Properties and ATRP Activity of Copper Complexes with Substituted Tris(2-pyridylmethyl)amine-Based Ligands

Synthesis, characterization, electrochemical studies, and ATRP activity of a series of novel copper­(I and II) complexes with TPMA-based ligands containing 4-methoxy-3,5-dimethyl-substituted pyridine arms were reported. In the solid state, Cu^I(TPMA*¹)­Br, Cu^I(TPMA*²)­Br, and Cu^I(TPMA*³)Br complexes were found to be distorted tetrahedral in geometry and contained coordinated bromide anions. Pseudo-coordination of the aliphatic nitrogen atom to the copper­(I) center was observed in Cu^I(TPMA*²)Br and Cu^I(TPMA*³)Br complexes, whereas pyridine arm dissociation occurred in Cu^I(TPMA*¹)­Br. All copper­(I) complexes with substituted TPMA ligands exhibited a high degree of fluxionality in solution. At low temperature, Cu^I(TPMA*¹)­Br was found to be symmetrical and monomeric, while dissociation of either unsubstituted pyridine and/or 4-methoxy-3,5-dimethyl-substituted pyridine arms was observed in Cu^I(TPMA*²)Br and Cu^I(TPMA*³)­Br. On the other hand, the geometry of the copper­(II) complexes in the solid state deviated from ideal trigonal bipyramidal, as confirmed by a decrease in τ values ([Cu^II(TPMA*¹)­Br]­[Br] (τ = 0.92) > [Cu^II(TPMA*³)­Br]­[Br] (τ = 0.77) > [Cu^II(TPMA*²)­Br]­[Br] (τ = 0.72)). Furthermore, cyclic voltammetry studies indicated a nearly stepwise decrease (Δ<i>E</i> ≈ 60 mV) of <i>E</i>_1/2 values relative to SCE (TPMA (−240 mV) > TPMA*¹ (−310 mV) > TPMA*² (−360 mV) > TPMA*³ (−420 mV)) on going from [Cu^II(TPMA)­Br]­[Br] to [Cu^II(TPMA*³)­Br]­[Br], confirming that the presence of electron-donating groups in the 4 (−OMe) and 3,5 (−Me) positions of the pyridine rings in TPMA increases the reducing ability of the corresponding copper­(I) complexes. This increase was mostly the result of a stronger influence of substituted TPMA ligands toward stabilization of the copper­(II) oxidation state (log β^I = 13.4 ± 0.2, log β^II = 19.3 (TPMA*¹), 20.5 (TPMA*²), and 21.5 (TPMA*³)). Lastly, ARGET ATRP kinetic studies show that with more reducing catalysts an induction period is observed. This was attributed to slow regeneration of Cu^I species from the corresponding Cu^II

A Silver Bullet: Elemental Silver as an Efficient Reducing Agent for Atom Transfer Radical Polymerization of Acrylates

Elemental silver was used as a reducing agent in the atom transfer radical polymerization (ATRP) of acrylates. Silver wire, in conjunction with a CuBr₂/​TPMA catalyst, enabled the controlled, rapid preparation of polyacrylates with dispersity values down to <i><i>Đ</i></i> = 1.03. The silver wire in these reactions was reused several times in sequential reactions without a decline in performance, and the amount of copper catalyst used was reduced to 10 ppm without a large decrease in control. A poly­(<i>n</i>-butyl acrylate)-<i>block</i>-poly­(<i>tert</i>-butyl acrylate) diblock copolymer was synthesized with a molecular weight of 91 400 and <i><i>Đ</i></i> = 1.04, demonstrating good retention of chain-end functionality and a high degree of livingness in this ATRP system

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

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 (<i>E</i>_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 <i>k</i>_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 <i>k</i>_a = 7.2 × 10⁶ M^–1 s^–1 was calculated with values of <i>K</i>_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

Benefits of Catalyzed Radical Termination: High-Yield Synthesis of Polyacrylate Molecular Bottlebrushes without Gelation

Catalyzed radical termination (CRT) in atom transfer radical polymerization (ATRP) of acrylates is usually considered as an unfavorable side reaction, as it accelerates termination and decreases chain-end functionality. CRT proceeds via a L/Cu^II–P_n organometallic intermediate and results in saturated chain-ends. Thus, CRT can help to suppress gelation in the synthesis of densely grafted poly­(<i>n</i>-butyl acrylate) molecular bottlebrushes using the “grafting-from” method by decreasing the fraction of chains terminated by conventional bimolecular radical combination. Molecular bottlebrushes by ATRP are typically prepared slowly in low yield and to limited monomer conversion to prevent radical combination, cross-linking, and gelation. Under conditions promoting CRT with highly active ATRP catalysts, a relatively high monomer conversion (>70%) was achieved without macroscopic gelation. CRT was favored using conditions that favored the formation of the L/Cu^II–P_n intermediate such as lower temperature and higher concentration of increasingly more active L/Cu^I catalysts. These conditions were beneficial for the fast and high-yield synthesis of polyacrylate molecular bottlebrushes, since they reduced the fraction of chains terminated by combination and prevented cross-linking of molecular bottlebrushes. High grafting density (>85%) and wormlike structures of molecular bottlebrushes were confirmed by side-chain cleavage and by molecular imaging via atomic force microscopy (AFM), respectively

Impact of Organometallic Intermediates on Copper-Catalyzed Atom Transfer Radical Polymerization

In atom transfer radical polymerization (ATRP), radicals (R^•) can react with Cu^I/L catalysts forming organometallic complexes, R–Cu^II/L (L = N-based ligand). R–Cu^II/L favors additional catalyzed radical termination (CRT) pathways, which should be understood and harnessed to tune the polymerization outcome. Therefore, the preparation of precise polymer architectures by ATRP depends on the stability and on the role of R–Cu^II/L intermediates. Herein, spectroscopic and electrochemical techniques were used to quantify the thermodynamic and kinetic parameters of the interactions between radicals and Cu catalysts. The effects of radical structure, catalyst structure and solvent nature were investigated. The stability of R–Cu^II/L depends on the radical-stabilizing group in the following order: cyano > ester > phenyl. Primary radicals form the most stable R–Cu^II/L species. Overall, the stability of R–Cu^II/L does not significantly depend on the electronic properties of the ligand, contrary to the ATRP activity. Under typical ATRP conditions, the R–Cu^II/L build-up and the CRT contribution may be suppressed by using more ATRP-active catalysts or solvents that promote a higher ATRP activity