21  US 9,411,230 B2  22  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.  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- 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 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.  -continued  0  (  ~s~  l)  0  ~  ~s~  l)  0  +l:J  0  0  0  fa  +s~  0  ~ 