Redox-Controlled Polymerization and Copolymerization - ACS Catalysis

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Redox Controlled Polymerization and Copolymerization Changle Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01096 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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Redox Controlled Polymerization and Copolymerization Changle Chen* CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, 230026, China ABSTRACT: Redox control is an attractive strategy for modulating polymerization processes as well as the compositions, microstructures, and properties of polymer products. This perspective summarizes recently reported catalytic systems that possess redox-switching capabilities, and their applications to the ring-opening polymerizations of cyclic esters, ring-opening-metathesis polymerizations of cyclic olefins, and coordination-insertion polymerizations of olefins. Redox-induced switching between catalytically active states enables the generation of unique polymer microstructures that are difficult to access otherwise.

Keywords: redox control, switchable polymerization, copolymerization, transition metal catalyst, olefin polymerization,

1. INTRODUCTION Switchable polymerization is a rapidly evolving research field, which has attracted a lot of attention.1-3 In such a process, a single catalytic species is used and its reactivity can be adjusted by an external stimulus. In this manner, the polymerization process, the polymer microstructures, and the polymer properties can be efficiently modulated. Different external stimuli have been reported in the literature, and these have led to allosteric,4 pH,5 electrochemical,6 photochemical,7,8 and mechanochemical control.9 Among these strategies, redox control is a fascinating concept, since many transition metals are naturally redox active, and switching between their redox states is usually easy and facile. In 1995, Wrighton et al. demonstrated that redox switching in a diphosphino-cobaltocene based rhodium complex can lead to different catalytic reactivity in cyclohexene hydrogenation reaction. 10 The application of redox-controlled concept in polymerization was first realized in 2006,11 and only began to attract wide attention after 2011. The great potential of this strategy has been elegantly demonstrated in several recent reports. This perspective aims to provide a comprehensive overview of this field with respect to different redox-active catalysts and their applications to a variety of polymerization reactions, including ring-opening polymerizations (ROPs) of cyclic esters, ring-opening-metathesis polymerizations

(ROMPs) of cyclic olefins, and coordination-insertion polymerizations of olefins. Special attention will be paid to redox-induced reactivity modulation in these polymerization reactions, as well as the underlying mechanisms. 2. RING-OPENING POLYMERIZATIONS OF CYCLIC ESTERS 2.1 Redox control through redox-active ligands. Redoxcontrolled polymerizations were first realized through the ROPs of cyclic esters, which have also been the mostextensively studied redox-controlled polymerization process. In 2006, Gibson, Long, and coworkers designed and prepared a titanium- bis(iminophenoxide) catalyst bearing two ferrocenyl units (Scheme 1a). 11 They demonstrated that the ferrocenyl units were reversibly oxidized with AgOTf (silver trifluoromethanesulfonate) and reduced with FeCp*2 (Cp* = pentamethylcyclopentadienyl). The neutral catalyst 1 was ~30 times faster than its oxidized counterpart 1ox during the ROP of rac-lactide (rac-LA). This difference was ascribed to decreased electron density at the titanium center following ferrocenyl oxidation. Importantly, in situ redox switching was achieved during the polymerization reaction, while wellcontrolled polymerization behavior was maintained. In 2011, Diaconescu et al. applied this strategy to ferrocenyl based yttrium- and indium alkoxide catalysts (Scheme 1b).12 Reversible oxidation and reduction of the ferrocenyl unit were

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achieved using FcBAF (ferrocenium tetrakis[3,5bis(trifluoromethyl)phenyl]borate) and CoCp2 (cobaltocene). In the case of yttrium, the neutral complex 2 catalyzed the ROP of L-lactide (L-LA) with 74% conversion after 3 h, while the oxidized counterpart 2ox was totally inactive. As such, switching between the catalytic “on” and “off” states was realized multiple times in this system. In comparison with 1, the closer proximity of the ferrocenyl unit to the metal center in 2 induced a more dramatic redox effect that influenced the catalytic properties of the system. The opposite effect was observed for the indium-containing catalyst 3; neutral 3 facilitated a 2% conversion after 1 d during the ROP of trimethylenecarbonate, while the oxidized counterpart 3ox catalyzed a 49% conversion. These results suggest that redox control is highly complex and involves more than simple electronic effects induced by the oxidations and reductions of the ligands. 2.2 Redox control through catalytically active metal centers. Modulating the oxidation state of the catalytically active metal center directly affects the electronic properties of the metal complex, which may more dramatically influence its catalytic properties. In 2011, Diaconescu et al. reported a cerium alkoxide complex supported by a Schiff-base ligand (Scheme 1c). 13 This system is different than the abovementioned two systems since the catalytically active cerium center can be directly oxidized and reduced using FcBAF and CoCp2, respectively. Neutral complex 4 mediated

