Closer Look at the Substituent Effects on the Copolymerization of Thionolactones by Radical Ring-Opening Polymerization

The negative impact of plastic waste on the environment is a serious issue for the future. Adding cleavable bonds to the polymer backbone can help to impart degradability properties. To this end, radical ring-opening polymerization (rROP) offers a very attractive way to achieve radical copolymerization with vinyl monomers. Thionolactones, one of the various cyclic monomers that can be used in rROP, are promising structures due to the efficiency of the C═S bond in acting as a radical acceptor. In this work, we used DFT calculations to better understand the effects of different substituents on the radical reactivity of thionolactones (dibenzo[c,e]oxepane-5-thione DOT derivatives) already described as more or less effective for copolymerization with vinyl monomers in order to adjust the reactivity of these thionolactones. To carry out these calculations, we focused on the value of the transfer constant k_tr and its relationship to the propagation constant k_p of the vinyl monomer. The calculations performed subsequently on 7-phenyloxepane-2-thione (POT) derivatives highlighted that electron-donating groups inserted in the para-position on the phenyl ring should improve the copolymerization efficiency with acrylate derivatives, and in contrast, electron-withdrawing groups should lead to more important compositional drift during styrene and acrylate copolymerization. Although POT derivatives bearing electro-donating groups could not be prepared, the preparation of those with CF₃ and NO₂ groups was successfully achieved. Experimental copolymerization of these two thionolactones with styrene and isobornyl acrylate is in good agreement with the calculations. This result confirmed the versatility and relevance of our calculation approach to account for the reactivity of thionolactones.