Tertiary amines which may be used in catalyctic quantities for the process according to the invention preferably correspond to the general formula: ##STR6## in which R1, R2 and R3 represent alkyl radicals having from 1 to 36 carbon atoms, more especially up to 18 carbon atoms, aryl radicals such as phenyl and cycloalkyl radicals having from 4 to 7 carbon atoms, more especially from 5 to 6 carbon a