Recent Developments in Chain-Growth Polymerizations of

Review pubs.acs.org/IECR

Recent Developments in Chain-Growth Polymerizations of Conjugated Polymers Melissa P. Aplan† and Enrique D. Gomez*,†,‡ †

Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States

‡

ABSTRACT: In this Review, we discuss recent developments and advancements in chain-growth polymerizations for conjugated polymers. Controlled synthesis methods will produce conjugated polymers with finely tuned properties and facilitate their development in new industrial applications. We begin with a discussion on the inherent differences between step-growth, chain-growth, and controlled chain-growth polymerization mechanisms. We emphasize that verification of polymerization mechanisms by kinetic analyses is necessary for the development of new controlled polymerizations. Next, we highlight recent advances in this rapidly growing field. Controlled syntheses of conjugated polymers usually employ a catalyst transfer mechanism, but techniques utilizing different controlled mechanisms are also beginning to emerge. We then highlight reported limitations that serve as a platform for future studies. Finally, to emphasize the potential of controlled chain-growth polymerizations, the synthesis of conjugated block copolymers by sequential chaingrowth reactions is briefly discussed.

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INTRODUCTION Semiconducting polymers may lead to a new generation of chemically versatile, mechanically flexible, solution-processed electronic materials. Incorporating aromatic repeat units to form fully conjugated backbones allows for extensive delocalization of π-electrons and efficient charge transport. Alkyl or alkoxy side chains are appended to the backbone, permitting solubility in organic solvents; this introduces the possibility for solution processing techniques including screen printing, inkjet printing, and roll-to-roll casting. Thus, conjugated polymers have potential applications in photovoltaics,1−9 flexible displays,10−14 thermoelectrics,15−20 and transistors.21−26 In the laboratory, it has been demonstrated that molecular weight,27−35 dispersity,36,37 and end-groups38−43 of synthesized conjugated polymers can have substantial effects on device performance. For example, in solar cells where the active layer is composed of poly[5-(2-hexyldodecyl)-1,3-thieno[3,4-c]pyrrole4,6-dione-alt-5,5-(2,5-bis(3-dodecylthiophen-2-yl)-thiophene)] (PTPD3T) as the donor and poly[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′bithiophene)] (P(NDI2OD-T2) as the acceptor, device efficiency increases by nearly a factor of 2 when Mn values of the donor and acceptor are individually optimized. Highest power conversion efficiencies of 3.2% are realized for intermediary molecular weights (∼40−50 kg/mol) of both the donor and acceptor polymers.35 In addition, the Yu group has established an inverse relationship between polymer dispersity (Đ) values and organic solar cell efficiencies. Increasing the dispersity (Đ) of poly(4,8-bis[(2-ethylhexyl)oxy]benzo[1,2b:4,5-b′]dithiophene-2,6-diyl-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl) (PTB7) leads to a red shift in absorbance, and, when incorporated in the photoactive layer, leads to devices with weaker exciton generation, stronger © 2017 American Chemical Society

bimolecular recombination, and lower charge carrier mobilities.37 Functional groups that terminate polymer chains as a result of common synthetic procedures (typically H or Br) can have detrimental effects when the materials are included in electronic devices. Bromine end groups in particular can quench photogenerated excitons, disrupt chain packing, and accelerate degradation.40−42 Alternatively, functional groups that are intentionally appended to the end of polymer chains can lead to devices with enhanced efficiencies. For instance, end-capping a donor polymer with an electron-accepting porphyrin moiety increases photovoltaic power conversion efficiency in devices from 1.5 to 4.5%.43 Creating the alternating double bond−single bond chemical structure of conjugated polymers often relies on cross-coupling reactions. As a consequence, synthesis is more commonly performed through polycondensation reactions that follow an uncontrolled step-growth mechanism. Nevertheless, to access the full potential of semiconducting polymers, the development of synthetic methods that follow a controlled chain-growth mechanism is necessary. Synthetic techniques that facilitate precise control over molecular weight, dispersity, and end groups will permit the optimization of polymer properties for industrial applications in electronic devices. Living chain-growth synthesis techniques also provide a method for end group functionalization of conjugated polymers.39,44−48 Furthermore, careful control of end groups will facilitate well-defined, fully conjugated block copolymers Received: Revised: Accepted: Published: 7888

March 11, 2017 May 16, 2017 June 13, 2017 June 13, 2017 DOI: 10.1021/acs.iecr.7b01030 Ind. Eng. Chem. Res. 2017, 56, 7888−7901

