Emulsion polymerization is a widely used technique to produce colloidal polymer particles (latex) despite its complex nature. The continued rise in the demand for emulsion polymers has raised questions on the effectiveness and sustainability of the current production methods based mainly on chain radical polymerization approaches of monomers such as (meth)acrylates or styrene. To date, several methods have been proposed towards the synthesis of step polymers with the key concerns being: achieving high monomer conversion rates, eliminating side reactions, and efficient byproducts removal. However, the technical challenges of step emulsion polymerization still pose a great threat to their advancement and large-scale industrial applications. The latest attempts to produce dispersion polymers from thiol-ene step polymerization in emulsion have also presented inadequate information regarding the fundamental mechanism dictating the particle formation, particle size distribution, and molecular weight progress, desirable to achieve high-molecular-weight products.
To address the above challenges, researchers at the University of Haute-Alsace in Mulhouse (France): Cuong Minh Quoc Le, Dr. Marc Schmutz and Dr. Abraham Chemtob studied the batch photo-initiated thiol-ene radical polyaddition of the aqueous emulsion prepared by two bifunctional monomers: diallyl phthalate and 2,2-(ethylenedioxy)diethanedithiol at room temperature. The underlying mechanisms dictating the particle formation were clarified in detail by determining the number of particles per unit volume of water depending on three factors: initiator concentration, reaction time, and surfactant. Their work is currently published in the research journal, Macromolecules.
The research team successfully synthesized linear polymer nanoparticles of about 60 nm in size under experimental conditions close to conventional chain emulsion polymerization, after only 20 minutes of polymerization at 385 nm. Additionally, thiol-ene polymerization exhibited the advantages of simultaneously obtaining both higher polymerization rates and desirable molecular weights. For instance, a high conversion rate above 99% and an average molecular weight of 14kDa were achieved. Furthermore, the process provided a clear understanding of the polymerization mechanism leading to particle formation.
The results also showed that the polymer particle formation depends on the precipitation of oligo-radicals even at higher surfactant concentrations above its critical micelle concentration. It was also noted that the end nucleation could occur until the monomer conversion of about 95%, while the solid dispersion content could reach 40%, all without affecting the achievable molecular weights. Unlike conventional thermally induced polymerization, the emulsion thiol-ene polymerization exhibited numerous advantages, including the ability to produce stable polysulfide latex without high energy homogenization and at low irradiance, making it suitable for enhancing the temporal control of the reactions and tunning the molecular weight through the energetic dosage of radiation.
In summary, the study presented a detailed investigation of thiol-ene emulsion step photopolymerization and reported a successful synthesis of linear polymers from two bifunctional monomers EDDT and DAP. Remarkably, thiol-ene polymerization exhibited numerous advantages including, stable polysulfide latex, high reaction rates, and the ability to form desirable particle sizes and molecular weights. Also, the underlying mechanisms leading to the formation of particles were clarified in detail. The results, according to Dr. Abraham Chemtob, will open up avenues for industrial and large-scale synthesis of linear polysulfide latexes at low irradiance and low energy homogenization.
Le, C. M. Q., Schmutz, M., & Chemtob, A. (2020). Ab Initio Batch Emulsion Thiol–Ene Photopolymerization. Macromolecules, 53(7), 2369-2379.