Red Light-Driven, Oxygen-Tolerant RAFT Polymerization Enabled by Methylene Blue

Photoinduced reversible-deactivation radical polymerization (photoRDRP) techniques enable the synthesis of well-defined polymers under mild conditions. However, oxygen inhibition remains a key challenge, requiring either complex deoxygenation methods or in situ oxygen-scavenging strategies. Here, we report a red light-driven, fully oxygen-tolerant reversible addition-fragmentation chain transfer (RAFT) polymerization mediated by methylene blue (MB⁺) as a photosensitizer and triethanolamine (TEOA) as an electron donor. This operationally simple method enables polymerization in fully open-to-air vials without stirring, even under direct sunlight, underscoring the robustness and accessibility of the system. Using MB⁺ in the presence of a sacrificial electron donor, the polymerization proceeds to high monomer conversions (>90%), with excellent temporal control, predictable molecular weights, and low dispersities (Đ < 1.3) across a wide range of conditions. The system is compatible with a broad scope of hydrophilic (meth)acrylamide and (meth)acrylate monomers, including those bearing charged and zwitterionic side groups. Notably, this methodology enables access to ultrahigh molecular weight (UHMW, >1,000,000) polymers under ambient conditions, an outcome rarely achieved in oxygen-tolerant RDRP. This metal-free, red light-driven photoRAFT platform offers a scalable, efficient, and biocompatible strategy for controlled polymer synthesis, with potential applications in bioconjugation, functional coatings, and high-throughput screening.