A polymer includes at least one monomeric unit represented by Formula I, II, or III: where X is CH2 or O； R1 is H, COOR2, COCH3, CONHR2, OR2 or SR2； L is a bond or a linker group； AA is a group derived from an active agent； each R2 is independently H, a cation, an alkyl, an alkenyl, an alkynyl, COCH3, CH2CH2OH, CH2CH2OR3, an amino acid, a small peptide, a large peptide, enzyme, protein, growth factor, cytokine, antibody, single chain antibody, antibody fragment, COCCH3═CH2, COCH═CH2, CH2CO2H, CH2CH2CO2H, CH2CH2SH, CH2CH2SR3 or (CH2CH2O)n′R4； n′ is an integer from 1 to 2000； each R3 is independently trityl, 4-methyltrityl or 2 pyridyl； each R4 is independently H, an alkyl, an alkenyl, an alkynyl, COCCH3═CH2, COCH═CH2, CH2CHO, CH2CH2CHO, CO2H, CO2R5, CH2CO2H, CH2CH2CO2H, CH2NH2, CH2NHR5, CH2N(R5)2, CH2CH2NH2, CH2CH2NHR, CH2CH2N(R5)2, SH, CH2CO2R5, or CH2CH2CO2R5； and each R5 is independently maleimide, a cation, an amino acid, a small peptide, a large peptide, protein, enzyme, growth factor, cytokine, antibody, single chain antibody, phosphate, sulfate, choline, or an activated ester.
