R1 is the radical of a conventional chain regulator or initiator which is free from active hydrogen atoms, R2 is identical or different and is an alkyl radical, substituted aryl radical, perfluoroalkyl radical or dialkylamino radical, R3 is identical or different and is an alkyl radical or aryl radical, R4 is an organic radical having at least one carboxylic acid or carboxylate function, X is oxyg