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Industrial & Engineering Chemistry Research synthesized by sequential chain-growth reactions. Driven by chemical incompatibility between the blocks, block copolymers self-assemble into well-defined microstructures.49,50 Taking advantage of this phenomenon, fully conjugated block copolymers have the potential to control the mesoscale morphology and interfacial structure, enhancing performance of organic electronic devices.51−58 In addition to linear homopolymers and block copolymers, controlled chain-growth polymerizations can be used to synthesize nonlinear fully conjugated polymers including graft copolymers,59−61 surface-grafted polymers,62−64 and branched polymers.65,66 Facile and carefully controlled synthesis of complex structures may extend potential applications of conjugated polymers to biomedical devices,67 water purification,68 and sensors.69−71 In this Review, we will first outline the defining characteristics of step-growth, chain-growth, and living chain-growth polymerizations. When attempting to develop a controlled polymerization reaction, it is necessary to confirm the mechanism using kinetic data. We then highlight recent advances in chain-growth polymerizations of conjugated polymers. The Review will emphasize reactions using the catalyst transfer mechanism but also include less common methods, such as gold-activated and monomer deactivation polymerizations. We then move to a discussion of reported limitations in chain-growth polymerizations. Careful analyses of unsuccessful reactions will be instrumental in the rational development of new syntheses. We also highlight the impact of chain-growth polymerizations using the synthesis of fully conjugated block copolymers as a case study and conclude with a brief outlook for conjugated polymers from living polymerizations.

Figure 1. Kinetics of step-growth vs controlled chain-growth polymerizations. (a) In a step-growth polymerization, any two molecular species can react and Xn grows according to 1/(1 − p). (b) In a chain-growth polymerization, only one end of a growing chain can react with monomers. When the rate of initiation is greater than the rate of propagation, and termination reactions are negligible or nonexistent, Xn grows linearly with p.

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than step-growth. A chain-growth mechanism consists of three individual reactions: initiation, propagation, and termination. The overall polymerization rate can be modeled as the net rate of the three competing reactions. The explicit rate expression will depend on nature of the individual reaction as well as the relative rates of initiation, propagation, and termination. If termination takes place without combination of two chains, such as through disproportionation, then Đ follows eq 2 as in step-growth polymerizations. If termination reactions require two chains ends to come together, such as through combination, then p Đ=1+ (3) 2 and Đ approaches 1.5 at high p. Some significant features that distinguish a chain-growth from step-growth mechanism are that monomers are consumed steadily throughout the reaction and high molecular weight polymers can be formed at low monomer conversions. Living polymerization is a subset of chain-growth polymerizations. A living polymerization is defined as a chain-growth reaction in which no termination or chain transfer reactions occur during the polymerization (p < 1). For optimal control over molecular weight with a narrow size distribution, there are additional criteria. First, the rate of initiation (ri) must be much greater than the rate of propagation (rp). If ri ≫ rp, initiation is effectively instantaneous and will be complete before propagation begins. This, combined with no chain termination or transfer reactions, will generate a constant number of polymer chains throughout the reaction. The second requirement is that all chains must be equally likely to react with a monomer throughout the polymerization; this ensures all chains grow at the same rate. When these conditions are satisfied, the resulting polymer molecular weight will follow a Poisson distribution. In

ESTABLISHING A CHAIN-GROWTH MECHANISM A polymerization can be classified into one of two categories based on the underlying reaction mechanism: either step-growth or chain-growth. The defining features that distinguish stepgrowth and chain-growth polymerizations originate in the reaction kinetics. This Review will highlight the key differences in the kinetics of step-growth versus chain-growth reactions and how these can be used to experimentally verify a polymerization mechanism. Detailed derivations can be found elsewhere.72−76 In step-growth reactions, polymer chains grow through reactions between any two molecular species. As illustrated in Figure 1a, two monomers can react, a monomer can react with a growing chain, or two growing chains can react. The overall polymerization rate is the sum of rates of all reactions between two molecular species. Assuming reactivity is independent of the size of the reacting molecular species, the number-average degree of polymerization (Xn) and dispersity (Đ) are expressed as 1 Xn = 1−p (1) Đ=1+p

(2)

