Development of an Efficient Thionolactone for Radical Ring-Opening Polymerization by a Combined Theoretical/Experimental Approach

The environmental impact of plastic waste has been a real problem for the past decades. The incorporation of cleavable bonds in the polymer backbone is a solution to make a commodity polymer degradable. When radical polymerization is used, this approach is made possible by the radical ring-opening polymerization (rROP) of a cyclic monomer that allows the introduction of a weak bond into the polymer backbone. Among the various cyclic monomers that could be used in rROP, thionolactones are promising structures due to the efficiency of the C=S bond to act as radical acceptor. Nevertheless, only few structures were reported to be efficient. In this work, we used DFT calculations to gain better understanding of the radical reactivity of thionolactones and in particular we focused on the transfer rate constant ktr value and its ratio with the propagation rate constant kp of the vinyl monomer. The closer to 1, the better the statistical incorporation of the two comonomers into the backbone. With this approach, we identified and prepared the 7-phenyloxepane-2-thione (POT) thionolactone. The copolymerization of styrene and various acrylate derivatives with POT led to statistical copolymers that were efficiently degraded under accelerated conditions (KOH in THF/MeOH, TBD in THF or mCPBA in THF), confirming the theoretical approach. The compatibility with RAFT polymerization was established as well as the homopolymerization behavior of POT. This theoretical approach paves the way to the in-silico design of new efficient thionolactones for rROP