The formation mechanism of monodisperse polymer latex particles in the emulsifier-free emulsion polymerizationof methyl methacrylate and butyl acrylate with potassium persulfate as initiator was investigated. A multi-step formationmechanism for the monodisperse polymer particles was proposed. The nucleation mechanism is considered to be the coagulation of the precursor particles by homogeneous nucleation when the primary particles reach a critical size with high surface charge density and sufficient stability.It had been proved by a special experiment that the early latex particles formed by the coagulation were stable.The primary particles grow by absorbing monomers and radicals in the polymerization system and then become colloidally unstable again due to the understandable decrease of particle surface charge density

which leads to the affregation of the growing particles and the formation of larger latex particles therefrom.After the nucleation period

the preferential aggregation of the smaller particles in the propagation process leads to the change of the particles towards a uniform size and narrower particle size distribution.The coexistence and competition of homogeneous nucleation

coagulation

propagation and aggregation result in the increase of the polydispersity index(U=D43/D10) in the first stage

then its decrease in the later stage because of the competition of propagation and aggregation

and the gradual formation of the monodisperse particles.