Mechanistic Investigation of ε‑Thiono-Caprolactone Radical Polymerization: An Interesting Tool to Insert Weak Bonds into Poly(vinyl esters)

The incorporation of heteroatoms into the backbone of commodity polymers prepared by radical polymerization is an elegant way to confer (bio)­degradability to such materials. This could be achieved via the radical ring-opening polymerization of cyclic monomers. Thionolactones have recently been identified as promising comonomers for radical ring-opening polymerization (rROP), but they react only with activated monomers such as acrylate and acrylamide derivatives. Herein, we describe two thionolactone monomers, oxepane-2-thione, i.e., ε-thiono-caprolactone (thCL), and 7-butyloxepane-2-thione, i.e., ε-thiono-decalactone (thDL), capable of performing radical ring-opening polymerization with less activated monomers such as vinyl acetate (VAc) to produce readily degradable copolymers via the insertion of thioesters containing repeating units. A thorough mechanistic investigation was performed to understand the reactivity of the two cyclic monomers. We identified the initiation as the main parameter to perform the polymerization of such thionolactone monomers. In a specific case, a poly­(ε-thCL) homopolymer was successfully obtained. Concerning the copolymerization with vinyl acetate, a variety of polymerizations with differing feed ratios were performed, and the degradability of the copolymers via aminolysis was examined by size exclusion chromatography (SEC). To demonstrate the usefulness of thionolactones, a variety of block copolymers containing thioester linkages involving N,N-dimethylacrylamide and VAc were also prepared and degraded.