Photoinduced Electron Transfer Reactions for Macromolecular

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Photoinduced Electron Transfer Reactions for Macromolecular Syntheses Sajjad Dadashi-Silab,† Sean Doran,† and Yusuf Yagci*,†,‡ †

Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey Center of Excellence for Advanced Materials Research (CEAMR) and Department of Chemistry, King Abdulaziz University, 21589 Jeddah, Saudi Arabia

‡

ABSTRACT: Photochemical reactions, particularly those involving photoinduced electron transfer processes, establish a substantial contribution to the modern synthetic chemistry, and the polymer community has been increasingly interested in exploiting and developing novel photochemical strategies. These reactions are efficiently utilized in almost every aspect of macromolecular architecture synthesis, involving initiation, control of the reaction kinetics and molecular structures, functionalization, and decoration, etc. Merging with polymerization techniques, photochemistry has opened up new intriguing and powerful avenues for macromolecular synthesis. Construction of various polymers with incredibly complex structures and specific control over the chain topology, as well as providing the opportunity to manipulate the reaction course through spatiotemporal control, are one of the unique abilities of such photochemical reactions. This review paper provides a comprehensive account of the fundamentals and applications of photoinduced electron transfer reactions in polymer synthesis. Besides traditional photopolymerization methods, namely free radical and cationic polymerizations, step-growth polymerizations involving electron transfer processes are included. In addition, controlled radical polymerization and “Click Chemistry” methods have significantly evolved over the last few decades allowing access to narrow molecular weight distributions, efficient regulation of the molecular weight and the monomer sequence and incredibly complex architectures, and polymer modifications and surface patterning are covered. Potential applications including synthesis of block and graft copolymers, polymer-metal nanocomposites, various hybrid materials and bioconjugates, and sequence defined polymers through photoinduced electron transfer reactions are also investigated in detail.

CONTENTS 1. Introduction 1.1. Photochemistry and Merging with Polymerization 1.2. Scope of the Review 2. Photoinitiation of Chain Polymerization by Photoinduced Electron Transfer Reactions 2.1. Free Radical Polymerization 2.1.1. Dye/Co-Initiator System 2.1.2. Type II Photoinitiation 2.1.3. Semiconducting Organic/Inorganic Compounds 2.1.4. Photoinduced Electron Transfer Reaction of Onium Salts 2.2. Cationic Polymerization 2.2.1. Direct Initiation 2.2.2. Free Radical Promoted Cationic Polymerization 2.2.3. Cationic Polymerization by Electron Transfer with Singlet and Triplet Excited States 2.2.4. Cationic Polymerization by Charge Transfer Complexes 2.2.5. Cationic Polymerization by Addition− Fragmentation Agents

© 2016 American Chemical Society

2.2.6. Photoinduced Living Cationic Polymerization 3. Step-Growth Polymerization by Photoinduced Electron Transfer Reactions 4. Polymer-Metal Nanocomposites by Photoinduced Electron Transfer Reactions 5. Block and Graft Copolymers by Photoinduced Electron Transfer Reactions 6. Controlled/Living Radical Polymerizations by Photoinduced Electron Transfer Reactions 6.1. Atom Transfer Radical Polymerization 6.1.1. Mechanistic Explanation 6.1.2. Monomer Compatibility 6.1.3. Ligands 6.1.4. Complex Architectures and SequenceControlled Polymers 6.1.5. Spatiotemporal Control 6.2. Reversible Addition−Fragmentation Chain Transfer (RAFT) Polymerization by Photoinduced Electron Transfer Reactions 7. Click Chemistry by Photoinduced Electron Transfer Reactions

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Received: October 5, 2015 Published: January 8, 2016 10212

DOI: 10.1021/acs.chemrev.5b00586 Chem. Rev. 2016, 116, 10212−10275

Chemical Reviews 7.1. Cycloaddition Click Reactions 7.2. Thiol−Ene/Yne Click Chemistry 8. Conclusion and Future Perspective Author Information Corresponding Author Notes Biographies References Note Added after ASAP Publication

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sional (3D) objects and patterned macromolecular structures. Controlled/living polymerizations together with other advanced chemical tools such as click chemistry have been integrated with photochemistry, opening up new avenues in the synthetic polymer community. Tailor-made polymers with precise structural design have been available in a wide range of topological and morphological varieties, including block, graft, star, gradient, and periodic copolymers, molecular brushes, various hybrid materials and bioconjugates, complex structures, and sequence-defined polymers. All these examples are just but a portion of great advancements and opportunities enabled by photoinitiated polymerization techniques. Considerable efforts have been dedicated toward designing and developing photoinitiation systems suitable for efficient photoinitiation of polymerizations. There are many types of chemically and optically different photoinitiators that can promote polymerization by forming initiating species according to their optical properties. The development has gone through using UV-based high-energy photoinitiating systems to sophisticated chemical and technological advancements allowing the use of very low-energy light sources such as light emitting diodes (LEDs). The polymer community has benefited a lot from advances in the related fields of organic synthesis and visible light photoredox as well as from emerging advanced technologies that enable polymer chemists to tailor novel macromolecular architectures.

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1. INTRODUCTION 1.1. Photochemistry and Merging with Polymerization

Man has always taken inspiration from nature during the course of innovation. Almost without exception, the great biosphere of Earth is powered by the energy from the sun, transmitted through light radiation, and harnessed in plants and other light harvesting organisms to drive forward the chemical transformations that make life possible. And so chemists in the modern age of science have researched to explore and advance the applicability of light as a reactant in synthetic organic chemistry and its multitude of related fields. The efficacy of light in affecting chemical transformations is due to electrons lying in molecular orbitals within molecules entering into excited states upon the absorption of light photons. The distribution of electrons across molecular orbitals is very different in the excited state compared with the ground state. A molecule entering into an electronically excited state can greatly change its chemical properties and reactivity. This can open avenues of reactivity for reactants that would not have been possible without electronic excitation by light. The fundamentals of this behavior have been described according to groundstate and excited-state potential-energy hypersurface topology.1 Photochemistry enables a powerful toolkit for a wide array of chemical transformations and is indeed unique in its distinctive ability to fulfill the energetic requirements for conducting such transformations that would be otherwise inaccessible with thermal counterparts. Typically, absorption of a mole of photons of UV light provides as 130 times greater energy as compared to ambient conditions. This huge amount of energy input provided by light would overcome energetic barriers for many chemical reactions to take place. And the fact that this energy for driving chemical reactions forward can be readily available from light, and ideally from inexhaustible sun light, holds great promise for sustainable development and a greener future. Thus, the green nature of photochemistry has been the fundamental inspiration for chemists in exploiting advantages of ubiquitous light and developing photochemical approaches.2−4 Photochemistry can be employed using photoinitiators to initiate chain processes such as radical or cationic polymerization reactions. The utilization of photochemistry, however, is not limited only to the photoinitiation of chain growth reactions. It is used in a broad sense of applications concerning initiation and control of polymerization, functionalization, decoration, and degradation of polymers and various application aspects. Photocuring and surface coatings have become common practice in many commercial and industrial applications. Biological applications of photopolymers have emerged in dental fillings, artificial bone generation, tissue engineering, and drug delivery arrays. Two-photon polymerization and spatially controlled polymerizations initiated by photochemical means allow fabrication of fine three-dimen-

1.2. Scope of the Review

In this contribution, we intend to review the fundamentals and applications of photoinduced electron transfer reactions for polymer synthesis. Synthesis of polymers by free radical, cationic, step-growth, and increasingly emerging controlled/ living radical polymerizations enabled by photoinduced electron transfer reactions are addressed in detail. A special emphasis will be made in drawing a detailed mechanistic explanation for photochemical events bringing about polymers. Furthermore, specific applications such as block and graft copolymer formation, metal−polymer nanocomposites, etc. will be explained as well. In this connection, we will focus our attention to discussing all aspects of photopolymerization reactions concerning electron transfer processes. As such, some systems operating in different modes of photoactivation other than electron transfer will not be involved in this contribution. Readers who are interested in other areas of photopolymerizations are directed to refer to previously published review articles and book chapters in this area.5−12

2. PHOTOINITIATION OF CHAIN POLYMERIZATION BY PHOTOINDUCED ELECTRON TRANSFER REACTIONS Photoinduced chain growth polymerization techniques constitute a considerable contribution to the synthesis of various polymer structures. Free radical and cationic polymerizations have been extensively applied in this context, whereas photoinduced anionic polymerization was limited in scope and application and only a few works have been reported in photoinitiated anionic polymerization. Efforts have been directed toward understanding the mechanistic explanations of the photochemical processes and designing novel photoinitiating systems to enhance and improve the efficiency of such reactions and expand their scope into new areas. The photoinduced electron transfer reactions bringing about radical 10213


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donor−acceptor concept with favorable results as compared with unlinked structures.33 It is well-known that in the presence of aliphatic amine coinitiators, α-amino radicals are generated through photoinduced electron and proton transfer to initiate free radical polymerization. However, in a recent study it was found that using methylene blue as the dye photosensitizer and a sacrificial sterically hindered amine co-initiator, namely N,N-diisopropylethylamine (DIPEA), no such radical formation was detected.34 In fact, it was suggested the strong basicity and sterically hindered nature of DIPEA allows it to serve as a twoelectron and one proton (2e−/1H+) donor, as opposed to oneelectron, one proton transfer reactions in known photochemical processes. A 2e−/1H+ transfer resulted in decomposition of the amine to nonradical products while generating colorless leucomethylene blue. No polymerization was initiated in the presence of methylene blue and DIPEA. However, adding an iodonium salt led to reoxidation of leuco-methylene blue regenerating the initial ground state dye and reduction of the iodonium salt to generate initiating phenyl radicals. Therefore, the radical polymerization of vinyl monomers was initiated. The mechanism of 2e−/1H+ transfer-initiated polymerization is illustrated in Scheme 2. The process involved rapid photoexcitation of methylene blue within seconds of irradiation and formation of charge-transfer exciplex by the DIPEA co-initiator which resulted in decomposition of the amine to nonradical (closed-shell) products and formation of leuco-methylene blue via 2e−/1H+ transfer. Thus, the energy of light was suggested to get stored in the leuco-methylene blue photoproduct without spontaneous radical formation and initiation of polymerization. Energy release was found to occur via redox reactions with the iodonium salt which proceeded in the dark and took a much longer time interval to complete. Despite their ability to generate initiating species and optical characteristics at higher wavelengths, the applicability of dyes in sensitization of polymerization is limited due to drawbacks concerning their poor stability and storage problems. As a result, dye-sensitized systems as compared to other photoinitiators have not been extensively applied in photopolymerization systems. 2.1.2. Type II Photoinitiation. 2.1.2.1. Two-Component Bimolecular Photoinitiators. According to their optical behavior in the formation of initiating species, photoinitiators are generally subdivided into two general classes of Type I or those capable of forming initiating radicals directly upon bond cleavage on absorption of light and Type II which generate radicals in conjunction with various co-initiators. Formation of initiating radicals in Type II photoinitiation occurs by absorption of light by a photoinitiator (ketone type photoinitiators, for example) which is followed by interaction with the co-initiator compounds resulting in radical generation.35,36 Benzophenone,37,38 thioxanthone,39−41 ketocoumarin,42 camphorquinone,43−46 anthraquinone,47,48 and their derivatives are examples of the most widely used Type II photoinitiators. The interaction of the excited photoinitiator with the co-initiator, depending on the nature of the co-initiator, may happen through different pathways, including hydrogen abstraction or electron transfer or an involvement of both electron transfer and hydrogen abstraction processes. Although in some cases the probability of quenching triplet photoinitiator with monomers should not be ruled out, the main pathway of formation of radicals is through the interaction with coinitiators. For instance, amines due to their good reductant

and cationic species to initiate corresponding polymerizations can be realized utilizing chromophore groups in the presence of co-initiator compounds. Therefore, understanding the interaction of chromophores and co-initiator compounds is important for a successful photopolymerization reaction, and various systems have been developed in this regard. 2.1. Free Radical Polymerization

