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

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 ( $k_p^{\mathrm{app}}$ ) of 0.809 min^-1, over four-fold higher than that of the existing dithiopyrazolylcarboxylate (DTP) with a $k_p^{\mathrm{app}}$ of 0.196 min^-1 in equivalent conditions under 425 nm (optimum for $k_p^{\mathrm{app}}$ ). This was achieved while maintaining low molecular weight dispersity (Đ < 1.1). DTI also exhibited better oxygen tolerance, maintaining 65.9% of its $k_p^{\mathrm{app}}$ (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.