NIR ‐induced upconversion‐assisted photopolymerization: Key factors, challenges, and future directions

NIR-induced upconversion-assisted photopolymerization has gained growing attention in the past two decades because of its numerous advantages over conventional UV/visible photopolymerization and two-photon polymerization processes. However, research in this area is still in its early stages. To extend the practical application of NIR-induced radiation curing, it is essential to optimize the factors affecting the photopolymerization reactions. Researchers have been constantly trying to improve these factors to tune the photo-physical characteristics (luminescence intensity and color) of upconversion particles (UCPs), enhance curing depths and degree of double bond conversion (DC), and investigate the application of UCPs in emerging fields. In this review, first, a brief discussion of the upconversion mechanisms and upconversion efficiency is provided. Then, a detailed discussion of the factors influencing the upconversion-assisted photopolymerization comprising UCP nature and characteristics, UCP content, presence of fillers/pigments/additives, laser intensity, photoinitiator content, and maximum absorption wavelength of photoinitiator is provided, and recent progress in improving these factors is presented. Finally, the advantages and drawbacks of the UC-initiated polymerization are discussed, and perspectives for future directions are suggested.