In eqs 1 and 2, p is the extent of reaction, defined as the fraction of functional groups that have reacted at a given time. Defining features of a step-growth mechanism are that monomers are consumed soon after the reaction begins, high molecular weight products are only formed as p approaches 1, and Đ approaches 2 as the reaction approaches completion (p = 1). In chain-growth polymerizations, a chain grows only by reaction of a monomer with the active end of a growing chain. The kinetics of chain-growth polymerization are more complex 7889

DOI: 10.1021/acs.iecr.7b01030 Ind. Eng. Chem. Res. 2017, 56, 7888−7901

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Industrial & Engineering Chemistry Research this Review, we will refer to such a mechanism as a “controlled chain-growth” polymerization (Figure 1b). In a controlled chaingrowth polymerization, all chains are initiated before propagation begins and grow at the same rate such that

Xn =

M0 p I

Đ=1+

Figure 2. In developing new chain-growth syntheses, the most common strategy is to convert an uncontrolled polycondensa-

(4)

IM 0 p 2

(I + M 0 p)

≈1+

I M0 p

(5)

M0 and I represent the initial amounts of monomer and initiator, respectively. Equation 4 leads to two important conclusions. First, assuming the reaction has gone to completion, the degree of polymerization can be calculated by the molar ratio of monomer to initiator. Second, the molecular weight increases linearly with extent of reaction. Furthermore, according to eq 5, dispersity is ≈1 for significant reaction conversion (approximately p ≥ 0.2, but this depends on M0 and I). Thus, a controlled chain-growth polymerization mechanism can be proven experimentally by (1) completing several reactions and demonstrating that the molecular weight of the product is linearly proportional to the ratio of monomer to initiator; or (2) monitoring the progress of a single reaction and confirming that the molecular weight of the product increases linearly with monomer conversion. An additional, albeit less conclusive, indication that a reaction follows a controlled chain-growth polymerization is that the dispersity value does not change as a function of monomer to initiator ratio or monomer conversion (assuming the conversion is high enough such that Đ ≈ 1 according to eq 5). Another experimental indication of a living chain-growth mechanism (but not necessarily a controlled chaingrowth mechanism) is a chain-extension reaction. Once an initial reaction has gone to completion, if molecular weight continues to increase linearly upon a second monomer addition, this indicates no termination has occurred and the reaction is living. Once the reaction has gone to completion, active chain ends can be terminated by the addition of an appropriate reagent with no effect on molecular weight. If the rate of termination, (rt) is nonzero during the reaction, but rt ≪ rp, it is possible that a significant conversion can be achieved before a substantial amount of termination occurs. Although, for eqs 4 and 5 to hold true, it must be assumed that rt ≈ 0. As discussed in the Introduction, performance of electronic devices can vary significantly with molecular weight, dispersity, and end-groups of the conjugated polymers that constitute the active layer. Controlled chain-growth polymerization reactions, in which the resulting molecular weight follows a Poisson distribution, provide the most precise control over these properties. As previously mentioned, conjugated polymers are typically synthesized by transition metal catalyzed cross-coupling polycondensation reactions; these usually proceed by a stepgrowth mechanism. Nevertheless, under the right reaction conditions, these polycondensation reactions can follow a controlled chain-growth mechanism; for example, when the catalyst does not diffuse away from a growing chain, but rather “transfers” from monomer to monomer as the polymerization proceeds. In the next two sections, we will discuss recent progress in chain-growth polymerizations of conjugated polymers.

Figure 2. Reported methods for chain-growth polymerizations of conjugated polymers. For all reactions, the success of a chain-growth mechanism is highly dependent on the monomer used. In most cases, the affinity of the monomer to the transition metal catalyst is critical.

tion reaction into a “catalyst transfer polymerization” (CTP). These reactions rely on a system-specific catalyst-monomer affinity that prevents the catalyst from diffusing away from a growing chain after reductive elimination. Kumada Catalyst Transfer Polymerization (KCTP). In 1999, McCullough and co-workers reported the synthesis of regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) by Kumada-type coupling with a [1,3-bis(diphenylphosphino)propane]nickel(II) chloride (Ni(dppp)Cl2) catalyst.77 Five years later, it was independently reported by McCullough and Yokozawa that this method of P3HT synthesis exhibits characteristics associated with a controlled chain-growth mechanism; molecular weight increases linearly with monomer conversion, molecular weight increases linearly with increasing M0/I, and Đ remains relatively low and steady throughout the reaction (Figure 3).78,79 It was

