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Photoredox Controlled Living Polymerization

GONG Honghong MA Mingxuan ZHOU Yang ZHAO Yucheng GU Yu CHEN Mao

引用本文:
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Photoredox Controlled Living Polymerization

    作者简介: Professor Chen Mao was born in Chongqing, China. After receiving the B.S. degree at Wuhan University, he undertook his Ph.D. at the same place under the supervision of Prof. Lei Aiwen and Prof. Zhang Xumu. Later, he joined Prof. Stephen L Buchwald's group at MIT from 2012 to 2014. In Oct 2014, he joined Prof. Jeremiah A Johnson's group at the same place, and was promoted to research scientist in June 2016. Now, he is working as a Thousand-Talent Professor at Fudan University, and his group name is PolyMao (http://chenmaofudan.wixsite.com/polymao). The main research directions of PolyMao group include exploration of novel polymerization methodologies based on photochemistry and late transition metal catalysis; development of continuous-flow technologies to facilitate automated and efficient polymer production; design of smart materials via the combination of polymer chemistry and organic synthesis.
    通讯作者: CHEN Mao, chenmao@fudan.edu.cn
  • 基金项目:

     2018M63200

     21704016

  • 中图分类号: O632;O633.1

Photoredox Controlled Living Polymerization

    Author Bio: Professor Chen Mao was born in Chongqing, China. After receiving the B.S. degree at Wuhan University, he undertook his Ph.D. at the same place under the supervision of Prof. Lei Aiwen and Prof. Zhang Xumu. Later, he joined Prof. Stephen L Buchwald's group at MIT from 2012 to 2014. In Oct 2014, he joined Prof. Jeremiah A Johnson's group at the same place, and was promoted to research scientist in June 2016. Now, he is working as a Thousand-Talent Professor at Fudan University, and his group name is PolyMao (http://chenmaofudan.wixsite.com/polymao). The main research directions of PolyMao group include exploration of novel polymerization methodologies based on photochemistry and late transition metal catalysis; development of continuous-flow technologies to facilitate automated and efficient polymer production; design of smart materials via the combination of polymer chemistry and organic synthesis.
    Corresponding author: CHEN Mao, chenmao@fudan.edu.cn
  • CLC number: O632;O633.1

  • 摘要: 近几年,光致氧化还原调控的可控自由基聚合得到了迅速发展,其适用单体范围广、反应条件温和,为合成聚合物和功能高分子材料提供了新方法。本综述对光致氧化还原调控活性聚合进行了总结与探讨,归纳整理了多种单体聚合的最新研究进展,为研究人员探索光催化聚合反应、设计合成功能高分子材料提供了新思路。
  • Figure 1.  Reaction mechanism for photo-controlled redical polymerization[25]

    Figure 2.  Selected photoredox catalysts used in photoredox-controlled radical polymerization of (meth)acrylates and (meth)acrylamides

    Figure 3.  Heterogeneous photoredox gel catalyst used in logical-controlled radical polymerization

    Figure 4.  Photoredox-controlled radical polymerizations of (meth)acrylates and (meth)acrylamides applied in different directions

    Figure 5.  One-pot synthesis of ABCDE multiblock copolymers with various segments

    Figure 6.  Automated synthesis of biohybrids using a DNA synthesizer

    Figure 7.  Photoredox-controlled radical polymerization of semifluorinated (meth)acrylates

    Figure 8.  Photoredox-controlled radical polymerization of vinyl ketones

    Figure 9.  Controlled cationic/radical polymerization with photoredox catalysts

    Figure 10.  Metal-free ring opening metathesis polymerization driven by light

    Figure 11.  Photoredox-controlled ring-opening polymerization of O-carboxyanhydrides

    Table 1.  Photoredox catalysts used in photoredox-CRP1)

    PC Eox* (PC·+/PC*) Eox(PC·+/PC) τf2)/ns τISC3)/μs φf2)/% φISC3)/% ref.
    Ir(ppy)3 -1.73 0.77 1.9 100 [49]
    Ir(ppy)3-2 -0.89 1.69 2.3 68 [75]
    Ru(bpy)32+ -0.81 1.29 1.1 100 [76]
    EY4) -1.60 0.72 2.1 48 32 [77]
    RB4) -0.68 1.09 0.5 9 77 [78]
    Perylene -1.87 0.98 5.5 5 000 3.6 2 [79-81]
    Ph-PTZ -2.10 0.68 4.5 420 1 [57, 70]
    4CzIPN -1.06 1.50 18 [82-85]
    BPBB-PTZ5) -1.94 0.76 4.7 61 [72]
    DN-DHPZ -1.64 0.23 [86-87]
    DBN-PXZ -1.80 0.65 480 [85, 88]
    1) Reduction potentials (V vs. standard calomel electrode (SCE)) of photoredox catalysts in MeCN; 2) Singlet state was determined at 298 K; 3) Triplet state was determined at 77 K; 4) Determined in methanol; 5) Determined in DMF; Ir(ppy)3-2: [Ir(dF(CF3)ppy)2(dtbbpy)](PF6); EY: Eosin Y; RB: Rose Bengal; 4CzIPN: 2, 4, 5, 6-tetra(9H-carbazol-9-yl)isophthalonitrile; BPBB-PTZ: 10-([1, 1′-biphenyl]-4-yl)-3, 7-bis(4-butylphenyl)-10H-phenothiazine; DN-DHPZ: 5, 10-di(naphthalen-1-yl)-5, 10-dihydrophenazine; DBN-PXZ: 3, 7-di([1, 1′-biphenyl]-4-yl)-10-(naphthalen-1-yl)-10H-phenoxazine
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  • 收稿日期:  2019-01-07
  • 刊出日期:  2019-06-01

