A Methylene Blue/Sodium Sulfinate Photo-Initiating System for the Red-Light Emulsion Photopolymerization of Methyl Methacrylate

The emulsion photopolymerization of methyl methacrylate (MMA) using red light is reported. The synthesis of poly(methyl methacrylate) (PMMA) latexes is initiated by irradiation of a new, very efficient red-absorbing methylene blue (MB)/sodium para-toluenesulfinate (NapTS) system in the red region (660 nm). Visible, and especially red light penetrates the dispersed medium imposed by the emulsion process better than ultra-violet photons (UVs). However, emulsion photopolymerizations require the development of fully water-soluble initiating systems activable by red light, which is still hugely challenging because red photons are much less energetic, therefore generally lead to less efficient photochemical processes. The influence of the surfactant concentration and the solids content is examined over particle sizes and polymerization kinetics. The polymerizations work for solids contents between 10 and 25 wt%, with narrowly distributed particles of average sizes ranging from 40 to 65 nm. Calculations of Mie scattering support the observation that at a given wavelength the larger the particles the lower the penetration. All latexes obtained are slightly blue, which are traced to the regeneration of MB when the latexes are exposed to dioxygen. The dye regeneration enables the reactivation of the photopolymerization process. In this work, the emulsion photopolymerization of methyl methacrylate (MMA) using red light and a fully water-soluble initiating system is reported. Red light penetrates better than UV light the dispersed medium of emulsion photopolymerization, and this enabled a more efficient process. The dye regeneration mechanism allows the reactivation of the photopolymerization process in successive experiments. image