| 摘要 |
A compound having formula (I) wherein R1, R2, R3, R4, R5 and R6 independently are hydrogen, C1-C10 alkyl, C5-C10 cycloalkyl, C6-C10 aryl or C7-C20 aralkyl; X is a difunctional group selected from the group consisting of C2-C20 alkyl, C5-C20 cycloalkyl, C6-C10 aryl, C8-C20 aryl alkyl, C4-C20 heteroalkyl or C10-C20 aryl heteroalkyl.
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