Useful monomer units include: ##STR1## in which R' represents a hydrogen atom or one of the group R3, --CH2 --O--R3, --CH2 --O--R4 --R3, --CH2 --N=(CH3)2, with R3 representing an alkyl or a cycloalkyl radical including particularly 1 to 16, preferably 1 to 5 carbon atoms and R4 representing a polyether radical of the general formula: