that the reactivity of compounds 1 - 5 was increased in the order: 5<4<2<1<3. The values of the entropy of activation ΔS^≠, enthalpy of activation ΔH ^≠ and free energy of activation ΔG^≠ of the reactions were estimated. The values of ΔG^≠ indicated that the thermal stability of compounds 1 - 3 with the same f(α) was increased in the order: 2<3<1.