Figure 3. Evidence of a controlled chain-growth mechanism for P3HT synthesis. (a) Mn increases linearly with conversion. (b) After all monomer has been consumed, Mn is given by the molar ratio of initial monomer to catalyst ([M]0/[Ni(dppp)Cl2]0). Adapted with permission from ref 79. Copyright 2004 American Chemical Society.

proposed, for the first time, that upon reductive elimination the nickel catalyst forms a π-complex with the electron-donating thiophene monomer. This complex prevents the catalyst from diffusing away from the growing chain and leads to an intramolecular oxidative addition, facilitating a chain-growth mechanism. The success or failure of a chain-growth mechanism is strongly dependent on the stability and lifetime of this catalyst association. The living chain-growth nature of P3HT polymerizations is a remarkable finding and has been extensively studied and

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CATALYST TRANSFER POLYMERIZATIONS There are numerous examples of chain-growth polymerizations for conjugated polymers. The various methods are listed in 7890

DOI: 10.1021/acs.iecr.7b01030 Ind. Eng. Chem. Res. 2017, 56, 7888−7901

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is likely that ri is slow enough relative to rp such that initiation is not complete when propagation begins. Because of this, Đ ∼ 1.004 is not possible. It is also possible that the associated complex formed upon reductive elimination was not ideal and did not always remain associated with the growing chain, causing some termination or chain-transfer reactions. This would cause Đ to increase slightly during the polymerization from 1.2 to 1.3. In practice, there will inevitably be several factors that prevent a reaction from following an ideal controlled chain-growth mechanism. Furthermore, if some amount of uncontrolled termination during the reaction is unavoidable, this will become increasingly evident at higher molecular weight increases as the reaction approaches completion. Thus, synthesis of high molecular weight conjugated polymers by a controlled chaingrowth mechanism is especially challenging. In recent years, KCTP reactions that proceed by a controlled chain-growth mechanism have also been demonstrated for other electron-rich monomers including furan,93 selenophene,94−96 tellurophene,97 pyrrole,98 thiazole,99,100 cyclopentadithiophene,101 and para-phenylene.102 Examples of electron-deficient monomers polymerized by a controlled chain-growth KCTP are very limited. Challenges associated with KCTP of electron-deficient monomers begin in the first step of the reaction; many electron-deficient monomers are incompatible with the metal−halogen exchange step required to form the organometallic monomer. The polymerization step is also challenging in that most electron-deficient monomers only have weak π-donation toward metal catalysts and do not form a stable associated complex.103 Yokozawa and co-workers synthesized the electron-deficient pyridine monomer with polymerization at the 3,5-position (meta type). It was demonstrated that Mn increased linearly with conversion, Mn increased linearly with the molar ratio of monomer to catalyst, and Đ remained relatively low (∼1.2) for all reactions. This was the first report of a controlled chain-growth mechanism of an electron-deficient monomer.104 We note that when the polymerization is moved to the 2,5-position (para type), a chain-growth mechanism could not be verified due to very low solubility.105 When solubility is increased by synthesizing a 2,5-pyridine-alt-3hexylthiophene polymer, the polymerization proceeds by an uncontrolled chain-growth mechanism.106 Recent work from the Seferos group may provide insight on how controlled chain-growth polymerizations can be developed for other electron-deficient monomers. Four nickel catalysts with electron-donating ligands were designed in an attempt to maximize stability of the associated π-complex and facilitate intramolecular oxidative addition.107,108 Polymerization proceeds with all catalysts. A nonliving chain-growth mechanism was observed for the two least electron-donating catalysts. Strong evidence for a controlled chain-growth mechanism was observed for the most electron-donating catalyst, [N,N′-dimesityl-2−3(1,8-naphthyl)-1,4-diazabutadiene] nickel(II) bromide (Ni(MesAn)Br2). The molecular weight of the product increases linearly with monomer consumption and the dispersity remains relatively constant at about 1.5 (Figure 4). Dispersity is higher than an ideal controlled chain-growth reaction, possibly due to relatively slow initiation. The second-most electron-donating catalyst, acenaphthylene(1,2-diylidene)bis(2-methoxyaniline nickel(II) bromide (Ni(OMeAn)Br2), exhibits evidence of an intermediate-type mechanism between chain-growth and controlled chain-growth. The molecular weight increased linearly only until a conversion of ∼40%. Dispersity was relatively constant, but higher than

