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.

Cyanoisopropyl radicals, generated thermally by the decomposition of azobis(isobutyronitrile) (AIBN), participate in reductive radical termination (RRT) under the combined effect of copper(I) complexes and proton donors (water, methanol, triethylammonium salts) in acetonitrile or benzene. The investigated copper complexes were formed in situ from [Cu I (MeCN)4] + BF4in CD3CN or Cu I Br in C6D6 using tris[2-(dimethylamino)ethyl]amine (Me6TREN), tris(2-pyridylmethyl)amine (TPMA) and 2,2'-bipyridine (BIPY) ligands. Upon keeping all other conditions constants, the impact of RRT is much greater for the Me6TREN and TPMA systems than for the BIPY system. RRT scales with the proton donor acidity (Et3NH + >> H2O > CH3OH), it is reduced by deuteration (H2O > D2O and CH3OH > CD3OD) and it is more efficient in C6D6 than in CD3CN. The collective evidence gathered in this study excludes the intervention of an outer-sphere proton-coupled electron transfer (OS-PCET), while an innersphere PCET (IS-PCET) cannot be excluded for coordinating proton donors (water and methanol). On the other hand, the strong impact of RRT for the non-coordinating Et3NH + in CD3CN results from the formation of an intermediate Cu I-radical adduct, suggested by DFT calculations to involve binding via the N atom to yield keteniminato [L/Cu-N=C=CMe2] + derivatives with only partial spin delocalization onto the Cu atom.

The vinyl acetate (VAc) radical polymerization initiated by V-70 at 30°C in the presence of [Co II (OPN)2] (OPN = deprotonated 9-oxyphenalenone), 1, leads to PVAc of lower molecular weight (MW) than expected for organometallic-mediated radical polymerization (OMRP), whether reversible termination or degenerate transfer conditions are used. This represents the first clear evidence of catalyzed chain transfer (CCT) in VAc polymerization. The bis-pyridine adduct [Co II (OPN)2(py)2], 2, shows a marginally lower polymerization rate and an increased CCT activity relative to 1, whereas the activity decreases with marginal effect on the polymerization rate upon addition of excess py. However, raising the temperature to 80°C (with AIBN as initiator) led to a low MW polymer even in the presence of a large py excess. The CCT was confirmed by 1 H NMR characterization of the chain ends and by a MALDI-TOF MS analysis of the recovered polymer. The collective trends are consistent with greater CCT activity for the 5-coordinate complex [Co II (OPN)2(py)] relative to 1 and 2. The presence of py association/dissociation equilibria relating these three complexes was confirmed by a 1 H NMR investigation.

Atom Transfer Radical Polymerization (ATRP), a metal‐catalyzed process, is a most powerful method for macromolecular engineering, producing polymers with targeted and low‐dispersity molar masses and with high chain‐end fidelity. This is due to the persistent radical effect, which dramatically reduces the spontaneous radical terminations, prolonging the lifetime of radical chains and the concurrent growth of all polymer chains. Two additional reaction modes that involve metals and organic radicals, however, may negatively affect the controlled polymerization. These are the Catalyzed Radical Termination (CRT) and the Reductive Radical Termination (RRT) and were shown to be particularly important for the polymerization of acrylate monomers. The scope and the mechanistic investigations carried out to elucidate how these processes interplay with ATRP and with each other are outlined in this Minireview.

Amine‐functionalized squaramides 1 and 2 were prepared and shown to be suitable polymerization organocatalysts for the controlled ring‐opening polymerization (ROP) of l‐lactide (l‐LA) in the presence of an alcohol source such as BnOH (which acts as an initiator) to afford chain‐length‐controlled and narrow‐dispersion poly(l‐lactide) (PLLA) under mild reaction conditions. The ROP experimental and polymer analysis data are consistent with the action of 1 and 2 as bifunctional hydrogen‐bonding (HB) catalysts that are able to activate both the lactide monomer and initiator BnOH thanks to their dual HB acceptor and donor properties. As a comparison, aminosquaramide 3, a direct analogue of 1 but a weaker HB donor because of the absence of electron‐withdrawing NH substituents, displays little lactide ROP activity, which highlights the key role of monomer activation through HB in the present systems. Unlike aminosquaramides 1 and 2, related monofunctional squaramides 4 and 5 are inactive in l‐LA ROP in the presence of BnOH, but the addition of NEt3, as an external HB acceptor, allows the ROP to proceed with the production of well‐defined PLLA. A cooperative dual activation with an activated monomer/activated chain‐end mechanism is most likely operative in the lactide ROP mediated by 1 and 2 in the presence of BnOH.

