Considering only the copolymerization at fPOT-NO2,0 = 0.05, reactivity ratios of rPOT-NO2 = 8.0 and rS = 0.2 were obtained. With these values, the copolymerization with fPOT-NO2,0 = 0.1 cannot be fitted perfectly (Figure ). Here, the assumption that both the vinyl and cyclic monomers can be processed via an irreversible propagation mechanism is probably no longer valid and thus the reversibility of the addition reaction on thionolactone is important and cannot be only considered as a part of the ktr value. Recently Johnson and coworkers highlighted that using common methods to determine reactivity ratios in the case of systeme whose one or two monomers undergo reversible propagation cannot be fitted properly. In the case of isobornyl acrylate -POT-NO2 copolymerization, only copolymerizations with low amount of POT-NO2 was possible (fPOT-NO2,0 ≤ 0.1), similarly to the copolymerization of styrene. Using fPOT-NO2,0 ≤ 0.1, the conversion of the thionolactone was very rapid and occurred with very low conversion of IBA (< 5 % for 70 % conversion of POT-NO2), confirming its incompatibility with acrylate derivatives (see ESI for details) as suspected from the DFT calculations. With the rapid consumption of POT-NO2 in these two copolymerization systems, we decided to investigate the homopolymerization of this thionolactone since we previously observed only very sluggish homopolymerization for POT. Operating at 80 or 120ºC, we cannot observe any conversion ruling out the possibility to obtain a pure polythioester. To explain this phenomenon, we calculated the different kinetic rate constants and also the ktr value for the radical addition of open POT-NO2 radical on POT-NO2 (Figure ). First, the open radical derived from POT-NO2 was found to add more rapidly than a styryl radical onto the POT-NO2 which confirms the extreme reactivity once this radical is generated. The most striking result concerns the selectivity of the β-scission that is totally shifted towards the release of the alkyl radical and the thionolactone (reverse addition) rather than the ring-opening (only 3% of ring-opening when the styryl radical resulting from reverse addition is NO2 substituted vs 90% for the unsubstituted styryl radical, Figure ). Whatever the comonomer, the low rate of ring-opening impedes its use in copolymerization. Lastly, the homopropagation rate constant (ktr, POT-NO2) value is two times lower than for POT. All these calculations are in good agreement with the experiments. At very low loading of POT-NO2 (fPOT-NO2,0 = 0.05), the copolymerization occurred with the cross-propagation between the ring-opened POT-NO2 radical and styrene, and its addition on POT-NO2 is not considered. In that case reactivity ratios are rPOT-NO2 = 8.0 and rS = 0.2. At fPOT-NO2,0 ≥ 0.1, the higher concentration of POT-NO2 makes the addition of the POT-NO2 radical on POT-NO2 and the reverse addition becoming predominant, inhibiting the consumption of both monomers. Consistently, with such a low homopropagation rate constant, POT-NO2 cannot be homopolymerized efficiently.