We have investigated the combination of the thermally auto-initiated free radical polymn. of styrene and pptn. polymn. in order to develop a fast and environmentally friendly approach to produce polystyrene. To achieve high reaction temps. in a short period of time, microwave irradn. was utilized as the heating source. Styrene was used without any purifn., e.g., without distn. or column filtration. Due to the auto-initiation of styrene at high temps. no radical initiator was required. Different water- or ethanol-to-styrene ratios were heated far beyond their b.ps. and at relatively high pressures for the auto-initiated polymn. of styrene. The obtained mol. wts. could be controlled by the ethanol-to-styrene ratio in the case of ethanol as the solvent although the monomer conversions were rather low under the applied conditions. Moreover, the effect of a com. available stable free nitroxide was investigated on the control over the polymn. It has been obsd. that it is possible to control the mol. wt. of the polymer by changing the ratio of styrene to free nitroxide (varied from 10:1 to 400:1) and moderate polydispersity indexes (PDI = 1.3 to 1.9) could be obtained. Finally, the developed polymn. processes only require a simple purifn. step due to the pptn. of the polystyrene in the reaction solvent.