The present disclosure provides a polymer, including a first repeating unit represented by formula (I), a second repeating unit represented by formula (II), and a third repeating unit represented by formula (III). The first repeating unit, the second repeating unit, and the third repeating unit are arranged in an alternating fashion, in a random fashion, or in discrete blocks. The molar ratio of the first repeating unit, the second repeating unit and the third repeating unit is m:n:o, and m:(n+o) is from 60:40 to 85:15. The definitions of a, R1, R2, A−, and R+ are as defined in the specification.