Therefore the process is carried cut in the presence of perfluoropolyethers having end groups containing bromine atoms comprised in the following general formula: ##STR1## wherein: T=F or CF3 ; m, n, and q are integers, including zero, m+n+q ranges from 2 to 100; A and B, either alike or different from each other, consist of one of the following groups: ##STR2## x being bromine or fluorine, provid