2.1.1. Dye/Co-Initiator System. Dye molecules have been employed as visible light photosensitizer compounds in photoinitiated polymerizations. They are capable of inducing electron transfer reactions in the photoexcited state in conjunction with suitable co-initiators.13 These electron transfer reactions convert the added co-initiator to active initiating sites. Dyes can be both reducing and oxidizing in the photoexcited state. And as such, they can be quenched in the photoexcited state by co-initiators through reductive or oxidative processes solely via photoinduced electron transfer reactions. In the reductive quenching, an electron donor is used as co-initiator which transfers an electron to the dye molecule to form electron-donor radical cation species and reduced dye molecules. On the other hand, in the oxidation quenching, an electron transfer takes place from the dye to an electron acceptor molecule oxidizing dye to radical cation while reducing the co-initiator compound (Scheme 1). Free radical Scheme 1. Photoinduced Electron Transfer Reactions of Dyes

polymerization can be initiated directly by radical cations (or initiating compounds resulting from their subsequent fragmentation) formed in the reductive quenching and in the presence of electron donor co-initiators. The oxidative quenching, on the other hand, is generally suitable for the initiation of cationic polymerizations where dye sensitizers are used to sensitize onium photoinitiators to form cationic species. This approach is described in detail in the following sections concerning photosensitization of onium salts. Examples of photoreducible dyes include xanthene dyes, such as rose bengal, eosine Y, and erythrosin B; acridinium dyes, such as acriflavine; phenazine dyes, such as methylene blue; and thiazene dyes, such as thionine. Several studies have appeared that used these systems to initiate free radical polymerization under visible light irradiation.14−31 As co-initiator, amines, for example, are commonly used as electron donor co-initiators. And iodonium salts are good examples of electron acceptor co-initiators. Moreover, halogen-containing compounds have also been used as co-initiator which can provide initiating radicals by electron transfer resulting in the C-halogen bond dissociation in an oxidative quenching.32 Allonas and co-workers reported linking electron acceptor groups to the structure of dye molecules 10214


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Scheme 2. Photoinduced 2e−/1H+ Transfer between Methylene Blue Dye and DIPEA Co-Initiatora

a

Evidence of photoredox cycle of methylene blue in the presence of amine and iodonium salt co-initiators leading to changes in the color of the solution. Reprinted from ref 34. Copyright 2014 American Chemical Society.

properties49 are commonly used as co-initiators in Type II photoinitiation. It is generally believed that amines interact with the excited state photoinitiator primarily by an electron transfer process forming an ion pair intermediate (or an exciplex) of photoinitiator radical anion and amine radical cation.50−52 Afterward, as depicted in Scheme 3, there follows a proton transfer, which results in the formation of an initiating radical derived from the amine co-initiator and a ketyl radical of photoinitiator. The latter is inactive toward adding to double bonds and rather tends to couple or terminate initiating radicals; however, radicals derived from the co-initiator induce the chain polymerization process. Similarly, various types of co-initiators have been proven to undergo photoinduced electron/proton transfer reactions with triplet photoinitiators. For example, thiols react with Type II photoinitiators in a similar manner to amines, which includes primarily, an electron transfer followed by a proton transfer to form sulfur-based free radicals capable of adding across double bonds and initiating free radical polymerization (Scheme 4).53−57 Sulfur-centered radicals are highly active to add to both allyl and vinyl double bonds exhibiting insensitivity toward oxygen inhibition. Thiol−ene photopolymerizations of multifunctional thiols and double bonds proceed in a step growthlike manner mediated by free radicals for the formation of polymer networks.58,59 The centrality of using thiols in photochemical processes deals with their substantial role in one of the most powerful tools of modern chemistry, that is the photoinduced thiol−ene click chemistry, which has been the center of research interest from many diverse areas.60,61 Alternatively, phosphorus-containing compounds were used as electron/hydrogen donor co-initiators in photopolymerization.62 A report from the laboratories of Lalevée and co-workers revealed that a hydrogen atom abstraction by triplet photo-

initiators was observed with phosphorus compounds bearing a labile hydrogen−phosphorus bond. The process was proven to occur without the involvement of any electron transfer reactions as no ion pair intermediates were detected in spectroscopic investigations. Whereas, photoinduced electron transfer of the triplet photoinitiator with those phosphorus compounds without any abstractable hydrogen was evidenced through the formation of phosphorus radical cation and ketone radical anion intermediates. The phosphorus radical cation intermediate was reported to further fragment to yield phosphorus-centered initiating radicals (Scheme 5). One way to manipulate and enhance the photoactivity of Type II photoinitiators is by introducing various functionalities onto the structure of the photoinitiator by which the absorption maxima, extinction coefficients, quantum yields, as well as adaptability in different media can be tuned on demand. For example, water-solubilizing agents have been introduced into the structure of benzophenones or thioxanthones to make them soluble in aqueous medium so that they easily initiate photopolymerization of water-soluble monomers (Chart 1).52,63−66 2.1.2.2. One-Component Bimolecular Photoinitiators. Additionally, one-component Type II photoinitiators consisting of both the chromophore core and co-initiator components have been synthesized and used in radical polymerizations. Without the need of additional co-initiators, one-component photoinitiators produce initiating radicals through an intramolecular and/or intermolecular photoinduced electron/ hydrogen transfer process between the chromophore and donator parts. Amines, thiols, and etherlike electron/hydrogen donors of low-molecular weight and polymeric structures involving, generally, benzophenone or thioxanthone chromophore groups have been synthesized as one-component photoinitiators. A derivative of benzophenone known as 10215


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Scheme 3. Schematic representation of Type II photoinitiation with Benzophenone as An Example of Ketone-Based Photoinitiator and a Tertiary Amine Co-Initiator

Scheme 4. Thiols as Co-Initiator in Type II Photoinitiation

Michler’s ketone bearing amine functionality, for example, is capable of generating initiating radicals upon irradiation through photoinduced intramolecular/intermolecular electron and proton transfers in a similar manner to two-component systems. Similarly, thiol derivatives of thioxanthone are capable of generating sulfur-centered radicals without the need of additional co-initiators (Scheme 6).67 Representative examples of one-component Type II photoinitiators are collected in Table 1. 2.1.2.3. Macrophotoinitiators. With the aim of facilitating solubility and compatibility of photoinitiators and overcoming some disadvantages of low-molecular weight photoinitiators, which may include migration of byproducts of the photolysis of photoinitiators in cured films due to high volatility, polymeric photoinitiators comprising photoinitiator species incorporated into a polymeric support have been utilized as well. Various techniques, including polymerization of suitable monomers, postmodification, and functionalization processes have been taken advantage of in producing such macrophotoinitiators.105 Postmodification of polystyrene as a polymeric backbone by treating with thiosalicylic acid, which led to the thioxanthonation of polystyrene, has been reported to prepare macrophotoinitiators with pendant thioxanthone groups.106 Additionally, this process can bring about water-soluble polystyrenebased polymeric photoinitiators by sulfonation during the thioxanthonation process in a one-pot manner.107 An amine coinitiator was necessary for polymerization to occur. 10216


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Scheme 5. Phosphorous Compounds as Co-Initiator in Type II Photoinitiated Polymerization (PI: Photoinitiator)

Scheme 6. Thiol-Derivative of Thioxanthone as OneComponent Type II Photoinitiator

Quite recently, our group reported the synthesis of thioxanthone-based polymeric networks as heterogeneous macrophotoinitiators.108 Using various cross-coupling processes such as Sonogashira−Hagihara or Friedel−Crafts alkylation techniques (the latter also referred to as “knitting”), conjugated microporous networks of thioxanthone with specific surface areas of up to 750 m2 g−1 were obtained using thioxanthone with different suitable comonomers. Dibromothioxanthone with triethynylbenzene were subjected to the Sonogashira− Hagihara coupling, which resulted in the formation of a

microporous network of thioxanthone with the pore size of 1.4 nm and microporosity of 500 m2 g−1. Using the Friedel−Crafts method, thioxanthone and benzene (or triphenylmethane) were “knitted” together. It was found that these macrophotoinitiators being two- or three-dimensional networks had strong absorption characteristics at visible regions which, it was reasoned, was due to the strong π interactions of the highly conjugated nature of the network. Free radical and cationic photopolymerizations were achieved in the presence of different co-initiators under visible or sun light irradiation. Reusability was found for all three types of microporous thioxanthone networks in both free radical and cationic polymerizations. The synthesis methodologies of one-component macrophotoinitiators mainly include step-growth109−114 and addition115 (co)polymerization of co-initiator/photoinitiator containing monomers, functionalization by click chemistry techniques,116 dendrimerization, and other functionalization methods. Examples of one-component macrophotoinitiators developed thus far are collected in Table 2. Alternatively, functionalization of supramolecular or hyper-branched polymeric structures containing amine or etheric functional groups as hydrogen donating groups117 with photoinitiator moieties have been used to form one-component macrophotoinitiators.118−120 As an example, dendritic or hyperbranched supramolecular amines were reacted with an epoxy functional thioxanthone molecule resulting in the formation of dendritic structures with thioxanthone end functional groups exhibiting

Chart 1. Representative Examples of Water-Soluble Type II Thioxanthone-Based Photoinitiators

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Table 1. One-Component Type II Photoinitiators Carrying Both the Chromophore and Co-Initiator Functionalities

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Table 1. continued

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Table 1. continued

particles to sensitize the free radical polymerization process. Both the photoinduced electron and hole species have been reported to contribute to the formation of initiating sites.141−143 In a related work from the authors’ laboratory, it has been found that both good reductant and oxidant coinitiators can be used with ZnO nanoparticles. The initiating radicals can be formed via reduction of the oxidant co-initiator by the photoinduced electrons or oxidation of reductant by the holes released upon irradiation of nanoparticles.144 A diphenyliodonium salt was used as the oxidant co-initiator that reduced to the diphenyliodonium radical, which further underwent decomposition giving phenyl radicals. On the other hand, triethylamine (TEA) as a reductant co-initiator interacted with the holes through the nonbonding electrons of the nitrogen atom, which was followed by a hydrogen atom abstraction from another amine molecule giving rise to the formation of initiating α-amino radicals. Polymerization of a water-soluble monomer, acrylamide, was reported in aqueous media in the presence of oxygen. Without the need of additional co-initiators, interaction of the electrons and holes

photoinitiation activity much higher than small molecular weight photoinitiating systems.121−123 Hyper-branched poly(ethylene imine) or dendritic poly(propyleneimine) could be used for this purpose. 2.1.3. Semiconducting Organic/Inorganic Compounds. The use of semiconducting materials of both organic and inorganic origins as photocatalysts has long been established in heterogeneous photocatalysis.135 In line with the concepts of sustainable chemistry, substantial applications of heterogeneous semiconductor materials have been achieved in a broad range of perspectives. Semiconductors are light sensitive materials that undergo excitation followed by the release of charge carriers.136,137 Such a manner of providing electron/hole species is taken advantage of leading to the triggering of an enormous array of chemical reactions.138−140 Metal oxide nanoparticles such as titanium dioxide (TiO2), zinc oxide (ZnO), and cadmium sulfur (CdS) among others, sized in the range of 1−100 nm, are typical examples of inorganic semiconductors. Early attempts to use these compounds in photopolymerization were carried out by using ZnO nano10220


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Table 2. Selected One-Component Type II Macrophotoinitiators

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Table 2. continued

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Table 2. continued

with water and oxygen molecules brought about initiating radicals revealing their potential applicability for overcoming oxygen inhibition problems. Scheme 7 depicts the mechanism of the formation of initiating free radicals by semiconducting ZnO nanoparticles in the presence of various co-initiators. Of great importance, their heterogeneity makes semiconductor nanoparticles efficient reusable photoinitiators as after the reaction they are easily recycled, refined, and applied in further reactions while preserving their reactivity after several usages.

