???[ORf3]q??? wherein Rf1 is a perfluorinated aliphatic group of 1 to 6 carbon atoms, for example up to 3 carbon atoms, Rf2 and Rf3 each independently represents linear or branched a perfluorinated alkylene of 1, 2, 3 or 4 carbon atoms and p and q each independently represent a value of 0 to 4 and wherein the sum of p and q is at least 1 and perfluorinated groups of the formula: Rf4???[ORf5]k???[O