this is achieved by using a family of specific nitroxides as additive application_number US 12243105 A description according to the present invention a nitroxide n of formula iii is preferably used in which r 1 r 2 r 3 and r 4 which may be identical or different represent a methyl ethyl propyl isopropyl butyl sec butyl or tert butyl radical r 5 r 6 r 7 r 8 and r 9 each represent a hydrogen atom and r 10 represents a linear branched or cyclic unsaturated hydrocarbon based group or an acrylate acrylamide methacrylate or methacrylamide group it is preferred to use in particular a nitroxide n containing an unsaturation represented by formula iv in which x represents a divalent group u2014 oc o u2014 u2014 nrbc o u2014 or u2014 p o or b o u2014 with r b having the same meaning as r and n is an integer which may range from 2 to 20 and preferably from 2 to 10 the position of the unsaturation on the group c n h 2n 1 is irrelevant but unsaturations located either in position 1 relative to x or in the terminal position are preferred according to the present invention compounds which may be used as free radical initiators are azo compounds and or organic peroxides which upon thermal decomposition produce free radicals which facilitate the curing crosslinking reaction among the free radical initiators used as crosslinking agents dialkyl peroxides and diperoxyketals are preferred a detailed description of these compounds will be found in encyclopedia of chemical technology 3rd edition vol 17 pages 27 to 90 1982 among the dialkyl peroxides the preferred initiators are dicumyl peroxide.