or Y' one or more of the fluorine atoms may be substituted by a halogen atom other than fluorine; wherein X and Z are the same or different and are selected from the group consisting of --(CF2)r COF, --(CF2)r OCF3, --(CF2)r COOH and Cr F2r+1-q Clq, wherein r is an integer from 1 to 12 and q is an integer from 0 to 25; wherein R1, R2, R3 and R4 are the same or different and are selected from the gr