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efficient and well-controlled L-LA polymerization, while its oxidized counterpart 4ox was totally inactive. This was the first example of a metal-based redox-controlled polymerization of a cyclic ester. In 2013, Okuda et al. reported an OSSO-type bis(phenolate)-based cerium system.14 In a similar fashion, the oxidation of cerium(III) to cerium(IV) inhibited the ROP of meso-lactide. In 2013, Byers et al. prepared the bis(imino)pyridine iron(II) bis(alkoxide) complex 5, which was reversibly converted into its oxidized form 5ox using FcPF6 (ferrocenium hexafluorophosphate) and reduced back using CoCp2 (Scheme 1d). 15 The iron(II)-based species 5 catalyzed 93% monomer conversion after 3 h at room temperature during the polymerization of rac-LA, while the oxidized iron(III) counterpart 5ox was inactive. As such, polymerization was turned “on” and “off” through in situ redox switching. In 2014, Lang et al. synthesized a pincer-type iron(III) trichloride complex 6ox, which was reversibly converted into its reduced form 6 using appropriate reducing and oxidizing reagents (Scheme 1e).16 In direct contrast to the 5/5ox system, the oxidized form was the active catalyst during the ROP of εcaprolactone (CL). These results further highlight the abovementioned complexity of redox-controlled polymerization catalysis.

Scheme 1. (a) Switchable ROP of rac-LA using Ti based 1/1ox system. (b) Redox switching in the Y (2/2ox) and In 3/3ox based systems. (c) Redox switching in the Ce based 4/4ox system. (d) Redox switching in the Fe based 5/5ox system. (e) Redox switching in the Fe based 6/6ox system


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2.3 Redox-controlled copolymerizations. The redoxcontrolled homopolymerizations of cyclic ester monomers, as well as the “on-off” switchable catalysis described above, are scientifically interesting. However, they are not particularly useful from a synthetic perspective. The concept of selective discrimination between two different monomers during redox switching is highly fascinating. Block copolymers can be synthesized by sequential addition of different monomers to a living polymerization system. The utilization of redox controlled strategy toward this purpose is advantageous since both monomers can be simultaneously added. In 2014, Diaconescu et al. reported a breakthrough using the [OSSO]-type bis(phenolato)-based titanium complex 7 (Scheme 2a).17 The pre-installed ferrocenyl unit was reversibly oxidized and reduced using AcFcBAF (AcFc = acetyl ferrocenium) and CoCp2. The neutral catalyst 7 exhibited higher activity for L-LA polymerization, while the oxidized counterpart 7ox showed higher activity for the polymerization of CL. During the 7-mediated copolymerization of these two monomers, 58% of L-LA was converted after 36 h at 100 °C, while CL was not consumed during this stage. However, CL was polymerized with 18% conversion after 2 h upon the addition of AcFcBAF at room temperature followed by heating at 100 °C. During the second stage, almost no LA consumption was observed. The resulting block copolymer was isolated and characterized in detail (Scheme 2b). However, significant amounts of L-LA were consumed when the catalyst was in the oxidized state for an extended period of time.

