. . " groups per 100 g of said compound and having the following general formula: ##STR3## wherein: R and R', which may be the same or different, are alkyl, alkenyl or arylR and R', which may be the same or different, are alkenyl, and n is an integer; (b) at least a catalyst selected from the group comprising inorganic and organic complexes of transition metals and organic salts of transition metals;" . .