67  US 9,904,167 B2  68  In General Formula (X),  Xa$_{1 }$ represents a hydrogen atom, a methyl group, a trif› luoromethyl group, or a hydroxymethyl group.  T represents a single bond or a divalent linking group.  Rx$_{1 }$ to Rx$_{3 }$ each independently represent a straight-chain  or branched alkyl group, or a monocyclic or polycyclic  cycloalkyl group. Here, two of Rx$_{1 }$ to Rx$_{3 }$ may bond with  each other to form a monocyclic or polycyclic cycloalkyl  group.  Examples of the divalent linking group as T include an  alkylene group, a -(COO-Rt)- group, and a -(O-Rt)› group. Here, Rt represents an alkylene group or a cycloalky› lene group.  Tis preferably a single bond or a -(COO-Rt)- group.  Here, Rt is preferably an alkylene group having  1  to 5 carbon  atoms, and a ----CH$_{2}$ - group or a -(CH$_{2}$ ) 3 - group are  more preferable.  The alkyl group as Rx$_{1 }$to Rx$_{3 }$ is preferably an alkyl group  having  1  to  4  carbon atoms such as a methyl group, an ethyl  group, an n-propyl group, an isopropyl group, an n-butyl  group, an isobutyl group, or at-butyl group.  The cycloalkyl group as Rx$_{1 }$ to Rx$_{3 }$ is preferably a  monocyclic cycloalkyl group such as a cyclopentyl group or  cyclohexyl group, or a polycyclic cycloalkyl group such as  a norbornyl group, a tetracyclodecanyl group, a tetracy- clododecanyl group, or an adamantyl group.  As the cycloalkyl group which may be formed by two of  Rx$_{1 }$ to Rx$_{3 }$ bonding with each other to form, a monocyclic  cycloalkyl group such as a cyclopentyl group or cyclohexyl  group, or a polycyclic cycloalkyl group such as a norbornyl  group, a tetracyclodecanyl group, a tetracyclododecanyl  group, or an adamantyl group is preferable.  In particular, Rx$_{1 }$ is a methyl group or an ethyl group, and  an aspect in which Rx$_{2 }$ and Rx$_{3 }$ bond with each other to form  the cycloalkyl group described above is preferable.  Specific examples are given below of the acid-decompos› able repeating unit; however, the present invention is not  limited thereto.  (In the formulae, Rx is H, CH$_{3}$ ,  CF$_{3}$ ,  CH$_{2}$OH, Rxa, and Rxb  are respectively alkyl groups having  1  to  4  carbon atoms)  -continued  Rxa  Rxb  Rx  I  ---f  CH2 -C  -,-- I  O=C  Rxa  b-tJo  Rx  I  ---f  CH2-C-,-- I  O=C  Rxa  Lt]  4  7  2 