The invention relates to novel cyclic and open-chain hydroxylamine esters and polymerizable compositions comprising these hydroxylamine esters and an ethylenically unsaturated monomer or oligomer. The invention also relates to use as polymerization initiators and to the use of known hydroxylamine esters selected from the group consisting of HALS compounds and the novel hydroxylamine esters for the controlled degradation of polypropylene and for achieving a controlled increase in the molecular weight of polyethylene.