In the formulae (i) and (ii), A represents an aliphatic or aromatic hydrocarbon radical, n is a whole number from 1 to 4, Rf represents a straight or branched perfluorinated chain containing 1 to 20 carbon atoms, B represents bivalent, possibly branched, chaining and may comprise sulfur, oxygen or nitrogen atoms, one of the symbols R represents a hydrogen atom and the other a hydrogen a