Unraveling the Roles of Amines in Atom Transfer Radical Polymerization in the Dark

Multidentate amines have been widely used as ligands (L) for Cu-catalysts in atom transfer radical polymerization (ATRP) and as electron donors in photochemically induced polymerizations. However, mechanistic aspects of the role of amines in ATRP in the dark have remained elusive. Herein, the structure-activity relationship and the related electron transfer reactions with Br-Cu^II/L complexes and/or with alkyl bromides (R-Br) were investigated for 25 amines. Amines function as electron donors and reducing agents for Br-Cu^II/L complexes via an outer sphere electron transfer (OSET) mechanism, enabling slow but continuous generation of Cu^I/L activators and inducing controlled ATRP. However, two amines, diazabicyclo(5.4.0)undec-7-ene (DBU) and 1,1,3,3-tetramethylguanidine (TMG), reduced Br-Cu^II/L faster, suggesting an inner sphere electron transfer (ISET) process. ATRP, starting with initial deactivators (Br-Cu^II/L) species, proceeded in the dark in the presence of an excess of tertiary amines, such as tris[2-(dimethylamino)ethyl]amine (Me₆TREN), 1,4-diazabicyclo[2.2.2]octane (DABCO), and TMG at room temperature and afforded polymers with low dispersities (Đ ≤ 1.15). With copper(II) triflate complex (Cu^II/L⁺², ^-(OTf)₂), which has a more positive reduction potential, ATRP proceeded at room temperature with several inexpensive secondary and tertiary amines including triethylamine (TEA) and dimethylethanolamine (DMAE). Interestingly, multidentate amines also served as direct R-Br activators at elevated temperatures (60 °C). In all cases, chains were initiated with R-Br and not by the amine radical cations as byproducts of electron transfer. Amines also enabled ATRP in the presence of residual air in flasks with a large headspace, underpinning them as a robust and accessible reducing agent for practical applications.