Scheme 2. (a) Redox switching in the titanium based 7/7ox system. (b) 7/7ox mediated switchable copolymerization of L-LA and CL. (c) 5/5ox mediated switchable copolymerization of L-LA and CHO. (d) Redox-triggered crosslinking reactions A zirconium analogue based on this ligand framework exhibited similar reactivity discrimination towards these two

monomers during redox switching; however, no copolymerization was realized.17 The neutral zirconium complex catalyzed L-LA polymerization, but the oxidized zirconium counterpart was unable to polymerize CL during the second stage, which was attributed to the strong coordination of the L-LA monomer to the oxidized zirconium species that blocked CL complexation to the metal. In 2016, Byers et al. reported another example of monomer discrimination during redox switching using the 5/5ox system (Scheme 2c). 18 The neutral complex 5 actively polymerized rac-LA, but was inactive toward cyclohexene oxide (CHO). In contrast, the oxidized counterpart 5 ox exhibited the opposite reactivity pattern. In the presence of both monomers, 5 selectively polymerized rac-LA, while 5ox selectively polymerized CHO. As such, copolymerization reactions led to the formation of block copolymers through in situ redox switching between the two oxidation states. This system can start from either the neutral catalyst 5 or its oxidized counterpart 5ox for the generation of block copolymers. Byers et al. further demonstrated a very elegant example of the application of this redox-controlled orthogonal reactivity (Scheme 2d). 19 They designed an epoxide-functionalized cyclic diester, in which the cyclic diester moiety undergoes chemoselective ROP catalyzed by 5. Upon oxidation to 5ox, polymerization of the epoxide moiety led to crosslinking of the pre-formed polymer. In a similar manner to that described above, the cross-linked polymer can be generated through 5oxinitiated epoxide polymerization, followed by redox switching to 5, which initiated cross linking reactions through cyclic diester polymerization. As a highlight to the usefulness of the redox switching method, the cross-linked polymers demonstrated significantly different thermal properties compared to poly(lactic acid). 2.4 Mechanistic understandings. The redox-controlled polymerizations and copolymerizations of cyclic esters are highly complicated, involving various reactions including monomer coordination, insertion, propagation, chain transfer, transesterification, and the competitive binding of two monomers, among others. 20 Consequently, performing mechanistic studies is highly challenging. Despite this, several research groups have expended serious efforts in this direction. In 2018, Cramer, Byers and coworkers used density functional theory (DFT) to study the electronic structures of the redox-active iron species 5 and 5ox.21 The computational results suggested that complex 5ox, which is a formally iron(III) complex, is most accurately described as a high-spin iron(II) center with approximately one unpaired electron residing on the alkoxide group. They also demonstrated that a formally iron(I) bis(imino)pyridine complex can be generated by the reduction of an iron(II) precursor. 22 Both experimental and computational studies suggest that the formally iron(I) species is best described as an iron(II) center coupled to a singly


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reduced bis(imino)pyridine ligand. It should be noted that the redox non-innocent feature of the bis(imino)pyridine ligand has been extensively studied and utilized in various catalytic reactions. 23 The active and indispensable role of the noninnocent ligand may be partially responsible for the conflicting experimental trends observed for the various redox-active polymerization systems discussed above. In theory, the abovementioned copolymerization systems should be able to generate triblock copolymers through the induction of a second redox switch. Indeed, a (LA)n(CL)m(LA)p triblock copolymer was generated using the 7/7ox system with two cycles of redox switching.24 However, more complicated situations are possible. In 2016, Diaconescu et al. reported the synthesis and characterization of ABA or BAB type triblock copolymers using an [ONNO]-type bis(phenolato)-based zirconium system (Scheme 3a). The oxidation and reduction of the 8/8ox system was reversible (Scheme 1a). While the neutral catalyst 8 was active for L-LA polymerization, it was inactive toward CHO. The oxidized counterpart 8ox was active for CHO polymerization but not for L-LA. However, the in situ one-pot switching between copolymerization modes in the presence of these two monomers was not possible, which was attributed to the side reactions between the AcFcBAF oxidant and CHO monomer, including [AcFc]+-induced cationic CHO polymerization, as well as some decomposition reactions. Therefore, the concomitant presence of the oxidant and CHO should be avoided; consequently, the monomers need to be sequentially added in order to prepare the triblock copolymer (Scheme 3b). Some further experimental and DFT investigations on this system indicated that the presence of one monomer significantly influences the polymerization behavior of the catalyst towards the other monomer, despite its orthogonal reactivity toward the two monomers during redox switching.25 Through a series of copolymerization studies, the authors probed the influences of concentration, block length, or switch on the polymerization rates of different monomers.