The reaction between [M(acac)3] (M = Fe, Co, acac = acetylacetonate) and the diamino-bis(phenol) proligands (2-OH-3,5t Bu2-C6H2CH2)2NCH2(2-NC5H4) (H21) and (2-OH-5t Bu-C6H3CH2)2NCH2CH2CH2N(CH3)2 (H22) afforded the [κ 4-(N2,O2)M(acac)] complexes [Fe(1)(acac)] (3), [Fe(2)(acac)] (4), [Co(1)(acac)] (5) and [Co(2)(acac)] (6) in moderate yields after crystallization. The proposed formulas were supported by various analytical data (ESI-MS, FT-IR, EA, NMR), and the molecular structure of all complexes was confirmed by X-ray diffraction studies. In each octahedral complex, the diamino-bis(phenolate) ligand (1 or 2) acts as a tetradentate ligand and the coordination sphere is completed by one chelating κ 2-O2 acac ligand. Although the previously reported ligand 1 and the new ligand 2 are very similar, their coordination to the Fe III (acac) or Co III (acac) moieties varies by the arrangement of the donor atoms. While the Ophenolate donor atoms of ligand 1 were found cis to each other in the Fe III complex 3, they are trans in the Co III complex 5, and vice versa for ligand 2. Since each ligand, 1 and 2, exhibits both configuration, this structural curiosity cannot be easily explained on the basis of steric factors, i.e. ortho-substituents or N,N'-linker length.

Abstract:The sterically bulky Ga(III) and In(III) (IPr*)MMe 3 adducts (1 and 2) and (SItBu)MMe 3 adducts (3 and 4) (M = Ga, In; IPr* = 1,3-bis{2,6-bis(diphenylmethyl)-4-methylphenyl}-1,3-dihydroimidazol-2-ylidene; SItBu = 1,3-bis(1,1-dimethylethyl)-imidazolidin-2-ylidene) were prepared and structurally characterized, allowing an estimation of the steric hindrance of such Lewis pairs (yields in 1-4: 92%, 90%, 73%, and 42%, respectively). While the IPr* adducts 1 and 2 are robust species, the more severely congested SItBu adducts 3 and 4 are more reactive and exhibit a limited stability in solution. Adduct (SItBu)GaMe 3 (3) reacts quickly with H 2 at room temperature to afford the corresponding aminal product, 1,3-di-tert-butylimidazolidine (5), along with free GaMe 3 . Such Frustrated Lewis Pair (FLP) reactivity constitutes the first instance of a H 2 activation involving a simple trialkyl GaR 3 species. Adduct 3 also mediates the ring-opening polymerization (ROP) of rac-lactide at room temperature to afford cyclic polylactide (PLA).

The novel pentadentate tetrapodal proligand 2,6-bis [(2-hydroxyphenyl)sulfanylmethyl]pyridine (1•H2) and its cobalt(II) complex [Co(1)] (2) were synthesized and characterized by several analytical (EA, ESI-MS) and spectroscopic methods (NMR or EPR, FT-IR), including X-ray crystallography for 1•H2. Cyclic voltammetry studies showed that 2 undergoes a reversible metal-based oxidation (Co II /Co III ). Complex 2 was designed to be applied to organometallic mediated radical polymerization (OMRP), however it exhibited an extremely poor solubility in non-coordinating solvents and several vinyl monomers (styrene, vinyl acetate and tert-butyl acrylate), which hampers its potential as moderator. Complex 2 has a high affinity towards Lewis bases, such as pyridine, leading to the clean formation of the monopyridine adduct 2•py, as confirmed by X-ray crystallography. In 2•py, ligand 1 is pentacoordinated to the Co II center, with the two thioether-phenolate (S,O) moieties oriented anti to each other, and the only free coordination site of the octahedron is completed by the additional pyridine, trans to the central pyridine linker of 1. The equilibrium between 2 and 2•py could be studied by 1 H NMR. Complex 2 could be cleanly and quantitatively oxidized to its diamagnetic iodo cobalt(III) analog [Co(1)I] (3), by simple reaction with iodine. The latter could then be subjected to a halide abstraction reaction, mediated by K[B(C6F5)], affording the cationic complex [Co(1)][B(C6F5)], 4. Highlights• Cobalt(II) complexes of a bis(thioether)-bis(phenolato)-pyridyl-based OSNSO-ligand.• Neutral (OSNSO)-cobalt(III) iodide and cationic (OSNSO)-cobalt(III) complexes.• XRD, NMR, CV and EPR studies.• Evaluation in radical polymerization.