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p-Substituted Tris(2-pyridylmethyl)amines as Ligands for Highly Active ATRP Catalysts: Facile Synthesis and Characterization.

We report a facile and efficient two-step synthetic route for p-substituted tris(2-pyridylmethyl)amine (TPMA) ligands to form Cu complexes with the highest activity in Atom Transfer Radical Polymerization (ATRP) to-date. The synthetic procedure followed a divergent approach, where the p-Cl substituents in tris(4-chloro-2-pyridylmethyl)amine (TPMA3Cl) were replaced in one-step and high yield by electron-donating cyclic amines (pyrrolidine (TPMAPYR), piperidine (TPMAPIP), and morpholine (TPMAMOR)) via nucleophilic aromatic substitution. The [CuII(TPMANR2)Br]+ complexes exhibited larger energy gaps between frontier molecular orbitals and >0.2 V more negative reduction potentials than [CuII(TPMA)Br]+, indicating >3 orders of magnitude higher ATRP activity. [CuI(TPMAPYR)]+ exhibited the highest reported activity for Br-capped acrylate chain ends in DMF, and moderate activity toward C-F bonds at room temperature. [CuI(TPMANR2)]+ could react with acrylate radicals forming organometallic species. However, this side reaction is suppressed in polymerization, because of the low equilibrium concentration of [CuI(TPMANR2)]+. Indeed, ATRP of n-butyl acrylate using only 10-25 part per million loadings of [CuII(TPMANR2)Br]+ exhibited excellent control.