The tris[(4-dimethylaminopyridyl)methyl]amine (TPMA) as a ligand for copper-catalyzed atom transfer radical polymerization (ATRP) is reported. In solution, the [Cu(TPMA)Br] complex shows fluxionality by variable-temperature NMR, indicating rapid ligand exchange. In the solid state, the [Cu(TPMA)Br][Br] complex exhibits a slightly distorted trigonal bipyramidal geometry (τ = 0.89). The UV-vis spectrum of [Cu(TPMA)Br] salts is similar to those of other pyridine-based ATRP catalysts. Electrochemical studies of [Cu(TPMA)] and [Cu(TPMA)Br] showed highly negative redox potentials (E = -302 and -554 mV vs SCE, respectively), suggesting unprecedented ATRP catalytic activity. Cyclic voltammetry (CV) in the presence of methyl 2-bromopropionate (MBrP; acrylate mimic) was used to determine activation rate constant k = 1.1 × 10 M s, confirming the extremely high catalyst reactivity. In the presence of the more active ethyl α-bromoisobutyrate (EBiB; methacrylate mimic), total catalysis was observed and an activation rate constant k = 7.2 × 10 M s was calculated with values of K ≈ 1. ATRP of methyl acrylate showed a well-controlled polymerization using as little as 10 ppm of catalyst relative to monomer, while side reactions such as Cu-catalyzed radical termination (CRT) could be suppressed due to the low concentration of L/Cu at a steady state.

The termination of acrylate radicals in atom transfer radical polymerization (ATRP) can involve either conventional bimolecular radical termination (RT) or catalytic radical termination (CRT). These processes were investigated using a poly(methyl acrylate)-Br macroinitiator under different initial conditions tuned to change the RT/CRT ratio. The polymers, obtained from alkyl halide chain-end activation by [Cu I (L)] + (L = tris[2-(dimethylamino)ethyl]amine (Me6TREN), tris(2-pyridylmethyl)amine (TPMA), or tris(3,5dimethyl-4-methoxy-2-pyridylmethyl)amine (TPMA* 3)) in the absence of monomer, were analyzed by size exclusion chromatography (SEC). RT-promoting conditions resulted in the increase of a shoulder with double molecular weight (MW) relative to the macroinitiator distribution, indicating that RT occurred predominantly via radical combination. Conversely, when CRT was promoted, the macroinitiator distribution did not shift, indicating a disproportionation-like pathway. The termination reactions for the TPMA system were further analyzed via PREDICI simulations, which showed the significant impact of mid-chain radicals, arising from backbiting, on the overall termination profile. In all cases, CRT and cross termination between secondary chain-end and tertiary mid-chain radicals contributed the most to the overall amount of terminated chains.

The external order in reagents for the activation of alkyl halides by Cu 0 was investigated in supplemental activator and reducing agents (SARA) ATRP. Using methyl 2-bromopropionate (MBrP) or ethyl a-bromophenylacetate (EBPA) and tris(2-(dimethylamino)ethyl)amine (Me 6 TREN) in DMSO and MeCN, it was determined that the rate of activation scaled with (S/V) 0.9 in both solvents. For MBrP, the rate was first order with respect to [MBrP] 0 until a saturation in the rate was observed around 33 and 110 mM in DMSO and MeCN, respectively. For EBPA, the reaction was also first order until a maximum rate was observed at 33 mM in DMSO, whereas an inverse order was observed for concentrations above 66 mM in MeCN. At saturated concentrations of MBrP, it was found that the rate increased linearly with respect to [Me 6 TREN] 0 for all systems but became asymptotic with a maximum rate of 2 3 10 26 and 4 3 10 25 M s 21 in DMSO and MeCN, respectively. Model polymerizations in the absence of ligand showed slow reaction rates, indicating the necessity for ligand. The results allow more accurate modeling and understanding of SARA ATRP under a large range of initiator concentrations.