content_image/893661.jpg content_image/893662.jpg
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] 

<img src='content_image/893661.jpg'>

(1) 

(2) 

(3) 

( 4 

(5) 

(6) 

(7) 

(8) 

(9) 

(10) 

<img src='content_image/893662.jpg'>

-continued 

(11) 

(12) 

(13) 

(14) 

(15) 

916) 

(17) 

(18) 

(19) 

(20) 

(21) 

(22) 

