An orthogonal O,S-CKA monomer for the introduction of thioester and/or thionoester functionalities by radical polymerization

The introduction of thioester linkages into the backbone of free-radical polymerization-produced polymers has received recently renewed interest due to their use in preparing (bio)degradable polymers. To access monomers capable of introducing the thioester functionality, a novel monothiolated analogue of the cyclic ketene acetal (CKA) class of radical ring-opening polymerization (rROP) monomers was synthesized and examined, specifically O,S-BMDO. Interestingly, O,S-BMDO offers an example of an orthogonal rROP monomer, being capable of introducing thioesters, thionoesters, or a mixture of both functional groups into the polymer backbone, with the ratio of these functional groups affected by the comonomer and the reaction conditions. This unusual effect was explored both experimentally and computationally. This report constitutes the first time a thionoester-containing repeating unit has been introduced directly by radical polymerization. In addition, the polymerization of O,S-BMDO can produce a rare poly(thionoester) homopolymer.