73  US 9,958,775 B2  74  Examples of A$_{2 }$ include the same as those described before  for Y$_{2 }$ in the general formula (Al).  A$_{2 }$ is preferably a hydrocarbon group (preferably having  20 or less carbon atoms, and more preferably having  4  to 12  carbon atoms), and more preferably at-butyl group, at-amyl  group, or a hydrocarbon group having an alicyclic structure  (for example, an alicyclic group itself, and a group in which  the alkyl group is substituted with an alicyclic group).  A$_{2 }$ is preferably a tertiary alkyl group or a tertiary  cycloalkyl group.  The alicyclic structure may be monocyclic or polycyclic.  Specifically, a monocyclo structure, a bicyclo structure, a  tricyclo structure, a tetracyclo structure, and the like, each  having 5 or more carbon atoms are included. The number of $_{15 }$ carbon atoms is preferably 6 to 30, and particularly prefer- ably 7 to 25. The hydrocarbon groups having these alicyclic  structures may have a substituent.  Examples of the alicyclic structures are shown below.  [Chem.42]  -continued  [p  O>  O>  a)  co  CD  co  co  CQ  Q<J  00  CX)  A  ~  ~  (1)  (2)  (3) ( 4 (5)  (6)  (7)  (8)  (9)  (10) (11)  (12)  (13)  (14)  (15)  916)  (17)  (18)  (19)  (20)  (21)  (22) 