US 10,698,314 B2  87  88  tible to deprotection with acid, typically ethoxyethoxy, prior  to polymerization, and the polymerization be followed by  deprotection with weak acid and water. Alternatively, the  hydroxyl group may be replaced by an acetyl, formyl,  pivaloyl or similar group prior to polymerization, and the  polymerization be followed by alkaline hydrolysis.  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.  The preferred recurring units  (f) include recurring units  having formula  (fl), recurring units having formula ( f2 ), and  recurring units having formula  (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.  Besides the recurring units described above, further recur› ring units (e) which are derived from styrene, vinylnaph› thalene, vinylanthracene, vinylpyrene, methyleneindene,  vinylpyridine, and vinylcarbazole may be incorporated in  the base polymer.  In a further embodiment, recurring units  (f) derived from  an onium salt having a polymerizable carbon-carbon double  bond may be incorporated in the base polymer. JP-A 2005- 084365 discloses sulfonium and iodonium salts having a  polymerizable carbon-carbon double bond capable of gen› erating a sulfonic acid. JP-A 2006-178317 discloses a sul- fonium salt having sulfonic acid directly attached to the  main chain.  Examples of the monomer from which recurring unit  (fl)  is derived are shown below, but not limited thereto.  RA  and  M- are as defined above.  Herein  RA  is each independently hydrogen or methyl. Z 1  is a single bond, phenylene,  ---O-Z 11 -,  or ----C(=O)› Z12-Z11-, wherein Z 11  is a straight, branched or cyclic  C$_{1 }$-C$_{6 }$ alkylene group or straight, branched or cyclic  C$_{2}$-C$_{6 }$ alkenylene group which may contain a carbonyl, ester, ether  or hydroxyl moiety, or phenylene group, Z$^{12 }$is ---0- or  -NH-. Rs1, Rs2, Rs3, Rs4,  Rss,  R66’ Rs7, and Rss are each  independently a straight, branched or cyclic C$_{1 }$-C$_{12 }$ alkyl  group which may contain a carbonyl, ester or ether moiety,  or a  CcC$_{12 }$ aryl,  C$_{7}$-C$_{20 }$ aralkyl, or mercaptophenyl group.  Z $^{2 }$is a single bond, -Z$^{21}$----C(=O)-O-, -Z$^{21}$---0-, or  -Z $^{21}$-O----C(=O)-, wherein Z $^{21 }$is a straight, branched  or cyclic C$_{1 }$-C$_{12 }$ alkylene group which may contain a car› bonyl, ester or ether moiety. Z $^{3 }$is a single bond, methylene,  ethylene, phenylene, fluorinated phenylene,  ---O-Z 31 -,  or  ----C(=0)-Z$^{32}$-Z 31 -,  wherein Z$^{31 }$is a straight, branched  or cyclic C$_{1 }$-C$_{6 }$ alkylene group or straight, branched or  cyclic C$_{2}$ -C$_{6 }$ alkenylene group which may contain a carbo› nyl, ester, ether or hydroxyl moiety, or a phenylene, fluori› nated phenylene or trifluoromethyl-substituted phenylene  group, Z 32  is -0- or -NH-. A$^{1 }$is hydrogen or trifluo› romethyl.  M- is a non-nucleophilic counter ion.  (fl)  (f2)  (fJ) 