Multistep Thermolysis Mechanisms of Azido-s-triazine Derivatives and Kinetic Compensation Effects for the Rate-Limiting Processes

The decomposition kinetics and mechanisms of 2-amino-4,6,-diazido-s-triazine and its derivatives have been investigated using thermogravimetry (TG) techniques and quantum chemical calculations. The kinetic compensation effect for their rate-limiting steps is found, which is compared with some other groups of materials. It has been found that the activation energy of 4,6-diazido-2-amino-1,3,5-triazine (DAAT) is about 99.7 kJ mol^-1, which is much lower than those of the first decomposition steps of its analogues. The activation energy of the first step of 4,6-diazido-N-nitro-1,3,5-triazine-2-amine (DANT) is much higher than the second one, while the third and fourth ones have almost the same activation energy (127.2 kJ mol^-1). The decomposition of DAAT does not follow any ideal model due to strong interaction of overlapped reactions. All four decomposition steps of DANT as well as the first step of 4,4′,6,6′-tetra(azido)hydrazo-1,3,5-triazine (TAHT) follow the same A2 model. The first two steps of TANDAzT decomposition are controlled by a "phase boundary controlled" mechanism, while the third and fourth steps roughly follow a "two-dimensional diffusion" model. It has been found that the first step of DANT decomposition is featured by scission of a NH-NO₂ bond via HONO elimination, which is very similar to nitramine decomposition. Decomposition of DAAT and DANT is featured by azido group scission, and the latter is greatly affected by elimination of HONO. As a comparison, the rate-limiting decomposition step of nitramine is featured by the bond rupture of N-NO₂, while it is O-NO₂ for nitric esters and C-NO₂ for aromatic nitrocompounds, following on different kinetic compensation lines. The rate-limiting steps for involved materials are scission of the azido group. Under the effect of the other functional groups, the isokinetic temperatures for azido group scission are almost identical, while their reaction rates are quite different.