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Herein Re $^{1 }$is each independently hydrogen, fluorine, methyl or trifluoromethyl. Re 2 is each independently hydrogen or a straight, branched or cyclic C$_{1 }$-C$_{20 }$ alkyl or fluoroalkyl group, or two Re $^{2 }$in a common monomer may bond together to form a ring with the carbon atom to which they are attached, and in this event, they together represent a straight, branched or cyclic C$_{2}$ -C$_{20 }$alkylene or fluoroalkylene group. Re 3 is fluorine or hydrogen, or Re 3 may bond with Re 4 to form a non-aromatic ring of 3 to 10 carbon atoms in total with the carbon atom to which they are attached. Re 4 is a straight, branched or cyclic C$_{1 }$-C$_{6 }$ alkylene group in which at least one hydrogen atom may be substituted by a fluorine atom. Res is a straight or branched C$_{1 }$-Ci, alkyl group in which at least one hydrogen atom is substituted by a fluorine atom. Alternatively, Re 4 and Res may bond together to form a non-aromatic ring with the carbon atoms to which they are attached. In this event, Re 4 , Res and the carbon atoms to which they are attached together represent a trivalent organic group of3 to 12 carbon atoms in total. Re $^{6 }$is a single bond or a C$_{1 }$-C$_{4 }$ alkylene. Re$^{7 }$is each independently a single bond, ---0-, or ----CRe$^{1}$Re 1 -. Re 8 is a straight C$_{1}$-C$_{4 }$ or branched C$_{1 }$-C$_{4 }$ alkylene group, or may bond with Rs 2 within a common unit to form a C$_{3}$-C$_{6 }$ non-aromatic ring with the carbon atom to which they are attached. Re 9 is a C$_{1 }$-C$_{30 }$ divalent hydrocarbon group which may contain a heteroatom. Re$^{10 }$is a linear perfluoroalkyl group of 3 to 6 carbon atoms, typically 3H-perfluoropropyl, 4H-perfluo› robutyl, 5H-perfluoropentyl or 6H-perfluorohexyl. Le is each independently ----C(=O)---0-, ---0-, or ----C(=O)› Re11----C(=O)---0-. Re 11 is a straight, branched or cyclic C$_{1 }$-C$_{10 }$ alkylene group. The subscripts are in the range: 0s(a’-l)sl, 0s(a’-2)sl, 0s(a’-3)sl, 0sb’sl, 0sc’sl, and 0<( a’-1 )+( a’-2)+( a’-3 )+b’+c’s 1. 

Examples of these units are shown below. Herein Ra $^{1 }$is as defined above. 

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