In the case where only one of R6 to R8 is a hydrogen atom or none of these is a hydrogen atom (e.g., in the case where R6 is a hydrogen atom and R7 and R8 each is an alkenyl group or where R6 to R8 each is an alkenyl group), then the alkenyl groups represented by the remaining two or all of R6 to R8 each is preferably a linear or branched alkenyl group having 2 to 16, more preferably 2 to 14 carbo