Light-Induced Copolymerization of Ethyl Lipoate and Vinyl Monomers: Increased Efficiency and Degradability

Radical ring-opening copolymerization of lipoic acid derivatives with vinyl monomers (VMs) is a versatile route to create degradable copolymers and enables tuning of vinyl polymer properties. Although thermally initiated systems with acrylates, styrene, and acrylamide are well established, photoinduced polymerization remains comparatively underexplored. In this work, we investigate the photopolymerization of ethyl lipoate (ELp) with diverse VMs, including butyl acrylate (BA), N,N-dimethylacrylamide (DMA), methyl methacrylate (MMA), N-vinylpyrrolidone (NVP), vinyl acetate (VAc), isobutyl vinyl ether (IBVE), and norbornene (NB) under 405 nm light. We show that efficient copolymerization is obtained with the use of the photoinitiator (diphenylphosphoryl)(mesityl)methanone (TPO). In its absence, ELp undergoes photolysis to produce only low-molecular-weight oligomers (Mn = 1–2 kg mol–1) with limited monomer conversion (≤20%). In contrast, with TPO, all comonomers, except for styrene and allyl alcohol, achieve efficient polymerization, yielding high molecular weight copolymers (Mn > 19 kg mol–1). In addition, we compared the TPO initiated photopolymerization at ambient temperature with AIBN-initiated conventional thermal polymerization at 70 °C. We found that ELp exhibits significantly higher conversion under photopolymerization. Monomers such as VAc, IBVE, and NB show much better incorporation when using photoinitiated polymerization conditions, whereas methyl methacrylate and styrene are more suitable for thermal polymerization. Using VAc as a representative case, we further examine how the ELp: VAc feed ratio influences copolymerization kinetics and final copolymer composition. Finally, the resulting VM–ELp copolymers undergo thiolate-promoted disulfide exchange, with Mn values decreasing significantly from 19–82 kg mol–1 to 1–2 kg mol–1. Finally, we evaluated the thermal properties of the copolymers. The VAc-co-ELp copolymer containing 15 mol % ELp shows comparable thermal stability to the corresponding VAc homopolymer. However, TGA analysis reveals that the copolymer undergoes a more complete thermal decomposition at lower temperatures. Collectively, this work highlights a substantial, untapped potential of photoinduced lipoate–vinyl copolymerization for creating degradable and functional polymeric materials.