Radical step-growth photopolymerization of dithiol−diene monomer miniemulsion is shown to be a highly efficient, robust, and versatile route to generate film-forming linear poly(thioether) latexes. At extremely fast rates, the process results in high-molecular-weight polysulfide products, exhibiting both semicrystalline and oxidation-responsive properties. Four key issues are addressed as regards the practical implementation of this novel UV-driven waterborne technology: the preparation of a photolatent and colloidally stable thiol−ene monomer miniemulsion, the identification of key experimental parameters controlling reaction kinetics and polymer microstructure, the characterization of film semicrystallinity, and the application of poly(thioether ester) latexes as dual-stimuli-responsive nanocarriers sensitive to both oxidation and hydrolysis.

At the forefront is the radical chain polymerization, that represents 40% of all processes for the production of commercial polymers, including some of the most common plastics such as polystyrene, acrylic or vinyl polymers. It is of high interest that the propagation step governing the chain-growth may be very fast, given that the propagation rate coeffi cient ( k p ) is for most monomers in the range of 10 2 -10 4 L mol −1 s −1 . Nevertheless, the paradoxical observation is that polymer chemists have always strived to limit reactivity. The highly exothermic nature of radical chain polymerizations, the high activation energies involved, and the tendency toward the gel effect combine to make heat dissipation and polymer architecture control very challenging on an industrial scale. To avoid uncontrolled acceleration of the polymerization rate that may cause disastrous "runaway" reactions, the conventional approach consists in limiting the concentration of free monomer within the reactor through semicontinuous operations, with obvious adverse consequences for productivity. [ 2 ] With this limitation in mind, we have developed a novel eco-effi cient, "fl ash," radical chain linear photopolymerization, [ 1 ] able to reduce reaction time scales from hours to seconds. Our synergetic approach integrates macrofl ow photochemistry and emulsion-type polymerization . The result is a practical "single pass" helix photochemical reactor for the continuous photopolymerization of The continuous photopolymerization of acrylate and methacrylate monomer miniemulsions (25% solids content) is investigated at room temperature in a compact helix minireactor. Using n-butyl acrylate, the process yields 95% conversion after only 27 s residence time, and gel-free high-molecular-weight products. Under optimized conditions, a 25-fold increase in effi ciency is obtained when compared to a batch photopolymerization. The reaction set-up offers a frugal process because of moderate irradiance (2.6 mW cm −2 ), photoinitiator concentration (0.75 wt%), and low-power UV-A fl uorescent lamp.

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Plugging is probably one of the most challenging issues facing further continuous polymerization process development. Starting with the photopolymerization of n-butyl acrylate miniemulsion in a continuous photoreactor composed by fluoropolymer coiled tubing, we show that three parameters have a critical role on the occurrence of plugging: solids content (≥ 30 wt%), surfactant concentration (≤ 1 wt%) and tubing diameter (≤ 1 mm). In contrast, monomer droplet stability, size and flow rate have a minimal impact. The use of nanodroplets, as individual reactors able to confine the solid products to these droplets, is in no way an efficient strategy to prevent channel clogging. Polymer adsorption occurs locally on macroscopic nucleation sites, where polymer build-up leads gradually to plugging. Based on interfacial tension measurements, we show adhesional wetting as the main trigger of plugging. In this process driven by the high cohesive energy of water, the monomer droplets not originally in contact with the reactor wall makes contact with that surface by displacing water, adhere to it, and polymerize.