Homolytic bond dissociation energy (BDE) of the (C-ON) bond for several N-alkoxyamines derived from 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and the corresponding (C-H) bonds were determined from quantum-mechanical calculations including the B3-LYP6-31G(d), B3-LYP/6-311++G(2df,p), UB3-LYP/6-311+G(3df,2p), and integrated IMOMO (G3:ROMP2/6-31G(d)) method. The investigated N-alkoxyamines were considered as models for dormant forms of propagating chains in the radical copolymerization process of styrene with acrylonitrile in the presence of TEMPO according to the terminal and penultimate model. The substituent effect on BDE was investigated. Radical stabilization energies (RSE) for radicals created from homolysis of the investigated N-alkoxyamines were calculated according to Ruchardt's method. Polar, steric, and stabilization effects on C-ON alkoxyamine bond homolysis were studied. A dramatically weakened C-ON bond in the alkoxyamine-containing two consecutive styrene units in the propagating chain was ascribed to geometric parameters characterizing energetically unfavorable conformation of the substituents. These phenomena can be regarded as the penultimate effect in the radical living/controlled copolymerization system of styrene with acrylonitrile. (C) 2008 Wiley Periodicals, Inc.