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In formula (A-3), RL 5 , RL 6 and RL 7 are each indepen- dently a monovalent hydrocarbon group, typically a C$_{1 }$-C$_{20 }$ straight, branched or cyclic alkyl group or C$_{2}$-C$_{20 }$ straight, branched or cyclic alkenyl group, which may contain a heteroatom such as oxygen, sulfur, nitrogen or fluorine. A pair ofRL 5 and RL 6 , RL 5 and Rn, or RL $^{6 }$and RL $^{7 }$may bond together to form a C$_{3}$ -C$_{20 }$aliphatic ring with the carbon atom to which they are attached. 

Exemplary groups of formula (A-3) include tert-butyl, triethylcarbyl, 1-ethylnorbornyl, 1-methylcyclohexyl, 1-eth- ylcyclopentyl, 2-(2-methyl)adamantyl, 2-(2-ethyl)adaman› tyl, and tert-pentyl. 

Other exemplary groups of formula (A-3) include those of the kg formulae (A-3)-1 to (A-3)-18. 

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-continued 

(A-3)-1 

(A-3)-2 55 

(A-3)-3 

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(A-2)-77 

-continued 

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(A-3)-4 

(A-3)-5 

(A-3)-6 

(A-3)-7 

(A-3)-8 

