either by the following formula: ##STR1## in which R' represents a hydrogen atom or 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 16, preferably 1 to 4 carbon atoms, Re representing a polyether radical of the general formula --(CH2 --CH2 --O)p --, p having a value from 1 to 100, notably from 1