A prepolymer prepared by condensation reaction between a first compound represented by Formula (1) below: Ar—H  (1), where Ar is composed of a crosslinkable moiety at one end, a moiety selected from the group —O—, —S—, —COO—, —CO—, —COS—, —SO2—, and —NH—, and one or two repeating units selected from the group: where A is carbon or nitrogen, and X is hydrogen or halogen; and a second compound that is an aromatic moiety.