Radical Cations of Phenoxazine and Dihydrophenazine Photoredox Catalysts and Their Role as Deactivators in Organocatalyzed Atom Transfer Radical Polymerization

Abstract

Radical cations of photoredox catalysts used in organocatalyzed atom transfer radical polymerization (O-ATRP) have been synthesized and investigated to gain insight into deactivation in O-ATRP. The stability and reactivity of these compounds were studied in two solvents, N,N-dimethylacetamide and ethyl acetate, to identify possible side reactions in O-ATRP and to investigate the ability of these radical cations to deactivate alkyl radicals. A number of other factors that could influence deactivation in O-ATRP were also probed, such as ion pairing with the radical cations, radical cation oxidation potential, and halide oxidation potential. Ultimately, these studies enabled radical cations to be employed as reagents during O-ATRP to demonstrate improvements in polymerization control with increasing radical cation concentrations. In the polymerization of acrylates, this approach enabled superior molecular weight control, a decrease in polymer dispersity from 1.90 to 1.44, and an increase in initiator efficiency from 78 to 102%. This work highlights the importance of understanding the mechanism and side reactions of O-ATRP, as well as the importance of catalyst radical cations for successful O-ATRP.

Aromatization as the driving force for single electron transfer towards C–C cross-coupling reactions

“…22,23 Furthermore, radical cations of diaryl dihydrophenazine has been generated using an external oxidant NOPF 6 , and the role of the oxidized form was studied towards ATRP reactions under photoexcitation. 24 However, the photoredox chemistry of dihydrophenazine in its anionic form has not been explored. In this report, we disclose how a deprotonated form of DPh can initiate electron transfer under photochemical conditions, and how the engendered radical can facilitate cross-coupling reactions via C-H functionalization.…”

Redox Properties of N,N′-Disubstituted Dihydrophenazine and Dihydrodibenzo[a,c]phenazine: The First Isolation of Their Crystalline Radical Cations and Dications

“…N,N′ -Disubstituted-dihydrophenazine derivatives are a class of highly electron-rich and redox-active building blocks and have attracted wide research interest due to their unique optical, electronic, magnetic, and chemocatalytic properties. For example, dihydrophenazine derivatives, especially N , N′ -diphenyldihydrophenazine ( DPPZ ), are easily able to form stable radical cation species under specific conditions, such as irradiation, heat, electrolysis, and chemical oxidation. − The easy to produce very persistent radical cation is one of the main advantages that makes DPPZ an emerging photocatalyst in various chemical transformations, especially visible light-driven reversible addition–fragmentation chain transfer (PET-RAFT) polymerization and metal-free atom transfer radical polymerization (ATRP). − The stable nature of the DPPZ radical cation also makes it ideal for the design of organic magnetic materials. , Notably, a number of studies have also illustrated that N , N′ -disubstituted-dihydrophenazines could undergo two single-electron oxidation processes accompanying the formation of radical cation and dication species (Scheme ). This special successive two-step, one-electron-transfer redox property renders DPPZ as an excellent active cathode material for the design of stable organic batteries. − However, compared to the well-studied radical cation of DPPZ , the dication species regarding to its real geometric and electronic structure and basic photophysical properties still remain a mystery.…”

Removal of photoredox catalysts from polymers synthesized by organocatalyzed atom transfer radical polymerization

“…As such, this method may not remove all PC from the polymer and often requires several successive precipitations to be performed, making it labor intensive. While investigating the radical cations (PC ·+ ) of O‐ATRP PCs, 6 we noticed these compounds can bind strongly to stationary phases employed in column chromatography such as silica (Figure S10). Based on this observation, we hypothesized this interaction could be used to purify polymers synthesized by O‐ATRP, first by oxidizing the PC to form PC ·+ , followed by a simple filtration through a plug to separate the PC ·+ from the polymer.…”

“…To reduce the frequency of termination reactions, a key step in O-ATRP is deactivation. 6 During deactivation, the PC Á+ reinstalls the bromine chain-end group on the polymer to create dormant polymer chains. This process lowers the concentration of radicals in solution, therefore decreasing the rate of termination.…”

“…However, the propagating chains can also react with each other through radical‐based termination reactions, creating dead polymer chains that can no longer be grown by O‐ATRP. To reduce the frequency of termination reactions, a key step in O‐ATRP is deactivation 6 . During deactivation, the PC · + reinstalls the bromine chain‐end group on the polymer to create dormant polymer chains.…”