The present application, among other things, provides technologies, e.g., reagents, methods, etc. for preparing oligonucleotides comprising phosphorothiotriesters linkages, e.g., oligonucleotides having the structure of IIIa, IIIb or IIIc. In some embodiments, provided methods comprise reacting an H-phosphonate of structure Ia or Ib with a silylating reagent to provide a silyloxyphosphonate, and reacting the silyloxyphosphonate with a thiosulfonate reagent of structure IIa or IIb to provide an oligonucleotide of structure IIIa or IIIb. In some embodiments, provided methods comprise reacting an H-phosphonate of structure Ic with a silylating reagent to provide a silyloxyphosphonate, reacting the silyloxyphosphonate with a bis(thiosulfonate) reagent of structure IVc to provide a phosphorothiotriester comprising a thiosulfonate group of structure Vc, and then reacting the phosphorothiotriester comprising a thiosulfonate group of structure Vc with a nucleophile of structure VIc to provide an oligonucleotide of structure IIIc.