A multimodule microwave reactor was used to radically polymerize styrene and Me methacrylate with benzoyl peroxide as initiator and poly(vinyl alcohol) as emulsifier at 200, 400, or 600 W for 10-90 min. Thermal polymn. was run for 60-240 min at 85-90 degrees C. Irradiation power 400 W and polymn. time 90 min gave the resp. polymers in 93% and 95% yields. Nonpolars (cyclohexane, 1,4-dioxane) vs. water as solvent ensured shorter (by resp. similar to 65 and 25%) polymn. time and tripled yield. The higher the power and the more the initiator (0.2-0.6 g), the higher the M of polystyrene (max. 198.4 kDa; max. diff. 20%) and the lower the M of PMMA).