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coordination to the metal center plays an important role. For example, DFT studies showed that the first LA insertion into the neutral and oxidized aluminum complex is facile. However, the reactivity of the insertion product toward a second monomer (such as CL and trimethylene carbonate) is much lower due to the chelation effect. For comparison, the insertion of CL or trimethylene carbonate monomer will not induce a significant reactivity change for the neutral metal species. Interestingly, the next insertion (propagation) can be facilitated by the chelation effect for the oxidized aluminum catalyst. In 2015, Long et al. reported an example that highlighted the potential complexity of a redox-controlled polymerization ox

system (Scheme 4).27 The redox switching between 9 and 9 was achieved reversibly using AcFcBAF and CoCp2. Complex 9 mediated slow L-LA ROP with 2% conversion after 3.5 h, ox

while the pre-oxidized 9 catalyzed over 50% conversion under the same conditions. It is interestingly that this system ox

exhibited the opposite reactivity trend to the titanium 1/1 system despite their similarities in ligand framework. Control experiments revealed that the catalyst displayed dramatically different reactivity towards this monomer depending on whether or not it was oxidized in the presence or absence of LLA. Detailed mechanistic studies supported a rapid switching in the coordination geometry of the ligand promoted by monomer coordination prior to any redox reaction. This further demonstrates the active role of non-innocent ligands in redox-controlled polymerization reactions.

Scheme 4. Redox switching in the Ti-based 9/9ox system

Scheme 3. (a) Redox switching in the Zr based 8/8ox system. (b) 8/8ox mediated switchable copolymerization of L-LA and CHO to prepare triblock copolymers In 2017, Diaconescu et al. studied the properties of a redoxactive aluminum complex during the ROPs of various cyclicester monomers. 26 Detailed experimental and DFT studies showed that redox switching changes the reaction profiles of these polymerization reactions, and that carbonyl-group

3. RING OPENING METATHESIS POLYMERIZATIONS 3.1 Redox controlled homopolymerization. In 2013, Plenio et al. reported the synthesis and characterization of eight N-Hoveyda-type ruthenium complexes bearing ferrocenyl moieties;28 specifically, complex 10 was studied in great detail (Scheme 5a). The neutral complex 10 mediated very slow ROMP of cis-cyclooctene (COE), with 4% conversion after 5 h. The oxidized counterpart 10ox is much more active, which consumes more than 96% of monomer under the same conditions. Most interestingly, ROMP activity can also be electrochemically switched in this system. When a constant current of 1 mA and a total charge of 0.385 C was applied to the solution, complex 10 was converted to 10ox, which catalyzed the ROMP of the COE monomer. The


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property change was attributed to the different electronic properties of the neutral and oxidized ferrocenyl units.

reduced activity during the ROMP of COD (12: kobs = 0.045 s-1; 12ox: kobs = 0.0012 s-1). 3.2 Redox controlled copolymerizations. From a synthetic perspective, access to a variety of catalytical “on” states is more useful than switching between “on” and “off” states. It would be more interesting if these different catalytical "on” states lead to different regio- or stereo-selectivities. In 2017, Bielawski et al. reported the first example of a redoxcontrolled ROMP copolymerization. 31 Reversible reductionand oxidation-switching between 13 and 13red was achieved with CoCp2 and FcPF6 (Scheme 6). The neutral catalyst 13 showed higher activity during the ROMP of COD than toward a substituted norbornene monomer (N-NB). After reduction, 13red exhibited the reverse reactivity toward these two monomers. By taking advantage of this reactivity difference, a series of copolymers with different compositions, microstructures, and physical properties was prepared using the in situ redox-controlled strategy. Computational studies showed that the oxidation state of the ligand affects the ratedetermining step in each polymerization reaction, leading to selective discrimination between these two monomers. Despite the reactivity difference, the copolymerization reactions were not particularly chemoselective. As a result, nonstatistical random copolymers rather than block copolymers can be synthesized with redox switching.

