Abstract: Based on the experiments, the standard enthalpy △H^Θ of the possible pyrolysis reactions of the carbon matrix precursor toluene was investigated by means of DFT method UB3LYP/ 3-21G^* (based on semi-empirical method UAM1 and ab initio method UHF/3-21G^*). The com putation results with UB3LYP/3-21G^* coincide with the experimental values well. Then, the mechanism for all types of the pyrolysis reactions of toluene was studied by UB3LYP/3-2lG^*. The geometries of the reactant and the product radicals were optimized, meanwhile, the standard thermodynamic parameters of the pyrolysis reaction at different temperatures (298, 773, 843, 963 and 1 073 K) were calculated. The thermodynamic computation result shows that when the pyrolysis temperature of toluene is lower than 963 K, the reaction path supported by thermody namics is that the C-H bond of the methyl on the benzene ring breaks and bitoluene form, while the temperature increases (about 1 073 K), the thermodynamic calculation result turns to sup port the reaction path producing phenyl radicals and methyl radicals. This mechanism is in accord with the experiments.

Key words: Carbon/carbon composites, Toluene, Pyrolysis mechanism, UB3LYP/3-21G^*