Surprisingly, we have now found that this problem can be solved by the use of peroxides of the general formula: ##STR3## in which R1 is a hydrogen atom, an alkyl radical containing up to 4 carbon atoms or a phenyl radical optionally substituted by halogen atoms or alkyl radicals containing up to 4 carbon atoms, R2, R3, R4 and R5, which can be the same or different, are hydrogen atoms or, together