US 9,164,380 B2  22  21  acyl group. The substituent preferably contains 1 to 10 carbon  atoms, more preferably 1 to  8  carbon atoms, and most pref› erably 1 to  5 carbon atoms.  Y 2  is preferably a linear or branched alkylene group, a  cyclic aliphatic hydrocarbon group, or a divalent linking  group containing a hetero atom.  When Y 2  represents a linear or branched alkylene group,  the alkylene 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,  within the description of the "divalent hydrocarbon group  which may have a substituent", for the linear or branched  aliphatic hydrocarbon group.  When Y 2  represents a cyclic aliphatic hydrocarbon group,  examples of the cyclic aliphatic hydrocarbon group include  the same cyclic aliphatic hydrocarbon groups as those men› tioned above, within the description for the "divalent hydro› carbon group which may have a substituent", for the "ali› phatic hydrocarbon group that includes a ring within the  structure".  As the cyclic aliphatic hydrocarbon group, a group in  which two or more hydrogen atoms have been removed from  cyclopentane, cyclohexane, norbornane, isobornane, ada› mantane, tricyclodecane or tetracyclododecane is particu› larly preferred.  When Y 2  represents a divalent linking group containing a  hetero atom, preferred examples of the linking group include  -0-, -C(=O)---0-, -C(=O)-, ---0----C(=O)› O-, ----C(=O)-NH-, -NH- (wherein H may be sub› stituted with a substituent such as an alkyl group or an acyl  group), -S-, -S(=O) 2 -, -S(=O) 2 ---0-, and groups  represented by a general formula -Y 21 ---0-Y 22 -,  -[Y2l_C(=O)---0Jm,-Y22_ or _y21---0-C(=O)› Y22 - [ wherein each ofY 21  andY 22  independently represents  a divalent hydrocarbon group which may have a substituent,  0 represents an oxygen atom, and m’ represents an integer of  0  to 3].  When Y 2  represents -NH-, this H may be substituted  with a substituent such as an alkyl group or an acyl group. The  substituent (the alkyl group, an acyl group or the like) pref- erably has 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)› OJm ,-Y 22  - 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 the divalent  hydrocarbon groups include the same groups as those  described above for the "divalent hydrocarbon group which  may have a substituent" within the description relating to the  divalent linking group.  Y 21  is preferably a linear aliphatic hydrocarbon group,  more preferably a linear alkylene group, still more preferably  a linear alkylene group of 1 to 5 carbon atoms, and most  preferably a methylene group or an ethylene group.  Y 22  is preferably a linear or branched aliphatic hydrocar› bon group, and is more preferably a methylene group, an  ethylene group or an 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.  In the group represented by the formula [Y 21 ----C(=O)- 0]m  ,-Y 22  -,  m’ represents an integer of Oto 3, preferably an  integer of Oto  2, more preferably 0 or 1, and most preferably  1. In other words, it is particularly desirable that the group  represented by formula -[Y 21 ----C(=O)-Olm ,-Y 22  - is a  group represented by a formula -Y 21 ----C(=O)-O› Y22 -.  Among such groups, groups represented by a formula  -(CH 2 )a,----C(=O)-O-(CH 2 )$_{6}$ ,- are particularly pre› ferred. In this formula, a’ is 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’ is 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.  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, for example, groups con› taining an ether linkage or ester linkage are preferable, groups  represented by one of the above formulas -Y 21 -O› y22_, -[Y21----C(=O)---O]m,-y22_ or _y21_ O ----C  (=O)-Y 22  - are more preferable, and groups represented  by either of the formulas -[Y 21 -C(=O)---0lm ,-Y 22  - or  -Y 21  -O----C(=O)-Y 22  - are particularly preferable.  Of the various possibilities described above, Y 2  is prefer› ably a linear or branched alkylene group, or a divalent linking  group containing 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 formula -[Y 21 ----C(=O)---0]m,-Y 22 -, or a  group represented by the above formula -Y 21 -O----C  (=O)-Y22 __  Specific examples of the structural unit (al 1) include struc› tural units represented by general formulas (al-1) to (al-4)  shown below.  [Chemical Formula 7]  k H 2  R}  c,..._I  C  I  O=C  "-o  I  X’  (al-1)  (al-2) 