The unseeded emulsion polymerization of styrene with n-nonyl mercaptan (nNM) as chain transfer agent (CTA) was investigated, with the aim of producing a PS latex of a low molecular weight polydispersity at high conversion and in short reaction times. To this effect, starved and minimum time semibatch reactions were investigated and compared to equivalent batch polymerizations. The high reactivity of nNM with respect to the monomer makes it feasible to implement a minimum time policy with intermediate addition of CTA only. The MWDs of the minimum time runs were intermediate between the broader distributions of the batch reactions and the narrower distributions of the starved experiments. The conversion profiles of minimum time experiments almost coincided with those of equivalent batch polymerizations. For controlling MWDs through semibatch operations, it seems preferable to use nNM instead of other (less reactive and more common) CTAs like tert-dodecyl mercaptan or CCl4. (C) 2001 Elsevier Science Ltd. All rights reserved.