Photoredox Controlled Living Polymerization

    通讯作者: CHEN Mao, chenmao@fudan.edu.cn
    作者简介: Professor Chen Mao was born in Chongqing, China. After receiving the B.S. degree at Wuhan University, he undertook his Ph.D. at the same place under the supervision of Prof. Lei Aiwen and Prof. Zhang Xumu. Later, he joined Prof. Stephen L Buchwald's group at MIT from 2012 to 2014. In Oct 2014, he joined Prof. Jeremiah A Johnson's group at the same place, and was promoted to research scientist in June 2016. Now, he is working as a Thousand-Talent Professor at Fudan University, and his group name is PolyMao (http://chenmaofudan.wixsite.com/polymao). The main research directions of PolyMao group include exploration of novel polymerization methodologies based on photochemistry and late transition metal catalysis; development of continuous-flow technologies to facilitate automated and efficient polymer production; design of smart materials via the combination of polymer chemistry and organic synthesis
  • State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200438, China
基金项目:   2018M63200 21704016

摘要: 近几年,光致氧化还原调控的可控自由基聚合得到了迅速发展,其适用单体范围广、反应条件温和,为合成聚合物和功能高分子材料提供了新方法。本综述对光致氧化还原调控活性聚合进行了总结与探讨,归纳整理了多种单体聚合的最新研究进展,为研究人员探索光催化聚合反应、设计合成功能高分子材料提供了新思路。

English Abstract

  • Photoredox controlled polymerization (photoredox-CP) involves the process of photosynthesis which is ubiquitous in nature and used to convert solar energy into chemical energy. Inspired by such a counterpart in nature, a variety of artificial photoreaction systems have been developed[1-3]. It is an interesting and meaningful attempt for chemists to covert solar energy into chemical energy stored in chemical bonds without extra energy conversion. With efforts of many years, photoredox catalysts (PCs) have been extensively studied for applications in inorganic, organic and material chemistry[4-5]. Particularly, photoredox catalysis has been developed as one of the most useful methods for the energy transition in processes such as water splitting[6-7] and carbon dioxide reduction[8]. Photoredox catalysts have also been applied in organic synthesis[9-11], dye-sensitized cells[12], organic light-emitting diodes[13-14] and photodynamic therapy[15].

    Recently, inspired by the seminal work performed by the groups of Macmillan[9], Stephenson[10] and Yoon[11] on the photoredox catalyzed organic synthesis, people have employed the photoredox strategy to promote polymerization with PCs exposing to light irradiation. Thanks to contributions made by polymer chemists all over the world, photoredox-CP derivatives of living radical polymerization (LRP), ring-opening metathesis polymerization (ROMP) and cationic polymerization have been accomplished[16-26]. It has brought many advantages to these reactions, such as spatiotemporal control, narrow molecular weight distributions (MWDs) at high conversions, high chain-end fidelity and mild reaction conditions etc.

    Indeed, photo-controlled radical polymerizations (photo-CRPs) are regarded as the earliest known CRP reactions[16-26]. From the mechanism perspective, many established photo-CRP reactions are based on a direct photochemical process (Fig. 1(a), i.e., photoinitiated carbon-heteroatom[27-31], carbon-metal[32-33], carbon-metalloid[34-35], or carbon-carbon[36] bond cleavages and others[37-38]). Organic/organometallic compounds of initiators/iniferters are directly used to absorb light and trigger the homolysis of the covalent bond upon photoexcitation[25]. The generated radical species can in turn initiate propagation[16-26].

    Figure 1.  Reaction mechanism for photo-controlled redical polymerization[25]

    In contrast, for example, in a photoredox-CRP (Fig. 1(b)), PC is employed to absorb light. After reaching its photoexcited state PC*, a photoinduced single-electron transfer from PC* to initiator/iniferter is normally proceeded to stimulate the generation of radical species, which can further initiate propagation. In a photoredox process, there's no need for an initiator/iniferter to directly absorb light. The undesired side reactions under UV light irradiation can be minimized, allowing the synthesis of polymers with high chain-end fidelity. By changing the types of PCs, different light sources, even having emission at the near-infrared/far-red wavelength[39], can be utilized in the photoredox-CP reaction. Moreover, benefiting from the nature of PCs, photoredox-CPs featuring with different advantages such as metal free, oxygen tolerance, mechanism interchange and others have been established in recent years.

