[In the formula (N5), R1 and R3 are the same or different and each is an alkylene group of a carbon number of 1 to 6, R2 and R4 are the same or different and each is a hydrogen, hydrocarbon group, or acyl group of a carbon number of 1 to 30, and n and m are integers from 0 to 50 satisfying n+m is 3 to 50.]