Research on solvent-free acrylic pressure-sensitive adhesives (PSAs) has tremendously grown over the last few decades due to the stringent regulations to control volatile organic compound emissions. They are mostly prepared in the presence of photoinitiators using high-energy UV light that causes several issues such as those associated with radiation safety. Herein, for the first time, solvent-free acrylic PSAs were prepared through visible-light-driven photocatalytic free radical polymerization. Importantly, we found that the use of N-vinyl-based monomers noticeably enhances the rate and conversion of polymerizations, thereby eliminating the need for additives (e.g. α-haloester and sacrificial electron donor) that are usually required for photoredox-mediated free radical polymerization, but concurrently needs an additional purification process to remove residues. Combined experiments and quantum chemical calculations suggest that N-vinyl-based monomers facilitate electron transfer between monomers and photocatalysts, which is responsible for the enhanced rate and converison of polymerization. Viscoelasticity, mechanical strength and adhesion performance of acrylic PSAs were well adjusted in a broad range by controlling the monomer composition, suggesting that our new method would replace the existing photoinitiated free radical polymerization.

Owing to their excellent properties, such as transparency, resistance to oxidation, and high adhesivity, acrylic pressure-sensitive adhesives (PSAs) are widely used. Recently, solvent-free acrylic PSAs, which are typically prepared via photopolymerization, have attracted increasing attention because of the current strict environmental regulations. UV light is commonly used as an excitation source for photopolymerization, whereas visible light, which is safer for humans, is rarely utilized. In this study, we prepared solvent-free acrylic PSAs via visible light-driven photoredox-mediated radical polymerization. Three α-haloesters were used as additives to overcome critical shortcomings, such as the previously reported low film curing rate and poor transparency observed during additive-free photocatalytic polymerization. The film curing rate was greatly increased in the presence of α-haloesters, which lowered the photocatalyst loadings and, hence, improved the film transparency. These results confirmed that our method could be widely used to prepare general-purpose solvent-free PSAs—in particular, optically clear adhesives for electronics.