reviewed throughout the past several years. As it stands, the nickel π-complex has never been isolated from a successful reaction. Despite this, compelling evidence for the CTP mechanism has been established in the literature.80,81 We will not discuss P3HT synthesis by KCTP or the mechanism in detail, but refer the reader to other excellent reviews on this topic.82−87 Soon after the development of KCTP for P3HT, efforts turned to using this method for the controlled synthesis of other conjugated polymers. When poly(9,9-dioctylfluorene) (PF8) is synthesized by Kumada coupling using a Ni(dppp)Cl2 catalyst, the polymerization proceeds by a chain-growth mechanism. The reaction does not proceed by a controlled chain-growth mechanism due to termination reactions, possibly caused by incomplete metal−halogen exchange of the Grignard reagent.88,89 Similar results were obtained for PF8 synthesis with [1,3-bis(diphenylphosphino)ethane]dichloro nickel(II) (Ni(dppe)Cl2), bis(triphenylphosphine) nickel(II) chloride (Ni(PPh3)2Cl2), and [1,3-bis(2,6-diisopropylphenyl)imidazol-2ylidene](3-chloropyridyl) palladium(II) (NHC ligated Pd); [1,1′-bis(diphenylphosphino)ferrocene] nickel(II) chloride (Ni(dppf)Cl2) will not polymerize fluorene at all.88,90 When nickel(II) acetylacetonate/diphenylphosphinopropane (Ni(acac)2/dppp) is used as the catalyst, polymerization of PF8 proceeds by a controlled chain-growth mechanism and is able to produce polymers with Mn ranging from 3 to 62 kg/mol. Mn increases linearly with conversion and all Đ values are about 1.2. When the PF8 octyl side chains are adjusted slightly to phenyl and triphenylamine groups, polymerizations with Ni(acac)2/ dppp result in dispersities ranging from ∼1.3 to 1.4, suggesting decreased control.91 If the Ni(acac)2/dppp ligand is modified to Ni(acac)2/dppp(m-Me), in which the dppp ligand has a methyl group on the meta position of the phenyl rings, this out-performs the Ni(acac)2/dppp system. PF8 can be synthesized in a controlled manner up to 91 kg/mol with Đ ∼ 1.2−1.3 throughout the polymerization. In contrast, Ni(acac)2/dppp(oMe), in which the methyl groups are incorporated on the ortho position of the phenyl rings, polymerizes PF8 by a nonliving chain-growth mechanism. Mn increases rapidly and then plateaus around 30 kg/mol. When the Ni(acac)2/dppp ligand is modified to Ni(acac)2/dppp(p-Me), the methyl groups are attached at the para position of the phenyl rings, results similar to the original Ni(acac)2/dppp system are obtained.92 Polyfluorene exemplifies a few very important aspects in the development of chain-growth syntheses for conjugated polymers. First, when developing a catalyst transfer polymerization of a new monomer, it is critical to distinguish the mechanism between step-growth, chain-growth, or controlled chain-growth through kinetic investigations. Second, we note that the mechanism depends strongly on the interplay between the catalyst and monomer, resulting in system-specific catalystmonomer interactions. For maximum control over end-groups and molecular weight, it is necessary to match carefully the catalyst/ligand to the monomer. This often requires extensive catalyst design and optimization. Despite the evidence for a controlled chain-growth polymerization, dispersity values are much greater than might be expected even when the highest degree of control is realized with the Ni(acac)2/dppp(m-Me) catalyst/ligand system. According to eq 5, PF8 synthesized by a controlled chain-growth mechanism that is 91 kg/mol should have Đ ∼ 1.004 (assuming the reaction has gone to completion). Yet, authors report Đ ∼ 1.2−1.3. In this reaction, dispersity does not change much with conversion, but is significantly higher than what eq 5 suggests. It 7891

