-CF2 Cl, --CFCl2, --CCl3, perfluoroalkyl of one to ten carbon atoms and perfluoroalkoxyalkyl of one to ten carbon atoms wherein one or more of the fluorine atoms may be substituted by a halogen atom other than fluorine; wherein n is an integer from 2 to 1,000; and wherein m is an integer from 0 to 1,000; provided that when R1, R2, R3 and R4 together are F, then Y and Y' cannot be both --C2 F4 -- .