Stable polystyrene latexes having controlled molecular weight distribution (MWD) with solids contents up to 40 wt% are obtained using potassium persulfate as initiator with the nitroxides 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (hydroxy-TEMPO) and 4-amino-2,2,6,6-tetramethylpiperidine-N-oxyl (amino-TEMPO). Using different concentrations of styrene (St) in the nucleation step, it is possible to create colloidal dispersions with particle size ranging from ≈57 to ≈390 nm, conversions up to ≈96%, nitroxide efficiencies (N Eff ) up to ≈0.76, and number average molecular weight (M n ) ranging from ≈10 000 to ≈85 000 g mol −1 . The effect of the monomer feed rate in the semibatch step is also studied. These polymerizations do not require a separate macroinitiator synthesis, since this is formed in situ during the heating ramp. A mathematical model is used to compare the mechanistic understanding of the system with the experimental results.