    As compared to photoinitiated systems without PCs, although a similar scope of initiators/iniferters (i.e., activated halides, perfluoroalkyl iodides, thiocarbonylthio compounds, trithiocarbonates) has been used in the photoredox systems, these alternative methods provide new approaches to polymer materials for various applications, such as surface fabrication, smart material engineering, biologically relevant transformation and so on.

    Since a number of excellent reviews on photopolymerization have been published[16-26], this review only highlights achievements of photoredox-CR reported from 2016 to present. Photo-initiated/controlled polymerizations from the perspectives of reaction mechanisms, reagents/catalysts, polymer frameworks, or applications have been summarized in pervious reviews, thus in this review we classify examples into different categories according to the used monomers.

    • (Meth) acrylates and (meth)acrylamides are derivatives of (meth)acrylic acid. They are typical monomers commonly used in free radical polymerization. Related polymers have found wide-spread applications in areas such as coating, adhesive, fiber, thermal plastic and so on. For example, poly(methyl methacrylate) (PMMA), due to the property of high light transmission, low cost and easy preparation, has been widely used in building, auto industry and optical device. The classical CRPs such as ATRP (atom transfer-radical polymerization)[40-41], RAFT (reversible addition-fragmentation chain-transfer) polymerization[42-45] and NMP (nitroxide mediated polymerization)[46-48] have provided elegant ways to access well-defined polymers of such monomers.

      While breakthroughs in photoredox-CRP methods with visible-light control[49-53], oxygen tolerance[50, 54-56], low/no metal contamination[57-59] and others[60-68] have been reported in the begining of 2010s. Recent efforts for the transformation of (meth)acrylates and (meth)acrylamides have mainly focused on the further development of metal-free systems, understanding related reaction mechanisms, synthesizing complex polymers, designing interesting reaction systems, as well as broadening the applications.

      In 2014, the groups of Hawker[57-58] and Miyake[69] reported the employment of phenylphenothiazine (Ph-PTZ) and perylene based organic PCs in the metal-free photoredox-ATRP reactions (Fig. 2). Matyjaszewski and co-workers[70] have recently provided a deeper understanding on the mechanism of metal-free ATRP with phenothiazine catalysts. Based on the dissociative electron-transfer (DET) theory, kinetic analysis of the activation exposing to light suggests that the activation step only occurs with a catalyst at its excited state. Supported by results from density functional theory (DFT), an associative electron transfer with a termolecular encounter is occurred in the deactivation step. To facilitate the metal-free ATRP under visible-light irradiation, they developed a new catalyst of phenyl benzo[b]phenothiazine[71]. Although the obtained molar weight distributions (MWD, D=1.3-1.7) are broader than the standard ATRP methods, the photoredox-ATRP displays switchable behavior as demonstrated by a four-cycle on/off experiments and the Mn increases linearly with conversions.

      Figure 2.  Selected photoredox catalysts used in photoredox-controlled radical polymerization of (meth)acrylates and (meth)acrylamides

      Very recently, Chen and co-workers[72] have further developed a number of PCs also based on the phenothiazine ring. They found that by substituting the phenothiazine core with multiple phenyl groups, much stronger visible-light absorptions are provided to the PCs without obvious change of their reductive potentials at the excited states. Moreover, the quantum yield of the excited catalyst is increased up to 0.61(Table 1). With these catalysts, the photoredox-CRP from aryl sulfonyl halide initiators driven by visible light (purple or white LED) has been successfully realized, allowing the preparation of poly(meth)acrylates and poly(meth)acrylamides possessing a broad scope of (hetero)aryl chain ends without metal-contamination concern[73-74]. Importantly, this method theoretically enables grafting from aromatic C—H bonds via an electrophilic aromatic substitution/organocatalyzed photoredox-CRP sequence, which would facilitate the functionalization of aromatic commodities with polymers.

      PC Eox* (PC·+/PC*) Eox(PC·+/PC) τf2)/ns τISC3)/μs φf2)/% φISC3)/% ref.
      Ir(ppy)3 -1.73 0.77 1.9 100 [49]
      Ir(ppy)3-2 -0.89 1.69 2.3 68 [75]
      Ru(bpy)32+ -0.81 1.29 1.1 100 [76]
      EY4) -1.60 0.72 2.1 48 32 [77]
      RB4) -0.68 1.09 0.5 9 77 [78]
      Perylene -1.87 0.98 5.5 5 000 3.6 2 [79-81]
      Ph-PTZ -2.10 0.68 4.5 420 1 [57, 70]
      4CzIPN -1.06 1.50 18 [82-85]
      BPBB-PTZ5) -1.94 0.76 4.7 61 [72]
      DN-DHPZ -1.64 0.23 [86-87]
      DBN-PXZ -1.80 0.65 480 [85, 88]
      1) Reduction potentials (V vs. standard calomel electrode (SCE)) of photoredox catalysts in MeCN; 2) Singlet state was determined at 298 K; 3) Triplet state was determined at 77 K; 4) Determined in methanol; 5) Determined in DMF; Ir(ppy)3-2: [Ir(dF(CF3)ppy)2(dtbbpy)](PF6); EY: Eosin Y; RB: Rose Bengal; 4CzIPN: 2, 4, 5, 6-tetra(9H-carbazol-9-yl)isophthalonitrile; BPBB-PTZ: 10-([1, 1′-biphenyl]-4-yl)-3, 7-bis(4-butylphenyl)-10H-phenothiazine; DN-DHPZ: 5, 10-di(naphthalen-1-yl)-5, 10-dihydrophenazine; DBN-PXZ: 3, 7-di([1, 1′-biphenyl]-4-yl)-10-(naphthalen-1-yl)-10H-phenoxazine

