Relation between Overall Rate of ATRP and Rates of Activation of Dormant Species

The rate of atom transfer radical polymerization (ATRP) depends on the rate constant of propagation (k(p)) and concentration of growing radicals. The latter is related to the ATRP equilibrium constant (K-ATRP) and concentrations of alkyl halides, activators, and deactivators. Activation of alkyl halides by Cu-I/L and deactivation of radicals by X-Cu-II/L are vital processes providing good control in ATRP. Rates of these reactions are typically identical throughout polymerization, since the ATRP equilibrium is maintained in essentially all ATRP systems. There are new ATRP processes carried out with ppm of Cu catalysts, such as activators regenerated by electron transfer (ARGET), initiators for continuous activator regeneration (ICAR), supplemental activators and reducing agents (SARA), and electrochemically or photochemically mediated ATRP (eATRP, photoATRP). In these processes, as in conventional radical polymerization (or in RAFT), concentration of radicals is established by balancing rates of radical generation (e.g., from thermal initiators, reduction rate or supplemental activation) and radical termination (i.e., reaching steady state). However, in these processes, the rate of activation of alkyl halides by Cu-I/L is still equal to the rate of deactivation of radicals by X-Cu-II/L. Can the rates of activation of alkyl halides (by Cu-I or by Cu-0) be directly related to the overall rate of ATRP? This report aims to clarify that rate of activation of alkyl halides by Cu species cannot be directly related to the overall rate of polymerization. There are many cases with the same rate of ATRP but dramatically different rates of activation and cases with similar activation rates but very different overall ATRP rates. Thus, both the analytical approach and PREDICI simulations clearly show that rates of normal ATRP with high catalyst loadings as well as rates of low ppm ATRP systems, such as ICAR ATRP and SARA ATRP, cannot be directly related with rates of activation of alkyl halides by Cu-I. In SARA ATRP, the activation of alkyl halides by Cu-I is always much faster than by Cu

Catalyzed radical termination in the presence of tellanyl radicals

International audienceThe decomposition of the diazo initiator dimethyl 2,2′‐azobis(isobutyrate) (V‐601), generating the Me2C.(CO2Me) radical, affords essentially the same fraction of disproportionation and combination in media with a large range of viscosity (C6D6, [D6]DMSO, and PEG 200) in the 25–100 °C range. This is in stark contrast to recent results by Yamago et al. on the same radical generated from Me2C(TeMe)(CO2Me) and on other X‐TeR systems (X=polymer chain or unimer model; R=Me, Ph). The discrepancy is rationalized on the basis of an unprecedented RTe.‐catalyzed radical disproportionation, with support from DFT calculations and photochemicaL V‐601 decomposition in the presence of Te2Ph2

Contribution of Photochemistry to Activator Regeneration in ATRP

With the recent interest in photochemically mediated atom transfer radical polymerization (ATRP), an interesting question arises: how significant are the photochemical processes in ATRP reactions that are supposed to be chemically controlled, such as initiators for continuous activator regeneration (ICAR) ATRP? A comparison of the rates of polymerization under ICAR ATRP conditions under ambient lighting and in the dark indicates negligible difference in the polymerization rate, under the conditions [MA]:[EBiB]:[TPMA*2]:[CuBr₂]:[AIBN] = 300:1:0.12:0.03:0.2 in anisole 50% (v/v) at 60 °C, where TPMA*2 is 1-(4-methoxy-3,5-dimethylpyridin-2-yl)-<i>N</i>-((4-methoxy-3,5-dimethylpyridin-2-yl)­methyl)-<i>N</i>-(pyridin-2-ylmethyl)­methanamine. This indicates that under typical ICAR conditions activator regeneration is almost exclusively due to the chemical decomposition of AIBN, not ambient lighting. To further investigate the effect of light on the activator regeneration, experiments were performed combining ICAR and photochemical processes in a 392 nm photoreactor of intensity 0.9 mW/cm². In this process, termed PhICAR (photochemical plus ICAR) ATRP, the overall rate of activator regeneration is the sum of the rates of activator regeneration by chemical (ICAR) decomposition of AIBN and the photochemical activator regeneration. At low AIBN concentrations (0.035 equiv with respect to ATRP initiator), the contribution of the photochemical processes in the 392 nm photoreactor is approximately 50%. At higher AIBN concentrations (0.2 equiv with respect to ATRP initiator), the contribution of photochemical processes to the overall polymerization drops to 15% due to the higher rate of chemically controlled processes

How are Radicals (Re)Generated in Photochemical ATRP?

