Semicontinuous monomer-starved emulsion polymerization as a means to produce nanolatexes: analysis of nucleation stage.

In this research, particle nucleation was decoupled from particle growth in the monomer-starved semicontinuous (micro)emulsion polymerization of styrene by close monitoring of the end of nucleation. This enabled us to exclude the effects of particle growth on nucleation and therefore unravel inherent features of nucleation in this process. Nanolatexes with average particle sizes as small as 15 nm were obtained. The average size of particles at the end of nucleation was found to be almost independent of surfactant concentration ([S]) but varied with the rate of monomer addition (Ra) to the 1/3 power. Nucleation time varied almost proportionally with [S](1.0). The sharpest particle size distribution was obtained at the lowest monomer feed rate used. The weight-average molecular weights (M̅w) of the polymer produced decreased with decreasing Ra. A simple correlation was developed which shows that the number-average molecular weight (M̅n) is proportional to Ra(1.0) but independent of [S] (i.e., [S](0.0)), which was in fair agreement with the experimental results. It is also shown that the polymer molecular weight is proportional to the average volume of particles; the smaller the particle, the lower the molecular weight.