Abstract: The present invention relates to selected glycidyl or carbonyl functional N-alkoxy-4,4-dioxy-polyalkyl-piperidine compounds forming an open chain or cyclic ketal structure, a polymerizable composition comprising a) at least one ethylenically unsaturated monomer and b) a glycidyl or carbonyl functional N-alkoxy-4,4-dioxy-polyalkyl-piperidine nitroxide initiator compound. Further aspects of the present invention are a process for polymerizing ethylenically unsaturated monomers and the use of glycidyl or carbonyl functional N-alkoxy-4,4-dioxy-polyalkyl-piperidine nitroxide initiators for radical polymerization.