US 2012/0003590 Al  27  Jan. 5, 2012  group; a sulfonamido group; an alkyl group such as methyl  group, ethyl group, propyl group, isopropyl group, n-butyl  group, sec-butyl group, hexyl group, 2-ethylhexyl group,  octyl group and dodecyl group; an alkoxy group such as  methoxy group, ethoxy group, hydroxyethoxy group, pro› poxy group, hydroxypropoxy group and butoxy group; an  alkoxycarbonyl group such as methoxycarbonyl group and  ethoxycarbonyl group; an acyl group such as formyl group,  acetyl group and benzoyl group; an acyloxy group such as  acetoxy group and butyryloxy group; and a carboxy group.  [0231]  The alkyl group is preferably an ethyl group, an  isopropyl group, an isobutyl group, a cyclohexylethyl group,  a phenylmethyl group or a phenylethyl group.  [0232]  The cycloalkyl group may be monocyclic or poly› cyclic. In the latter case, the cycloalkyl group may be a  crosslinked cycloalkyl group. That is, in this case, the  cycloalkyl group may have a bridged structure. Incidentally, a  part of carbon atoms in the cycloalkyl group may be substi› tuted with a heteroatom such as oxygen atom.  [0233]  The monocyclic cycloalkyl group is preferably a  cycloalkyl group having a carbon number of3 to 8. Examples  of such a cycloalkyl group include a cyclopropyl group, a  cyclopentyl group, a cyclohexyl group, a cyclobutyl group  and a cyclooctyl group.  [0234]  The polycyclic cycloalkyl group includes, for  example, a group having a bicyclo, tricyclo or tetracyclo  structure. The polycyclic cycloalkyl group is preferably a  cycloalkyl group having a carbon number of 6 to 20.  Examples of such a polycyclic cycloalkyl group include an  adamantyl group, a norbornyl group, an isoboronyl group, a  camphanyl group, a dicyclopentyl group, an a-pine! group, a  tricyclodecanyl group, a tetracyclododecyl group and an  androstanyl group.  [0235]  The aralkyl group in Li, L$_{2 }$ and Z 2  includes, for  example, an aralky I group having a carbon number of 7 to 15,  such as benzyl group and phenethyl group.  [0236]  These aralkyl groups may further have a substituent.  Preferred substituents include an alkoxyl group, a hydroxyl  group, a halogen atom, a nitro group, an acyl group, an acy› lamino group, a sulfonylamino group, an alkylthio group, an  arylthio group and an aralkylthio group. Examples of the  aralkyl group having a substituent include an alkoxybenzyl  group, a hydroxybenzyl group and a phenylthiophenethyl  group. The carbon number of the substituent which these  aralkyl groups may have is preferably 12 or less.  [0237]  The 5- or 6-membered ring which may be formed by  combining Z$^{2 }$and L$_{1 }$with each other includes, for example, a  tetrahydropyran ring and a tetrahydrofuran ring. Among  these, a tetrahydropyran ring is preferred.  [0238]  Z 2  is preferably a linear or branched alkyl group.  Thanks to this configuration, the effects of the invention are  more successfully brought out.  [0239]  Specific examples of the repeating unit represented  by formula (Al) are illustrated blow, but the present invention  is not limited thereto. 