. . "Examples of the alkyl group include methyl group, ethyl group, propyl group, and butyl group; examples of the substituent group introduced thereto include sulfonic acid group, carboxyl group, and hydroxyl group; and examples of the divalent connecting group include --O--, --NHCO--, --NHSO2 --, --NHCOO--, --NHCONH--, --COO--, --CO--, and --SO2 --.)" . . .