is especially advantageous to use as control agents xanthates dithiocarbamates or dithiocarbazates advantageously use is made as control agent in step e0 of xanthate compounds for instance o ethyl s 1 methoxycarbonyl ethyl xanthate of formula ch 3 ch co 2 ch 3 s c u2550 s och 2 ch 3 and more generally the reactive stabilizer of step e1 preferably bears such groups a control agent that is particularly suited to the implementation of step e0 is the compound sold by the company rhodia under the name rhodixan a1 the sources of free radicals that may be used in steps e0 and e1 the sources of free radicals used in steps e0 and e1 may be identical or different and are typically identical for practical reasons a source of free radicals that is suitable both in step e0 and in step e1 is a polymerization initiator of redox type which comprises two agents namely an oxidizing agent and a reducing agent which may be introduced simultaneously or consecutively according to an advantageous embodiment the reducing and oxidizing agents are introduced separately which makes it possible to delay the initiation of polymerization until the second agent has been introduced advantageously a step using such a redox agent is performed i.