group in which the only ring structure is the lactone ring is  called a monocyclic group, whereas a lactone-containing  cyclic group containing other ring structure is called a poly› cyclic group regardless of the structure of the other rings. The  lactone-containing cyclic group may be either a monocyclic 5  group or a polycyclic group.  US 9,235,123 B2  49  50  The lactone-containing cyclic group in the constituent unit  (a2) is not particularly limited, and any optional lactone› containing cyclic group can be used. Specifically, examples  thereof include groups represented by the following general  formulae (a2-r-1) to (a2-r-7), respectively.  [Chemical formula 22]  In the formulae, each Ra’ 21  independently represents a  hydrogen atom, an alkyl group, an alkoxy group, a halogen  The -S0$_{2}$--containing cyclic group in R" is the same as  a -S0$_{2}$--containing cyclic group to be described later.  The carbonate-containing cyclic group in R" is the same as  a carbonate-containing cyclic group to be described later.  Specifically, examples thereof include groups represented by  general formulae (ax3-r-1) to (ax3-r-3), respectively.  Examples of the lactone-containing cyclic group in R"  include the same groups as those represented by the general  formulae (a2-r-1) to (a2-r-7), respectively.  In the case where R" is a cyclic alkyl group, the number of  carbon atoms thereof is preferably 3 to 15, more preferably  4  to 12, and most preferably 5 to  10.  Specifically, examples  thereof include a group in which one or more hydrogen atoms  are eliminated from a monocycloalkane, which may or may  not be substituted with a fluorine atom or a fluorinated alkyl  group; and a group in which one or more hydrogen atoms are  eliminated from a polycycloalkane such as a bicycloalkane, a  tricycloalkane, and a tetracycloalkane. More specifically,  examples thereof include a group in which one or more  hydrogen atoms are eliminated from a monocycloalkane such  as cyclopentane and cyclohexane; and a group in which one or  more hydrogen atoms are eliminated from a polycycloalkane  such as adamantane, norbornane, isobornane, tricyclodecane,  and tetracyclododecane.  In the case where R" is a linear or branched alkyl group, the  number of carbon atoms thereof is preferably  1  to  10,  and  more preferably  1  to 5. Above all, R" is especially preferably  a methyl group or an ethyl group.  The alkyl group in R" may be linear, branched, or cyclic,  and the number of carbon atoms thereof is preferably  1  to 15.  In -COOR" and ---OC(=O)R" in Ra’ 21 , each R" is a  hydrogen atom, an alkyl group, a lactone-containing cyclic  group, a carbonate-containing cyclic group, or a -S0$_{2}$ -› containing cyclic group.  Examples of the halogenated alkyl group in Ra’$^{21 }$include a  group in which some or all hydrogen atoms of the alkyl group  in Ra’$^{21 }$are substituted with a halogen atom. The halogenated  alkyl group is preferably a fluorinated alkyl group, and espe› cially preferably a perfluoroalkyl group.  Examples of the halogen atom in Ra’$^{21 }$include a fluorine  atom, a chlorine atom, a bromine atom, and an iodine atom,  with a fluorine atom being preferable.  The alkoxy group in Ra’$^{21 }$is preferably an alkoxy group  having  1  to 6 carbon atoms. The alkoxy group is preferably  linear or branched. Specifically, examples thereof include a  group in which the alkyl group exemplified above as the alkyl  group in Ra’$^{21 }$and an oxygen atom (-0-) are connected to  each other.  In the general formulae (a2-r-1) to (a2-r-7), the alkyl group  in Ra’ 21  is preferably an alkyl group having  1  to 6 carbon  atoms. The alkyl group is preferably linear or branched. Spe› cifically, examples thereof include a methyl group, an ethyl  group, a propyl group, an isopropyl group, an n-butyl group,  an isobutyl group, a tert-butyl group, a pentyl group, an iso› pentyl group, a neopentyl group, and a hexyl group. Of these,  a methyl group or an ethyl group is preferable, with a methyl  group being especially preferable.  atom, a halogenated alkyl group, a hydroxyl group,  --COOR", ---OC(=O)R", ahydroxyalkyl group, or a cyano  group; R" represents a hydrogen atom, an alkyl group, a  lactone-containing cyclic group, a carbonate-containing  cyclic group, or a -S0$_{2}$--containing cyclic group; A" rep› resents an alkylene group having  1  to 5 carbon atoms, which  may contain an oxygen atom (---0-) or a sulfur atom  (-S-), an oxygen atom, or a sulfur atom; n’ represents an  integer of  O  to 2; and m’ represents  O or 1.  (a2-r-1)  (a2-r-2)  (a2-r-3)  Ra’21)jl  I’  Ra,21~ A"  0  0  (a2-r-4)  (a2-r-5)  (a2-r-6)  0  (a2-r-7) 