either by the following formula: ##STR4## in which R' represents one of the groups Ra, --CH2 --O--Ra, --CH2 --O--Re--Ra, --CH2 --N???(CH3)2, with Ra representing an alkyl or cycloalkyl radical comprising notably 1 to 12, preferably 1 to 4 carbon atoms, Re representing the polyether radical of the general formula --(CH2 --CH2 --O)p --, p having a value from 1 to 10,