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Herein R 114 is each independently hydrogen, fluorine, methyl or trifluoromethyl. R $^{118 }$is each independently hydro› gen or a straight, branched or cyclic C$_{1 }$-C$_{20 }$ alkyl or fluo› roalkyl group, or two R $^{115 }$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 fluoroalky› lene group. R$^{116 }$is fluorine or hydrogen, or R$^{116 }$may bond with R$^{117 }$to form a non-aromatic ring of 3 to 10 carbon atoms in total with the carbon atom to which they are attached. R$^{117 }$is a straight, branched or cyclic C$_{1}$-C$_{5 }$ alky› lene group in which at least one hydrogen atom may be substituted by a fluorine atom. R $^{118 }$is a straight or branched C$_{1 }$-C$_{10 }$alkyl group in which at least one hydrogen atom is substituted by a fluorine atom. Alternatively, R 11 7 and R 118 may bond together to form a non-aromatic ring with the carbon atoms to which they are attached. In this event, R 117 , R 118 and the carbon atoms to which they are attached together represent a trivalent organic group of2 to 12 carbon atoms in total. R $^{119 }$is a single bond or a C$_{1 }$-C$_{4 }$ alkylene. R 120 is each independently a single bond, ---0-, or -CR$^{114}$R 114 -. R $^{121 }$is a straight or branched C$_{1}$-C$_{4 }$ alky› lene group, or may bond with R 115 within a common monomer to form a C$_{3}$ -C$_{6 }$ non-aromatic ring with the carbon atom to which they are attached. R 122 is 1,2-ethylene, 1,3-propylene, or 1,4-butylene. Rf is a linear perfluoroalkyl group of3 to 6 carbon atoms, typically 3H-perfluoropropyl, 4H-perfluorobutyl, 5H-perfluoropentyl, or 6H-perfluoro› hexyl. X $^{2 }$is each independently----C(=O)-O-, -0-, or -C(=O)-R$^{123}$----C(=O)---0-. R 123 is a straight, branched or cyclic C$_{1 }$-C$_{10 }$alkylene group. The subscripts are in the range: 

0s(a’-1)<1, 0s(a’-2)<1, 0s(a’-3)<1, 

0<(a’-l)+(a’-2)+(a’-3)<1, 0sb’<l, 0sc’<l, and 

0<(a’-1 )+(a’-2)+(a’-3)+b’+c’sl. 

[0120] Examples of these units are shown below. 

-continued 

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