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

Abstract

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.

Figure 1

Amines as reducing agent for ATRP compared to conventional chemical and external reducing agents. The arrow represents the effectiveness of amines as reducing agents for ATRP using the CuBr₂/TPMA or Cu(OTf)₂/TPMA complex. DBU: 1,8-diazabicyclo(5.4.0)undec-7-ene; DACH: 1,2-diaminocyclohexane; CYC: 1,4,8,11-tetraazacyclotetradecane; Me₆TREN: tris[2-(dimethylamino)ethyl]amine; TREN: tris(2-aminoethyl)amine; DABCO: 1,4-diazabicyclo[2.2.2]octane; TMG: 1,1,3,3-tetramethylguanidine; DCHMA: N,N-dicyclohexylmethylamine; DHA: dihexylamine; DMAE: N,N-dimethylaminoethanol; PMDETA: N,N,N′,N″,N′′-pentamethyldiethylenetriamine; TEOA: triethanolamine; TMED: tetramethylethylenediamine; HMTETA: 1,1,4,7,10,10-hexamethyltriethylenetetramine; DMAEMA: 2-(dimethylamino)ethyl methacrylate; TEA: triethylamine; PYR: pyrrolidine; TMT: 1,3,5-trimethyl-1,3,5-triazinane; PIP: piperidine; TPMA: tris(2-pyridylmethyl)amine; TMPD: N,N,N′,N′-tetramethyl-p-phenylenediamine; MDMA: 4-methoxy-N,N-dimethylaniline; PG: N-phenylglycine; PTH: phenothiazine, BPY: 2,2‘-bipyridyne. The conversion and dispersity values under each amine are according to Table 3 and Table 4.

Figure 2

(A) Kinetics of ATRP of MA with amines. (B) Evolution of Đ and M_n,GPC with conversion. Evolution of GPC traces for polymerization with (C) (Br–Cu^II/Me₆TREN)⁺Br^– and Me₆TREN (with deoxygenation), (D) (Br–Cu^II/Me₆TREN)⁺Br^– and Me₆TREN (ambient), and (E) (Cu^II/TPMA)^2+-(OTf)₂ and DMAE (with deoxygenation). GPC traces of (F) poly(methyl acrylate) synthesized at different target DPs (50–1200) with (Br–Cu^II/Me₆TREN)⁺Br^– and Me₆TREN (with deoxygenation), (G) PMA precursor before and after chain extension with (Br–Cu^II/Me₆TREN)⁺Br^– and Me₆TREN (with deoxygenation). Reaction conditions: [MA]₀/[EBiB]₀/[CuBr₂ or Cu(OTf)₂]₀/[Me₆TREN or TPMA]₀/[amines]₀ = 100/1/0.02/0.02/0.4 in DMSO at 23 °C, [MA]₀ = 5.8 M.

Figure 3

Mechanistic analysis of ATRP with amines by evaluating alternative reaction pathways. (a) supplemental activation of R-Br with amines at 60 °C, (b) no initiation of polymerization from amines without R-Br, (c) no polymerization was observed with equimolar amines.

Figure 4

(A) General mechanism for ATRP in the presence of amines in the dark and (B) possible chemical events of oxidized amines in ATRP. ISET: inner sphere electron transfer. OSET: outer sphere electron transfer; XBC: halogen bonding complex; XAT: halogen atom transfer.