Copper(0)-mediated reversible-deactivation radical polymerization

Copper(0)-mediated reversible-deactivation radical polymerization (Cu(0)-mediated RDRP) is a member of the class of reversible-deactivation radical polymerization.^[1] As the name implies, metallic copper is employed as the transition-metal catalyst for reversible activation/deactivation of the propagating chains responsible for uniform polymer chain growth.

History of Copper-mediated RDRP

Although copper complexes (in combination with relevant ligands) have long been used as catalysts for organic reactions such as atom transfer radical addition (ATRA) and copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC), copper complex catalyzed RDRP was not reported until 1995 when Jin-Shan Wang and Krzysztof Matyjaszewski introduced it as atom transfer radical polymerization (ATRP).^[2][3] ATRP with copper as catalyst quickly became one of the most robust and commonly used RDRP techniques for designing and synthesizing polymers with well-defined composition, functionalities, and architecture. Due to some inherited drawbacks, such the persistent radical effect (PRE),^[4] several advanced ATRP techniques have been developed, including activators regenerated by electron transfer (ARGET) ATRP^[5] and initiators for continuous activator regeneration (ICAR) ATRP.^[6]

One intriguing catalyst, metallic copper, has also been applied to these modified ATRP systems. The polymerization using Cu(0) and suitable ligands was introduced for the first time by Krzysztof Matyjaszewski in 1997.^[7] However, then, in 2006, the Cu(0) - mediated RDRP of MA (in combination with tris(2-(dimethylamino)ethyl)amine(Me6TREN) as ligand in polar solvents) was reported, with a very different mechanism, single electron transfer living radical polymerization (SET-LRP) postulated by Virgil Percec.^[8] Initiated by this mechanistic difference, many of research articles were published during the recent years aimed to shed a light on this specific polymerization reaction and the discussion of the mechanisms has been a very striking event in the field of polymer science.^{[9][10][11][12]}

Discussion of the mechanism

Supplemental activator and reducing agent atom-transfer radical polymerization (SARA ATRP)

In the case of RDRP reactions in the presence of Cu(0), one of the mechanistic models proposed in the literature is called the supplemental activator and reducing agent atom-transfer radical polymerization (SARA ATRP).^[10][13][14] The SARA ATRP is characterized by the traditional ATRP reactions of activation by Cu(I) and deactivation by Cu(II) at the core of the process, with Cu(0) acting primarily as a supplemental activator of alkyl halides and a reducing agent for the Cu(II) through comproportionation. There is minimal kinetic contribution of disproportionation because Cu(I) primarily activates alkyl halides and activation of all alkyl halides occurs by inner sphere electron transfer (ISET).

Single electron transfer living radical polymerization (SET-LRP)

Another model is called single-electron transfer living radical polymerization (SET-LRP), where Cu(0) is the exclusive activator of alkyl halides - a process that occurs by outer sphere electron transfer (OSET). The generated Cu(I) disproportionates ‘spontaneously’ into highly reactive ‘nascent’ Cu(0) and Cu(II) species, instead of participating in the activation of alkyl halides, and there is minimal comproportionation.^[8][15]

Copper(0)-mediated reversible-deactivation radical polymerization (Cu(0)-mediated RDRP)

One unique experimental phenomenon in the Cu(0)-mediated RDRP systems with Me₆TREN/DMSO as ligand/solvent is that the existence of an apparent induction period in the early stage and the absence of this induction period was observed by adding extra Cu(II) to the reaction system or employing PMDETA as ligand.^{[9][16][17][18]} This intriguing phenomenon cannot be explained either by SARA ATRP or SET-LRP, thereby, another mechanism: copper(0)-mediated reversible-deactivation radical polymerization (Cu(0)-mediated RDRP) mechanism was proposed by Wenxin Wang.^[16]

The Cu(0)-mediated RDRP mechanism showed that induction period is originated from the accumulation of soluble copper species during that initial unstable stage. It was demonstrated that Cu(I) act as a powerful activator even under its disproportionation favored conditions (in Me₆TREN/DMSO system), whilst Cu(0) also can activate dormant species to some extent and both disproportionation and comproportionation coexist. In other words, the mechanism lies between SET-LRP and SARA ATRP. The overall effect of disproportionation and comproportionation depends on the thermodynamic and kinetic condition of the experiments (such as equilibrium constant and the concentrations of Cu(I) and Cu(II) in nonpolar and polar solvents).

It should be noted that two equilibriums coexist under real polymerization conditions – activation/deactivation equilibrium (DEACT) and disproportionation/comproportionation equilibrium (DISP). Even if the solvent and ligand thermodynamically favor disproportionation over comproportionation, the relative concentrations of Cu(I) and Cu(II) species may not approach the disproportionation equilibrium ratio ([Cu(II)]/[Cu(I)]²=k_disp) because this ratio may be far from the one in the activation/deactivation equilibrium. For instance, although DMSO and Me₆TREN are commonly used as solvent and ligand favoring activation with Cu(0) and disproportionation (with a relatively high k_act0 and k_disp), the preferred activator (Cu(0) or Cu(I) species) and the extent of the disproportionation depend on both the relevant reaction rate constants and the relative concentrations of the copper species during polymerization. The synergistic effect of the two equilibriums results in a more complicated mechanism and we cannot isolate them from each other as they are in a complex system.^[15]

Understanding this synergistic effect is crucial to understand the existence of the induction period. As the relative concentrations of different copper species are far from the polymerization equilibrium (activation/deactivation) ratio, the disproportionation equilibrium is thermodynamically favoured and dominates the mutual conversion of the different valent copper spices in the initial stage, resulting the accumulation of the dissolved copper species. Once the [Cu(I)]/[Cu(II)] ratio approach to a critical value (i.e. the polymerization equilibrium ratio), the polymerization would be accelerated and the induction period is ceased. Therefore, if the dissolved copper species – either Cu(I) or Cu(II) – are added, the initial concentration ratio is changed and the polymerization equilibrium is favoured, resulting an instant polymerisation.^[16]

See also

Reversible-deactivation radical polymerization
Atom-transfer radical-polymerization

References