(57) [summary] The present invention provides a compound of formula (I), (II) or (III) ## STR1 ## [In the formula, R ₁ and R ₂ are each independently of the other hydrogen, an alkyl group of 1 to 18 carbon atoms, an alkenyl group having 3 to 18 carbon atoms, an alkynyl group or a phenyl group having 3 to 18 carbon atoms a is an unsubstituted or nO _2, halogen atom, an amino group, a hydroxy group, a cyano group, a carboxy group, an alkoxy group having 1 to 4 carbon atoms, an alkylthio group having 1 to 4 carbon atoms, C 1 -C to represent those substituted with 4 alkyl amino or diamino group (alkyl of 1 to 4 carbon atoms); a is a stable free nitroxyl radical a that is bonded to a carbon atom via its oxygen atom - it is formed can group; Y is O, NR ₃ or _CHR 3 -X a (wherein X _a is O, represents S or _NR 3.) represent; R ₃ is a hydrogen atom, C 1 -C 18 alkyl group, an alkenyl group having 3 to 18 carbon atoms, an alkynyl group or a phenyl group having 3 to 18 carbon atoms, unsubstituted or nO _2, halogen atom, an amino group, a hydroxy group, a cyano group, carboxy group, an alkoxy group having 1 to 4 carbon atoms, an alkylthio group having 1 to 4 carbon atoms, optionally substituted (C 1 -C 4 alkyl) alkylamino group or a di-C 1 -C 4 by an amino group each as a; Q ₁ represents at least one derived organic group or inorganic group from a compound having a functional group capable of reacting with a hydroxy group; Q ₂ is one or polyfunctional alcohols, mono- or poly the number of and n is 1 to 20; functionality amino alcohols, one or a multifunctional amine, one or multifunctional mercaptan, represents one or multifunctional phenol or one or a multifunctional organic radical derived from thiophenol the stands; however, in the formula (I), when n represents _1, Q 1 is ## STR2 ## The not represented, or when n represents 2, R ₁ is and represents a H R ₂ represents -CH ₂ -O- tert-butyl, A is 2,2,6,6-tetramethylpiperidine or it does not represent 2,2,6,6-tetramethyl-piperidine-4-carboxylic acid. It relates to a compound represented by]. The composition subject of the present invention comprising at least one ethylenically unsaturated monomer and said compound further relates to polymerization methods and then the resulting (co) polymer.