Nanoparticles of different composition have also been reported to initiate free radical polymerization in a quite similar mechanism, involving the interaction of the photoinduced electron or hole species with initiating components, including additional co-initiators, solvent, or monomer molecules.145−150 The optical characteristics of these nanoparticles are generally governed and controlled by adjusting the shape, size, and surface modification of nanoparticles.151 It has been recently found that magnetic iron oxide nanoparticles functionalized with carboxylic acid efficiently initiated free radical polymerization. Carboxylic acid groups used for stabilizing nanoparticles interacted with the photogenerated holes, resulting in a decarboxylation process to form initiating free radicals.152 A similar decarboxylation process was also reported using additional carboxylic acid co-initiators in the presence of nonfunctionalized TiO2 nanoparticles.147 Similar to inorganic nanoparticles, semiconducting organic materials have been developed as heterogeneous photoorganocatalysts for a wider range of catalytic reactions. For example, mesoporous graphitic carbon nitride (mpg-C3N4) composed of carbon and nitrogen is highly photosensitive with a large specific surface area and has been proposed as a promising potential heterogeneous organocatalyst for a diverse range of redox reactions.153 The use of mpg-C3N4 in photoinitiated polymerization was based on photoinduced electron/hole release by mpg-C3N4 under visible light irradiation. In the presence of an oxidant such as amine, initiating free radicals were generated.154 Of great importance, the heterogeneous nature of mpg-C3N4 made it possible to reuse it in further reactions by retaining catalytic activity in each cycle. Similar strategy has been undertaken with conjugated microporous polymers as heterogeneous photocatalysts.155

Scheme 7. Photoinitiated Free Radical Polymerization Using Semiconductor Nanoparticles: Interaction of the Photoinduced Electron and Hole Species with Different CoInitiators

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are used so as to oxidize terminating ketyl radicals while generating new initiating radical species.157−159 Neckers and co-workers studied photopolymerization of bithiophenefunctionalized gold (or silver) nanoparticles, which was based on an intramolecular electron transfer between thiophene and onium salt leading to bithiophene radical cation species. In the absence of a vinyl monomer, a rapid aggregation of nanoparticles was observed as a result of a combination of thiophene radical cation species,160−163 whereas in the presence of an acrylate monomer, free radical polymerization was initiated (Scheme 9).163,164

Upconverting nanoparticles (UCNPs) are worth mentioning in this connection. These materials are highly photosensitive at near-infrared (NIR) region and upon excitation emit UV or visible light. Being photosensitive at NIR or IR regions, these nanoparticles are of high potential for bioapplication since they can penetrate in living tissues without causing any harmful effects. Beyazit, Haupt, and co-workers were the first to apply UCNPs in localized photoinduced polymerization to the surface modification of the particles making them more hydrophilic suitable for many bioapplications.156 Emitting light at UV and visible region, the UCNPs upon excitation under 980 nm LED illumination were able to sensitize conventional photoinitiating systems to initiate the polymerization process. Bezophenone/TEA or eosin Y/TEA combinations were used as UV or visible light photoinitiating components, respectively. Polymerization of a series of watersoluble monomers was successfully initiated this way leading to the formation of a polymeric hydrophilic shell around the nanoparticles, and the use of functional monomers enabled further modification by attaching biocompatible functional groups (Scheme 8).

Scheme 9. Photopolymerization by Gold Nanoparticles: Photoinduced Electron Transfer Reactions of Bithiophene Functionalized Gold Nanoparticles

Scheme 8. Photoinduced Polymerization by Upconverting Nanoparticles

2.2. Cationic Polymerization

2.2.1. Direct Initiation. Photoinitiated cationic polymerization of corresponding monomers such as epoxides and vinyl ethers is widely utilized in various UV-curing and other commercial applications.165,166 Unlike free radical polymerizations, cationic polymerization does not suffer from oxygen inhibition and so polymerizations can be conducted in air. The use of onium salts in cationic photopolymerization has long become a common practice in relevant commercial applications and academic research arenas.167−170 Iodonium,171 sulfonium,171−173 phosphonium,174,175 and pyridinium176,177 salts among others178,179 are commonly applied onium salt photoinitiators. These salts contain a heteroatom with a cationic center and an inorganic metal complex anion as the counteranion part such as BF4−, PF6−, and SbF6−. Due to their photosensitivity albeit at relatively short wavelengths, onium salts undergo a direct photolysis upon irradiation through a heterolytic or homolytic bond cleavage giving rise to the formation of cation or radical cation species, respectively. These resulting charged species in some cases may not initiate cationic polymerization of target monomers. However, in the presence of a hydrogen donor, they become highly active to initiate cationic polymerization by Brønsted acids formed through a hydrogen abstraction process. The photolysis of onium salts is presented in the example of diphenyliodonium salt in Scheme 10. A variety of iodonium, pyridinium, sulfonium, phosphonium, and many other types of onium salt photoinitiators developed and successfully used for cationic photopolymerization are given in Chart 2. Phenacyl onium salts are a particular class of onium salt photoinitiators. They can be synthesized by the reaction of phenacyl halide compounds with the corresponding heteroatom containing nucleophiles followed by an anion-exchange process with potassium or sodium salts containing nonnucleophlic counteranions to yield the final product (Scheme

2.1.4. Photoinduced Electron Transfer Reaction of Onium Salts. Photoexcited state photoinitiators can be good oxidant or reductant species compared with their ground state form and can be quenched through either a reduction or oxidation process, depending on the characteristics of the present co-initiator. In contrary to hydrogen abstraction wherein the triplet photoinitiator is quenched through a reduction process, as observed with amine co-initiators, an onium salt co-initiator can quench the excited photoinitiator in an oxidation manner through photoinduced electron transfer processes. Onium salts are well-known co-initiators in Type II photoinitiated polymerizations having good redox properties; nevertheless, they are primarily used as photoinitiator in cationic systems (vide infra). In the process, a photoinduced electron transfer from the photoexcited photoinitiator to the onium salt is encountered that forms a photoinitiator radical cation and a neutral onium radical with which both cationic and radical photopolymerizations can be initiated, respectively. In addition to their use as co-initiator for free radical photopolymerization, onium salts are also used as additives in Type II photoinitiated polymerization reactions. Ketyl radicals formed in such systems, as discussed earlier, are capable of terminating propagating radicals. To suppress this disadvantage, onium salts 10224


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This process is referred to as free radical promoted cationic polymerization. The radicals can be generated using various photochemical means or other thermal methods195,196 as well. This technique allows conducting cationic polymerization initiated by onium salts at higher wavelength using even visible light radical photoinitiators. Therefore, it is possible to tune and broaden the spectral sensitivity of those systems by carefully selecting the photoinitiator. For example, Type I photoinitiators which afford easily oxidizable free radicals by unimolecular bond cleavage are excellent promoters of cationic photopolymerization. The general mechanism of the free radical promoted cationic polymerization using a benzoin Type I photoinitiator and iodonium salt is outlined in Scheme 14. There have been numerous studies using benzoin-type photoinitiators for the promotion of cationic polymerization in the presence of onium salts.197−205 Acylphosphine oxide photoinitiators have been used to oxidize with onium salts to afford phosphonium cations.206−208 Acylgermane-based209 photoinitiators which form germyl radicals upon visible light irradiation have been shown to get oxidized by onium salts generating germanium cations for the initiation of the cationic polymerization of a variety of appropriate monomers.210−213 Phosphites have been utilized as reducing co-initiators with iodonium salts to initiate visible light cationic polymerization. The mechanism involves the formation of phosphonaryl radicals and subsequent redox reactions with iodonium salt to form cationic species. Azo-based compounds such as phenylazoisobutyronitrile are used as photoinitiators that afford phenyl radicals upon decomposition under visible light irradiation. The phenyl radical adds to the phosphite compounds yielding arylphosphoranyl radicals. The interaction of phosphoranyl radicals with the iodonium salt was proposed to proceed via an electron transfer to the iodonium salt resulting in the oxidization of phosphoranyl radicals to give arylphosphonium cationic species. The phosphonium cations initiated cationic polymerization of cyclic monomers, including cyclohexene oxide and tetrahydrofuran by methylation of the monomer and formation of arylphosphonate product.214,215 Due to their high nucleophilicity, phosphites can also act as inhibitor of polymerization through chain transfer reactions. Therefore, the concentration of phosphites is required to be carefully balanced to ensure efficient reaction with radicals and subsequently with iodonium salts and yet must not be so great to inhibit the polymerization reaction. Later studies revealed that modification of phosphites with halogen-substituted alkoxy groups reducing their nucleophilicity could suppress any inhibitory activity of phosphite compounds to allow an efficient polymerization process (Scheme 15).216 Promotion of cationic polymerization by free radicals produced by Type II photoinitiators has been investigated. As Type II photoinitiators thioxanthone, benzophenone, and camphorquinone and related structures have been employed in this connection. In these systems, co-initiators interacting with the excited state photoinitiators are necessary to form electron-donor free radicals.94,217−219 In such systems utilizing Type II photoinitiators for cationic polymerization, the use of amines as co-initiator should be carefully considered as aliphatic amines due to the fact that their strong basicity may terminate propagating cationic chains and prevent polymerization.220 Aromatic amines are suitable co-initiators for this purpose. Additionally, o-phthaldehyde was employed to promote the cationic polymerization process by the formation of biradicals through intramolecular hydrogen abstraction in the excited

Scheme 10. Photolysis of Diphenyliodonium Hexafluorophosphate as a Typical Example of Onium Salts

11).180 These compounds are highly photosensitive, and their photolysis mechanism follows the same trend as other onium salts outlined previously. Although being extensively applied for cationic photopolymerization, phenacyl onium salts are also capable of initiating free radical as well as Zwitterionic polymerizations. Many studies have been reported on the use of pyridinium, anilinium, sulfonium, and other phenacyl-based photoinitiation systems.181−191 An interesting study revealed a tautomerization behavior associated with the phenacyl benzoylpyridinium salts which resulted from the formation of keto−enol isomers upon irradiation.184 This behavior was found to result in a change in the optical characteristics of the photoinitiator extending its absorption maxima to visible light regions (Scheme 12). In a recent study, a polymeric photoinitiator of phenacylpyridinium salt was synthesized using a polystyrene-b-poly(2vinylpyridine) support, which was obtained by a living anionic polymerization of styrene and 2-vinylpyridine monomers. Phenacyl bromide was reacted with the pyridine moieties present in the block copolymer support to form phenacylpyridinium salt after an anion-exchange process with potassium hexafluoroantimonate, and the resulting polystyrene-b-poly(2vinyl phenacylpyridinium) salts were used to photoinduce both free radical and cationic polymerization of corresponding monomers (Scheme 13).192 Moreover, photolysis of phenacyl groups provided a means to photoswitching the behavior of block copolymers from cationic to neutral states. 2.2.2. Free Radical Promoted Cationic Polymerization. A practical disadvantage most often associated with onium salts is their photoactivity laying mainly at short wavelength of the UV spectrum between 230 and 300 nm. Such photoactivity requires the use of high-energy irradiation sources to accomplish the photolysis process, which would consequently render it unsuitable for certain purposes. To overcome such problems, indirect photolysis through electron transfer processes has been proposed.193,194 As stated previously, onium salts can easily be reduced by electron-donating radical species. As a result, free radicals with electron-donating properties are oxidized by the onium salts to form carbocation species suitable for the initiation of cationic polymerizations. 10225


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Chart 2. Common Onium Salts Used in Photopolymerization Reactions

state (Scheme 16).221 Subsequent oxidation of the biradicals by pyridinium salt and further decomposition to yield Bronsted acid initiated cationic polymerization. As previously shown, electron transfer reactions from the radicals formed this way to the onium salt oxidize radicals to cationic species that are capable of initiating cationic polymerization of various monomers. It is worth noting that in those systems using Type II photoinitiators in combination with different types of co-initiators in the presence of onium salts, there is also the possibility of the interaction of the photoexcited photoinitiator with both co-initiator and onium salts as well via reduction or oxidation pathways, respectively. However, this possibility is highly dependent on the characteristics of the photoinitiator and onium salts to directly interact in the excited state. Two-photon absorption mechanism was also applied in free radical promoted cationic polymerizations. Benzodioxinone and naphtadioxinone were shown to act as two-photon absorption photoinitiators, whereby a desirable photoinitiator (i.e.,

benzophenone) could be formed in a stepwise two-photon absorption process. Absorption of the first photon by benzodioxinone or naphtadioxinone and their subsequent photolysis released benzophenone photoinitiator.222,223 The second photon absorption was by the benzophenone formed in situ to generate radicals through successive electron transfer and proton abstractions. Subsequent interaction with the iodonium salt and electron transfer reactions resulted in the formation of initiating cationic species, as well as regenerating ground-state benzophenone. The mechanism of the process is outlined in Scheme 17. Neckers and co-workers and others used various dye molecules in conjunction with amine co-initiators to induce free radical promoted cationic polymerization by onium salts under visible light.220,224,225 In this regard, silane compounds were particularly interesting co-initiators for the promotion of cationic polymerization. Silane co-initiators interacting with the excited state bimolecular photoinitiators, produce silyl radicals that can easily be oxidized by onium salts to form active 10226


