Sivaprakash Shanmugam

A Robust and Versatile Photoinduced Living Polymerization of Conjugated and Unconjugated Monomers and Its Oxygen Tolerance

Controlled/living radical polymerization techniques have transformed polymer chemistry in the last few decades, affording the production of polymers with precise control over both molecular weights and architectures. It is now possible to synthesize almost an infinite variety of macromolecules using nonspecialized equipment, finding applications in high-tech industry. However, they have several shortcomings. Until recently, living radical polymerizations could not be controlled by an external stimulus, such as visible light, pH, mechanical, chemical, etc. Moreover, they are usually sensitive to trace amounts of oxygen in the system. In this Article, we report a photoinduced living polymerization technique, which is able to polymerize a large range of monomers, including conjugated and unconjugated monomers, using ultralow concentrations of an iridium-based photoredox catalyst (typically 1 ppm to monomers) and a low energy visible LED as the light source (1-4.8 W, λ(max) = 435 nm). The synthesis of homopolymers with molecular weights ranging from 1000 to 2,000,000 g/mol was successfully achieved with narrow molecular weight distributions (M(w)/M(n) < 1.3). In addition, chain extensions of poly(methacrylate)s, poly(styrene), poly(N-vinyl pyrrolidinone), poly(vinyl ester)s, and poly(acrylate)s were performed to prepare diblock copolymers. The reusability of the catalyst was demonstrated by the synthesis of a decablock polymer by multiple chain extensions. Most importantly, this process was employed to prepare well-defined polymers and multiblock copolymers in the presence of air.

Copper-Mediated Living Radical Polymerization (Atom Transfer Radical Polymerization and Copper(0) Mediated Polymerization): From Fundamentals to Bioapplications

Radical Polymerization and Copper(0) Mediated Polymerization): From Fundamentals to Bioapplications Cyrille Boyer,*,†,‡ Nathaniel Alan Corrigan,‡ Kenward Jung,‡ Diep Nguyen,‡ Thuy-Khanh Nguyen,‡ Nik Nik M. Adnan,†,‡ Susan Oliver,†,‡ Sivaprakash Shanmugam,‡ and Jonathan Yeow†,‡ †Australian Centre for Nanomedicine, and ‡Centre for Advanced Macromolecular Design (CAMD), School of Chemical Engineering, University of New South Wales, Sydney 2052, Australia

Photocatalysis in organic and polymer synthesis

This review, with over 600 references, summarizes the recent applications of photoredox catalysis for organic transformation and polymer synthesis. Photoredox catalysts are metallo- or organo-compounds capable of absorbing visible light, resulting in an excited state species. This excited state species can donate or accept an electron from other substrates to mediate redox reactions at ambient temperature with high atom efficiency. These catalysts have been successfully implemented for the discovery of novel organic reactions and synthesis of added-value chemicals with an excellent control of selectivity and stereo-regularity. More recently, such catalysts have been implemented by polymer chemists to post-modify polymers in high yields, as well as to effectively catalyze reversible deactivation radical polymerizations and living polymerizations. These catalysts create new approaches for advanced organic transformation and polymer synthesis. The objective of this review is to give an overview of this emerging field to organic and polymer chemists as well as materials scientists.

Exploiting Metalloporphyrins for Selective Living Radical Polymerization Tunable over Visible Wavelengths

The use of metalloporphyrins has been gaining popularity particularly in the area of medicine concerning sensitizers for the treatment of cancer and dermatological diseases through photodynamic therapy (PDT), and advanced materials for engineering molecular antenna for harvesting solar energy. In line with the myriad functions of metalloporphyrins, we investigated their capability for photoinduced living polymerization under visible light irradiation over a broad range of wavelengths. We discovered that zinc porphyrins (i.e., zinc tetraphenylporphine (ZnTPP)) were able to selectively activate photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of trithiocarbonate compounds for the polymerization of styrene, (meth)acrylates and (meth)acrylamides under a broad range of wavelengths (from 435 to 655 nm). Interestingly, other thiocarbonylthio compounds (dithiobenzoate, dithiocarbamate and xanthate) were not effectively activated in the presence of ZnTPP. This selectivity was likely attributed to a specific interaction between ZnTPP and trithiocarbonates, suggesting novel recognition at the molecular level. This interaction between the photoredox catalyst and trithiocarbonate group confers specific properties to this polymerization, such as oxygen tolerance, enabling living radical polymerization in the presence of air and also ability to manipulate the polymerization rates (kp(app) from 1.2-2.6 × 10(-2) min(-1)) by varying the visible wavelengths.

Organo-photocatalysts for photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization

In this article, we are investigating a series of organo-dyes, including methylene blue, fluorescein, rhodamine 6G, Nile red and eosin Y, to perform a visible light-mediated controlled/“living” radical polymerization of methacrylates. We demonstrate that eosin Y and fluorescein were efficient catalysts to activate a photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) mechanism. The concentration of eosin Y was varied from 10 to 100 ppm with respect to monomers. This polymerization technique yielded well-defined (co)polymers with a good control of the molecular weights ranging from 10 000 to 100 000 g mol−1 and low polydispersities (PDI < 1.30). A variety of functional monomers, including N,N-dimethylaminoethyl methacrylate, hydroxyl ethyl methacrylate, pentafluorophenyl methacrylate, glycidyl methacrylate, oligoethylene glycol methyl ether methacrylate (OEGMA), and methacrylic acid, were successfully polymerized. Finally, the addition of a tertiary amine, such as triethylamine, afforded the polymerization in the presence of air via a reductive quenching cycle. Different diblock polymethacrylate copolymers, i.e. PMMA-b-POEGMA and PMMA-b-PMMA, were prepared to demonstrate the high end group fidelity.