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mers have been synthesized by externally initiated KCTP, enhancing control over molecular weight. External initiation can also be used as a method for end group functionalization; all chains will be functionalized with the Ar2 moiety of the initiator. If Ar2 contains functional groups capable of further reaction, external initiation can provide a synthetic route for more complex structures including block copolymers,113,114,116 surface-grafted polymers,110 and polymer brushes.111 Interestingly, there is evidence that external initiation can enhance control not only by ensuring unidirectional growth, but also by enhancing the rate of initiation relative to the rate of propagation. To achieve a controlled chain-growth mechanism, it is necessary that ri/rp ≫ 1. The McNeil group investigated the effect of different external initiators on the polymerization of a para-phenylene monomer by KCTP.102 During the reductive elimination step, the associated π-complex increases the electron density on the catalyst. DFT calculations reveal that when these charges are able to delocalize onto the Ar2 moiety of the external initiator, the activation free energy associated with the initial reductive elimination step is stabilized and the rate of initiation increases. Furthermore, the enhanced charge delocalization is unique to the Ar2 moiety of the external initiator and thus, the rate of propagation is not effected. By selecting the most appropriate ligand, a phenyl ring functionalized with trifluoroethoxy and dimethylamine substituents, ri/rp is increased. Control over the polymerization of para-phenylene is enhanced and dispersity values of ∼1.1 are achieved. Nevertheless, this does not affect the potential for chain transfer and termination reactions to occur. Thus, although control is enhanced, dispersity values remain higher than those of a truly ideal controlled chaingrowth polymerization. Suzuki Catalyst Transfer Polymerization (SCTP). Palladium-catalyzed Suzuki polycondensation is one of the most common methods used to synthesize conjugated polymers. In 2007, it was demonstrated that Suzuki coupling can also proceed by a controlled chain-growth mechanism. Yokozowa and coworkers reported the controlled chain-growth synthesis of PF8 from the asymmetric monomer 2-(7-bromo-9,9-dioctyl9Hfluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane using a palladium catalyst with a bulky electron-donating ligand, (tertbutylphosphine)phenylpalladium(II) bromide (tBu3PPd(Ph)Br).118 Molecular weight increased linearly with conversion as well as the molar ratio of monomer to catalyst. The reactive Pd complex is more likely to undergo oxidative addition than diffuse away from the growing polymer chain, thereby promoting a chain-growth mechanism.119 Unlike KCTP, the monomer does not have to be generated in situ, eliminating difficulties associated with incomplete metal−halogen exchange and unreacted Grignard reagents. In general, SCTP is relatively less controlled than KCTP and is more successful with monomers that consist only of carbon and hydrogen; the reasons for these trends have yet to be elucidated. Controlled chain-growth SCTP has been reported for fluorene,118,120,121 para-phenylene,122 and thiophene120,123,124 monomers. Analogous to palladium-catalyzed KCTP, an unconventional nickel-catalyzed SCTP was recently developed by Noonan and co-workers. 125 P3HT and poly(3-hexylesterthiophene) (P3HET) were synthesized using Ni(dppp)Cl2 and 1,3-bis[2,6-bis(1-methylethyl)phenyl]-1,3-dihydro-2H-imidazol-2ylidene(triphenylphosphine) nickel(II) chloride (Ni(PPh3)IPrCl2), respectively. A controlled chain-growth mechanism was indicated by molecular weight modulation with catalyst

Figure 4. Plot of Mn and Đ as a function of monomer consumption in the polymerization of electron-deficient benzotriazole using the Ni(MesAn)Br2 catalyst. A linear relationship between Mn and monomer consumption combined with a constant Đ throughout the reaction suggests a controlled chain-growth mechanism. Adapted with permission from ref 108. Copyright 2014 American Chemical Society.

expected for a controlled polymerization with values around 2. This work exemplifies the requirement in catalyst transfer polymerization for careful design and pairing of the catalyst and monomer. Traditionally, KCTP reactions incorporate a nickel catalyst. Nevertheless, there is at least one example of palladium-catalyzed KCTP. P3HT and poly(2,5-bis(hexyloxy)phenylene) (PPP) were successfully synthesized by Pd-catalyzed KCTP and shown to follow a controlled chain-growth mechanism.90 For both thiophene and para-phenylene monomers, Mn increases linearly with conversion and monomer/catalyst ratio. Although, under the same reaction conditions, polymerization of fluorene follows a nonliving chain-growth mechanism. Significant chain termination reactions were observed by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS) end-group analysis, possibly due to incomplete metal−halogen exchange. This promising result suggests more polymers may be synthesized by Pd-catalyzed KCTP, and that changing the metal species away from nickel may broaden the scope of KCTP. KCTP is most easily performed by in situ initiation. This is when the polymerization begins by addition of a NiL2X2 (or PdL2X2) catalyst, such as Ni(dppp)Cl2, to the MgX−Ar1−Br monomer following the metal−halogen exchange step. External initiation of KCTP can enhance control over the polymerization. An external initiator typically takes the form of Ar2−NiL2Br. The polymerization begins with a transmetalation step between the Ar2−NiL2Br initiator and a MgX−Ar1−Br monomer, followed by reductive elimination, followed by an intramolecular oxidative addition to the Ar1−Br bond. Polymerization then proceeds according to the typical CTP mechanism; in principle, all chains will have an Ar2 group on one end. External initiation ensures unidirectional growth. Assuming Ar2 does not contain an aryl-halide bond, there is no opportunity for the catalyst to undergo oxidative addition except at the Ar1− Br bond. Bidirectional growth, which can occur when polymerization is initiated in situ by a NiL2X2 catalyst, will decease control over molecular weight and increase dispersity.109 Thiophene,103,110−117 selenophene,112 and para-phenylene102 mono7892

