Provided herein is a method comprising: (a) obtaining a mixture of multiple sets of oligonucleotides, wherein the oligonucleotides within each set each comprise a terminal indexer sequence and can be assembled to produce a synthon; and (b) hybridizing the oligonucleotide mixture to an array, thereby spatially-separating the different sets of oligonucleotides from one another. In some embodiments the method may comprise (c) contacting the array with a solution, thereby producing, for each feature bound by the oligonucleotides, a discrete droplet comprising the feature and, optionally, placing an immiscible liquid over the droplets, thereby producing, for each feature bound by the oligonucleotides, a discrete reaction chamber defined by a droplet. The method may further comprise incubating the array under conditions by which a synthon is assembled in each of the reaction chambers. Other embodiments are also provided.