) but at the same time the selectivity between β-scission and inverse radical addition decreased, leading to higher kp/ktr values. This led to an unexpected low kp/ktr value for thionolactone 8. With this result, we presumed that increasing the stabilization of the release radical could modify the kp/ktr value. We thus designed thionolactone 9 that possesses a pendant phenyl group at the released radical position, ie. in epsilon position to enhance the stabilization of the released radical (styryl instead of benzyl like radical) and thus increase the selectivity of the fragmentation. Upon calculation, the thionolactone 9 presents the closest kp/ktr for copolymerization both with styrene and methyl acrylate. Compared to 8, the β-scission is clearly favored by the better stabilizing effect of the radical and the selectivity between βscission and inverse radical addition is strongly increased. To confirm experimentally these calculations, the thionolactone 9 was then synthesized and its (co)polymerization with vinyl monomers has been investigated, as described below.