Visible-Light Emulsion Photopolymerization of Acrylates and Methacrylates: Mechanistic Insights and Introduction of a Simplified Sulfur-Based Photoinitiating System

The emulsion photopolymerization under visible light of acrylate and methacrylate monomers is reported. We previously introduced an NHC-borane-based system that was efficient for the emulsion photopolymerization of styrene [acridine orange (AO)/disulfide/NHC-borane]. This system relies on the disulfide photoreduction from the excited AO, followed by H-atom abstraction from the NHC-borane by the thiyl radical generated from the disulfide, eventually forming initiating NHC-boryl radicals. For the photopolymerization of (meth)acrylates, the boron-based photoinitiating system was simplified; as under LED illumination, the water-soluble disulfide alone is able to generate thiyl radicals that could initiate the polymerizations. With this disulfide/blue light system, MMA efficiently polymerized in emulsion, with solid contents up to 40%. The simplified system led to smaller particles than the full photoinitiating boron-based system, but the latter gave slightly better particle size distributions. Compared to styrene, the particle sizes attained for MMA were generally smaller. The disulfide photoinitiator gives access to a wide range of poly(meth)acrylic latexes, but it does not initiate the photopolymerization of styrene. We suggest that the propagation rate prevails on monomer solubility in the case of (meth)acrylates, which explains the efficiency of the photopolymerization with a much weaker initiating system. The method can also lead to the production of random copolymer latexes, such as P(MMA-co-St) and the film-forming P(MMA-co-BA). The fact that the disulfide is able to lead to the former further supports the predominant role of the propagation rate on the polymerization. Finally, the polymerization allows for temporal control. It proceeds under light and stops in the dark.

Automated summary

This study reports on the emulsion photopolymerization of acrylate and methacrylate monomers under visible light using a disulfide photoinitiating system. The simplified boron-based system was efficient in initiating polymerization reactions and producing latexes with different particle sizes. Furthermore, the method allows for the production of random copolymer latexes with temporal control of the polymerization process.