The invention provides for methods of manufacturing an oligonucleotide comprising a pentavalent phosphate triester. In particular, the method comprises providing a 5' blocked-nucleoside, deblocking the 5' blocked-nucleoside to form a 5' OH-nucleoside, coupling the 5' OH-nucleoside with a phosphoramidite to form and oligonucleotide comprising a trivalent phosphite triester; and oxidizing the oligonucleotide comprising a trivalent phosphite triester to the oligonucleotide comprising a pentavalent phosphate triester. In some embodiments, the wash between any of the steps above is with at least one solvent wash comprising a toluene.