Photopolymerization of thick pigmented systems still remains challenging due to the light screening effect of the pigments. Here, we present a facile method based on upconversion nanoparticles (UCNPs)-assisted photochemistry to achieve efficient photopolymerization and improved curing depth of pigmented systems. Under a 980-nm laser irradiation, UCNPs are able to convert NIR light into UV and visible light to activate photoinitiators for the initiation of polymerization. Influencing factors on photopolymerization were systematically investigated. With optimal parameters, 25.5 mm of photopolymerization depth combined with 70% of maximal double bond conversion was obtained. The peak temperature of 120.4 degrees C during UCNPs-assisted photopolymerization is comparable with or lower than that of some reported frontal photopolymerization applied to prepare functional composite polymeric materials. Both indentation hardness and reduced modulus of the photocured materials using UCNPs as internal lamps were higher than those of the reference cured under traditional blue LED light. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 994-1002