Microwave-mediated miniemulsion polymerization of styrene with consecutive periods of heating and cooling exhibits - compared to continuously heated polymerizations - a very unique behavior. For medium-hydrophobic azo-initiators under optimized conditions, polymer radicals survive the heating pulse and grow during the cooling period to ultrahigh molecular weights > 10(7) g/mol. This "surviving radical effect" is accompanied by unexpectedly high conversion after the first polymerization cycle, comprising of a temperature pulse of less than 10 s duration and a subsequent cooling period. Besides the ultrarapid heating by the microwaves, the surviving radical effect is purely thermal in nature and can be explained by the elemental reactions of radical heterophase polymerizations.