C.,optionally under excess pressure, characterised in that the reaction is carried out in the presence of a dihalogenated disulphide corresponding to formula (III) ##STR3## in which X represents halogen, such as chlorine and bromine,R represents hydrogen, a C.sub.1 -C.sub.4 -alkyl radical, a C.sub.5 -C.sub.10 -cycloalkyl radical, a C.sub.7 -C.sub.14 -aryl radical and the total number of carbon ato