Photoinduced organocatalyzed atom-transfer radical polymerization (O-ATRP) is a controlled radical polymerization technique that can be driven using low-energy, visible light and makes use of organic photocatalysts. Limitations of O-ATRP have traditionally included the need for high catalyst loadings (1000 ppm) and the narrow scope of monomers that can be controllably polymerized. Recent advances have shown that N , N -diaryl dihydrophenazine (DHP) organic photoredox catalysts (PCs) are capable of controlling O-ATRP at PC loadings as low as 10 ppm, a significant advancement in the field. In this work we synthesized five new DHP PCs and examined their efficacy in controlling O-ATRP at low ppm catalyst loadings. We found that we were able to polymerize methyl methacrylate at PC loadings as low as 10 ppm (relative to monomer) while producing polymers with dispersities as low as Đ = 1.33 and achieving initiator efficiencies ( I* ) near unity (102%). In addition to applying these PCs in O-ATRP, we carried out a thorough investigation into the structure–property relationships of the new DHP PCs reported herein and report new photophysical characterization data for previously reported DHPs. The insight into the DHP structure–property relationships that we discuss herein will aid in the elucidation of their ability to catalyze O-ATRP at low catalyst loadings. Additionally, this work sheds light on how structural modifications affect certain PC properties with the goal of bolstering our understanding of how to tune PC structures to overcome current limitations in O-ATRP such as the controlled polymerization of challenging monomers.
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Open-air green-light-driven ATRP enabled by dual photoredox/copper catalysis
Photoinduced atom transfer radical polymerization (photo-ATRP) has risen to the forefront of modern polymer chemistry as a powerful tool giving access to well-defined materials with complex architecture. However, most photo-ATRP systems can only generate radicals under biocidal UV light and are oxygen-sensitive, hindering their practical use in the synthesis of polymer biohybrids. Herein, inspired by the photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization, we demonstrate a dual photoredox/copper catalysis that allows open-air ATRP under green light irradiation. Eosin Y was used as an organic photoredox catalyst (PC) in combination with a copper complex (X–Cu II /L). The role of PC was to trigger and drive the polymerization, while X–Cu II /L acted as a deactivator, providing a well-controlled polymerization. The excited PC was oxidatively quenched by X–Cu II /L, generating Cu I /L activator and PC˙ + . The ATRP ligand (L) used in excess then reduced the PC˙ + , closing the photocatalytic cycle. The continuous reduction of X–Cu II /L back to Cu I /L by excited PC provided high oxygen tolerance. As a result, a well-controlled and rapid ATRP could proceed even in an open vessel despite continuous oxygen diffusion. This method allowed the synthesis of polymers with narrow molecular weight distributions and controlled molecular weights using Cu catalyst and PC at ppm levels in both aqueous and organic media. A detailed comparison of photo-ATRP with PET-RAFT polymerization revealed the superiority of dual photoredox/copper catalysis under biologically relevant conditions. The kinetic studies and fluorescence measurements indicated that in the absence of the X–Cu II /L complex, green light irradiation caused faster photobleaching of eosin Y, leading to inhibition of PET-RAFT polymerization. Importantly, PET-RAFT polymerizations showed significantly higher dispersity values (1.14 ≤ Đ ≤ 4.01) in contrast to photo-ATRP (1.15 ≤ Đ ≤ 1.22) under identical conditions.
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- Award ID(s):
- 2000391
- NSF-PAR ID:
- 10401720
- Date Published:
- Journal Name:
- Chemical Science
- Volume:
- 13
- Issue:
- 39
- ISSN:
- 2041-6520
- Page Range / eLocation ID:
- 11540 to 11550
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)ATRP (atom transfer radical polymerization) is one of the most robust reversible deactivation radical polymerization (RDRP) systems. However, the limited oxygen tolerance of conventional ATRP impedes its practical use in an ambient atmosphere. In this work, we developed a fully oxygen-tolerant PICAR (photoinduced initiators for continuous activator regeneration) ATRP process occurring in both water and organic solvents in an open reaction vessel. Continuous regeneration of the oxidized form of the copper catalyst with sodium pyruvate through UV excitation allowed the chemical removal of oxygen from the reaction mixture while maintaining a well-controlled polymerization of N -isopropylacrylamide (NIPAM) or methyl acrylate (MA) monomers. The polymerizations of NIPAM were conducted with 250 ppm (with respect to the monomer) or lower concentrations of CuBr 2 and a tris[2-(dimethylamino)ethyl]amine ligand. The polymers were synthesized to nearly quantitative monomer conversions (>99%), high molecular weights ( M n > 270 000), and low dispersities (1.16 < Đ < 1.44) in less than 30 min under biologically relevant conditions. The reported method provided a well-controlled ATRP ( Đ = 1.16) of MA in dimethyl sulfoxide despite oxygen diffusion from the atmosphere into the reaction system.more » « less
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Abstract A key challenge of photoregulated living radical polymerization is developing efficient and robust photocatalysts. Now carbon dots (CDs) have been exploited for the first time as metal‐free photocatalysts for visible‐light‐regulated reversible addition–fragmentation chain‐transfer (RAFT) polymerization. Screening of diverse heteroatom‐doped CDs suggested that the P‐ and S‐doped CDs were effective photocatalysts for RAFT polymerization under mild visible light following a photoinduced electron transfer (PET) involved oxidative quenching mechanism. PET‐RAFT polymerization of various monomers with temporal control, narrow dispersity (
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Abstract A key challenge of photoregulated living radical polymerization is developing efficient and robust photocatalysts. Now carbon dots (CDs) have been exploited for the first time as metal‐free photocatalysts for visible‐light‐regulated reversible addition–fragmentation chain‐transfer (RAFT) polymerization. Screening of diverse heteroatom‐doped CDs suggested that the P‐ and S‐doped CDs were effective photocatalysts for RAFT polymerization under mild visible light following a photoinduced electron transfer (PET) involved oxidative quenching mechanism. PET‐RAFT polymerization of various monomers with temporal control, narrow dispersity (
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Abstract The photocatalyst Zn(II)
meso ‐tetra(4‐sulfonatophenyl)porphyrin (ZnTPPS) is found to substantially accelerate visible‐light‐initiated (red, yellow, green light) single unit monomer insertion (SUMI) ofN ,N ‐dimethylacrylamide into the reversible addition–fragmentation chain transfer (RAFT) agent, 4‐((((2‐carboxyethyl)thio)carbonothioyl)thio)‐4‐cyanopentanoic acid (RAFT1), in aqueous solution. Thus, under irradiation with red (633 nm) or yellow (593 nm) light with 50 mpm (moles per million mole of monomer) ZnTPPS at 30 °C, the rate enhancement provided by photoinduced energy or electron transfer (PET) is ≈sevenfold over the rate of direct photoRAFT‐SUMI (without catalyst), which corresponds to achieving full and selective reaction in hours versus days. Importantly, the selectivity, as judged by the absence of oligomers, is retained. Under green light at similar power, higher rates of SUMI are also observed. However, the degree of enhancement provided by PET‐RAFT‐SUMI over direct photoRAFT‐SUMI as a function of catalyst concentration is less and some oligomers are formed.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2SC04210J