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Scheme 11. Synthesis of Phenacyl Onium Salt Photoinitiators and Representative Examples

Scheme 13. Phenacyl Pyridinium Salts Based on a Block Copolymer Support as Photoinitiator for Free Radical and Cationic Polymerizations

cationic species. The silyl radicals are considered insensitive toward oxygen and can act as oxygen scavengers. Those silane compounds with a labile hydrogen atom form silyl radicals through hydrogen abstraction by the photoinitiator and the disilane structures without any abstractable hydrogen, photoinduced electron transfer results in Si−Si bond dissociation bringing about active silyl radicals.226−231 Lalevée, Fouassier, and co-workers have developed elegant approaches for induction of cationic polymerization in a free radical promoted manner using three component initiating systems. Transitional metal complexes such as RuII or IrIII have

been used in combination with silane or other co-initiators to form initiating cation species with the aid of onium salt oxidants. The transitional metal complexes are highly photoactive at visible light regions and undergo photoexcitation under soft irradiation conditions. In the process, the photoexcited metal complex RuII* species can be quenched in an oxidative manner by the iodonium salt resulting in the oxidation of the metal complex to RuIII and formation of phenyl radicals. These two components failed to efficiently initiate cationic polymerization of epoxides. However, in the presence of silane, the phenyl radicals abstracted the labile

Scheme 12. Photoinduced Keto-Enol Tautomerism of the Phenacyl Benzoylpyridinium Salta

a

Reprinted from ref 184. Copyright 2006 American Chemical Society. 10227


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electron transfer either to the onium salt or through interaction with the oxidized RuIII species. The latter process also regenerates the initial ground state photocatalyst as well. Initiation of cationic polymerization was effectively achieved using this three-component system based on transitional metal photocatalyst and silane co-initiators.232−234 The mechanism of the process is illustrated in Scheme 18. Various other systems benefiting from the photoinduced electron transfer reactions enabled by transitional metal photoredox catalysis including Ir, Ru, Fe, Cu, etc., and silane or similar co-initiators have been reported for the promotion of cationic polymerization.230,235−243 It is worth mentioning the use of substituted vinyl halide compounds as promoters of cationic polymerization. The carbon-halide bond present in these compounds is readily cleavable upon irradiation at corresponding wavelengths giving rise to the formation of vinyl radicals. An electron transfer then is considered to take place, yielding vinyl cationic and bromine anionic species.244 Moreover, oxidization of these vinyl radicals by sulfonium or pyridinium salts was employed to form carbocations capable of initiating cationic polymerization (Scheme 19).245 2.2.3. Cationic Polymerization by Electron Transfer with Singlet and Triplet Excited States. Photosensitization is one such efficient indirect approach for the photolysis of onium salts.246 Photosensitizers being photochemically active at higher wavelength at near UV or visible light regions are expected to interact in the excited state with onium salts through various pathways resulting in the photolysis of the onium salts. As a result of photosensitization of onium salts through photoinduced electron transfer, radical cations of the photosensitizer and neutral onium radical species are generated. The radical cation so-formed may directly initiate cationic polymerization, or it can interact with a hydrogen donor compound to form a Brønsted acid capable of initiating cationic polymerization reactions (Scheme 20).247 The photolysis of onium salts by photosensitization is thermodynamically feasible only if the free energy changes (ΔG) of the photoinduced electron transfer process are negative. This can be estimated using the Rehm−Weller equation:

Scheme 14. Photoinduced Free Radical Promoted Cationic Polymerization by Type I Photoinitiator

Scheme 15. Phosphite Co-Initiators in Visible Light Free Radical Promoted Cationic Polymerization

Scheme 16. Photoinduced Free Radical Promoted Cationic Polymerization by Using o-Phthaldehyde and a Pyridinium Salt

ox red ΔGet = fc [E1/2 (D/D·+) − E1/2 (A/A·−)] − E* + ΔEc •+ red •− where fc is the Faraday constant, Eox 1/2(D/D ) and E1/2(A/A ) are redox potentials of the donor (D) and acceptor (A) compounds, respectively, E* is the excited state energy of the sensitizer (singlet or triplet), and ΔEc is the Coulombic stabilization energy. The free-energy changes of the electron transfer (ΔGet) of some photoinitiators and photosensitizers with frequently used onium salts, calculated by the Rehm−Weller equation, are given in Table 3. Different types of photoinitiators, polynuclear aromatic, highly conjugated compounds, and dyes exhibiting light sensitivity at near UV and visible light regions have been reported as favorable photosensitizers in this regard. Earlier attempts in photosensitization of onium salts were carried out by Crivello using dye molecules.170 Acridinium, benzothiazolium, and hematoporphyrin type dyes with strong absorption characteristics at higher wavelengths of visible light were used for this purpose (Chart 3). Relatively poor solubility of these dyes in monomer solutions was encountered. Photosensitiza-

hydrogen of the silane bringing about silyl radicals. At this point, the silyl radicals are oxidized to initiating cationic species through two possible pathways. Oxidation can happen by 10228


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Scheme 17. Free Radical Promoted Cationic Polymerization by Stepwise Two-Photon Absorption Mechanism

Scheme 18. Free Radical Promoted Cationic Polymerization by Transitional Metal Complexes and Silanes in the Presence of Onium Salts

Scheme 19. Photoinduced Cationic Polymerization by Substituted Vinyl Ethers: Formation of Cationic Species by Electron Transfer between (a) Radical Fragments or (B) Oxidation by Onium Salts

Scheme 20. Photosensitization of an Onium Salt for the Initiation of Cationic Polymerization (PS: Photosensitizer)

tion of iodonium salts was effectively achieved with these compounds; however, they were inefficient for the sensitization of sulfonium salts. Electron-rich polynuclear aromatics such as perylene, pyrene, anthracene, and their derivatives have been shown to 10229


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Table 3. Redox Potentials of Photosensitizers and the Free Energy Change of Electron Transfer with Some Onium Salts Calculated by the Rehm-Weller Equationa

a

E*t : triplet excited energy, E*s : singlet excited energy.

Chart 3. Dye-Photosensitizers for Onium Salts

Chart 4. Polynuclear Aromatic Compounds As ElectronTransfer Photosensitizer

studies have been reported on photosensitization of onium salts based on photoinduced electron transfer reactions for the initiation of cationic polymerization.218,289−296 It was recently shown that ferrocenium salts can promote sensitization of onium salts via photoinduced electron transfer.297 Photoinduced electron transfer between the excited ferrocenium salt and iodonium salt resulted in the photolysis of the iodonium salt producing ferrocenium radical cations and onium radicals. The radical cation could either undergo further photolysis producing the arene ligand and an active Lewis acid or react with the phenyl radicals to form protonic acid species. The Lewis acid or protonic acid species so-formed were responsible for the initiation of cationic polymerization. Fullerene (C60 or buckyball) is another such efficient photosensitizer for onium salt photoinitiators to initiate cationic polymerization. It has been shown that photoinduced electron transfer to generate

photosensitize onium salts (Chart 4).248−261 The ΔG values for the electron transfer sensitization of onium salts by polynuclear aromatic compounds have been found to be quite large and negative, indicating their potential for use as photosensitizers. Substitution was proposed to overcome problems concerning poor solubility of these compounds. Phenothiazine and its derivatives are another class of efficient photosensitizers with excellent reducing properties.262−265 In a similar manner, thioxanthones,266−271 carbazoles,272−275 curcumin,276,277 quinoxaline and related structures,278−281 and highly conjugated derivatives of thiophene242,282−288 have been used as long wavelength photosensitizers. Moreover, several other 10230


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initiating cationic species could be efficiently promoted by fullerene under visible light and in the presence of onium salts.298 Upon photoexcitation of fullerene in the presence of onium salts, an exciplex was formed which was followed by electron transfer from the fullerene to the onium salt, generating fullerene radical cation while reducing onium salts. A variety of monomers including cyclohexene oxide, isobutyl vinyl ether, and N-vinylcarbazole were successfully polymerized using this method. Silver hexafluourophosphate (AgPF6) proved as a more efficient oxidant for fullerene than diphenyliodonium salt as the free energy changes for the electron transfer in the presence of AgPF6 were more negative and higher monomer conversions were achieved. Fullerene could be used in chlorobenzene solvent or it could alternatively be attached to polystyrene to improve its solubility in organic monomers without the need for using solvents. Onium salts are most often used in three-component photoinitiation systems. These systems consist of a photoinitiator (or photosensitizer), a hydrogen donor (an amine, generally), and an onium salt co-initiator.299−306 With regard to three-component photoinitiators, triplet state photoinitiator simultaneously reacts with both the amine and onium salt coinitiators through hydrogen abstraction and electron transfer reactions, respectively. The aim of these systems is to enhance the efficiency and rate of photopolymerization reactions. The consequence of hydrogen abstraction from the amine coinitiator brings about an amine-based radical capable of initiation of radical polymerization. A ketyl radical is also formed which in the presence of the onium salt is oxidized to form a protonic acid and a neutral onium radical while regenerating the ground state photoinitiator. Considering the interaction of the triplet photoinitiator with the onium salt, a neutral onium radical and photoinitiator radical cation are formed, in which the latter can sometimes initiate cationic polymerization with the former capable of initiating radical polymerization. In the presence of an amine co-initiator, the formed radical cation abstracts a hydrogen atom yielding a protonic acid and amine-derived radical and, again, regenerating ground state photoinitiator. The general mechanism of this three-component photoinitiation is summarized in Scheme 21. The ability to initiate polymerization of both free radical and cationic monomers, converting noninitiating species to active, initiating components (as in the oxidation of terminating ketyl radicals to generate protonic acid and initiating radical), and more importantly, regeneration of the ground state photoinitiator are the advantages of three-component photoinitiation systems making them highly advantageous for many applications. 2.2.4. Cationic Polymerization by Charge Transfer Complexes. Pyridinium salts have been shown to be capable of forming ground state charge transfer complexes with electron-rich donors such as methyl- and methoxy-substituted benzene derivatives.307,308 These complexes absorb at relatively high wavelengths, where other components are virtually transparent. For example, the complex formed between Nethoxy-4-cyano pyridinium hexafluorophosphate and 1,2,4trimethoxybenzene exhibited absorption maxima at 420 nm. The absorption maxima of the two components individually are 270 and 265 nm for the pyridinium salt and trimethoxybenzene, respectively. It was found that the charge transfer complexes formed between pyridinium salts and methyl- and methoxy-substituted benzene could act as photoinitiators for the cationic polymerization of cyclohexene oxide and 4-

Scheme 21. Three-Component Photoinitiation of Free Radical and/or Cationic Polymerizations

vinylcyclohexene oxide. The photoinitiation mechanism primarily involved formation of charge transfer complexes between the two components with subsequent electron transfer bringing about a neutral pyridinium-based radical and radical cation of the benzene derivative. The radical cations formed were considered effective initiating species. The overall mechanism is depicted in Scheme 22. Moreover, since a Scheme 22. Cationic Polymerization through Photoinduced Electron Transfer in Charge Transfer Complexes

proton scavenger had no influence on the rate of polymerization, the initiation by Bronsted acid which could be possibly formed by an interaction with hydrogen donor components was excluded. Notably, the charge transfer complexes described above were found applicable for the photoinitiation of epoxide monomers but not for the photoinitiation of other monomers such as vinyl ethers and N-vinylcarbazol monomers. Theses monomers were observed to polymerize in the dark upon addition of the complexes. 2.2.5. Cationic Polymerization by Addition−Fragmentation Agents. Another way to achieve free radical promotion 10231


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of cationic polymerization is by an addition−fragmentation technique.309 This technique enables photosensitization of allyl functional onium salts. It involves the addition of a radical species generated by photochemical or thermal means to the double bond of the allyl group of the onium salt. The radical adduct so-formed tends to fragment to yield onium radical cation species. The cationic polymerization can be initiated by the resulting onium radical cation after further fragmentation, or in the presence of a hydrogen donor it may abstract a hydrogen atom and produce Bronsted acids capable of initiating cationic polymerization. The overall process is depicted in Scheme 23. A variety of mono- or biallyl sulfonium,

Scheme 24. Controlled Living Cationic Polymerization in the Presence of Lewis Acids