      Table 1.  Photoredox catalysts used in photoredox-CRP1)

      Phenazines and phenoxazines have been found in natural products. Synthesized phenazines and phenoxazines have been studied for their antitumor, antibiotic, antifungal activities, as well as their potential applications in OLED, dye-sensitized solar cells and so on[89]. Upon using perylene as a catalyst, Miyake and coworkers[89] have developed two new series of PCs based on dihydrophenazines and phenoxazines. When the modified dihydrophenazines[87] and phenoxazines[90] were employed in the photoredox-ATRP (white LED), polymers with MWD (D=1.1-1.4) from low to moderate levels were achieved using activated alkyl bromide initiators under visible-light irradiation. The photo-switchable nature with both types of PCs was verified by on/off experiments accomplished through periodically exposing reactions to light.

      Miyake's group[90] also systematically investigated the differences of photoredox catalysts. Although phenothiazines, dihydrophenazines and phenoxazines have shown comparable strong reductive potentials (~2 V vs SCE) at their lowest lying triplet excited state, phenothiazines adopt a bent boat conformation at the ground and triplet states, while dihydrophenazines and phenoxazines constantly adopt planner conformations. However, all these three types of catalysts presented the planner conformations at the radical cation states as supported by calculation, suggesting a higher reorganization energy is required for phenothiazines during the single-electron transfer process. Moreover, when the triplet excited state of 1-naphthalene substituted phenothiazine, dihydrophenazine and phenoxazine were mapped with the electrostatic potentials, related results suggest that electron density is transferred to the naphthalene substituent from either dihydrophenazine or phenoxazine core upon photoexcitation, while electron density still localizes on the phenothiazine heterocycle[90]. The observed charge transfer from ground to excited state is reminiscent of the metal-to-ligand charge transfer[9] observed in Ir- and Ru-based photoredox complexes, and is suggested to be favorable for the photoredox-CRP.

      Due to the electron-rich nature, conjugated thiophene derivatives were employed as electron donors upon photoexcitation. Yagci and co-workers[91] reported photoredox-ATRP from ethyl 2-bromoisobutyrate in the presence of thienothiophene derivatives (TT-TPA), resulting in PMMA with Mn=1.53×104-1.20×105 kDa and D=1.33-1.83. These thienothiophene PCs cover the UV and visible spectral regions below 500 nm. Although cyclic voltammetry information is not provided for these catalysts, DFT calculation results suggest that the reduction of the initiator is favorable from the excited singlet state species. The dependency of this reaction on light irradiation was proved by periodically switching this reaction between light on and off for 3 times.

      An and coworkers[64] developed a supramolecular PC, which was made up of cucurbit[7]uril and Zn(Ⅱ) meso-tetra(4-naphthalylmethylpyridyl) porphyrin to form a host-guest complex (CB[7]@ZnTPOR), preventing aggregation of ZnTPOR in water. It provides an interesting example of using supramolecular interaction to regulate the reaction rate of photo-CP using trithiocarbonate (TTC) iniferters.

      Heterogeneous catalysts are widely used in academia and industry. When they are used in photochemistry transformations, the photoexcitation only occurs on the surface of particles where light can easily reach. For a place where light cannot penetrate, there's no photoexcitation, not to mention temporal control with such catalysts. Johnson and co-workers[92] recently developed a gel photoredox catalyst based on polymeric network, allowing phenothiazine rings covalently bonded on to a gel (Fig. 3). Because the major composition of this gel is made of poly(N-isopropylacrylamide) (PNIPAAM), the gel PC shows lower critical solution temperature (LCST) behavior in water. It introduces multiple external switches including temperature "low/high", light "on/off" and catalyst presence "in/out" to achieve logic gating of CRP reactions. While the new gel catalyst is able to achieve CRP under heterogeneous conditions, facilitating catalyst recycle, it also offers a straightforward approach to realize multiplexed external switching of polymerization without necessary addition of exogenous reagents. With the TTC units enriched polymer network, Johnson's group[93-95] has also developed photo-controlled smart materials with life like properties of reproduction based on photoredox polymerization in the presence of Ph-PTZ catalyst.

