preferably an alkenyl group containing 3 carbon atoms, cycloalkyl groups containing 5 to 6 carbon atoms, methylsubstituted cycloalkyl groups containing 6 to 7 carbon atoms, a phenyl group, a cresyl group and a benzyl group, R' is a hydrogen atom or a methyl group, a plurality of R' groups being the same or different, and n is a positive integer of 1 to 9.