Scheme 5. (a) Redox switching in the Ru based 10/10ox system, and its application in ROMP of COE. (b) Redox switching in the Ru based 11/11ox system, and its application in ROMP of COD. (c) Redox switching in the Ru based 12/12ox system In 2013, Bielawski et al. showed that the commercially available ruthenium catalyst 11 can be reversibly oxidized to 11ox using 2,3-dichloro-5,6-dicyanoquinone (DDQ) and reduced back using FeCp*2 (Scheme 5b). 29 Complex 11 mediated fast ROMP of cis,cis-1,5-cyclooctadiene (COD), while 11ox exhibited significantly reduced activity (kobs = 4.8 × 10-3 s-1 vs. 6.0 × 10-5 s-1). As such, redox-controlled switchable polymerization was achieved. In this system, the reduced catalytic activity following oxidation was proposed to be due to the precipitation of the oxidized catalyst, which was much less soluble than the neutral catalyst. Using a biphasic organicsolvent/ionic-liquid mixture (C6H6/CH2Cl2/[BMI][PF6], BMI=1-butyl-3-methylimidazolium), the oxidation of 11 removed over 99.9% of the ruthenium species from the organic phase, which provides an alternative strategy for the removal of residual metal species from the final product. In 2013, Bielawski et al. designed a ferrocenyl-based Grubbs-type ruthenium catalyst 12 that can be reversibly oxidized and reduced (Scheme 5c).30 In contrast to the 10/10ox system, the oxidation of the ferrocenyl unit led to greatly

Scheme 6. Redox switching in the Ru based 13/13red system, and its application in copolymerizations of COD with a substituted norbornene monomer (N-NB)

4. COORDINATION-INSERTION POLYMERIZATION

OLEFIN

4.1 Realization of redox controlled olefin polymerization using single component catalysts. Compared with the ROP of cyclic esters and the ROMP of cyclic olefins, the redoxcontrolled polymerization of ethylene is experimentally more


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challenging, since the monomer is a gas that requires continuous feeding. Moreover, ethylene pressures of above 1 atm are sometimes required to achieve high polymerization activity, which complicates the in situ addition of the oxidant and reductant. As a matter of fact, the concept of modulating polymerization processes using redox control in was originally applied to ethylene polymerization, which preceded the redoxcontrolled ROP of lactide. 32 - 34 Between 2003 and 2006, Gibson et al. prepared several nickel, palladium, iron, and cobalt complexes bearing ferrocene-containing ligands (Scheme 7a, 14, 15, and 16). These metal complexes can be reversibly oxidized and reduced using common oxidants and reductants. The neutral metal complexes catalyzed ethyleneoligomerization or polymerization processes in the presence of methylaluminoxane (MAO) as cocatalyst. Unfortunately, their oxidized counterparts exhibited identical properties under the same conditions. The aluminum cocatalyst was proposed to be responsible for reducing the oxidized species back to its neutral form during polymerization.

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Scheme 7. (a) Redox switchable nickel, palladium, iron and cobalt metal complexes. (b) Redox switching in the palladium based 17/17ox system, and its application in ethylene polymerization and copolymerizations with MA and NB

