Methods for efficient preparation of drug-polymer (or oligomer) conjugates which are useful in the preparation of particles, including microparticles and nanoparticles, for delivery of the drug in vivo for therapeutic applications. The invention additionally provides certain drug-polymer and drug-oligomer conjugates which are useful in the preparation of particles for delivery of the drug in vivo. The invention also provides nanoparticles of this invention prepared by nanoprecipitation using drug-polymer/oligomer conjugates of the invention. The drug conjugates are formed during polymerization of the polymer or oligomer in which the drug is employed as an initiator of the polymerization of the monomers which form the polymer and/or oligomer. More specifically, the drug conjugates are formed by ring-opening polymerization of cyclic monomers in the presence of an appropriate ring-opening polymerization catalyst and the initiator (the drug). The method is particularly useful for formation of polymer/oligomer conjugates with drugs and other chemical species containing one or more hydroxyl groups or thiol groups.