Scheme 23. Cationic Photopolymerization by Addition− fragmentation Technique In a conceptually distinct approach, Perkowski and coworkers utilized a photoredox concept to carry out controlled/ living cationic polymerization.327 They used 2,4,6-tri(p-tolyl)pyrylium tetrafluoroborate as a photoinitiator in combination with methanol to initiate the cationic polymerization of 4methoxystyrene. Oxidation of the monomer by the photoexcited state photoinitiator was suggested to induce the process generating a styrenyl radical cation species. Methanol was utilized as a reversible chain transfer agent to regulate the concentration and propagation of cationic chain ends. A cationic species formed was captured by methanol generating a proton capable of protonating the additional monomer. This process of nucleophilic addition of methanol to cationic species and subsequent protonation of monomers took place until methanol was totally consumed. As a result, methoxy-captured dormant species and active cationic chains were formed. The methoxy group serving as the chain transfer agent was able to form a transient intermediate of oxonium ion by capturing an active cationic chain end. The oxonium intermediate further fragmented to form active cation and dormant species in a reversible manner, thus regulating the propagation of cationic chains and establishing control over the cationic polymerization process. Polymers with controlled molecular weight properties and narrow distributions were obtained in this way. Additionally, the livingness of the resulting polymers was demonstrated by successful chain extension experiments.

phosphonium, and pyridinium salts have been investigated for the initiation of cationic polymerization using the addition− fragmentation technique.175,310−319 2.2.6. Photoinduced Living Cationic Polymerization. In order to establish control over the cationic polymerization systems, Lewis acids have been employed. These compounds enable cationic polymerization of vinyl ether monomers in a controlled/living manner by coordinating with the propagating cationic chains. Mechanistically, a photochemically generated Brönsted acid adds to the vinyl ether monomer to form a halogen-terminated adduct. The Lewis acid used coordinates with the so-formed halogen-containing adduct ensuring the stabilization of the carbocation center of propagating chains. As a result, the cationic polymerization proceeds in a controlled/ living manner with suppressed chain-breaking reactions due to the coordination of Lewis acids, leading to polymers with controlled molecular weight properties (Scheme 24). Various approaches have been reported in this regard which include direct photolysis of onium salts,320−323 free radical promoted,204 photosensitization,324 and other approaches such as the use of substituted vinyl halides325,326 to generate the initiating protonic acid species.

3. STEP-GROWTH POLYMERIZATION BY PHOTOINDUCED ELECTRON TRANSFER REACTIONS Photoinduced electron transfer reactions have been exploited in step-growth polymerization of conjugated monomers as well. For example, interacting photoexcited state thiophene species with electron acceptor compounds such as dinitrobenzene or carbon tetrachloride promoted electron transfer reactions between the thiophene and electron donor compounds.328−330 This resulted in the formation of corresponding thiophene radical cation species which were then coupled to form polythiophene. Potassium dichromate (Cr(VI)), which is a photochemically active catalyst, was also used to photochemically initiate polymerization of thiophenes. The photoexcited Cr(VI) was suggested to form a complex with the monomer which then resulted in a charge-separation reaction, leading to the formation of radical cation species of the thiophene monomer.331 Other photochemical strategies concerning photoinduced electron transfer reactions of onium salts and thiophene 10232


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derivatives have also been developed by our group.332,333 It was shown that an electron transfer from the radical cations formed during the photolysis of the iodonium salt to thiophene was responsible for the initiation of the process. Radical cation species of thiophene were formed that could then undergo dimerization by proton release followed by rearrangement of aromaticity.334 The dimer thus formed participated in further similar reactions with the iodonium radical cation and subsequently with the thiophene radical cation to further polymerization (Scheme 25). Polymerization took place until a

well be taken advantage of for initiating cationic polymerization of appropriate monomers. An interesting approach to photoinduced step-growth polymerization has been recently proposed by Ravoo and coworkers who used a photocatalytic system based on TiO2 nanoparticles to carry out step-growth polymerization.336 They took advantage of photoinduced electron/hole charge carriers from nanoparticles to induce reduction/oxidation of ethanol amine to initiate its step-growth polymerization. They used an alcohol-functionalized substrate to grow polymer brushes by a microcontact printing method. It was proposed that the photogenerated hole species could oxidize ethanol amine (or the alcohol groups present on the surface of the substrate) to the corresponding aldehyde. This aldehyde was then reacted with another ethanol amine molecule to produce an imine compound, which was finally reduced by the photogenerated electrons to the secondary amine. The amine molecules grown as such can re-enter the catalytic cycle and consequently form polyethylenimine in a step-growth manner mediated by TiO2 photocatalysis. The overall process is illustrated in Scheme 26. Using this novel method in such photoinduced step-growth polymerization, the authors were able to prepare microcontact printed polymer brushes. A similar approach could also be applied to the synthesis of polypyrrole using TiO2-based nanostructures as both photocatalyst and a template for the polymerization of pyrrole using photoinduced charge carriers.148,337

Scheme 25. Photoinduced Electron Transfer Reactions for Step-Growth Polymerization of Thiophene

4. POLYMER-METAL NANOCOMPOSITES BY PHOTOINDUCED ELECTRON TRANSFER REACTIONS Photochemical generation of metallic nanoparticles is arguably one of the most important implementations of photoinduced electron transfer reactions, which has been the subject of various studies in recent years.338 It is argued that for the generation of metal nanoparticles, photochemical means are considered advantageous over other methodologies in that they offer control of the rate of nanoparticle formation and the possibility of spatiotemporal control. In principle, photochemical means are used for the formation of free radicals that are able to reduce metal ions via electron transfer reactions to form metal nanoparticles. Due to their favorable redox potentials, ketyl radicals availed from both Type I and Type II photoinitiators are promising means for the formation of many types of nanoparticles.339−342 In the presence of free radical or cationic monomers, simultaneous photoinitiation of corresponding free radical or cationic polymerizations and the formation of metal nanoparticles afford a polymeric matrix containing embedded metal nanoparticles. Nanocomposites of homogeneously distributed metal nanoparticles within the polymeric matrix can thus be obtained by photochemical means. In this regard, gold nanoparticles, for example, were

dark precipitate was formed on the inner surface of the reaction tube preventing reaction to continue further. Iodonium, sulfonium, and pyridinium salts were able to promote stepgrowth polymerization of thiophene derivatives. Similar strategies could be applied to the efficient photochemical synthesis of a variety of thiophene derivatives using various types of onium salt photoinitiators (Chart 5).282,335 In the presence of highly conjugated thiophene derivatives, due to their strong light sensitivity at higher wavelengths, they were able to sensitize the photolysis of onium salts in the photoexcited state under visible light irradiation. In this case also, photoinduced electron transfer between the photoexcited thiophene and onium salt induced photolysis of the salt, yielding thiophene radical cation species. Photosensitization of onium salts by conjugated thiophene derivatives is wellestablished for the initiation of free radical and cationic polymerization (vide supra). Notably, proton release in the course of coupling of thiophene radical cation species could

Chart 5. Thiophene Derivatives Polymerized by Photoinduced Induced Electron Transfer Reactions

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Scheme 26. Photoinduced Step-Growth Synthesis of Polyethyleneimine Brushes by Using Semiconducting Photocatalysisa

a

AFM measurements of patterned brushes with profile height. Reprinted with permission from ref 336. Copyright 2015 Royal Society of Chemistry.

Scheme 27. In Situ Formation of Nanoparticles Embedded in Polymer Network by Photoinduced Electron Transfer Reactionsa

a

TEM images of Au nanoparticles embedded in a crosslinked poly(ethylene glycol) diacrylate network with different concentrations of Au3+ solution: (a) 1 wt % and (b) 5 wt %. Reprinted with permission from ref 349. Copyright 2008 Royal Society of Chemistry.

synthesized in situ using AuCl4− as the metal ion precursor and a Type I photoinitiator in the presence of a poly(ethylene

glycol) diacrylate (PEGDA) monomer. The photoinitiator affords two initiating and reducing ketyl radicals by which the 10234


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generating phenyl radicals. Free radical polymerization was initiated by the phenyl radicals. In the presence of a metal ion, photoinduced electron transfer took place from the excited RuII* complex to the silver ion, generating silver nanoparticles.. Furthermore, the phosphine-centered radicals generated from the bond scission of the ligand in the RuII complex underwent oxidation to reduce metal ions to metallic nanoparticles. Upon addition of a hydrogen donor solvent such as tetrahydrofuran, the formation of metal nanoparticles was accelerated due to the fact that the phenyl radicals abstracted a hydrogen atom from the hydrogen donor generating easily oxidizable radicals by the metal salt used. Also cationic polymerization could be successfully initiated by the cations formed during the reduction of the salt. Lalevée and co-workers have developed sophisticated strategies for the synthesis of novel metallic nanoparticles that do not exist in a salt form to incorporate in polymeric networks. A series of zirconium (Zr) and titanium (Ti) nanoparticles were formed using zirconocene dichloride (Cp2ZrCl2) or titanocene dichloride (Cp2TiCl2) compounds, respectively, in the presence of a Type I photoinitiator such as DMPA. Conducting the experiments in the presence of oxygen, they observed significant reduction of oxygen inhibition mediated by those organometallic compounds. In fact, peroxyl radicals formed by the reaction of oxygen with the photogenerated radicals from the DMPA photoinitiator were proposed to interact with Cp2ZrCl2 (or Cp2TiCl2) to convert nonactive peroxyl radicals to new initiating radicals for initiating free radical polymerization. Thus, significant enhancement of the rate of polymerization in the presence of oxygen was achieved by the added organometallic compounds being used as additives to Type I photoinitiators. The interaction of the peroxyl radicals with the organometallic compounds through a bimolecular homolytic substitution reaction led to the formation of metal-based structures, which finally resulted in the generation of metallic nanoparticles embedded in the polymeric matrix.356−358 In a quite similar approach, they investigated a series of different Ti-based complexes containing Ti−O bonds as additives toward reduction of oxygen inhibition for the free radical polymerization initiated by a bisacylphosphine oxide photoinitiator, as well as photochemical generation of metallic nanoparticles.359,360 The key to these systems is the formation of metal-based radical compounds which act as peroxyl radical scavengers, and multiple addition of peroxyl radicals leads to the formation of metal nanoparticles. Various applications have been encountered for such nanocomposite systems that contain metal nanoparticles in polymeric networks. Of particular interest have been silvercontaining composites owing to the antibacterial activity of silver nanostructures. For instance, acrylamide-based hydrogels with swelling−deswelling properties were photochemically synthesized with simultaneous photogeneration of silver nanoparticles, which exhibited antibacterial activity against a series of standard bacterial samples.361 Surface modification of a biopolyester, namely poly(3-hydroxybutyrate-co-3-hydroxyvalerate), was photochemically conducted to form antibacterial materials containing silver nanoparticles.362 For this purpose, butan-2-one as a ketone-type photoinitiator was used which was capable of abstracting a hydrogen atom from the backbone of the to induce "grafting from" of the methacrylic acid monomer. The carboxylic functionality of grafted polymers served as stabilizing agents for the immobilization of silver nanoparticles generated via photoinduced electron transfer

initiation of polymerization and formation of gold nanoparticles were achieved, respectively (Scheme 27). Formation of crosslinked networks enables long run stabilization of the resulting nanoparticles. The formation of gold nanoparticles could be detected by the evolution of the typical surface plasmon absorption band for gold nanoparticles centered at about 600 nm. There are many studies reporting on photochemical synthesis of gold, silver, and other metal nanoparticles via photoinduced electron transfer while concomitantly carrying out radical polymerization to afford metal nanoparticles embedded polymer matrices.343−348 Also a 2,7-diaminofluorene derivative dye was found to simultaneously initiate free radical polymerization and nanoparticles formation via photoinduced electron transfer.350 A self-quenching mechanism was proposed to form dye radical cation species which then could abstract a hydrogen atom from the poly(ethylene glycol) monomer forming radical species capable of initiating radical polymerization. In the presence of silver ions, photoinduced electron transfer from the excited state dye directly to the silver ions corresponded to the formation of silver nanoparticles. Adding a hydrogen donor compound like an amine significantly increased the rate and efficiency of both polymerization and nanoparticle formation. Initiation of the polymerization was predominantly through the interaction of the dye/amine compounds via consecutive electron and hydrogen transfers forming initiating α-aminoalkyl radicals. Due to the stabilization role of the amine molecules, samples formed in the presence of amines had uniform size distribution, while in the absence of the amine some agglomeration of the nanoparticles was observed. The key to the formation of metal nanoparticles embedded in polymer matrices via such photochemical reactions is the ability to control the kinetics of the process and the morphology of the resulting nanoparticles. The concentration and type of the photoinitiator employed, the initial concentration of metal ion can directly affect the process and thus the morphology of nanoparticles. Cationically polymerizable monomers have also been used in combination with metal ions to simultaneously form nanocomposites of metal nanoparticles embedded in polymeric matrices. For example, using a cleavable photoinitiator such as 2,2-dimethoxy-2-phenyl acetophenone (DMPA) which produces strong electron donor radicals by bond cleavage under irradiation, the reduction of silver ions could be achieved to form silver nanoparticles. In the meantime, upon donation of an electron, the radicals oxidize to produce active cationic species, which are capable of initiating cationic polymerization in a similar manner to free radical promoted cationic photopolymerizations. A variety of multifunctional cationic monomers including epoxy and vinyl ether monomers have been polymerized via this approach.351−354 An alternative approach involves the use of photosensitizer compounds such as highly conjugated thiophenes. Sensitization through electron transfer processes from the excited photosensitizer to the metal ion salt induces the formation of metallic nanoparticles, while the radical cations of the oxidized photosensitizer are used to initiate cationic polymerization of epoxy monomers.284 A photoredox complex, namely tris(triphenylphosphine)ruthenium(II) dichloride (Ru(PPh3)3Cl2), was shown to promote the in situ synthesis of metallic nanoparticles, while initiating cationic or free radical polymerizations to afford nanocomposite structures.355 The irradiation of the Ru(PPh3)3Cl2 complex resulted in the P-Ph bond scission, thus 10235


