| abstract |
wherein A1 and A2 are each independently a hydroxy group or a hydrogen atom, provided that A1 and A2 are not hydrogen atoms at the same time, R1 is a hydrocarbon group having at most 30 carbon atoms, R2 to R6 are each independently a hydrogen atom or a hydrocarbon group having at most 30 carbon atoms, and m and n are each independently an integer of from 0 to 10. |