The invention pertains to a compound of formula (I), (II) or (III) wherein R1 and R2 are independently of each other hydrogen, C1-C18alkyl, C3-C18alkenyl, C3-C18alkinyl or phenyl which are unsubstituted or substituted by NO2, halogen, amino, hydroxy, cyano, carboxy, C1-C4alkoxy, C1-C4alkylthio, C1-C4alkylamino or di(C1-C4alkyl)amino; A is a group capable of forming a stable free nitroxyl radical A., which is bound via its oxygen atom to the carbon atom; Y is O, NR3 or CHR3-Xa, wherein Xa is O, S or NR3; R3 is hydrogen, C1-C18alkyl, C3-C18alkenyl, C3-C18alkinyl or phenyl which are unsubstituted or substituted by NO2, halogen, amino, hydroxy, cyano, carboxy, C1-C4alkoxy, C1-C4alkylthio, C1-C4alkylamino or di(C1-C4alkyl)amino; Q1 is an organic or inorganic radical, derived from a compound having at least one functional group being capable of reacting with a hydroxy group; Q2 is an organic radical derived from a mono or polyfunctional alcohol, mono or polyfunctional aminoalcohol, mono or polyfunctional amine mono or polyfunctional mercaptane, mono or polyfunctional phenol or mono or polyfunctional thiophenol; and n is a number from 1 to 20; with the provison that in formula (I) if n is 1, Q1 is not (a), or if n is 2, R1 is H, and R2 is -CH2-O-tert.butyl, A is not 2,2,6,6-tetramethylpiperidine or 2,2,6,6-tetramethylpiperidine-4-carboxylic acid. Further subjects of the invention are a composition comprising above compounds and at least one ethylenically unsaturated monomer, process for polymerization and the (co)polymers obtained therefrom.