“Bacteriochlorophyll a” Promotes Photoinduced Controlled Radical Polymerization

The advancement of photochemical processes for macromolecular tailoring has provided polymer chemists with new opportunities to construct advanced and complex polymer structures. The unique abilities of such light-mediated reactions enable establishing spatiotemporal control over the polymerization reaction as well as conducting polymerization under mild and environmentally-friendly conditions by utilizing the energy of light to drive forward chemical transformations. In this connection, it is highly desirable to utilize such photochemical processes that operate efficiently with high-wavelength, low-energy light. The use of high-energy UV light in some cases may result in undesirable side-reactions which render their applicability in biological conditions. To overcome these drawbacks, novel photochemical strategies working at higher wavelength with efficient low-energy light sources have been the focus of recent research in this area.

Recently a group of researchers from the University of New South Wales, Australia, has developed a new strategy to carry out photoinduced living radical polymerization utilizing high-wavelength, low-energy near-infrared (NIR) and far-red light sources. Shanmugam, Boyer and co-workers have reported the use of a biocatalyst, namely bacteriochlorophyll a, with light harvesting capabilities which absorbs light in the NIR and far-red regions, as efficient photoredox catalyst for reversible addition-fragmentation chain transfer (RAFT) polymerization process. Bacteriochlorophyll a is a photosynthetic bacteria which enables anoxygenic photosynthesis in dark, deep-sea waters by absorbing NIR and far-red light and utilizing sulfide, elemental sulfur, or hydrogen instead of water as an electron donor. The authors took advantage of bacteriochlorophyll a to carry out RAFT polymerization with low-energy NIR LED light sources, diminishing some undesirable side-reactions as a result of using high-energy UV and visible light to produce well-defined polymers with controlled molecular weight properties and high chain end fidelity.

PET-RAFT Mechanism
The proposed mechanism for photoinduced RAFT polymerization

In the process, bacteriochlorophyll a is used as a photoredox catalysis which facilitates photoinduced electron transfer reaction under NIR LED irradiation reducing the RAFT agent to generate initiating radicals. The propagating radicals can be deactivated by the oxidized bacteriochlorophyll a regenerating initial ground-state photocatalyst and the RAFT agent containing polymer chain. This photocatalytic cycle can be re-initiated further to grow polymer chains in a controlled manner. Furthermore, the NIR light being capable of penetrating depth was used to successfully conduct polymerization with the light source screened by a paper barrier, which reveals the potential of this process for biomedical applications where deep penetration is required without causing damage to living tissues.

Screened PET-RAFT Scheme
Photoinduced RAFT polymerization with a screened NIR light source

Read more on this work: 

Light-Regulated Polymerization under Near-Infrared/Far-Red Irradiation Catalyzed by Bacteriochlorophyll a, Angew. Chem. Int. Ed.2015, 10.1002/anie.201510037

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