In previous studies, we used the transfer rate constant ktr (3) as identified in scheme 2. It integrates the three steps of the addition-fragmentation process and follows the equation (Scheme 2). This constant was compared to the theoretical vinyl monomer propagation rate constant computed with similar level of theory to precisely assess the reactivity of the comonomer pair by determining the reactivity ratio rvinyl. In these systems, the thionolactone is typically added as a minor comonomer or an additive (less than 10 mol%), meaning that the growing macroradicals are predominantly polyvinyl radicals. Therefore, the most likely event is the competition of the vinyl radical addition either to the vinyl monomer (kp) or to the thionolactone (ktr). This method allowed us to reduce computation time and avoid determining the thionolactone reactivity ratios rthionolactone. 13, 16, 18 Scheme 2. Transfer rate constant ktr and the corresponding equation.