During ethylene polymerization, 17 was 4–6 times more active than 17ox, and produced polyethylene with 3–5 times higher molecular weight. We attributed this to the electronwithdrawing properties of the oxidized ferrocenyl unit. The molecular weight trend is consistent with the literature studies on the electronic effect of the ligand in phosphine-sulfonate palladium systems. 39 , 40 Mechanistic studies showed that the oxidized catalyst 17ox is more prone to chain transfer than the neutral catalyst 17. Chain-transfer reactions generate palladium hydride species 41 that are more likely to undergo decomposition, leading to lower activity. Reduced activity and polymer molecular weight were observed for the oxidized catalyst 17ox during the copolymerization of ethylene with methyl acrylate (E-MA copolymerization). In addition, 17ox led to lower MA incorporation, which was probably due to increased poisoning of the more-electrophilic palladium center by the MA comonomer following ferrocenyl oxidation. The neutral catalyst 17 was inactive during norbornene oligomerization, while the oxidized counterpart 17ox exhibited appreciable activity. As such, redox-controlled switchable catalysis was achieved in this system. Once again, these observations are consistent with a redox-induced ligand electronic effect.42 4.2 Redox controlled olefin polymerization in various catalytic systems. Brookhart-type α-diimine nickel and palladium catalysts have been extensively investigated in ethylene polymerization and copolymerization reactions. 43 - 46 In 2016, Long et al. reported that the reduction of the αdiimine nickel complex 18 using 0–1 equiv. of CoCp2 resulted in a decrease in branching density of up to 30% during ethylene polymerization in the presence of an aluminum cocatalyst (Scheme 8a).47 In contrast to the systems based on 14–16, the reducing power of the aluminum cocatalyst was avoided through the reduction of the catalyst, and the reduction was proposed to be more likely ligand-centered. Reduction/oxidation reversibility in this system was not investigated. Subsequently, they demonstrated that comonomer incorporation levels can be modulated in this system through redox control during the copolymerizations of ethylene with α-olefins.48

This issue can be straightforwardly addressed through the use of a single-component olefin polymerization catalyst that does not require a cocatalyst. 35 In 2015, we reported a successful example of redox-controlled olefin polymerization.36 Phosphine-sulfonate palladium is one of the most extensively studied olefin polymerization catalysts; it possesses excellent catalytic properties for the copolymerizations of ethylene with polar comonomers. 37 Following literature procedures, 38 we prepared several ferrocene-containing phosphine-sulfonate palladium catalysts (Scheme 7b, 17). The reversible interconversion between 17 and the oxidized counterpart 17ox was achieved using AgOTf and CoCp2.

Scheme 8. (a) Reduction of an α-diimine nickel complex 18, and its application in ethylene polymerization and copolymerization with α-olefin. (b) Some α-diimine nickel complexes 19 bearing ferrocenyl unit


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This strategy is very sensitive to the ligand structure. In a related α-diimine nickel system (Scheme 8b, 19), CoCp2 reduction effectively modulated the properties of the catalyst when the substituents (R) were acenaphthyl, but was ineffective when unsubstituted or Me-substituted. 49 The ferrocenyl moiety can be oxidized, but this did not lead to any different behavior during ethylene polymerization, which was ascribed to the presence of the MAO type cocatalyst, as was observed for systems involving 14–16. In 2016, Diaconescu et al. reported a neutral ferrocene-based palladium complex with reversible oxidation and reduction capabilities. 50 The oxidized complex 20ox polymerized norbornene and some substituted norbornenes, while the neutral complex 20 was inactive (Scheme 9a). A ferroceneoxidation-induced electronic effect was proposed as the key reason for the observed reactivity difference. In 2017, we used the redox active carbene ligand developed by Bielawski et al. to prepare some palladium complex (Scheme 9b, 21).51 The neutral complex 21 actively polymerized norbornene and alkyne types monomers, while the reduced counterpart 21red exhibited very low activities. Switchable polymerization was realized by the addition of reductants and oxidants.

22, 22+, and 222+ showed different catalytic activities and generated polyethylene with different molecular weights and different topologies during the polymerization of ethylene. Specifically, stepwise oxidation led to more branchedpolyethylene microstructures, which was ascribed to the ligand electronic effect.53- 56 Polyethylenes with bimodal or trimodal GPC curves were generated through the in-situ addition of different amounts of oxidant during 22-catalyzed ethylene polymerizations. The catalytic properties of this system were also redox-controlled in a stepwise fashion during ethylene copolymerizations with MA, norbornene, and substituted norbornene comonomers.

