DFT calculations. The first part of this study was to understand the reactivity of thionolactones via DFT calculations. Our approach is based on the determination of rate constant of the different steps of the radical ring-opening polymerization, that is the addition of the propagating radical onto the C=S bond, the reverse addition that releases the same radical and the original thionolactone and the β-scission of the radical intermediate that leads to the incorporation of the thioester bond in the polymer chain and the release of a new propagating alkyl radical (Figure ). and the ratio between ktr and the propagation rate constant kp of styrene and acrylate derivatives calculated using the same approach for the experimentally prepared thionolactones (80 °C) In a previous study, we introduced the use of the transfer rate constant ktr that is a complex rate constant combining the three steps and describing the addition-fragmentation process. This constant was then compared with the propagation rate constant of the vinyl monomer as a good way to evaluate the reactivity of the comonomer pair (Figure ) by determining the rvinyl reactivity ratio. Indeed, in these materials, the cyclic monomer is usually inserted as an additive and thus the rvinyl reactivity ratio is already very informative. Thereby, we could save computational time and avoid computing all reactivity ratios. We showed in particular the negative Arrhenius behavior of such ratio that was experimentally proven in the case of the DOT/styrene copolymerization.