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Scheme 28. Polymer-Bound Onium Salts for Photochemically Formation of Block Copolymers

mers.366,367 Photochemical approaches have been demonstrated as efficient methods in promoting such transformations.368−370 This approach is based on polymerization of a monomer by one mechanism, and, if necessary, endfunctionalization, to obtain macroinitiators used for the initiation of the second monomer through another differing mechanism. The functional groups on one or both ends or middle of prepared polymers are activated by photoinduced electron transfer processes generating suitable sites for polymerization of other monomers. One approach deals with the use of thermal initiators functionalized with photoactive groups. This method is particularly appropriate for combination of radical and cationic polymerizations for the synthesis of block copolymers. As such, first, the radical polymerization of a monomer is conducted thermally. Afterward, the formed polymers with photolabile groups are irradiated in the presence of suitable compounds or co-initiators to generate cationic sites that are capable of initiating cationic polymerization. In this connection, an azo initiator functionalized with photoactive benzoin groups at both ends was used to thermally initiate the radical polymerization of styrene.200 By doing so, a polystyrene macrophotoinitiator was synthesized with benzoin-functionalized end groups.371 Irradiation of the macrophotoinitiator resulted in the bond scission of the benzoin group and radical formation which in the presence of oxidant onium salts were oxidized to carbocation by electron transfer processes. Polymerization of cyclohexene oxide monomer could then be initiated in a radical promoted manner affording polystyrene-b-poly(cyclohexene oxide) copolymers. The sequence of the blocks could be arranged by changing the sequence of polymerizations, meaning that the azo initiator could be first irradiated in the presence of onium salts to initiate cationic polymerization of cyclohexene oxide containing an azo linkage in the main chain. On heating up, the latter polymer free radicals were formed to polymerize styrene. Macrophotoinitiators containing Type I photoinitiator functionalities are useful for the formation of block copolymers in the presence of onium salts. The role of onium salt is to

using antraquinone as the photoinitiator and an amine coinitiator. Capping the formed silver nanoparticles by the carboxylic functional groups resulted in the stabilization and homogeneity of these nanoparticles. The obtained material containing silver nanoparticles showed successful antibacterial activity. Finally, polymeric nanocomposites containing gold nanoparticles and glucose oxide as a biomolecule enzyme were formed and investigated as efficient platforms for the biosensing system.363 The so-called ex situ techniques have also been applied for the generation of polymeric networks containing embedded metal nanostructures. The ex situ techniques utilize preprepared metallic nanostructures, which are generally stabilized and functionalized with photoinitiators to initiate polymerization processes. Silver nanorods or spherical nanoparticles functionalized with thioxanthone have been reported in this regard.364,365

5. BLOCK AND GRAFT COPOLYMERS BY PHOTOINDUCED ELECTRON TRANSFER REACTIONS Complex macromolecular architectures such as block and graft copolymers have been obtained using various photochemical approaches. Indeed, such copolymeric structures consisting of various segments each possessing different properties have unique and novel physiochemical properties that make them suitable for many emerging applications. The advent and development of controlled/living polymerizations has paved the way for versatile syntheses of a variety of different copolymers. The advancement in this area is dealt with in the subsequent sections, whereby photoinduced electron transfer reactions have been shown to combine with controlled/living radical polymerizations for the synthesis of complex macromolecular structures. In this section, however, we will review block copolymerization of monomers from chemically different families that are generally homopolymerized through different conventional polymerization mechanisms. This process in general is referred to as mechanistic transformation, which allows copolymerization of chemically different mono10236


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to initiate radical polymerization of the second monomer.396 Obviously, such well-defined polymers by ATRP could be used as macrophotoinitiators to initiate cationic polymerization as well.397 The radicals formed upon abstraction of the chain end bromine functionality of the polymers by •Mn(CO)5 radicals could be oxidized by a photoinduced electron transfer in the presence of onium salts to generate cationic sites capable of initiating cationic polymerization. Cationic polymerization of epoxides and vinyl ethers was conducted to afford block copolymers by combination of ATRP and free radical promoted cation polymerization methods. In a recent study, Mn2(CO)10 was used as a visible light photoinitiator to form polyethylene-g-poly(cyclohexene oxide) (PE-g-PCHO) by a combination of ring opening metathesis polymerization (ROMP) and free radical promoted cationic polymerization techniques.398 Photogenerated radicals by the manganese photoinitiator abstracted bromine functionality of a brominated polyethylene chain forming radical sites on the backbone of the polymer. These radicals were then oxidized to carbocation in the presence of an iodonium salt to initiate the cationic polymerization of cyclohexene oxide. Another similar approach used dichloromethane as the halogen source to form oxidizable carbon-centered radicals by manganese decacarbonyl photoinitiator.399 Several other approaches taking advantage of the photochemistry of Mn2(CO)10 and halide compounds have been developed for the synthesis of complex macromolecular structures such as block, graft, hyperbranched, star, etc. copolymers.400−407 These complex copolymeric structures have found use in various biological applications and controlled drug delivery systems.402,404,406 A combination of anionic and cationic polymerizations was also used to form graft copolymers of polyethers.408 First, poly(ethylene oxide-co-ethoxyl vinyl glycidyl ether) was synthesized by anionic ring-opening copolymerization of corresponding monomers. The vinyl ether moieties were then photochemically converted to protic acid groups by using diphenyliodonium iodide photoinitiator. Living cationic polymerization of isobutyl vinyl ether was then initiated in the presence of zinc iodide catalyst by a grafting-from method in the dark.

oxidize photogenerated macroradicals to form carbocation centers.372−375 Use of polymeric co-initiators in conjunction with Type II photoinitiators is also applied for block or graft copolymerization. In this technique, radicals are formed by a successive electron/proton transfer process of the photoinitiator and polymeric co-initiator, yielding suitable sites for the initiation of second polymerization.376−383 The immobilization of various onium salts on polymeric supports384 has been reported as an efficient route to macrophotoinitiators for both cationic and free radical photopolymerizations. This can generally be achieved using postmodification processes on certain polymer chains, so they may act as the supporter to incorporate onium salt functionality. In doing so, problems concerning poor solubility of some onium salts in certain media as well as drawbacks of small molecule onium salt photoinitiators can be overcome to some degree. Perhaps the most significant attraction of developing such macrophotoinitiators, however, is their resulting ability to photochemically generate copolymers of different topologies through mechanistic transformation processes385 enabled by onium salt functionalities. In this regard, for example, pyridinium-functionalized polytetrahydrofuran (PTHF) macrophotoinitiators were prepared by quenching the oxonium groups of the parent PTHF polymer polymerized by living cationic polymerization thus placing the pyridinium salts at chain ends of the polymer (Scheme 28).386−388 Using these pyridinium-functionalized PTHF as macrophotoinitiator, which decomposed on irradiation to form polymeric alkoxy radicals, initiation of a monomer polymerizable by a free radical mechanism [e.g., methyl methacrylate (MMA)] was achieved, leading to the formation of block copolymers containing PTHF and PMMA blocks. Similarly, polystyrene prepared by atom transfer radical polymerization was used as a support to immobilize pyridinium salts.389 The bromine end group functionality of polystyrene was replaced with alkoxy pyridinium salts with a non-nucleophilic counteranion hexafluoroantimonate. The same procedure was applied to synthesize graft copolymers using a macrophotoinitiator with pendant pyridinium salt groups. A copolymer of polystyrene and polychloromethylstyrene was used as the polymeric support to convert chlorine groups into pyridinium functionalities by a substitution reaction. The resultant polystyrene with pendant pyridinium groups was used to form graft copolymers of a grafting-from method.390 Transitional metal carbonyls such as rhenium carbonyls (Re2(CO)10) or, particularly, dimanganese decacarbonyl (Mn2(CO)10) are an interesting class of visible light photoinitiators. Its photodecomposition through Mn−Mn bond dissociation forms Mn-centered radicals that are able to abstract halogen atoms from alkyl halide compounds.391 This process forms carbon-centered radicals by electron transfer processes that are capable of initiating a variety of radical polymerizations, or it can be turned to suitable sites for other polymerization modes. Block and graft copolymerization has been successfully carried out utilizing the photochemistry Mn2(CO)10 in the presence of halide-containing polymers.392−395 Polymers prepared by ATRP containing bromine end groups could be used as a polymeric halide source to generate radicals by means of Mn2(CO)10. Upon formation of •Mn(CO)5 radicals, the bromine present in the end chain of polystyrene was abstracted, forming radical sites. These sites were then used

6. CONTROLLED/LIVING RADICAL POLYMERIZATIONS BY PHOTOINDUCED ELECTRON TRANSFER REACTIONS Merging photochemistry with controlled polymerization techniques has revolutionized the synthesis of well-defined macromolecular architectures. Many attempts have been directed toward adaptation of existing controlled/living polymerization methodologies with photochemical means, which in turn result in photochemically induction, mediation, and control of these processes.409−415 Going one step further, novel-controlled polymerization techniques have emerged that are solely mediated and controlled by light. Copper-mediated controlled polymerization techniques including atom transfer radical polymerization (ATRP)416,417 and single-electron transfer-living radical polymerization (SET-LRP),418,419 reversible addition−fragmentation chain transfer (RAFT),420 and nitroxide-mediated polymerization (NMP)421 are increasingly becoming well-established photochemical approaches for macromolecular architecture building via photochemical methods. A compelling feature of photoinitiated controlled polymerizations is the ability of establishing on-demand control 10237


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of the process both in terms of temporally manipulating the polymerization reaction as well as gaining spatial control. Thus, fabrication of 3D polymeric structures and functional patterned surfaces can be obtained in a controlled manner using photochemistry, which would have been otherwise inconceivable. In the following sections, various approaches for the synthesis of macromolecular structures via controlled polymerization techniques enabled by photoinduced electron transfer reactions are highlighted. Special emphasis is put on explaining and providing a better understanding of the mechanism of the reactions. And applications for the synthesis of various macromolecular architectures using special catalytic systems are discussed. Moreover, a comprehensive list of monomers, initiators, ligands, chain transfer agents, solvents, etc. used in photoinduced ATRP and RAFT processes is given, and the influence of particular components is discussed.

