group, R3 and R4 each are a hydrogen atom or a C1 -C18 hydrocarbon group, R5 and R6 each are a hydrogen atom or a C1 -C18 hydrocarbon group and R7 and R8 each are a hydrogen atom or a C1 -C18 hydrocarbon group and may be bonded to each other to form one or more cyclic hydrocarbon group.