Abstract: This invention relates to a process for the radical homo-or copolymerization of acrylonitrile with one or more copolymerizable monomers under reaction conditions such that the monomers are present predominantly in a gaseous form of an adsorbed form and polymerization of the monomers, which are presumably adsorbed, takes place in the solid phase (solvent-free polymerization) and results in pulverulent PAN-(co)polymers. The process is preferably carried out in the presence of small quantities of a mediator substance such as dimethylformamide which does not alter the pulverulent consistency of the PAN-polymers but leads considerably more rapidly and with much improved yields to freshly formed polyacrylonitrile polymers.