The invention relates to a polymer comprising structural units of formula and formula wherein R1, R2, R5 and R6 are independently hydrogen, a methyl group, or —COOH, only one of R1 and R2 or R5 and R6 is —COOH; R3 and R7 are independently hydrogen, or a methyl group; R4 is —COOH, —CONH2 or —OH, when R4 is —OH, R1, R2 and R3 are respectively hydrogen; and R8 is —COO, —CONH, or —O—, when R8 is —O—, R5, R6 and R7 are respectively hydrogen. Methods for preparing and using the polymer are also described herein.