taining 1 to 7 carbon atoms, branched-chain alkyl containing 1 to 7 carbon atoms, alkenyl containing 2 to 7 carbon atoms, cycloalkyl containing 5 to 6 carbon atoms, methyl-substituted cycloalkyl containing 6 to 7 carbon atoms, phenyl cresyl and benzyl R' is a hydrogen atom or methyl, a plurality of R' groups may be the same or different, and n is a positive integer of 1 to 9.