Disclosed is a process for pressurizing ethylenically unsaturated monomers to 200-5000 bar in the absence of a polymerization initiator, which comprises effecting said pressurizing in the presence of nitroxyl compounds of the formula I where R1 and R2 are singly C1-C4-alkyl or combine with the joining carbon atom to form a 5- or 6-membered saturated hydrocarbon ring, R3 is C1-C4-alkyl, and R4 is hydrogen or C1-C12-alkyl. Also disclosed are a process for preparing copolymers by such pressurization and subsequent polymerization and copolymers obtainable thereby.