This invention relates to xanthogen disulphides of the formula ##STR1## in which R is the same or different and denotes the following groups: ##STR2## wherein X = H, alkyl, aryl, aralkyl or halogenn = 1 to 20,A process for preparing them, a process for polymerizing dienes and .alpha.-olefines in the presence of these particular xanthogen disulphides and to vulcanizable rubber mixtures consisting of an uncross-linked benzene soluble chloroprene homopolymer or copolymer which is prepared in the presence of said xanthogen disulphides.