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R 4 " represents an alkyl group, a halogenated alkyl group, an aryl group or an alkenyl group which may have a substitu› ent. 4 

The alkyl group for R " may be any oflinear, branched or cyclic. 

The linear or branched alkyl group preferably has 1 to 10 carbon atoms, more preferably 1 to 8 carbon atoms, and most preferably 1 to 4 carbon atoms. 

The cyclic alkyl group preferably has 4 to 15 carbon atoms, more preferably 4 to 10 carbon atoms, and most preferably 6 to 10 carbon atoms. 4 

As an example of the halogenated alkyl group for R ", a group in which part of or all of the hydrogen atoms of the aforementioned linear, branched or cyclic alkyl group have been substituted with halogen atoms can be given. Examples of the aforementioned halogen atom include a fluorine atom, a chlorine atom, a bromine atom and an iodine atom, and a fluorine atom is preferable. 

In the halogenated alkyl group, the percentage of the num› ber of halogen atoms based on the total number of halogen atoms and hydrogen atoms (halogenation ratio(%)) is pref› erably 10 to 100%, more preferably 50 to 100%, and most preferably 100%. Higher halogenation ratio is preferable because the acid strength increases. 4 

The aryl group for R " is preferably an aryl group of 6 to 20 carbon atoms. 4 

The alkenyl group for R " is preferably an alkenyl group of 2 to 10 carbon atoms. 4 

With respect to R ", the expression "may have a substitu› ent" means that part of or all of the hydrogen atoms within the aforementioned linear, branched or cyclic alkyl group, halo- 

-continued 

genated alkyl group, aryl group or alkenyl group may be substituted with substituents (atoms other than hydrogen atoms, or groups). 4 

R " may have one substituent, or two or more substituents. 

Examples of the substituent include a halogen atom, a hetero atom, an alkyl group, and a group represented by the formula x $^{00 }$-Q8 - (in the formula, Q 8 represents a divalent linking group containing an oxygen atom; and X$^{00 }$represents a hydrocarbon group of3 to 30 carbon atoms which may have a substituent). 

Examples of halogen atoms and alkyl groups as substitu› ents for R 4 " include the same halogen atoms and alkyl groups as those described above with respect to the halogenated alkyl group for R 4 ". 

Examples ofhetero atoms include an oxygen atom, a nitro› gen atom, and a sulfur atom. 8 8 

In the group represented by formula x$^{00}$-Q -, Q repre› sents a divalent linking group containing an oxygen atom. 8 

Q may contain an atom other than an oxygen atom. Examples of atoms other than oxygen include a carbon atom, a hydrogen atom, a sulfur atom and a nitrogen atom. 

Examples of divalent linking groups containing an oxygen atom include non-hydrocarbon, oxygen atom-containing linking groups such as an oxygen atom ( an ether bond; ---0-), an ester bond (-C(=O)---0-), an amido bond (-C(=O)-NH-), a carbonyl group (-C(=O)-) and a carbonate bond (--0-C(=O)---0-); and combinations of the aforementioned non-hydrocarbon, hetero atom-contain› ing linking groups with an alkylene group. 

Specific examples of the combinations of the aforemen› tioned non-hydrocarbon, hetero atom-containing linking groups and an alkylene group include -R 91 ---0-, -R 92 - O----C(=O)-, -C(=O)-O-R 93 -O----C(=O)- (in the formulas, each of R 91 to R 93 independently represents an alkylene group). 91 93 

The alkylene group for R to R is preferably a linear or branched alkylene group, and preferably has 1 to 12 carbon atoms, more preferably 1 to 5, and most preferably 1 to 3. 

Specific examples of alkylene groups include a methylene group [----CH$_{2}$ -]; alkylmethylene groups such as -CH (CH 3 )-, -CH(CH$_{2}$CH 3 )-, ----C(CH 3 )$_{2}$-, -C(CH 3 ) (CH$_{2}$CH 3 )-, -C(CH 3 )(CH$_{2}$CH$_{2}$CH 3 )- and ----C(CH$_{2}$CH 3 )$_{2}$-; an ethylene group [----CH$_{2}$CH$_{2}$-]; alky- $^{!ethylene groups such as ----CH(CH}$3)CH$_{2}$ -, -CH(CH 3 )CH (CH 3 )-, ----C(CH 3 )$_{2}$CH$_{2}$ - and ----CH(CH$_{2}$CH 3 )CH$_{2}$-; a trimethylene group (n-propylene group) [-CH$_{2}$CH$_{2}$CH$_{2}$-]; alkyltrimethylene groups such as -CH (CH 3 )CH$_{2}$CH$_{2}$ - and ----CH$_{2}$CH(CH 3 )CH$_{2}$-; a tetrameth- ylene group [----CH$_{2}$CH$_{2}$CH$_{2}$CH$_{2}$-]; alkyltetramethylene groups such as ----CH(CH 3 )CH$_{2}$CH$_{2}$CH$_{2}$ - and -CH$_{2}$CH (CH 3 )CH$_{2}$CH$_{2}$-; and a pentamethylene group [-CH$_{2}$CH$_{2}$CH$_{2}$CH$_{2}$CH$_{2}$-]. 1 

Q is preferably a divalent linking group containing an ester linkage or ether linkage, and more preferably a group of -R 91 -O-, -R $^{92}$---0-C(=O)- or -C(=O)-O› R93 ---0-C(=O)-. 8 

In the group represented by the formula X$^{00}$-Q -, the hydrocarbon group for X$^{00 }$is the same as defined for X 0 in formula (1) explained above in relation to the structural unit (a0-2). 00 

In the present invention, as X , a cyclic group which may have a substituent is preferable. 

The cyclic group may be either an aromatic hydrocarbon group which may have a substituent, or an aliphatic cyclic group which may have a substituent, and an aliphatic cyclic group which may have a substituent is preferable. 

