oup consisting of --F, --Cl, --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 or Y'