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

Abstract 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.