We report a dispersion polymerization method based on thiol–Michael addition reactions for the preparation of cross-linked, narrow dispersity microparticles with well-defined, tunable physicochemical properties. Polymerization between pentaerythritol tetra­(3-mercapto­propionate) (PETMP) and trimethylol­propane triacrylate in methanol was chosen as a model system, with the addition of triethylamine as a catalyst and polyvinyl­pyrrolidone as a stabilizer. The formation of microparticles took place within seconds at ambient conditions, as a result of a polymerization driven phase transition from dissolved monomers to precipitated polymers. The particle size was found to be affected by the amount of catalyst, the monomer concentration, and the monomer/polymer solubility in the reaction media. Monodispersity was achieved within a range of particle diameters from 1.6 to 4.3 μm, as determined both by scanning electron microscopy and dynamic light scattering. The reaction kinetics were studied by Fourier transform infrared spectroscopy by analyzing aliquots withdrawn from the reaction system at various reaction time points. Nearly quantitative conversions were achieved within 6 h for stoichiometric systems and 1 h for off-stoichiometric systems, both initiated with triethylamine. By utilizing photolabile bases as the reaction catalyst, phototriggered formation of the microparticles was demonstrated with ultraviolet irradiation. Monodisperse particles were formed with hexylamine and 1,1,3,3-tetramethyl­guanidine, both with 2-(2-nitrophenyl)­propyloxy­carbonyl as the UV-labile photocage. Furthermore, as a demonstration of the versatility of this method, microparticles were prepared from copolymerizations between PETMP and four types of diacrylates with varied backbone structures. With increased backbone rigidity, the microparticle glass transition temperature increased from −36 to 8 °C. This method provides a platform for the realization of the nearly ideal step-growth networks in microscale, with highly tunable backbone structures, robust thermal transitions, and intrinsic functionalization capacity.