Scheme 29. In Situ Generation of Activator Catalyst for ATRP by Means of Various Techniques

molecular weight properties of the polymers. This enhancing of the rate and efficiency of the process achieved on irradiation was attributed to the acceleration of the activation of the innersphere complex between the catalyst and the alkyl chloride, [CuCl/bipy and R-Cl], or perhaps through the activation of the catalyst alone by light. Photochemically (re)generation of active catalyst species has been realized by using various photochemical approaches. These include the use of additional photoinitiators, photosensitizers, or photocatalysts to interact with CuII compounds while reducing them to form an activator catalyst. Additionally, many Cu complexes are highly photosensitive and have been proven to undergo excitation on irradiation and further redox reactions with the ligands or other compounds present in the reaction media have been confirmed. This behavior, of course, is highly dependent on the nature of Cu compounds and the type of ligand used to form complexes with Cu. The consequence of these reactions is the formation of free radicals and reduction of deactivator CuII complexes to form activator CuI. The reduction process can be through many different photochemical mechanisms. Thus, the process in which the reduction of CuII complexes takes place is often referred to as either indirect (in the case of using additional photoinitiator or photocatalyst compounds) or direct (as in the case of not using additional photoinitiators) photogeneration of the activator catalyst for ATRP. It was found that the irradiation of a solution of copper(II) bromide/N,N,N′,N″,N″-pentamethyldiethylenetriamine (CuBr2/PMDETA) under UV light resulted in the disappearance of the characteristic absorption band of the CuBr2/ PMDETA complex centered around 640 nm.438 This further confirmed the reduction of CuII complexes to CuI through the interaction of the photoexcited ligand with the central metal ion and unimolecular Cu-halogen bond cleavage. As a result, the bulk polymerization of MMA was successfully achieved using equivalent amounts of CuBr2/PMDETA with respect to the ATRP initiator irradiated under UV light. The process showed the characteristics of a living/controlled polymerization system, including a linear relationship between the monomer consumption and polymerization time, evolution of molecular weight according to the monomer conversion, and excellent chain end fidelity. Polymers with narrow molecular weight distributions were synthesized having good agreement with theoretical molecular weight values. Moreover, another study revealed the enhancement of the same process by adding small amounts of methanol probably due to the improved solubility of Cu facilitated by methanol and its likely part in the reduction of CuII.439 The level of CuII catalyst loading was successfully decreased to 50−100 ppm levels in 25% anisole as the solvent in a well-

6.1. Atom Transfer Radical Polymerization

6.1.1. Mechanistic Explanation. 6.1.1.1. Copper-Based Systems. ATRP is a reversible-deactivation radical polymerization method which uses a transition metal complex as a catalyst to establish control over the polymerization process.422−424 By reaction of the transition-metal catalyst with alkyl halide compounds used as the initiator, active initiating species are formed in a reversible manner. Copper is the most widely used catalyst for ATRP reactions. The general mechanism of the ATRP process is based on the reaction of a copper catalyst in its lowest oxidation state (CuI) with the initiator alkyl halide (R−X) to form active radical species capable of initiating the radical polymerization process. Deactivation through back halogen transfer brings about dormant species. Thus, as a consequence of activation−deactivation processes enabled by the transition-metal catalyst, control is established over the entire polymerization process. In addition to Cu compounds, other transition metal complexes, including Ru, Fe, Mo, etc. have also been efficiently employed to initiate and control radical polymerization.425,426 The development of ATRP has been focused on designing and developing new catalyst systems that operate more efficiently under mild and environmentally benign conditions. This has been mainly based on the in situ (re)generation of activator catalyst species, which in turn has led to significant developments in broadening the scope of the process as well as reducing its costs. For example, catalyst loading has been drastically reduced to ppm amounts;427 tolerance to oxygen has been attained;428 and applications under biologically relevant conditions429,430 have been realized. To generate the activator catalyst in situ, several methods have been undertaken. Reducing agents,427,431,432 various radical initiators,433,434 electrochemically redox processes,435 coppercontaining nanoparticles,436 and photochemical approaches are some techniques undertaken for the generation of activator catalyst (Scheme 29). With regard to photochemical techniques, considerable enhancement of the conventional ATRP process was observed when irradiating under light. Guan and co-workers investigated the influence of irradiation on the conventional ATRP of methyl methacrylate (MMA) employing a copper(I) chloride/ bipyridine (CuCl/bipy) catalyst with the ratio of [MMA]/ [RCl]/[CuCl]/[bipy]:100/0.1/0.3/1.437 While conducting the reaction in the dark reached a 41% reaction within 16 h, employing light to the same reaction conditions resulted in complete conversion of monomer with good control over the 10238


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controlled manner.440,441 However, decreasing the catalyst loading further to 25 ppm resulted in the loss of control. Matyjaszewski and co-workers along with others investigated the photoinduced ATRP under visible or sunlight irradiation with ppm of Cu catalyst without the use of any conventional photoinitiator or reducing agent.442,443 A series of ligand components including PMDETA, tris(2-pyridylmethyl) amine (TPMA), and its modified counterpart tris((4-methoxy-3,5dimethylpyridin-2-yl)methyl)amine (TPMA*) was used to study the effect of the ligand. Mechanistically, photoinduced electron transfer of CuII/L on irradiation resulted in photoreduction to CuI and a halogen radical. In the presence of an ATRP initiator, the formed CuI activator initiated the ATRP process, while halogen radicals formed during photoreduction were also able to initiate chain polymerization. Irradiation was conducted using various light sources. Polymerizations at 392 nm with LED provided a faster polymerization rate and wellcontrolled results as compared to the 450 nm. Sunlight irradiation also resulted in a well-controlled process, however, irradiations at 632 nm failed to initiate the polymerization, indicating inefficiency of the photoactivation at low energy ranges. Furthermore, in a recent study investigating the applicability of such photocatalytic system in aqueous media, the authors found that utilization of a low amount of catalyst resulted in a poor-controlled process, though high concentrations of CuII catalyst gave well-defined polymers.444 The poor control over the system in low concentrations of catalyst was attributed to the poor stability of the CuII deactivator in aqueous media, which could be overcome by supplementing additional halide-containing salt to the system. As a result, utilizing the CuII/TPMA catalyst as low as 22 ppm with halide salt resulted in a well-controlled process yielding water-soluble polymers with narrow molecular weight distributions and high chain end fidelity in aqueous media. Indeed, many different interactions are conceivable in such photochemical systems. Therefore, to elucidate the exact mechanism of the photochemical ATRP would require detailed consideration of every possible pathway. The group of Matyjaszewski and co-workers thus set up a detailed study of the mechanism of the photogeneration of activators.445 The contribution and influence of every component present in the system that participates in photochemical reactions leading to the generation of the activator catalyst and finally initiation of polymerization was investigated. It was found that the use of equivalent amounts of ligand with respect to Cu failed to reduce CuII species under irradiation at 390 nm and, subsequently, no polymerization was achieved. However, supplementing the ligand in excess amounts with respect to Cu (with a 6:1 ratio) induced the reduction of CuII under irradiation and polymerization was then successfully initiated in a well-controlled manner. This further confirmed the necessity of the presence of uncoordinated ligand species for successful CuII reduction and initiation of polymerization. Therefore, they concluded that the failure of the system to initiate polymerization where no free ligands were available in the reaction media, rules out the possibility of the photolysis of the CuII complex to CuI and the halogen radical as the dominant reaction pathway. The involvement of free, uncoordinated amine ligands was responsible for the photoinitiation processes. This was further proved by a control experiment using a copper(II) triflate (CuII(OTf)2) catalyst instead of CuBr2 with the excess amounts of ligand available. The reaction in the presence of the CuII(OTf)2 catalyst gave similar results to those

obtained utilizing the CuBr2 system. Since there was no Cuhalogen bond in CuII(OTf)2 to cleave, this observation further ruled out the homolytic Cu halogen bond cleavage upon irradiation as the only dominant mechanism. Interestingly, the nature of the excess amounts of the amine ligand was found to be independent of the type of the amine ligand. Rather it was the concentration of the tertiary amine groups that was important to promote photochemical initiation of ATRP. For this purpose, TEA was used instead of excess amounts of Me6TREN, and almost the same polymerization results were achieved in both cases. Together with experimental results and simulation studies, they drew various photochemical pathways with different contributions in the photoinitiation mechanism. The dominant reduction mechanism was suggested to be through free tertiary amine species (either uncoordinated tertiary amine ligand or other tertiary amine compounds) by a photoinduced electron transfer mechanism generating the CuI complex and amine-centered radical cation. This mechanism resembled a photochemically mediated ARGET ATRP process. A proton transfer by the amine radical cation was suggested to form protonated amine and free radical species capable of initiating polymerization.446 Photochemical generation of radicals by alkyl halide, ligand, or ligand and monomer was also suggested to be involved in the initiation of the reaction though with very low portions. This mechanism was similar to the photochemical ICAR ATRP process. The general proposed mechanism is depicted in Scheme 30, which was suggested to Scheme 30. Proposed Mechanism of Photoinitiated ATRP by Matyjaszewski and Co-Workers

predominantly involve photochemically mediated ARGET ATRP plus a small contribution by photochemical ICAR ATRP processes. Control over the reaction was established by activation−deactivation reactions as in classical ATRP. With the experimental results and kinetic simulations, the authors were able to evaluate the contribution of these reaction pathways during the photochemical ATRP process. As explained in the text, the dominant reaction between the CuII and free amine ligands contributes 90% to the radical (re)generation and Cu reduction. The direct formation of radicals involving alkyl halide or ligand was calculated at about 1%. The contribution of reaction between ligand and macromolecular alkyl halide was found to be 8% of the activator regeneration. Figure 1 shows the fraction contributions of the explained mechanisms. Haddleton and co-workers developed an elegant approach for the synthesis of functional polymers via photoinitiated LRP.447 A CuBr2/Me6TREN complex was used as the catalyst, which under irradiation at UV or even visible light was found to photochemically initiate and control the polymerization of a 10239


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UV−vis spectroscopy was used to investigate the behavior of the polymerization system under irradiation. The CuBr2/ Me6TREN (1:6) solution in DMSO exhibited a strong characteristic absorption band at 950 nm as well as 750 nm, which was attributed to the d-d transitions of the d9 CuII complex. Irradiation of this solution under UV light for 90 min resulted in significant reduction of the intensity of the absorption bands, indicating the successful reduction of the CuII complex. Approximately about 75% reduction of CuII was achieved. However, continuing irradiation resulted in no further reduction of the CuII absorption band. Even after long irradiation times (up to 72 h), it remained as such. Moreover, changing in the experimental conditions had effects on the optical behavior of the system. For example, in the presence of alkyl halide, no reduction was detected after a 90 min irradiation. This behavior was attributed to two possible mechanisms. It was suggested that in the presence of alkyl halide it could undergo interaction with the reduced CuI complex, thus oxidizing it back to the initial CuII state. Moreover, the photoexcited free ligand species was suggested to preferably interact with the alkyl halide instead of inducing the reduction of CuII; this was a more reasonable explanation because in the presence of monomer almost the same behavior was detected (there was no significant reduction in the absorption of CuII) (Figure 2). So based on UV−vis spectroscopic investigations and systematic control experiments, they proposed an initiation mechanism based on the photoexcitation of the free, uncoordinated Me6TREN species which was followed by interaction with the alkyl halide. The reaction of the photoexcited Me6TREN with the alkyl halide was through an outer-sphere-single-electron-transfer mechanism resulting in the homolysis of the alkyl halide. This further gave rise to the formation of the required initiating radical and Me6TREN radical cation species with a Br− counterion. While initiating polymerization, the resultant radicals were proposed to interact with CuII species, reducing them to CuI and

Figure 1. Fraction contributions of activator regeneration from each reaction considered for the simulated polymerization of methyl acrylate with CuBr2/Me6TREN irradiating with 392 nm LEDs. Reprinted from ref 445. Copyright 2014 American Chemical Society.

series of vinyl monomers. Irradiating a solution of CuBr2/ Me6TREN in DMSO (50%) containing the acrylate monomer in the presence of alkyl halide under visible light initiated the polymerization in a well-controlled manner. An induction period of 3 h was observed, after which a linear relationship between monomer consumption and reaction time demonstrated a well-controlled/living process. Remarkably, the rate of the polymerization process was considerably enhanced when irradiating the system under UV light at 360 nm. Polymerizations proceeded within a 15 h period under daylight, whereas using the artificial UV light, the system was reached to completion within 90 min irradiation. No polymerization was achieved when the CuBr2/Me6TREN catalyst was used in a 1:1 ratio. However, increasing the ratio of the Me6TREN specie from 1 to 2, 3, and 6 (with respect to CuBr2) successfully initiated and controlled polymerization of acrylates. This indicated that the presence of free ligand compounds were essential for the reduction and initiation of polymerization.

