Sunlight-driven photoinitiating systems for photopolymerization and application in direct laser writing

Abstract

Currently, there are only a few industrial and academic works focused on photopolymerization conducted under natural light. To address this challenge, six new dyes are synthesized as photosensitizers and combined with additives (an amine and an iodonium salt) to create three-component photoinitiation systems. These systems can efficiently initiate both the free radical polymerization (FRP) of an acrylate monomer and the cationic polymerization (CP) of an epoxy monomer. Remarkably, the FRP process, facilitated by these systems under natural sunlight, required only a low loading of dyes (0.1 wt% relative to the monomer), and the conversion obtained with dye-B1 as the photosensitizer can reach 90% within 5 minutes. It highlights the viability of natural sunlight as an efficient light source for polymerization processes. More interestingly, dye-B1 can effectively initiate metal-free CP of an epoxy monomer. The chemical mechanism underlying photopolymerization is comprehensively investigated through a combination of theoretical calculations, photolysis experiments, fluorescence quenching experiments, cyclic voltammetry (CV), and electron spin resonance spin trapping (ESR-ST) experiments. Leveraging the exceptional photoinitiation ability of dye-B1, the corresponding three-component photoinitiating system is applied to 3D printing, achieving high-precision 3D patterns via direct laser writing (DLW).