Compounds of the formula ##STR1## in which R represents hydrogen or a methyl group,Y.sup.1 represents an optionally branched alkylene radical with 1 to 6 C atoms,Y.sup.2 represents an optionally branched alkylene radical with 2 to 8 C atoms,X denotes oxygen or an --NH-- group,R.sup.1 and R.sup.2 are identical or different and denote hydrogen, or an alkyl, alkenyl, cycloalkyl, cycloalkenyl, aryl, aralkyl or alkylaryl group which has 1 to 11 C atoms and is optionally substituted by one or more,hydroxyl, amino, epoxide, urethane, urea, ester or ether groups, orR.sup.1 and R.sup.2 together form a 3-membered to 6-membered ring, which optionally contains nitrogen, oxygen or sulphur as hetero-atoms,R.sup.3 has the meaning of R.sup.1 or represents a group ##STR2## or R.sup.2 and R.sup.3, together with the ##STR3## group, form a 5-membered or 6-membered ring, which optionally contains oxygen as a further hetero-atom, andR.sup.4 and R.sup.5 are identical or different and represent hydrogen, an alkyl or alkenyl group which has 1 to 10 C atoms and is optionally substituted by halogen, or halogen,function as activators in the free radical polymerization of olefinically unsaturated monomers.