The polymerization mechanism of photochemically mediated Cu-based atom-transfer radical polymerization (ATRP) was investigated using both experimental and kinetic modeling techniques. There are several distinct pathways that can lead to photochemical (re)­generation of Cu^I activator species or formation of radicals. These (re)­generation pathways include direct photochemical reduction of the Cu^II complexes by excess free amine moieties and unimolecular reduction of the Cu^II complex, similar to activators regenerated by electron-transfer (ARGET) ATRP processes. Another pathway is photochemical radical generation either directly from the alkyl halide, ligand, or via interaction of ligand with either monomer or with alkyl halides. These photochemical radical generation processes are similar to initiators for continuous activator regeneration (ICAR) ATRP processes. A series of model experiments, ATRP reactions, and kinetic simulations were performed to evaluate the contribution of these reactions to the photochemical ATRP process. The results of these studies indicate that the dominant radical (re)­generation reaction is the photochemical reduction of Cu^II complexes by free amines moieties (from amine containing ligands). The unimolecular reduction of the Cu^II deactivator complex is not significant, however, there is some contribution from ICAR ATRP reactions involving the interaction of alkyl halides and ligand, ligand with monomer, and the photochemical cleavage of the alkyl halide. Therefore, the mechanism of photochemically mediated ATRP is consistent with a photochemical ARGET ATRP reaction dominating the radical (re)­generation

Disproportionation or combination? The termination of acrylate radicals in ATRP

International audienceThe 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 [CuI(L)]+ (L = tris[2-(dimethylamino)ethyl]amine (Me6TREN), tris(2-pyridylmethyl)amine (TPMA), or tris(3,5-dimethyl-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 midchain radicals, arising from backbiting, on the overall termination profile. In all cases, CRT and cross-termination between secondary chain-end and tertiary midchain radicals contributed the most to the overall amount of terminated chains

Effect of ligand structure on the Cu-II-R OMRP dormant species and its consequences for catalytic radical termination in ATRP

International audienceThe kinetics and mechanism of catalytic radical termination (CRT) of n-butyl acrylate (BA) in MeCN in the presence of Cu complexes with tridentate and tetradentate ligands was investigated both theoretically and experimentally. The tetradentate TPMA, TPMA*(1), TPMA*(2), TPMA*(3), and the newly synthesized tridentate N-propyl-N,N-bis(4-methoxy3,5-dimethylpyrid-2-ylmethyl)amine (BPMA*(Pr)) as well as tridentate BPMA(Me) were used as ligands. L/(CuX2)-X-II (X = Cl or OTf) complexs were characterized by cyclic voltammetry (CV), UV-vis-NIR, and X-ray diffraction. Polymerization of BA initiated by azobis(isobutyronitrile) (AIBN) in MeCN in the presence of a L/Cu-I complex showed higher rates of CRT for more reducing L/Cu-I complexes. The ligand denticity (tri- vs tetradentate) had a minor effect on the relative polymerization kinetics but affected the molecular weights in a way specific for ligand denticity. Quantification of the apparent CRT rate coefficients, kappa(app)(CRT) showed larger values for more reducing L/Cu-I complexes, which correlated with the L/Cu-II-R (R = CH(CH3)(COOCH3)) bond strength, according to DFT calculations. The bond strength is mostly affected by the complex reducing power and to a lesser degree by the ligand denticity. Analysis of kinetics and molecular weights for different systems indicates that depending on the ligand nature, the rate-determining step of CRT may be either the radical addition to L/Cu-I to form the L/Cu-II-R species or the reaction of the latter species with a second radical

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