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hydrogen atom may be substituted by a fluorine atom. R $^{118 }$is a straight or branched C $_{1 }$-C $_{10 }$alky 1 group in which at least one hydrogen atom is substituted by a fluorine atom. Alterna› tively, R$^{117 }$and R$^{118 }$may bond together to form a non-aro› matic ring with the carbon atoms to which they are attached. 5 In this event, R 117 , R $^{112 }$and the carbon atoms to which they are attached together represent a trivalent organic group of 2 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$^{121 }$isastraightorbranchedC$_{1}$-C$_{4}$ alkylene 10 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 of 3 to 6 15 carbon atoms, typically 3H-perfluoropropyl, 4H-perfluo› robutyl, 5H-perfluoropentyl, or 6H-perfluorohexyl. X $^{2 }$is each independently ----C(=O)---0-, ---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 20 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’s 1. 

Exemplary units are shown below. 

<img src='content_image/885427.jpg'>

<img src='content_image/885428.jpg'>

