More preferred compounds are a compound represented by the formula (I) wherein each of R1 and R2 is a methyl group; R3 is a hydrogen atom; each of R4 and R5, which may be the same or different, is a hydrogen atom or a methyl group; each of Y's, which may be the same or different, is a hydrogen or halogen atom, or an alkyl, haloalkyl or haloalkoxyl group; R6 is an alkyl, haloalkyl or cycloalkyl gro