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[0072] In another preferred embodiment, the base polymer may further comprise recurring units ( d) selected from units of indene, benzofuran, benzothiophene, acenaphthylene, chromone, coumarin, and norbornadiene, or derivatives thereof. Suitable monomers are exemplified below. 

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[0073] Besides the recurring units described above, further recurring units ( e) may be incorporated in the base polymer, examples of which include styrene, vinylnaphthalene, vinylanthracene, vinylpyrene, methyleneindene, vinylpyri› dine, and vinylcarbazole. 

[0074] In a further embodiment, recurring units (f) derived from an onium salt having a polymerizable unsaturated bond 

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may be incorporated in the base polymer. JP-A 2005-084365 discloses sulfonium and iodonium salts having a polymer› izable unsaturated bond capable of generating a sulfonic acid. JP-A 2006-178317 discloses a sulfonium salt having sulfonic acid directly attached to the main chain. 

[0075] The preferred recurring units (f) are recurring units having the following formulae (fl), (f2) and (f3). These units are simply referred to as recurring units (fl), (f2) and (f3), which may be used alone or in combination of two or more types. 

[0076] Herein RA is each independently hydrogen or methyl. Z$^{1 }$is a single bond, phenylene group, ---O-Z 12 -, or ----C(=0)-Z$^{11}$-Z 12 -, wherein Z 11 is -0- or -NH-, and Z$^{12 }$is a C$_{1 }$-C$_{6 }$ alkylene, C$_{2}$ -C$_{6 }$ alkenylene or phenylene group, which may contain a carbonyl, ester bond, ether bond or hydroxyl moiety. Z 2 is a single bond, or -Z$^{21}$-O----C(=O)-, wherein Z 21 is a C$_{1}$-C$_{12 }$ alkylene group which may contain a carbonyl moiety, ester bond or ether bond. Z 3 is a single bond, methylene, ethylene, phe› nylene or fluorinated phenylene group, ---O-Z 32 -, or ----C(=0)-Z $^{31}$-Z32 -, whereinZ$^{31 }$is-O-or-NH-, and Z 32 is a C$_{1 }$-C$_{6 }$ alkylene, phenylene, fluorinated phe› nylene, trifluoromethyl-substituted phenylene, or C$_{2}$-C$_{6 }$ alk› enylene group, which may contain a carbonyl, ester bond, ether bond or hydroxyl moiety. "A" is hydrogen or trifluo› romethyl. 

[0077] R$^{21 }$to R$^{28 }$are each independently a C$_{1 }$-C$_{20 }$mon› ovalent hydrocarbon group which may contain a heteroa› tom. Any two ofR 23 , R $^{24 }$andR$^{25 }$or any two ofR26 , R $^{27 }$and R$^{28 }$may bond together to form a ring with the sulfur atom to which they are attached. 

[0078] The monovalent hydrocarbon group may be straight, branched or cyclic and examples thereof are as exemplified above for R 4 to R 6 in formula (A). The sulfa- 

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