The concentration of solvent that will break up the complex will usually prevent reversed phase binding. There is one additional charge for each additional base so the separation is driven by the net charge of the molecule. Some bases add positive charges so not all oligos of the same length will have the same net charge. Anion exchange has higher capacity than reversed phase. Solvent increases ionic interactions.
Solvents break up complexes while increasing the binding to the anion exchange.
Uncharged reaction products fall through anion exchange in solvent. Initial steep gradients followed by shallow linear gradients give the best analytical separations in the shortest time. Initial steep gradients followed by shallow linear gradients to a hold point give the best preparative separations in the shortest time. At preparative loads the oligos elute earlier than the analytical run. Phosphodiesters may need only aqueous systems with chaotropic salts.