ene group or alkylmethylene group. The alkyl group within  the alkylmethylene group is preferably a linear alkyl group of  1 to 5 carbon atoms, more preferably a linear alkyl group of 1  to 3 carbon atoms, and most preferably a methyl group.  fluorinated alkyl group of 1 to 5 carbon atoms, and is most  preferably a hydrogen atom or a methyl group.  There are no particular limitations on X  1  as long as it is an  acid-dissociable group. Examples include the aforemen› tioned tertiary alkyl ester-type acid-dissociable groups and  acetal-type acid-dissociable groups, and of these, a tertiary  alkyl ester-type acid-dissociable group is preferable.  In the general formula (al-0-2), R is the same as defined  above.  X 2  is the same as defined for X 1  in formula (al-0-1).  Although there are no particular limitations on the divalent  linking group for Y 2 , preferred examples include divalent  hydrocarbon groups which may have a substituent, and diva› lent linking groups containing a hetero atom. Specific  examples of these divalent hydrocarbon groups which may  have a substituent, and divalent linking groups containing a  hetero atom include the same divalent linking groups as those  described above for R 0 $_{-}$1  in the general formula (a0-0-1)  within the description relating to the structural unit (a0).  Y 2  is preferably a linear or branched alkylene group, a  divalent alicyclic hydrocarbon group, or a divalent linking  group containing a hetero atom.  When Y 2  is a linear or branched alkylene group, the alky› lene group preferably contains 1 to 10 carbon atoms, more  preferably 1 to 6 carbon atoms, still more preferably 1 to 4  carbon atoms, and most preferably 1 to 3 carbon atoms.  Specific examples include the same linear alkylene groups  and branched alkylene groups as those mentioned above for  the linear or branched aliphatic hydrocarbon group within the  description of the divalent linking group for R 0 $_{-}$1 . When Y 2  is  a divalent alicyclic hydrocarbon group, examples of the ali- cyclic hydrocarbon group include the same alicyclic hydro› carbon groups as those mentioned above as the "aliphatic  hydrocarbon group that includes a ring within the structure"  within the description of the divalent linking group for R 0 $_{-}$1 .  Specific examples of the alicyclic hydrocarbon group  include groups in which two or more hydrogen atoms have  been removed from cyclopentane, cyclohexane, norbomane,  isobornane, adamantane, tricyclodecane and tetracyclodode› cane.  When Y 2  is a divalent linking group containing a hetero  atom, preferred examples of the divalent linking group  include -0-, ----C(=O)-O-, ----C(=O)-, -O----C  (=0)-0-, ----C(=O)-NH-, -NH- (wherein H may  be substituted with a substituent such as an alkyl group or acyl  group), -S-, -S(=O) 2 -, -S(=O) 2 ---0-, and groups  represented  by  general  formulas  -Y 21 ---0-Y 22 -[Y2l_C(=O)---0Jm-Y22_  and  _y21_O----C  (=O)-Y 22 - [wherein each ofY 21  and Y 22  independently  represents a divalent hydrocarbon group which may have a  substituent, 0 represents an oxygen atom, and m’ represents  an integer of Oto 3].  When Y 2  represents -NH-, the H may be substituted  with a substituent such as an alkyl group or acyl group. The  substituent (the alkyl group or acyl group or the like) prefer› ably contains 1 to 10 carbon atoms, more preferably 1 to 8  carbon atoms, and most preferably 1 to 5 carbon atoms.  In the formulas -Y 21 ---0-Y 22 -, -[Y 21 -C(=O)- 0]m-Y22- and-Y 21 -O----C(=O)-Y 22 -, each ofY 21  and Y 22  independently represents a divalent hydrocarbon  group which may have a substituent. Examples of this diva› lent hydrocarbon group include the same groups as those  described above for the "divalent hydrocarbon group which  may have a substituent" for R 0 $_{-}$1 . Y 21  is preferably a linear  aliphatic hydrocarbon group, more preferably a linear alky› lene group, still more preferably a linear alkylene group of 1  to 5 carbon atoms, and most preferably a methylene group or  ethylene group.  Y 22  is preferably a linear or branched aliphatic hydrocar› bon group, and is more preferably a methylene group, ethyl- Specific examples of the structural unit (al 1) include struc› tural units represented by general formulas (al-1) to (al-4)  shown below.  In the group represented by formula -[Y 21 ----C(=O)- OJm -Y 22  -, m’ represents an integer of Oto 3, and is pref› erably an integer of 0 to 2, more preferably 0 or 1, and most  preferably 1. In other words, the group represented by for› mula -[Y 21 ----C(=O)-Olm-Y 22 - is most preferably a  group represented by a formula -Y 21 ----C(=O)-O› Y22 -. Among  such groups,  groups  represented by  -(CH 2 )a,C(=O)---O-(CH 2 )$_{6}$ ,-areparticularlypreferred.  In this formula, a’ represents an integer of 1 to 10, preferably  an integer of 1 to 8, more preferably an integer of 1 to 5, still  more preferably 1 or 2, and most preferably 1. b’ represents an  integer of 1 to 10, preferably an integer of 1 to 8, more  preferably anintegerofl to 5, still more preferably 1 or 2, and  most preferably 1.  36  US  9,104,101 B2  35  [Chemical Formula 14]  (al-1)  J H 2  R>  c,.._I  C  I  O=C  \ $_{0 }$ I  X  J~~1>  I  O=C  \ $_{0 }$ I ,  I  2’  R-C-R  I  0  A--- CH2  ~n  y  The divalent linking group containing a hetero atom for Y 2  is preferably an organic group composed of a combination of  at least one non-hydrocarbon group and a divalent hydrocar› bon group. Among such groups, linear groups containing an  oxygen atom as the hetero atom, such as groups containing an  ether linkage or an ester linkage, are preferred, a group rep› resented by one of the above formulas -Y 21 -O-Y 22 -,  -[Y21----C(=O)---O]m-Y22_ or _y21_O----C(=O)- Y22 - is more preferred, and a group represented by the  formula -[Y 21 ----C(=O)---0]m-Y 22 - or the formula  -Y 21 -O----C(=O)-Y 22 - is particularly desirable.  Among the above possibilities, Y 2  is preferably a linear or  branched alkylene group, or a divalent linking group contain› ing a hetero atom, and is more preferably a linear or branched  alkylene group, a group represented by the above formula  -Y 21 -O-Y 22 -, a group represented by the above for› mula -[Y 21  ----C(=O)-Olm - Y 22  -or  a group represented  by the above formula -Y 21 ---0-C(=O)-Y 22 -. 