      Figure 3.  Heterogeneous photoredox gel catalyst used in logical-controlled radical polymerization

      Moreover, Boyer and co-workers[96] previously developed a photoredox-CRP method logically gated with light and pH in the presence of a water soluble zinc porphyrin catalyst. The addition of acid/base is required to switch the growth of polymer chains. Bergbreiter[97], Zhang[55], Boyer[98] and co-workers developed polymer- or nanoparticle-bonded PCs to promote the catalyst recyclability for homogenous/heterogeneous photoredox-CRP. With these catalysts, polymerizations have shown good temporal control upon switching light irradiation between on and off.

      Besides the above-mentioned compounds, fluorescein[53], benzaldehyde[67] and others[68, 99-100] have also been investigated as PCs for the photoredox-CRP of (meth)acrylates and (meth)acrylamides. Interestingly, Qiao and co-workers observed a photo-enhancement of RAFT polymerization under fume hood light irradiation[101] and certain RAFT agents (e.g., 2-cyano-2-propyl dodecyl trithiocarbonate) can be used as PCs for CRP under visible light[102].

      On the basis of the previously reported photoredox-CRPs since early 2010s[25], polymer chemists further explored a variety of transformations and applications.

      After establishing a number of system for the photoredox-CRP of (meth)acrylates and (meth)acrylamides from thiocarbonylthio and trithiocarbonate compounds[39, 50-51], Boyer and co-workers keep making efforts on topics including oxygen tolerant systems[103-104], near-infrared light controlled polymerizations[39, 105], photopolymerization-induced self-assembly (photo-PISA)[106-109], reactions under flow conditions[110-111], the precise synthesis of oligomers[112-113], and others (Fig. 4)[114].

      Figure 4.  Photoredox-controlled radical polymerizations of (meth)acrylates and (meth)acrylamides applied in different directions

      In the oxygen tolerant systems[103-104, 115], after the photoexcitation of PC, the PC* can transfer the oxygen from ground state to the singlet state, which is consumed by monohydroascorbate ion to form hydrogen peroxide and dehydroascorbate, allowing oxygen tolerance.

      For the photo-PISA studies, while PISA is a useful tool to efficiently prepare polymeric nanoparticles with different morphologies, photo-PISA facilitates high degrees of temporal control over the dispersion polymerization by managing wavelength and intensity of light under mild conditions[106-109].

      As we know, molecular weight distribution has a significant influence on the polymer physical properties. By adjusting parameters of flow rates, intensity and wavelength of light under flow conditions, Boyer and co-workers[111] have prepared polyacrylates and polyacrylamides with controllable MWDs.

      An appealing topic for polymer chemists is the regulation of monomer sequence of macromolecules, allowing replications of the complexity of natural bio-macromolecules, such as DNA, proteins and so on. Different from the precise synthesis of oligomers by the sequential insertion of different monomers into RAFT reagents[112], Hawker and co-workers[116] have synthesized multiblock polyacrylates consisting of hydrophobic, hydrophilic and semi-fluorinated segments via the photoredox-ATRP (Fig. 5). In this transformation, polymers with ABCDE, EDCBA and EDCBABCDE sequence is successfully prepared with more than 98% monomer conversions through iterative monomer additions, affording polymers with D < 1.16. This method provides a particle platform for the preparation of tailored polymer materials from readily available starting substances.

      Figure 5.  One-pot synthesis of ABCDE multiblock copolymers with various segments

      Grafting synthetic polymers onto the surfaces of live cells can potentially manipulate their phenotype and underlying cellular processes. Hawker and coworkers[117] reported a RAFT-based grafting-from strategy for directly modifying the surfaces of live yeast and mammalian cells via photoredox-CRP using Eosin Y as a PC under the irradiation of a 465 nm light (D < 1.3). Compared with conventional grafting-to methods, Hawker's method significantly improves the grafting efficiency, allowing the active manipulation of cellular phenotypes.

      The system consisting of CuBr2/Me6TREN (tris[2-(dimethylamino)ethyl]amine)/alkyl bromide/UV light was studied for copper-mediated photo-ATRP by Matyjaszewski[118-119], Haddleton[120-121] and co-workers. A photoredox single-electron transfer process is suggested as a key mechanism step. Recently, Haddleton's group[122] applied this system into the synthesis of a well-defined polymeric prodrug of the antimicrobial lipopeptide colistin. Remarkably, influences on either structure or activity of the colistin were not observed during the transformation, and most active colistin can be recovered in vitro within two days. The in vitro biological analyses have shown that the prepared substances present good antibacterial properties. The photopolymerization provides a useful platform towards polymer bioconjugates.

      On the basis of the DNA synthesizer technique, Matyjaszewski and coworkers[123] have prepared well-defined homopolymers, diblock copolymers, and biohybrids by Auto-ATRP (Fig. 6). Due to the mild reaction conditions including ambient temperature, tolerance to air and no need of adding reducing agents or radical initiators, photo-ATRP (365 nm UV light) was selected over other ATRP methods. With this method, both hydrophobic and hydrophilic monomers were grafted from DNA, providing an efficient approach to prepare polymer-biohybrids.