Light-Regulated Polymerization under Near-Infrared/Far-Red Irradiation Catalyzed by Bacteriochlorophyll a.

Photoregulated polymerizations are typically conducted using high-energy (UV and blue) light, which may lead to undesired side reactions. Furthermore, as the penetration of visible light is rather limited, the range of applications with such wavelengths is likewise limited. We herein report the first living radical polymerization that can be activated and deactivated by irradiation with near-infrared (NIR) and far-red light. Bacteriochlorophyll a (Bachl a) was employed as a photoredox catalyst for photoinduced electron transfer/reversible addition-fragmentation chain transfer (PET-RAFT) polymerization. Well-defined polymers were thus synthesized within a few hours under NIR (λ=850 nm) and far-red (λ=780 nm) irradiation with excellent control over the molecular weight (M(n)/M(w)<1.25). Taking advantage of the good penetration of NIR light, we showed that the polymerization also proceeded smoothly when a translucent barrier was placed between light source and reaction vessel.

Stereo-, Temporal and Chemical Control through Photoactivation of Living Radical Polymerization: Synthesis of Block and Gradient Copolymers

Nature has developed efficient polymerization processes, which allow the synthesis of complex macromolecules with a perfect control of tacticity as well as molecular weight, in response to a specific stimulus. In this contribution, we report the synthesis of various stereopolymers by combining a photoactivated living polymerization, named photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) with Lewis acid mediators. We initially investigated the tolerance of two different photoredox catalysts, i.e., Ir(ppy)3 and Ru(bpy)3, in the presence of a Lewis acid, i.e., Y(OTf)3 and Yb(OTf)3, to mediate the polymerization of N,N-dimethyl acrylamide (DMAA). An excellent control of tacticity as well as molecular weight and dispersity was observed when Ir(ppy)3 and Y(OTf)3 were employed in a methanol/toluene mixture, while no polymerization or poor control was observed with Ru(bpy)3. In comparison to a thermal system, a lower amount of Y(OTf)3 was required to achieve good control over the tacticity. Taking advantage of the temporal control inherent in our system, we were able to design complex macromolecular architectures, such as atactic block-isotactic and isotactic-block-atactic polymers in a one-pot polymerization approach. Furthermore, we discovered that we could modulate the degree of tacticity through a chemical stimulus, by varying [DMSO]0/[Y(OTf)3]0 ratio from 0 to 30 during the polymerization. The stereochemical control afforded by the addition of a low amount of DMSO in conjunction with the inherent temporal control enabled the synthesis of stereogradient polymer consisting of five different stereoblocks in one-pot polymerization.

Selective Photoactivation: From a Single Unit Monomer Insertion Reaction to Controlled Polymer Architectures

Here, we exploit the selectivity of photoactivation of thiocarbonylthio compounds to implement two distinct organic and polymer synthetic methodologies: (1) a single unit monomer insertion (SUMI) reaction and (2) selective, controlled radical polymerization via a visible-light-mediated photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) process. In the first method, precise single unit monomer insertion into a dithiobenzoate with a high reaction yield (>97%) is reported using an organic photoredox catalyst, pheophorbide a (PheoA), under red light irradiation (λmax = 635 nm, 0.4 mW/cm(2)). The exceptional selectivity of PheoA toward dithiobenzoate was utilized in combination with another catalyst, zinc tetraphenylporphine (ZnTPP), for the preparation of a complex macromolecular architecture. PheoA was first employed to selectively activate a dithiobenzoate, 4-cyanopentanoic acid dithiobenzoate, for the polymerization of a methacrylate backbone under red light irradiation. Subsequently, metalloporphyrin ZnTPP was utilized to selectively activate pendant trithiocarbonate moieties for the polymerization of acrylates under green light (λmax = 530 nm, 0.6 mW/cm(2)) to yield well-defined graft co-polymers.

Organic photocatalysts for cleaner polymer synthesis

Metal-free catalysts enable synthesis of polymers for biomedical and electronics applications The material properties of synthetic polymers can be tuned by changing their chain length and branching and the way in which monomer units repeat. For example, high-density polyethylene, which has little chain branching, is a stiff polymer used for food containers and drain pipes, whereas low-density polyethylene, which has more chain branching, is flexible and used to make grocery bags and bottles for chemicals. Polymers are usually made through thermal polymerization, but recent efforts focusing on green chemistry have led to a push toward using solar energy to drive chemical reactions. On page 1082 of this issue, Theriot et al. (1) report on metal-free visible-light photocatalysts that produce well-defined polymers free of metal contamination through radical polymerization.

Synthesis of Discrete Oligomers by Sequential PET‐RAFT Single‐Unit Monomer Insertion

Uniform synthetic polymers with precisely defined molar mass and monomer sequence (primary structure) have many potential high-value applications. However, a robust and versatile synthetic strategy for these materials remains one of the great challenges in polymer synthesis. Herein we describe proof-of-principle experiments for a modular strategy to produce discrete oligomers by a visible-light-mediated radical chain process. We utilize the high selectivity provided by photo-induced electron/energy transfer (PET) activation to develop efficient single unit monomer insertion (SUMI) into reversible addition-fragmentation chain-transfer (RAFT) agents. A variety of discrete oligomers (single unit species, dimers, and, for the first time, trimers) have been synthesized by sequential SUMI in very high yield under mild reaction conditions. The trimers were used as building blocks for the construction of uniform hexamers and graft copolymers with precisely defined branches.