Very few step polymer latexes have been produced by aqueous emulsion polymerization and never with a clear picture of the operating mechanism. Therefore, finding new means to expand this widely used process industrially beyond chain radical polymerization is highly desirable. Herein, we report the successful photo-initiated thiolene radical polyaddition of an emulsion based on two bifunctional monomers diallyl phthalate and 2,2-(ethylenedioxy)diethanedithiol. After 20 min of irradiation (lambda(max) = 385 nm, 3.7 mW cm(-2)), polymerization resulted in a conversion above 99%, a coagulum-free poly(thioether) latex with a solid content of 10 wt %, a number-average molecular weight of 14 kDa, and an average particle diameter of 60 nm. Reaction kinetics, particle size distribution, and molecular weight progress have been studied in detail. To clarify the mechanism dictating particle formation, the number of particles per unit volume of water has been determined depending on reaction time and surfactant and initiator concentration. The nucleation mode is found to depart significantly from conventional chain radical emulsion polymerization. Polymer particle formation mainly proceeds through the precipitation of oligo-radicals (homogeneous nucleation) even when the concentration of the surfactant exceeds its critical micelle concentration. The end of nucleation (interval I) occurs until a monomer conversion of about 95%. The dispersion's solid content can reach as high as 40%, and the molecular weights remain reasonable (>4 kDa) even if the functional groups are stoichiometrically imbalanced. In contrast to conventional thermally induced polymerization, temporal control is demonstrated through sequential "on"/"off' cycles, and tuning of molecular weights is achieved by precise energetic dosage of radiation. This new step photopolymerization in emulsion thus opens an avenue for the synthesis of linear polysulfide latexes without high energy homogenization and at low irradiance.