      Figure 6.  Automated synthesis of biohybrids using a DNA synthesizer

      As a meaningful attempt, Tang and co-workers[55] recently reported the photoredox-ATRP of three methacrylates derived from renewable biomass (i.e., soybean oil, rosin acid and furfural) using the Ph-PTZ catalyst.

    • Semi-fluorinated (meth)acrylates are a group of monomers belonging to the acrylic acid family. Because of the unique properties of fluorinated polymers, such as high hydrophobicity, tunable lipophilicity, low refractive index, and high stability to heat and chemical, they have attracted increasing attentions in both academia and industry[124-129]. Semi-fluorinated poly(meth)acrylates are used as coatings for buildings, ships, cars and optical fibers, additives for papers and textiles, and so on. Precisely synthesized block copolymers with semi-fluorinated segments show interesting self-assemble behaviors. However, (semi)fluorinated polymers are not directly available from nature. The development of synthetic methods toward semi-fluorinated poly(meth)acrylates is thus of great interests[130-132].

      Recently, Hawker and co-workers[133] reported the employment of the CuBr2/Me6TREN/alkyl bromide/UV light system in the photoredox-ATRP of semi-fluorinated (meth)acrylates (Fig. 7(a)). This method allows the preparation of semi-fluorinated polymers with an unprecedented monomer scope (3 to 21 fluorine atoms on each monomer). For all examples, above 90% monomer conversions are achieved, endowing polymers with narrow MWDs (D=1.04-1.24) and Mn value of up to 1.86×104. Moreover, the high-level controllability of the chain-growing process is illustrated by chain-extension and light on/off experiments. Due to the fact that most fluorinated alcohol solvents such as TFE (trifluoroethanol), TFP (2, 2, 3, 3-tetrafluoro-1-propanol) and TRFP (trifluoro-2-propanol) will transesterificate with semi-fluorinated (meth)acrylates at room temperature as promoted by amine (e.g., Me6TREN)[133], Hawker and co-workers chose TFMP (2-trifluoromethyl-2-propanol), which has much higher steric hindrance than others, to eliminate undesired transesterification in the presence of Me6TREN. The successful elimination of side reactions is demonstrated by experiments of the matrix-assisted laser desorption/ionization time-of-flight mass (MALDI-ToF-MS) and the proton nuclear magnetic resonance (1H-NMR) spectrometry.

      Figure 7.  Photoredox-controlled radical polymerization of semifluorinated (meth)acrylates

      Besides the development of novel methodologies to access semi-fluorinated polymers, Hawker and co-workers[134-135] also applied metal-free photoredox-ATRP to access semi-fluorinated brush architectures from flat/curved surfaces, allowing the preparation of patterned surfaces with different topographies and chemical compositions starting from uniform monolayers.

      On the basis of the photoredox-CRP from TTC iniferters and RAFT polymerization of semi-fluorinated (meth)acrylates[26, 136-137], Chen's group[138-139] (Fig. 7(b)) developed an organocatalyzed photoredox-CRP of semi-fluorinated (meth)acrylates driven by visible light (white LEDs). This method results in the productions of a variety of semi-fluorinated poly(meth)acrylates with narrow MWDs (D=1.04-1.27) and monomer conversions from 85% to above 99% in DMSO solvent. While using non-fluorinated TTC iniferters provides polymers with limited initiation efficiencies and shoulder peaks in the gel permeation chromatography (GPC) profiles, the employment of semi-fluorinated TTC and perfluorinated alkyl iodides results in well-defined polymers with no discernible shoulder peak in the GPC traces. Furthermore, combining the well-demonstrated advantages of flow chemistry[140] and their previous experiences in this field[141-144], a continuous-flow technique was first established to streamline the manufacturing of semi-fluorinated block copolymers[138].

    • Polyacrylonitrile (PAN) is a versatile polymer used in the production of many high-tech/daily materials, such as military aircraft, missile, filtration membranes, advanced fibers, and fishing rods. Acrylonitrile belongs to the acrylic acid family, where the carboxylic acid group is replaced by the nitrile group. The commercial PAN production is mainly based on free radical polymerization. To produce PAN with narrow MWDs and good chain-end fidelity, for example, Cu-mediated ATRP[145-148] have been developed as reliable methods.

      In 2015, Pan et al[149] reported the preparation of PAN with the metal-free ATRP using Ph-PTZ under UV light irradiation. When up to 72% monomer conversion is realized, PAN is produced with Mn=1.36×104 and D=1.53. Following this work, Chen and co-workers[150] reported the employment of benzenediazonium tetrafluoroborate as initiators in the Eosin Y catalyzed photoredox polymerization of acrylonitrile exposing to blue LEDs. While C—Br or C—S bond cleavage is commonly suggested to (re)generate carbon-centered radical species in the photoredox-CRP, a C—F bond cleavage is proposed for this process.