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chain-growth process. Nevertheless, the authors note that the reaction does not proceed by a living chain-growth mechanism, likely due to chain termination and reinitiation processes.131 The activated zinc method was further developed and shown to polymerize a bithiophene perylene diimide monomer using the analogous radical-anion monomer.132 Interestingly, polymerization would not proceed with any nickel catalyst tested, but worked well with bis(acetonitrile)dichloropalladium(II)/tertbutylphosphine (Pd(CH3CN)2/PtBu3). Again, a nonliving chain-growth mechanism was established; molecular weight does not follow a linear trend with monomer consumption, but high molecular weight products are seen at low levels of conversion. Interestingly, when this Pd(CH 3 CN) 2 /P tBu 3 catalyst is used with the bithiophene naphthalene diimide monomer, the reaction does not follow a chain-growth mechanism at all; the molecular weight is unaffected by the ratio of monomer to initiator.133 The development of the active zinc method provides an excellent opportunity to gain a deeper understanding of catalyst transfer polymerizations for electrondeficient monomers and further emphasizes the importance of careful monomer/catalyst pairing.

loading and chain extension of either P3HT or P3EHT to form a block copolymer. One exciting development in the field of SCTP came from the Kiriy group with the successful chain-growth polymerization of an alternating fluorene-benzothiadiazole monomer.126 Kinetic studies confirmed a chain-growth reaction mechanism as monomer concentration decreased gradually throughout the reaction, relatively high molecular weights were produced at low monomer conversions, and dispersity remained relatively low and constant (∼1.1−1.3). Nevertheless, a controlled chaingrowth mechanism could not be established and only molecular weights up to 10 kg/mol were obtained. These results are especially important as most high-performing conjugated polymers utilize this alternating electron-rich, electron-deficient architecture; they provide a foundation for further experiments toward a living chain-growth mechanism for SCTP of highperforming alternating copolymers. Stille Catalyst Transfer Polymerization (StCTP). Similar to Suzuki polycondensation, palladium-catalyzed Stille polycondensation is a popular method used to make conjugated polymers. Until recently, this method had only been used to prepare conjugated polymers by an uncontrolled step-growth mechanism. P3HT was prepared from the asymmetric monomer, 2-bromo-5-trimethylstannylthiophene and a controlled chaingrowth method confirmed by kinetic studies.127 For two sequential monomer loadings, the molecular weight increased linearly with monomer conversion and dispersity remained fairly constant at ∼1.2. These results conclusively demonstrate controlled chain-growth behavior with the absence of termination reactions. As with SCTP, StCTP uses asymmetric functionalized monomers and avoids challenges arising with in situ monomer synthesis. Negishi Catalyst Transfer Polymerization (NCTP). Negishi catalyst transfer polymerization (NCTP) was used to synthesize poly(dithienosilole) (PDTS) in a quasi-living chaingrowth mechanism.128 The molecular weight of the product is dependent on the ratio of monomer to initiator. Nevertheless, the dependence of molecular weight on conversion suggests only modest control, possibly due to chain-termination. Interestingly, dithienosilole cannot be polymerized in a controlled manner by KCTP due to incomplete metal−halogen exchange of Grignard reagents with the dithienosilole monomer.128,129 Further optimization may increase control over the polymerization and yield a controlled chain-growth mechanism. Zn Activated Catalyst Transfer Polymerization. An unusual, but exciting catalyst transfer polymerization method for highly electron-deficient monomers was recently reported by the Kiriy group. In an attempt to synthesize a bithiophene naphthalene diimide (TNDIT) polymer using NCTP, the expected organo-zinc derivative did not form upon the addition of active zinc to the dihalogenated naphthalene diimide monomer. Instead, an unusual radical-anion monomer was produced.130 Upon addition of a Ph-Ni(dppe)-Br catalyst, the radical-anion monomer polymerized rapidly. A chain-growth mechanism is apparent, as adjusting the catalyst load results in moderate control over the product molecular weight. Furthermore, high molecular weight polymers are formed at low values of conversion, precluding a step-growth mechanism. A catalyst transfer polymerization mechanism was indicated by 31P NMR that demonstrated the presence of a Br-TNDITNi(dppe)-Br complex. Such a complex would be the result of the CTP initiation. In addition, the end-groups are determined by the catalyst used, further verifying an initiation followed by a

