Synthesis and Characterization of the Most Active Copper ATRP Catalyst Based on Tris[(4-dimethylaminopyridyl)methyl]amine

Abstract

The tris[(4-dimethylaminopyridyl)methyl]amine (TPMA^NMe2) as a ligand for copper-catalyzed atom transfer radical polymerization (ATRP) is reported. In solution, the [Cu^I(TPMA^NMe2)Br] complex shows fluxionality by variable-temperature NMR, indicating rapid ligand exchange. In the solid state, the [Cu^II(TPMA^NMe2)Br][Br] complex exhibits a slightly distorted trigonal bipyramidal geometry (τ = 0.89). The UV-vis spectrum of [Cu^II(TPMA^NMe2)Br]⁺ salts is similar to those of other pyridine-based ATRP catalysts. Electrochemical studies of [Cu(TPMA^NMe2)]²⁺ and [Cu(TPMA^NMe2)Br]⁺ showed highly negative redox potentials (E_1/2 = -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_a = 1.1 × 10⁶ M^-1 s^-1, 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_a = 7.2 × 10⁶ M^-1 s^-1 was calculated with values of K_ATRP ≈ 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^I-catalyzed radical termination (CRT) could be suppressed due to the low concentration of L/Cu^I at a steady state.