The present invention generally relates to polymers and macromolecules, in particular, to block polymers useful in particles such as nanoparticles. One aspect of the invention is directed to a method of developing nanoparticles with desired properties. In one set of embodiments, the 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. In some cases, the nanoparticle may contain a drug. The moiety, in some embodiments, may have a molecular weight greater than about 1000 Da; for example, the moiety may include a polypeptide or a polynucleotide, such as an aptamer. The moiety may also be a targeting moiety, an imaging moiety, a chelating moiety, a charged moiety, or a therapeutic moiety. Another aspect of the invention 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 aspects of the invention are directed to methods using such libraries, methods of using or administering such polymeric conjugates, methods of promoting the use of such polymeric conjugates, kits involving such polymeric conjugates, or the like.