A method includes producing libraries of nanoparticles having highly controlled properties, which can be formed by mixing together two or more macromolecules in different ratios. One or more of the macromolecules may be a polymeric conjugate of a moiety to a biocompatible polymer. The nanoparticle may contain a drug. The moiety may include a polypeptide or a polynucleotide, such as an aptamer. The moiety may be a targeting moiety, an imaging moiety, a chelating moiety, a charged moiety, or a therapeutic moiety. Another aspect is directed to systems and methods of producing such polymeric conjugates. In some embodiments, a solution containing a polymer is contacted with a liquid, such as an immiscible liquid, to form nanoparticles containing the polymeric conjugate. Other methods use such libraries, use or administer such polymeric conjugates, or promote the use of such polymeric conjugates. Kits involving such polymeric conjugates are also described.