the present disclosure concerns biologically active materials , particularly materials that comprise a biodegradeable polymer linked to a biologically active agent . the disclosure further concerns materials known as polymer - drug conjugates that typically contain a therapeutic agent , for instance a bioactive cytotoxic drug linked to a polymer backbone . the linkage typically is a convalent linkage . however , in some embodiments the disclosure concerns other polymer conjugates including those where the biologically active agent is an imaging agent , such as a tyrosinamide , a diagnostic agent , or a targeting agent , such as biotin .