      In addition, in 2017, Li et al[151] reported the high-molecular-weight PAN using photo-RAFT. The polymerization was carried out under mild conditions such as blue light-emitting diode light and ambient temperature. Polymers with molecular weight, Mn, as high as 2.863×105 could be obtained at optimized conditions.

    • Compared with the acrylates, vinyl ketones (VKs) are poorly studied in CRPs. Because of the high reactivity of the ketone group at versatile reaction conditions, poly(vinyl ketone)s (PVKs) present great potentials in post-polymerization modifications[152]. To produce PVKs with precise MWDs and block copolymer structures, CRPs including ATRP, RAFT polymerization and NMP reactions have been reported. However, previous examples still have limitations such as coordination of ketone groups to metal complexes in ATRP, side reactions in NMP/RAFT reaction at elevated temperature, and lack of temporal control.

      Combining the RAFT polymerization of VKs with photoredox polymerization, Hawker's group[152] reported the photoredox-CRP of a variety of VKs including alkyl and phenyl derivatives using a Eosin Y catalyst under visible light (Fig. 8), generating PVKs with MWDs as low as 1.09 (Mn=8.9×103). It allows reactions in the presence of air with the addition of amine as a reductant. Additionally, this method is scalable as demonstrated by the multi-gram scale synthesis of PVKs. The high end-group fidelity is demonstrated by the chain-extension experiments with different monomers including phenyl vinyl ketone, methyl acrylate and N, N-dimethyl acrylamide.

      Figure 8.  Photoredox-controlled radical polymerization of vinyl ketones

    • Polystyrene (PS) is one of the most widely used plastics with a production scale of million tons per year. Living radical, anionic, cationic polymerization or metal complexes-mediated polymerization is capable of producing PS and its derivatives with narrow MWDs. Compared with these methods, the photoredox polymerization of styrenes received much less progress.

      In 2015, You, Nicewicz and co-workers[153] reported a metal-free visible-light (450 nm LEDs) initiated living cationic polymerization of 4-methoxystyrene using 2, 4, 6-tri(p-tolyl)pyrylium tetrafluoroborate as a photo-initiator. Interestingly, catalyst amount of methanol was used to achieve narrow MWDs (D= 1.1-1.3) by regulating propagation via reversible chain transfer.

      The Spokoyny group[154] has developed perfunctionalized icosahedral dodecaborate clusters of B12(OCH2Ar)12. These novel compounds have shown very strong oxidative capability upon excitation exposing by visible light (450 nm LEDs). For instance, when Ar is C6F5, the reductive potential at excited state is about 2.98 V (vs SCE). With this cluster, a series of PS derivatives were prepared through a cationic polymerization mechanism. Furthermore, a less activated olefin of isobutylene is also polymerizable using this photooxidant, resulting in highly branched poly(isobutylene).

      Upon using pyrene as a PC, Yagci and co-workers[155] investigated the polymerization of styrene from ethyl 2-bromopropionate with a light exposure at 350 nm. However, only less than 20% monomer conversion is achieved within 2 h, resulting in PS with Mn ≈2×103 and D=1.32.

    • Poly(vinyl ether)s (PVEs) are featured with good elasticity and excellent resistance to many chemicals. Many PVEs are miscible with water. PVEs and their copolymers find many applications such as adhesives, surface coatings, lubricants, elastomers, fibers, textile finishes and so on. While both cationic and radical polymerizations are capable of producing PVEs, the later method normally results in polymers with low molar masses.

      Although living cationic polymerization has been developed as a powerful method to prepare PVEs with narrow MWDs[156], spatiotemporal control would be difficult without an external stimulus. Based on controlled cationic RAFT polymerization[157-158] and photoredox catalysis[159], Fors and co-workers[160] have developed a living cationic polymerization regulated by visible light (450 nm LED) using a pyrylium PC at room temperature (Fig. 9(a)). This method allows the preparation of a variety of PVEs with well controlled MWDs (D=1.16-1.30) and excellent chain-end fidelity. For the first time, the cationic polymerization can be temporally controlled with light. Further detailed studies have elucidated elementary steps of the mechanism as shown in Fig. 9(a)[161]. An oxidation of RAFT agent is followed by the C—S bond cleavage of the resulting radical cation species, generating dithiocarbamate radical species and a reactive carbon cation. The former dithiocarbamate radical reacts with another RAFT agent and reaches a fast equilibrium with thiuram disulfide dimer; the latter carbon cation initiates polymerization of vinyl ethers. After the dithiocarbamate radical is reduced by PC·, the generated sulfur-centered anion deactivates the propagating chain, producing polymers at dormant state.

      Figure 9.  Controlled cationic/radical polymerization with photoredox catalysts

      Interestingly, on the basis of two types of PCs with oxidative and reductive capabilities respectively at the excited states and a TTC agent, Fors and co-workers[160] recently realized living radical and cationic polymerization in a one-pot setup (Fig. 9(a) and 9(b)), leading to a light-triggered switch with the monomer scope of methyl acrylate and isobutyl vinyl ether. This unique method allows the synthesis of polymers with tunable structures under identical solution conditions via the simple selection of the wavelength of light.

