Thermal decomposition of (V-(l-cyanocyclohexyl)pentamethyleneketenimine (RR'), the ketenimine from azocyanocyclohexane, in chlorobenzene (100°) in the presence of radical scavengers gives pressure dependent observed activation volumes which decrease from ca. +13 cc/mol to ca. +5 cc/mol with increasing pressure. With no scavenger, AK*obsd is about +5 cc/mol and pressure independent. In the absence of scavengers, the pressure dependence of decomposition of RR' in terf-butylbenzene and cumene parallels that in chlorobenzene without and with scavengers, respectively. Products include dicyanobicyclohexyl, cyanocyclohexane, 1-cyanocyclohexene, and products resulting from reaction of cyanocyclohexyl radicals with the solvents cumene and toluene. Quantitative product data as a function of pressure and the kinetic data support the conclusions that observed activation volumes for decomposition of one-bond initiators are substantially larger than the true activation volume for scission of a single chemical bond and that the latter may be ca. +5 cc/mol for RR'. When geminate radicals recombine to form the initiator from which they arose (Scheme I), the kinetic parameters characterizing decomposition of Scheme I k A-B ?=*
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