Fabricating monodispersed polymer latex particles with ∼300 nm size at high monomer concentrations by batch macroemulsion polymerization remains significantly challenging because of latex stability. In this study, we developed a novel approach based on in situ charge neutralization to prepare 40 wt% solid content latex containing monodispersed sub-300 nm latex particles. The cationic initiator 2,2′-azobis(2-methylpropionamidine)dihydrochloride (AIBA) was used to induce in situ charge neutralization in the particle nucleation period and shield the negative charges of surfactant sodium dodecyl sulfate molecules to further reduce the electrostatic repulsion between primary particles, resulting in primary particle coagulation nucleation. The primary particle coagulation promoted particle number decreased to ∼1016 L−1, and the average particle size increased to ∼100 nm at a very low monomer conversion (