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group), or a cyclic alkoxy group having a carbon number of3 to 8 (such as cyclopentyloxy group and cyclohexyloxy group). 

A structural moiety where any one ofR$_{1}$c to R$_{5}$c is a linear or branched alkyl group, a cycloalkyl group, or a linear, branched or cyclic alkoxy group is preferred, and a structural moiety where the sum of carbon numbers ofR$_{1}$c to R$_{5}$c is from 2 to 15 is more preferred. Thanks to such a structural moiety, the solvent solubility is more enhanced and production of particles during storage can be suppressed. 

The aryl group as R$_{6}$c and R 7 c is preferably an aryl group having a carbon number of 5 to 15, and examples thereof include a phenyl group and a naphthyl group. 

In the case where R$_{6}$c and R 7 c combine to form a ring, the 15 group formed by combining R$_{6}$c and R 7 c is preferably an alkylene group having a carbon number of 2 to 10, and examples thereof include an ethylene group, a propylene group, a butylene group, a pentylene group and a hexylene group. Also, the ring formed by combining R$_{6}$c and R 7 c may 20 contain a heteroatom such as oxygen atom in the ring. 

Examples of the alkyl group and cycloalkyl group as Rx and RY are the same as those of the alkyl group and cycloalkyl group in Rlc to R7c• 

Examples of the 2-oxoalkyl group and 2-oxocycloalkyl 25 group include a group having >C=O at the 2-position of the alkyl group or cycloalkyl group as R$_{1}$c to R7c- 

Examples of the alkoxy group in the alkoxycarbonylalkyl group are the same as those of the alkoxy group in R$_{1}$c to Rsc• The alkyl group is, for example, an alkyl group having a carbon number of 1 to 12, preferably a linear alkyl group having a carbon number of 1 to 5 (e.g., methyl group, ethyl group). 

The ally! group is not particularly limited but is preferably an unsubstituted ally! group or an ally! group substituted with a monocyclic or polycyclic cycloalkyl group. 

The vinyl group is not particularly limited but is preferably an unsubstituted vinyl group or a vinyl group substituted with a monocyclic or polycyclic cycloalkyl group. 

The ring structure which may be formed by combining Rx and RY with each other includes a 5-or 6-membered ring, preferably a 5-membered ring (that is, tetrahydrothiophene ring), formed together with the sulfur atom in formula (ZI-3) by divalent Rx and RY ( e.g., methylene group, ethylene group, propylene group). 

Each of Rx and RY is preferably an alkyl group or a cycloalkyl group, having a carbon number of 4 or more, more preferably 6 or more, still more preferably 8 or more. 

Specific examples of the cation moiety in the structural unit (ZI-3) are illustrated below. 

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<img src='content_image/895148.jpg'>

-continued 

