The invention relates to the use of O-imino-iso-urea compounds as source of radicals to polymerizable compositions comprising these O-imino-iso-urea and to new O-imino-iso-urea compounds. The O-lmino-isoureas compounds are compounds of the Formula (I), wherein n is 1, 2, 3 or 4, R100 and R101 are independently H, C1-18 alkyl, C3-C12cycloalkyl, C6-C14aryl, C1-C14heteroaryl, C7-C15aralkyl, C2-C14heteroaralkyl, Cyano, or R100 and R101 form together with the carbon to which they are attached a mono or polycyclic C3-C18 carbocyclic or C1-C18 heterocyclic ring; R102 and R103 are independently C1-C18alkyl, C3-C12cycloalkyl, C6-C14aryl, C6-C14aryl once or more than once substituted by C1-C18alkyl; C7-C15aralkyl, (CH3)3Si—; or R102 and R103 are C1-C18alkyl, C3-C12cycloalkyl, C6-C14aryl, C7-C15aralkyl or R102 and R103 are C1-C18alkyl, C3-C12cycloalkyl which are interrupted or substituted by O or by N containing groups selected from C1-C18alkylamino, bis(C1-C18alkyl)amino or tris(C1-C18alkyl)ammonium; R104 if n is 1 is H, C1-C18alkyl, C3-C12cycloalkyl, C7-C14aralkyl, C6-C14aryl or acyl selected from the group consisting of the following acyls —C(═O)—H, —C(═O)—C1-C18alkyl, —C(═O)—C2-d18alkenyl, —C(═O)—C6-C14aryl, —C(═O)—C2-C18alkenyl-C6-C14aryl, —C(═O)—O—C1C18alkyl, —C(═O)—O—C6-C14aryl, —C(═O)—NH—C1-C18alkyl, —C(═O)—NH—C6-C14aryl and —C(═O)—N(C1C18alkyl)2; or R102 and R104 if n is 1 form together with the nitrogen atom to which they are attached a 5 to 12 membered ring which may contain additional heteroatoms, R104 if n is more than 1 is di-, tri-, tetra-C1-C18alkylidene, diacyls, triacyls or tetraacyls and salts thereof.