The derivatives of hign-N heterocycles, e.g., tetrazole and triazole, containing long catenated nitrogen atom chains are of great interest as promising green energetic materials. However, these compounds often exhibit poor thermal stability and high impact sensitivity. Kinetics and mechanism of the primary decomposition reactions are directly related to these issues. In the present work, with the aid of highly accurate CCSD(T)-F12 and DLPNO-CCSD(T) quantum chemical calculations, we obtained reliable bond dissociation energies and activation barriers of thermolysis reactions for a number of N-rich heterocycles. We studied all existing 1,1′-azobistetrazoles containing an N10 chain, their counterparts with the 5,5′-bridging pattern, and the species with hydrazo- and azoxy-bridges, which are often present energetic moieties. The insights into the mechanism of primary decomposition reactions and related kinetics and sensitivity issues will be discussed in detail. This work is supported by the Russian Science Foundation (project 22-13-00077). The support by the Supercomputer Centers of Irkutsk (ISCC) and Novosibirsk State University is also acknowledged.