The present disclosure provides a solid phase method of making oligonucleotides via sequential coupling cycles including at least one coupling of a dinucleotide dimer subunit to a free 3'-terminal group of a growing chain. The oligonucleotides include at least two nucleoside subunits joined by a N3'→P5' phosphoramidate linkage. The method may include the steps of (a) deprotecting the protected 3' amino group of a terminal nucleoside attached to a solid phase support, said deprotecting forming a free 3' amino group； (b) contacting the free 3' amino group with a 3'-protected amino-dinucleotide-5'-phosphoramidite dimer in the presence of a nucleophilic catalyst to form an internucleoside N3'→P5' phosphoramidite linkage； and (c) oxidizing (e.g., sulfurizing) the linkage. The compositions produced by the subject methods may include a reduced amount of one or more (N-x) oligonucleotide products. Also provided are pharmaceutical compositions including the subject oligonucleotide compositions.
