This invention relates to a process for the free-radical polymerization of monomers in gas phase or suspension processes using a supported, polymerization-initiating system comprising a) one or more polymerization-initiating components which initiate and start free-radical polymerization, b) a support, onto which said polymerization-initiating component is applied, and c) optionally, one or more modifiers which have an influence upon reaction kinetics and/or product properties.