A dianhydride monomer having a large side chain R is provided. The large side chain would interrupt the symmetry and regularity of diamine monomer. The diamine monomer has the general formula shown as formula (I) below: The functional group R includes α-substitution cycloalkene having at least a tertiary carbon atom, cycloalkene having at least a tertiary carbon atom, cycloalkane having at least a tertiary carbon atom, α-substitution phenyl, phenyl, α-substitution naphthalyl, naphthalyl, α-substitution phenanthrenyl, phenanthrenyl, α-substitution anthracenyl, anthracenyl, α-substitution adamantyl, adamantyl, α-substitution adamantyl and adamantyl.