Open‐Air, Ultrafast Photoiniferter Polymerization Enables 3D Printing with Low‐Viscosity Resins

Abstract Photo‐curing 3D printing is fundamentally challenged by the formation of heterogeneous networks with uneven cross‐linking densities and unreacted monomer residues, which compromise the integrity and performance of printed parts. While photo‐controlled radical polymerization (photo‐RDRP) offers a path to precise network control, it is often limited by sluggish kinetics, necessitating viscous resins to compensate for slow curing rates. To overcome this, we employed a computer‐guided strategy to design a new photoiniferter, dithioindazolylcarboxylate (DTI). Computational analyses predicted DTI would be an efficient and oxygen‐tolerant photoiniferter. Under 415 nm light (10 mW cm −2 ), DTI enabled the photoiniferter polymerization of methyl acrylate with an apparent propagation rate () of 0.809 min −1 , over four‐fold higher than that of the existing dithiopyrazolylcarboxylate (DTP) with a of 0.196 min −1 in equivalent conditions under 425 nm (optimum for ). This was achieved while maintaining low molecular weight dispersity ( Đ < 1.1). DTI also exhibited better oxygen tolerance, maintaining 65.9% of its (with respect to that under nitrogen) under an air‐purging condition (1 mL 30 s −1 ), as compared to 13.3% for DTP under the same condition. This enabled ultrafast digital light processing (DLP) 3D printing at just 8 s per layer using routine low‐viscosity acrylic resins, producing high‐resolution, complex centimeter‐scale prototypes. The technology also enabled photo‐curing of anticorrosion coatings with an adhesion strength of 13.53 MPa and 720 h resistance to acetic acid salt spray.