The first part of this study aimed at better understanding the effects of substituents on the reactivity of certain thionolactones reported in the literature through DFT calculations. Our method relies on evaluating the rate constants of various steps in the rROP process. This methodology has been previously described and includes the addition of propagating vinyl radicals onto the C=S bond (kadd) (1), the reverse addition that releases the same radical along with the initial thionolactone (kadd) (1), and the β-scission constant (kβ) (2) of the radical intermediate (Scheme 1). This final step results in the incorporation of the thioester linkage into the polymer chain and the formation of a new propagating alkyl radical. Scheme 1. Elementary steps relevant to the rROP of substituted thionolactones with vinyl monomers and the corresponding rate constants, with kadd (1) as the addition rate constant, k-add (1) as the reverse addition rate constant, kβ (2) as the fragmentation rate constant, and kp as the propagation rate constant.