Scheme 10. Accessing multiple catalytic “on” states in a specially designed α-diimine-Pd system, and its applications to ethylene polymerization, and copolymerizations with MA and norbornene derivatives

Scheme 9. (a) Redox switching in the Pd-based 20/20ox system, and its application to the polymerization of norbornene derivatives. (b) Redox switching in the Pdbased 21/21ox system, and its application in the polymerization of norbornene derivatives and an alkyne monomer 4.3 Stepwise redox control. In 2017, we extended the concept of redox-controlled olefin polymerization to an αdiimine palladium system. 52 Complex 22 bearing two ferrocenyl units was prepared and characterized (Scheme 10). Most interestingly, the two ferrocenyl units can be oxidized in a stepwise fashion. We generated carbonyl complexes during this stepwise oxidation. The shifts observed for the CO infrared stretching frequencies (from 2133 to 2136, and 2139 cm-1) during the stepwise oxidation provided direct evidence in support of the hypothesis that ferrocenyl oxidation increases the electron-withdrawing capability of the ligand. Complexes

5. CONCLUSIONS AND OUTLOOKS In summary, the redox-control strategy has been successfully realized in three types of polymerization reactions: the ROPs of cyclic esters, the ROMPs of cyclic olefins, and the coordination-insertion polymerizations of olefins. In each category, several redox-active catalytic systems were designed and prepared; their applications to both homopolymerizations and copolymerizations have been studied, and examples of both “on-off” switching and monomer discrimination have been demonstrated. Despite these significant developments, many challenges, opportunities, and possibilities remain in this field. It should be noted that redox controlled strategy has been widely used in radical polymerization, 57 and cationic polymerization fields.58 However, those kinds of reactions are not included, since they are fundamentally different than the ones described throughout this perspective. The application of redox-control strategy for other types of polymerization reactions is attractive. For example, some redox active catalysts for the ROPs of epoxides have been reported. These and some further developed catalysts may be applicable for the redox controlled copolymerizations of epoxides with CO259 or cyclic anhydrides.60 The redox-control strategy has an intrinsic disadvantage: the requirement of externally added oxidants and reductants. This complicates experimental procedures, and sometimes the


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oxidant/reductant may react with the catalyst or monomers. The electrochemical strategy shown for the 10/10ox system is an attractive alternative, which has actually recently been realized using the iron based 5/5ox system.61 Furthermore, the addition of a photoreductant that reduces/oxidizes the catalyst upon exposure to light may also address this issue.62 Along with this, the development of systems that respond to multiple stimuli is highly fascinating. As discussed above, there are many examples of opposite reactivity trends exhibited by neutral and oxidized catalysts for the same transition metals bearing similar ligand structures. A detailed mechanistic understanding and the eventual ability to rationally design and predict redox-control behavior are highly desired but are also highly challenging objectives. Currently, there are very few examples of redox-controlled copolymerizations, among which even fewer systems possess monomer-discriminating capabilities. It would also be of great interest if redox switching can be used to change the stereoselectivity of lactide ROP, or the regioselectivity of cyclic-olefin ROMP. The rational design of redox-control systems that discriminate between monomers in terms of both activity and selectivity remains a great challenge in this field.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] Notes The authors declare no competing financial interests. ASSOCIATED CONTENT ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (NSFC, 21690071 and 51522306) and the Fundamental Research Funds for the Central Universities. REFERENCES (1) Leibfarth, F. A.; Mattson, K. M.; Fors, B. P.; Collins, H. A.; Hawker, C. J. External Regulation of Controlled Polymerizations. Angew. Chem., Int. Ed. 2013, 52, 199-210. ( 2 ) Blanco, V.; Leigh, D. A.; Marcos, V. Artificial Switchable Catalysts. Chem. Soc. Rev. 2015, 44, 5341-5370. ( 3 ) Teator, A. J.; Lastovickova, D. N.; Bielawski, C. W. Switchable Polymerization Catalysts. Chem. Rev. 2016, 116, 1969-1992. (4) Yoon, H. J.; Kuwabara, J.; Kim, J. -H.; Mirkin, C. A. Allosteric Supramolecular Triple-Layer Catalysts. Science 2010, 330, 66-69. (5) Balof, S. L.; P’Pool, S. J.; Berger, N. J.; Valente, E. J.; Shiller, A.M.; Schanz, H. J. Olefin Metathesis Catalysts

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