Jan. 13, 2011  32  US 2011/0008728 Al  -continued  (a2-5-6)  (a2-5-3)  -continued  (a2-5-4)  (a2-5-5)  [Chemical Formula 28]  (a5-1)  [0178]  In the component (Al), as the structural unit ( a2),  one type of structural unit may be used, or two or more types  may be used in combination.  [0179]  As the structural unit ( a2), at least one structural unit  selected from the group consisting of formulas ( a2- l) to ( a2- 5) is preferable, and at least one structural unit selected from  the group consisting of formulas (a2-l) to (a2-3) is more  preferable. Of these, it is preferable to use at least one struc› tural unit selected from the group consisting of structural  units represented by formulas (a2-l-l), (a2-l-2), (a2-2-l),  (a2-2-7), (a2-3-l) and (a2-3-5).  [0180]  In the component (Al), the amount of the structural  unit (a2) based on the combined total of all structural units  constituting the component (Al) is preferably 5 to 60 mo! %,  more preferably 10 to 50 mo! %, and still more preferably 20  to 50 mo!%. When the amount of the structural unit (a2) is at  least as large as the lower limit of the above-mentioned range,  the effect of using the structural unit (a2) can be satisfactonly  achieved. On the other hand, when the amount of the struc› tural unit (a2) is no more than the upper limit of the above› mentioned range, a good balance can be achieved with the  other structural units.  [0181]  (Structural Unit (a5))  [0182]  The structural unit (a5) is a structural unit repre› sented by general formula (a5-l) shown below.  0  0  0 