可见光驱动的有机催化原子转移自由基聚合外文翻译资料

 2022-08-07 14:28:23

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Organocatalyzed atom transfer radical polymerization

driven by visible light

Jordan C. Theriot,1 Chern-Hooi Lim,1,2 Haishen Yang,1 Matthew D. Ryan,1

Charles B. Musgrave,1,2,3 Garret M. Miyake1,3*

Atom transfer radical polymerization (ATRP) has become one of the most implemented methods for polymer synthesis, owing to impressive control over polymer composition and associated properties. However, contamination of the polymer by the metal catalyst remains a major limitation. Organic ATRP photoredox catalysts have been sought to address this difficult challenge but have not achieved the precision performance of metal catalysts. Here, we introduce diaryl dihydrophenazines, identified through computationally directed discovery, as a class of strongly reducing photoredox catalysts. These catalysts achieve high initiator efficiencies through activation by visible light to synthesize polymers with tunable molecular weights and low dispersities.

Over the past two decades, atom transfer radical polymerization (ATRP) (1–4) has matured into one of the most powerful methodologies for precision polymer synthesis (5). Strict control over the equilibrium between a dormant alkyl halide and an active propagating radical dictates a low concentration of radicals and minimizes bimolecular termination to achieve controlled polymer chain growth (6). ATRP has historically relied on transition-metal catalysts to mediate this equilibrium and polymerize monomers with diverse functionality into macromolecules with controlled molecular weight (MW), low MW dispersity (ETH;), defined chemical composition, and complex architecture (7).

The caveat of traditional ATRP has been that the transition-metal catalysts present purification challenges for the polymer products and impede their use in biomedical and electronic applications (8). Despite substantial strides in lowering catalyst loading (9, 10) and facilitating purification (11), organocatalyzed methods remain highly desirable for circumventing the need for metal removal, reducing toxicity concerns, and avoiding interference with electronic systems. Organocatalyzed variants of ATRP by use of alkyl iodide initiators have been established, although they are not a broadly applicable replacement for metal-catalyzed ATRP (12–14).

Our interest in this field originated in 2013 with the discovery that perylene could serve as an organic visible-light photoredox catalyst (PC) to mediate an ATRP mechanism with alkyl bromide initiators, albeit with less control over the polymerization than has become the benchmark for traditional metal-catalyzed ATRP (15–17). Our ongoing work has striven to establish organocatalyzed ATRP (O-ATRP) for the synthesis of polymers with the precision of traditional ATRP, using visible-light PCs to realize energy-efficient polymerization methods that eliminate a major limitation of ATRP. Although photoredox catalysis has been established for decades, visible-light photoredox catalysis has drawn increasing attention by presenting the opportunity to harness solar energy to mediate chemical transformations under mild conditions (18, 19). Phenyl phenothiazine derivatives have since also proven effective as PCs for the ATRP of methacrylates (20) and acrylonitrile (21) but require irradiation by ultraviolet light and leave much room for improvement for generating polymers with higher molecular weights and lower dispersities coupled with increased initiator efficiency.

Our proposed mechanism of photoredox O-ATRP posits reversible electron transfer (ET) from the photoexcited PC in order to reversibly activate an alkyl bromide initiator (Fig. 1C). In addition to the requirement that the excited triplet state 3PC* possess sufficiently strong reducing power to activate the initiator, a delicate interplay must be balanced between the stability of the radical cation 2PCbull; and its capacity to oxidize the propagating radical so as to efficiently deactivate the propagating polymer and yield a controlled radical polymerization.

Computationally directed discovery (22, 23) inspired us to focus on 5,10-diphenyl-5,10- dihydrophenazines as a potential class of PCs for O-ATRP (Fig. 1B). The phenazine core is shared by several biologically relevant molecules that serve as redox-active antibiotics (24, 25), whereas synthetic derivatives have drawn interest in organic photovoltaics (26–28) and organic ferromagnets (29, 30). We hypothesized that an appropriate union between the excited-state reduction potential (E0*) and the stability of the radical cation PCbull; resulting from ET to the initiator would be required for the production of polymers with controlled MW and low ETH;. As such, we investigated electron-donating (OMe, 1), neutral (H, 2), and electron-withdrawing (CF3, 3, and CN, 4) moieties on the N-pheny

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