The copolymerizations of POT-CF3 with styrene and with isobornyl acrylate, to determine the reactivity ratios, were carried out in anisole (50 mol%) at 80°C, initiated with 0.05 equivalents of AIBN (Figure ). The reactivity ratios of POT-CF3 with styrene confirmed the trend predicted by DFT calculations. We obtained reactivity ratios of rPOT-CF3 = 1.15 and rS = 0.29, indicating a less favorable situation than with the unmodified POT for achieving random polymerization. Thus, POT-CF3 shows higher reactivity than POT during copolymerization with styrene. The reactivity ratios of POT-CF3 with isobornyl acrylate confirmed this trend, with values of rPOT-CF3 = 70 and rIBA = 0.25 (Figure ). This demonstrates that the reactivity of POT-CF3 is in this case significantly higher than that of POT during copolymerization with isobornyl acrylate, hampering its use with acrylate derivatives due to compositional drift. The values of these reactivity ratios confirm the hypotheses based on our DFT method: adding an electron-withdrawing substituent to the α position of the aryl group significantly influences the reactivity of the thionolactone, making it more reactive during copolymerization with styrene or acrylate. To further corroborate this trend, we also investigated the copolymerization behavior of POT-NO2, which is supposed to be even more reactive than POT-CF3 in these systems. We carried out the copolymerization of POT-NO2 (f POT-NO2,0 = 0.05, 0.1 and 0.2) with styrene and observed that the copolymerization was not possible with fPOT-NO2,0 above 0.1, and that for fPOT-NO2,0 = 0.1, the copolymerization was drastically slowed down, reaching an overall conversion of only 20% after 5h at 80 °C.