Triplet‐Enhanced Photoiniferter Polymerization: Ultrafast, Oxygen‐Tolerant, and Biocompatible Synthesis of Well‐Defined Polymers Under Visible Light

Photoiniferter polymerization offers an attractive, additive-free strategy for precision polymer synthesis but is often constrained by a trade-off between speed and control, oxygen sensitivity, and the need for high-energy light. In this study, we introduce a triplet-enhanced photoiniferter strategy that overcomes these limitations. Through computationally guided design, we developed methyl 2-((3-methoxypyrazole-1-carbonothioyl)thio)propanoate (MOP), a photoiniferter exhibiting exceptional photophysical properties: a large triplet energy (E_T = 43.5 kcal mol^-1), a remarkable triplet quantum yield (Φ_T = 82.9%), and a low C─S bond cleavage barrier (8.5 kcal mol^-1). These attributes collectively enable ultrafast polymerization kinetics via rapid photolysis while preserving excellent control through enhanced reversible deactivation and faster degenerative chain transfer. This triplet-enhanced photoiniferter polymerization (TEPP) achieves high monomer conversion within minutes under ambient, open-to-air conditions while maintaining low dispersity (Đ < 1.1), with intrinsic oxygen tolerance conferred by efficient triplet oxygen sensitization to singlet oxygen. We further demonstrate the versatility and biocompatibility of this system through high-throughput synthesis, sunlight-driven polymerization, and in situ polymerizations that achieve >95% cell viability. This work establishes a powerful and versatile platform for precision polymer synthesis under mild, ambient conditions, with broad potential in surface coating, biomaterials, and additive manufacturing.