From Lipoic Acid to 1,2-Dithianes: Expanding Radical Ring-Opening to Less-Activated Monomers Such as Vinyl Acetate

Polymers containing cleavable functionality along the backbone represent a path to reduce the environmental persistence of commodity rubbers and plastics. One route to installing cleavable functionality involves the radical ring-opening polymerization of five-membered 1,2-dithiolanes such as α-lipoic acid, which introduces disulfide bonds into vinyl polymer backbones. However, the ring strain and electronics of 1,2-dithiolanes generally restrict reactivity to only favor copolymerization with more-activated comonomers, such as acrylate and styrene derivatives. Here, we show that the six-membered cyclic disulfide, 1,2-dithiane overcomes this limitation under simple thermal free-radical conditions. We demonstrate that 1,2-dithiane copolymerizes efficiently with the less-activated monomer vinyl acetate. In sharp contrast to α-lipoic acid and its derivatives, 1,2-dithiane exhibits nearly ideal copolymerization across different feed ratios, affording high conversions of both monomers (>90%), tunable molar masses (M_n ≈ 10-100 kg mol^-1), and scalability (>20 g). Moreover, the 1,2-dithiane scaffold is synthetically versatile: a 1,2-dithiane-4,5-diol was synthesized from dithiothreitol as a building block for further derivatization. By varying dithiane functionality and loading, poly(dithiane-co-vinyl acetate) copolymers span semicrystalline to elastomeric mechanical properties and, critically, embed backbone-cleavable sulfur motifs (including monothioacetal units) even at low (<1 mol %) loadings. This operationally simple reaction highlights the key influence of ring size in the copolymerization behavior of disulfide-containing monomers and demonstrates the practical advantages of using 6-membered cyclic disulfides as renewable building blocks for creating degradable vinyl copolymers derived from inexpensive, industrially relevant feedstocks.