Mechanistic Insights into Regioselectivity and Its Evolution in On-Surface Polymerization

Surface-catalyzed polymerization is crucial in both chemical science and industrial manufacturing, yet achieving regioselective radical polymerization on the surface remains challenging. Here, we demonstrate the regioselective Ullmann polymerization of nonsymmetrical 2,8-dibromoquinoline (DBQ) on an Au(111) surface. By combining scanning tunneling microscopy, density functional theory calculations, and kinetic modeling, we reveal the regioselectivity and its evolution with surface temperature at the molecular level. At 348-368 K, DBQ monomers primarily form covalent dimers through energetically favored head-to-head (HtH) coupling. As the temperature increases to 390-473 K, oligomers and long polymer chains are formed, with less favored head-to-tail (HtT) linkages emerging and eventually dominating over HtH linkages. Such regioselectivity evolution from HtH to HtT is suggested to be related to a sequential monomer addition mode and a shift in the distribution of reactive sites at the end and tail of the polymer chains during polymerization. This result provides molecular-level mechanistic insights into the regiochemistry of surface-catalyzed polymerization.