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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. 

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] 

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In the formulae, each Ra’ 21 independently represents a hydrogen atom, an alkyl group, an alkoxy group, a halogen 

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. 

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. 

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. 

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. 

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. 21 

In -COOR" and ---OC(=O)R" in Ra’ , 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. 

The alkyl group in R" may be linear, branched, or cyclic, and the number of carbon atoms thereof is preferably 1 to 15. 

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. 

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. 

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. 

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. 

The -S0$_{2}$--containing cyclic group in R" is the same as a -S0$_{2}$--containing cyclic group to be described later. 

