AbstractThe synthesis of a total of twelve DNA fragments on the solid supports cellulose or polystyrene via a phosphotriester approach is described. Dimer and trimer blocks, protected at the 5′‐end with the levulinoyl group, were used for chain extension in the presence of the activating agent (2,4,6‐triisopropylbenzenesulfonyl)‐3‐nitro‐1,2,4‐triazole or (mesitylenesulfonyl)‐3‐nitro‐1,2,4‐triazole. The yields of the coupling procedures are normally high and the final unprotected DNA fragments released from the solid support could easily be isolated by Sephadex G50