Figure 2. UV−vis changes of the CuBr2/Me6TREN system under various conditions. Reprinted from ref 447. Copyright 2014 American Chemical Society. 10240


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affording dormant polymer chains. The reduced CuI complex was believed to undergo some possible reactions, including reaction with amine radical cations or disproportionation processes. The proposed mechanism is depicted in Scheme 31. The effect of ligand was crucial in that excess amount of the

initiated LRP process using high-resolution mass spectrometry technique. 450 Combined pulsed-laser polymerization (PLP)451−453 technique and electrospray-ionization mass spectrometry (ESI-MS)454 were used to initiate the polymerization and analyze the resultant compounds, respectively. Each of the participating compounds, namely CuBr2/Me6TREN and ethyl α-bromoisobutyrate (EBiB), was independently irradiated in a solution of MMA in DMSO to draw a clear idea of whether and to what extent they participate in the photoinitiation process. Due to the low monomer conversions obtained, the authors were able to give an insight into the photoinitiation and earlier stages of polymerization processes. No initiation was observed when irradiating MMA alone in DMSO. However, pulse-laser irradiation of EBiB in MMA/DMSO ([EBiB]: [MMA] = 1:50) in the absence of Cu catalyst initiated the polymerization of MMA. A C−Br bond cleavage of the ATRP initiator upon irradiation was suggested to the formation of initiating radicals. Analysis of poly(methyl methacrylate) (PMMA) formed in this course revealed that the polymerization was initiated by both alkyl and bromine radicals. All compounds detected in ESI-MS are shown in Scheme 32. Irradiation of Me6TREN in a solution of MMA/DMSO and in the absence of Cu and ATRP initiator also initiated polymerization. Mass spectroscopy analysis confirmed polymerization initiated by Me6TREN radicals as well as radicals formed on MMA. Photoexcited Me6TREN species were able to abstract a hydrogen atom from the α-position of the ligand, forming an initiating free radical. On the other hand, this hydrogen abstraction could well be achieved by MMA, leading to the formation of initiating free radicals on MMA. Moreover, irradiation of a CuBr2/Me6TREN solution in MMA/DMSO revealed that the polymerization was exclusively initiated by Me6TREN radicals, and no initiation by MMA radicals (as in the former case) was detected. The initial green color of the CuII solution turned transparent after the irradiation, indicating the successful reduction of CuII complexes to CuI. Photoexcited Me6TREN species instead of abstracting a hydrogen atom from MMA were shown to reduce CuII to CuI by electron transfer reactions and form amine radical cations as well as bromine radicals, which could each contribute to the initiation or reduction processes. Irradiation in the presence of EBiB, Me6TREN, and MMA in DMSO, but in the absence of CuBr2,

Scheme 31. Proposed Mechanism of Photoinitiated LRP by Haddleton and Co-Workers

ligand was required to initiate the polymerization. More importantly, using TREN which was similar to Me6TREN in structure gave almost the same result as that of Me6TREN. However, a loss of control was observed in the case of PMDETA as the molecular weight distribution of the polymers were quite high with respect to Me6TREN (1.27 and 1.05, respectively) with low monomer conversions (48% vs 98%, respectively). And no polymerization was initiated in the presence of bpy, indicating the crucial role of aliphatic tertiary amine groups in the photoinitiation process. The same group has also designed new Cu catalyst precursors, including copper(II) formate448 and copper(II) gluconate,449 to conduct photoinduced LRP. The mechanism was proposed to follow reduction of CuII by similar photoinduced electron transfer processes of counteranion species acting as reducing agents with CuII to activate the catalyst and initiate the polymerization. Recently, Barner-Kowollik, Haddleton, and co-workers attempted to elucidate the exact mechanism of the photo-

Scheme 32. Chemical Structures Found in ESI-MS, Resulting from Various Experimental Conditions

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initiated the polymerization by alkyl and Me6TREN radicals, whereas no polymers were detected polymerized by bromine or MMA radicals. The bromine radicals were suggested to participate in termination or hydrogen abstraction from the amine. Interestingly, upon irradiation of EBiB, CuBr2, and MMA in DMSO in the absence of Me6TREN ligand, no polymerization was observed. The formed alkyl and bromine radicals were believed to rapidly interact with the CuII reducing it to CuI. While reducing CuII to CuI, EBiB and elemental bromine (Br2) were also formed under these conditions. The initial green color of the solution of CuBr2 turned to pale yellow, indicative of the reduction of CuII and formation of Br2 species. Finally, in the presence of all components (i.e., MMA, EBiB, CuBr2, and Me6TREN in DMSO solution), the polymerization was initiated only by the alkyl radicals and no polymers initiated by bromine, Me6TREN, or MMA radicals were detected. A well-controlled polymerization process was established under these conditions with polymers having narrow molecular weight distributions (350 nm undergoes conversion to the unmasked strained cyclooctyne species, which thereafter can react in the above-described Cu-free azide cyclooctyne click reaction (Scheme 47).

applicability of the photoinduced thiol−ene click reaction toward the macromolecular synthesis of complex polymeric architectures. Hawker and co-workers have demonstrated the thiol−ene click reaction’s utility for rapid complex dendrimer formation.627−630 Du Prez and co-workers have used the thiol− ene click reaction to fashion diblock and star-shaped polymers.571 The photoinduced thiol−ene click reaction relies on the use of a photoinitiator to provide radical species upon light irradiation that can then bring about a radical hydrogen abstraction which in turn leads to a thiol radical species which can undergo the coupling process as can be observed in Scheme 49.

Scheme 47. Photoinduced Cu-Free Cyclooctyne-Azide Click Reaction

Scheme 49. Photoinduced Thiol−ene Reaction Mechanism

7.2. Thiol−Ene/Yne Click Chemistry

Thiol−ene/yne reactions bear all the characteristics required to be termed click chemistry. They are high yielding under mild conditions producing highly regioselective products exhibiting largely anti-Markovinkov selectivity, and their work-ups are hassle-free with no chromatography required to remove small molecule side-products. The reactants involved are tolerant to a variety of functional groups lending the process with orthogonality with respect to other chemical transformations. They are carried out with benign catalysts and solvents and can often be insensitive toward molecular oxygen (Scheme 48).

Our group has thoroughly investigated the influence of the type of photoinitiator on the photoinitiation of thiol−ene click chemistry for functionalization of polymers.61 Thiol or allylmodified polystyrenes with controlled molecular weight properties were successfully functionalized by an appropriate alkene or thiol functionality in the presence of either Type I or Type II photoinitiators with excellent yields achieved in both systems. Type I photoinitiation appeared as slightly more efficient than the other one as higher conversions were obtained by Type I photoinitiators. Thiol groups also served as electron/proton transfer co-initiator for Type II photoinitiators.631−633 The photoinduced thiol/ene has been recently employed by the group of Bowman and co-workers for the synthesis of sequence defined polymers.634 A series of nucleobase functional monomers were polymerized using thiol click reactions to form polymers in a sequence-specific manner. The monomers were structurally analogous to oligonucleotides in DNA and comprised of a thiol and an allyalmine or a thiol and an acrylamide functional groups suitable for thiol−ene click or thiol-Michael reactions, respectively. Thiol−ene photopolymerization was used to form nucleobase-containing sequencecontrolled homo polymers as well as diblock copolymers. A combination of thiol−ene and thiol-Michael approaches led to more specific and robust strategies in synthesizing DNA-like macromolecules such as organogels with reversible cross-links. The process allowed incorporating a wide range of functional groups into the polymer structure in a sequence-controlled manner. Recently investigations have taken place with regards to the photoinduced electron transfer thiol−ene reaction photocatalyzed by photoredox catalysts.635−637 For example, when employing Ru(bpz)3(PF6)2 as the photoredox catalyst, it is possible to carry out thiol−ene chemistry with visible light

Scheme 48. Addition of Thiol to An Alkene Moiety by Either Radical or Anionic Catalytic Processes

Thiol−ene/yne chemistry is defined as the addition of a primary thiol radical species to an alkene or alkyne moiety. The reaction can proceed by two separate mechanisms, either by the free radical mediated or Michael addition processes. When the free radical approach is used, the transformation is labeled the thiol−ene/yne reaction; however, when the transformation is catalyzed and proceeds through an anionic pathway, it is labeled the thiol Michael addition. The photoinduced thiol−ene/yne click reaction has now become well-established. It lends all the expected advantages of a photochemical process to the reaction such as mild reaction conditions and spatiotemporal control. Groups such as Hoyle,618 Bowman,619,620 Lowe621 and others622−626 greatly expanded the sphere of knowledge as it applies to the 10256


Chemical Reviews

Review

8. CONCLUSION AND FUTURE PERSPECTIVE Learning to mimic the natural phenomena that drive chemical transformations will significantly advance our understanding of and ability in constructing structurally complex building blocks and molecules in all areas of science. Every moment that passes by, an intricate set of chemical reactions are taking place in living organisms using powerful yet simple toolkits provided by nature that are able to assemble the essential complexity of life. Photosynthesis is an intriguing example of such complex chemical transformations. It smoothly occurs in plants and is based on the efficient utilization of the energy of sunlight as it is converted into chemical energy. This process involves a series of photoinduced electron transfer processes that result in products that are the essential components of fueling life on Earth. We are yet to completely master and mimic this process in order to reach its efficiency and complexity. Nevertheless, we have made significant strides, and in recent years, technological advancements have enabled us to establish molecular complexity through artificial molecular tailoring.640 Photochemical reactions, particularly those involving photoinduced electron transfer reactions, establish a substantial contribution to the modern synthetic chemistry, and the polymer community has been increasingly interested in exploiting and developing novel photochemical strategies. As described in detail throughout this review, these reactions are readily utilized in almost every aspect of macromolecular architecture synthesis involving initiation, control of the reaction kinetics and molecular structures, functionalization, and decoration, etc. Chain growth polymerizations including different modes of free radical and cationic polymerizations have long been established and expanded upon substantially, together with interesting contributions in step-growth polymerization of various systems. Merging with controlled/living polymerizations, photochemistry has opened up new fascinating and powerful avenues for macromolecular synthesis. Construction of various polymers with incredibly complex structures and specific control over the chain topology, as well as providing the opportunity to manipulate the reaction course through spatiotemporal control, is one of the unique abilities of such photochemical reactions. Two-photon polymerization microfabrication is a very exciting development which avails of the spatial control potential of photochemical polymerization used for 3D printing with ultrafine microresolution.641 This has already found application in such fields as micro- and nanophotonics, microelectromechanical systems, microfluidics, biomedical implants, and microdevices. Photoinitiating systems for controlled polymerizations which focus on low energy, longer wavelength visible light, and even sunlight have grown in prevalence. Previously, these processes have been centered on ATRP and the photoredox catalyst approach using Cu and Ir. More recently for both cationic and free radical promoted cationic photopolymerization, the use of cheap low-energy LEDs (blue, red, green, and purple laser diodes) has been implemented.

irradiation and a catalyst loading of typically 0.25 mol % with quantitative yields obtained in certain cases. Direct photooxidation of thiols with the photocatalyst led to thiyl radicals through photoinduced electron and proton transfers promoting the click reaction; however, this direct path was found to be slow in rate. The reduced state Ru(bpz)3+ could be regenerated in the presence of oxygen to initiate further chain processes. The rate of the reaction could be effectively accelerated by employing aromatic amines such as anilines as electron-transfer mediators between the photocatalyst and thiol component. The amine mediators were oxidized to the corresponding amine radical cation by an excited state photocatalyst. These amine radical cations were capable of interacting with thiols via consecutive electron- and proton-transfer reactions generating thiyl radicals (Scheme 50). Scheme 50. Photoredox Catalyzed Thiol−Ene Reaction

The authors of the above work highlighted the photoredox catalyzed thiol−ene reaction’s excellent efficiency, where 99% yield was obtained for many substrates. Also it was shown to be large in scope where the system could be applied to a variety of thiols and alkenes with positive results under mild conditions using a visible wavelength light source. The photoredox thiol−ene system has also been applied to macromolecular architectures for polymer postfunctionalization and step-growth addition polymerization (Scheme 51).638,639 It was reported that polybutadiene and poly(allyl methacrylates) could be postfunctionalized with thiol side groups by photoredox thiol−ene reaction with high conversion obtained within minutes, typically times of

Photoinduced Electron Transfer Reactions for Macromolecular

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