The present invention relates to selected 1-alkoxy-2,2 diethyl-6,6 dimethyl piperidine and 1-alkoxy-2,6 diethyl-2,3,6 trimethyl piperidine derivatives which are substituted in the 4 position by an oxygen or nitrogen atom; a polymerizable composition comprising a) at least one ethylenically unsaturated monomer and b) said piperidine derivatives. Further aspects of the present invention are a process for polymerizing ethylenically unsaturated monomers, and the use of 1-alkoxy-2,2 diethyl-6,6 dimethyl piperidine and 1-alkoxy-2,6 diethyl-2,6 dimethyl piperidine derivatives which are substituted in the 4 position by an oxygen or nitrogen atom for controlled polymerization. The intermediate N-oxyl derivatives, a composition of the N-oxyl derivatives with ethylenically unsaturated monomers and a free radical initiator, as well as a process for polymerization are also subjects of the present invention.