      Yagci and co-workers[162] reported a novel photoinitiated system for living cationic polymerization, of which the mechanism is differ from the photoredox. The reaction between Mn2(CO)10 and benzyl bromide under visible-light irradiation is employed to generate the alkyl radical. This species is further oxidized by diphenyliodonium bromide to generate alkyl cationic species, which can initiate propagation of vinyl ether. By reacting with the bromide anion, the propagating polymeric chain changes to dormant species. The living nature of this polymerization shows the first order kinetics reaction.

    • With the development of transition-metal carbene complexes, ring-opening-metathesis polymerization (ROMP) has received great attentions[163-165]. Polymers obtained through ROMP have been investigated for applications such as drug delivery, high-performance plastics and ion exchange.

      In 2015, Boydston and co-workers[166-167] reported the first metal-free method for controlled ROMP. Using pyrylium salt PC and vinyl ether initiator, poly(norbornene) is obtained with Mn of 6.02×104 (D=1.3-1.7) under visible-light irradiation (Fig. 10). Temporal control over the chain growth is demonstrated by a five-time on/off cycle of light irradiation. For the reaction mechanism, the photoexcited catalyst reacts with a vinyl ether initiator to give a radical cation species and PC· via a single-electron transfer. The radical cation initiates polymerization of norbornene. The propagating chain is reduced by PC·, resulting in PC at the ground state and poly(norbornene) with a vinyl ether group at the end of the chains, which can be re-activated upon oxidation by PC*. Later, for this organocatalyzed ROMP, the Boydston group further investigated the applicable scope of functional groups on norbornene[168], influences of different photoredox catalysts[169], tacticity of the obtained poly(norbornene)[170], living characteristics, as well as bidirectional polymer growth from bifunctional olefin initiators[171]. This organocatalyst involved method provides a new access towards ROMP with capability of spatiotemporal control and none metal-contamination concern.

      Figure 10.  Metal-free ring opening metathesis polymerization driven by light

    • Different from polymers with carbon backbones as mentioned above, poly(α-hydroxy acid)s are based on ester backbones. These biodegradable polymers are applied in clothing, packaging, biomedicine and other areas.

      Feng and Tong[172] reported a method that combines photoredox Ir/Ni catalysis with Zn-alkoxide for the ring-opening polymerization of O-carboxyanhydrides (Fig. 11), leading to isotactic poly(α-hydroxy acid)s with Mn up to 1.41×105 and narrow MWDs (D < 1.1). The reaction performed at low temperatures (-15 ℃) is critical to eliminate undesired side reactions. First order kinetics and the synthesis of block copolymers demonstrate the livingness nature of this reaction. The proposed catalytic mechanism involves that an oxidative addition of the Ni(0) complex with an O-carboxyanhydride molecule generates a nickel-contained cyclic intermediate followed by decarboxylation, transmetalation with an organo-zinc complex and single-electron oxidation by the excited Ir(Ⅲ)-catalyst. The generated Ni(Ⅲ) species further undergoes reductive elimination to produce Zn-alkoxide terminus for chain propagation and Ni(Ⅰ) intermediate. This intermediate is reduced by Ir(Ⅱ) generated in the previous step, resulting in Ni(0) and Ir(Ⅲ) at the ground state.

      Figure 11.  Photoredox-controlled ring-opening polymerization of O-carboxyanhydrides

    • Although the investigations on photoredox-controlled polymerizations are just started less than ten years, recent efforts made in this area have led to many exciting advances. It allows the spatiotemporal control over the chain-growing process for a broad variety of monomers, including those (i.e., vinyl ethers, norborenes, O-carboxyanhydrides) are challenging for well-established photo-controlled polymerizations. The understanding of the polymerization mechanism and inspiration from other areas (i.e., organometallic chemistry, organic chemistry, photo chemistry, and chemical engineering) leads to new ideas for photoredox-controlled polymerization, there by facilitating more progresses in synthetic methods and applications.

      However, there are still some challenges in this field that need to be addressed. First of all, while PCs are used to tune the wavelength of absorbed light, most PCs contained solutions are not colorless. Steps are thus required to remove the color of PCs from the resulting polymer materials. Second, scalable continuous-flow techniques have been developed due to the fact that visible light has better penetration capability through common glassware and solution, but the successful transformation still relies on the solubility of monomers and polymers. Moreover, compared with controlled/living polymerization, monomers for photoredox-CP are normally limited to a narrow scope as summarized here. For other monomers, such as epoxides[173] and N-vinylcarbazole[174-175], the controlled chain-growing process has not yet been achieved though they are reactive under photoinitiated conditions.

      In the future, on the basis of the extensive investigations on photopolymerization, the emergence of the photoredox catalysis offers an alternative approach for accelerating further improvements and innovations in polymer science. Based on this approach, the photo-controlled polymerization can not only be precisely controlled at molar mass and molecular architecture, but also be controlled over the chain growth, selectivity/programmability on monomer scope, and be performed with negligible metal contamination and other fantastic possibilities.

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