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BEYOND CATALYST TRANSFER POLYMERIZATIONS Converting step-growth polycondensation reactions into CTPs is certainly the most common method to develop controlled chain-growth syntheses for conjugated polymers. Nevertheless, there are reports of techniques that do not follow the CTP mechanism. In an effort to minimize processing steps and circumvent the metal−halogen exchange required for many CTP reactions, the Luscombe group developed a method to synthesize P3HT by C−H functionalization of the thiophene at the 5-position.134 This was done by aurylating 2-bromo-3-hexylthiophene with chloro(tritert-butylphosphine)gold(I). The resulting monomer can be polymerized with [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene](3-chloropyridyl)palladium(II) dichloride (Pd-PEPPSI-iPr). A controlled chain-growth mechanism was demonstrated with kinetic experiments including a chainextension reaction and monitoring a linear increase in molecular weight as a function of monomer conversion. Intriguingly, most of the P3HT product was terminated with H end groups on both ends, suggesting polymerization does not proceed by a catalyst transfer process. The authors note that the mechanism has yet to be elucidated and further investigations are ongoing. A clever method for controlled chain-growth polymerization of conjugated polymers by monomer deactivation was developed by Koeckelberghs and co-workers. This was initially demonstrated for the synthesis of P3HT using a 2-dicyclohexylphosphino-2′,6′-diisopropoxy-1,1′-biphenyl)[2-(2-aminoethyl)phenyl]palladium(II) chloride (Pd(RuPhos) catalyst in a Negishi-type coupling reaction.135 In the metal−halogen exchange step, an aryl-Br bond is converted to an aryl-ZnBr bond. The ZnBr group deactivates the aromatic monomer to the Pd(RuPhos) catalyst and thus, the catalyst can only undergo oxidative addition to the aryl-Br bond at the end of a growing chain. After reductive elimination, the catalyst will diffuse away and proceed with an intermolecular oxidative addition at the terminal aryl-Br bond of another chain. Nevertheless, because the catalyst can only react with the end of a growing chain, it is a chain-growth mechanism. A controlled chain-growth polymerization mechanism was established for thiophene, fluorene, and selenophene by monitoring Mn as a function of conversion 7893

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Table 1. Monomers Used in Chain-Growth Polymerizations of Conjugated Polymers chain-growth method

Figure 5. Linear relationship between Mn and conversion provides evidence of a controlled chain-growth mechanism for the polymerization of P3HT using the monomer deactivation method. Reprinted with permission from ref 135. Copyright 2011 John Wiley and Sons.

used to synthesize P3HT with controlled degrees of regioregularity,136 gradient copolymers of thiophene and fluorene,137 and triblock copolymers of thiophene, fluorene, and selenophene.138 Table 1 summarizes all of the monomers discussed in this Review that have been successfully polymerized by a chaingrowth mechanism; chemical structures are illustrated in Figure 6. Other than KCTP, most methods are limited to only a few different monomers. Future efforts will likely increase the variety of monomers that can be polymerized by a chain-growth mechanism. Rational design of new monomer/catalyst systems will likely be inspired by reports of successful reactions, but also guided by insight into unsuccessful reactions.

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REPORTED LIMITATIONS: LEARNING OPPORTUNITIES In this rapidly growing field, failed experiments provide excellent opportunities to deepen our understanding as to why some reactions succeed, while others do not. Although negative results often go unpublished, there are a few examples in the literature (Figure 7) that are reviewed in this section. Thienothiophene (TT) is a popular building block of highperforming conjugated polymers. Thus far, attempts to polymerize an alkylthiolated TT by a chain-growth mechanism have proved unsuccessful.139 Authors demonstrated that in a Kumadatype coupling reaction with several different nickel catalysts, no polymer was formed. They were able to isolate a nickel−TT complex that was highly stable in solution. Although it has been shown that increasing the stability of the catalyst-monomer associated pair can facilitate a chain-growth mechanism, these results suggest an upper-limit to the stability of the complex.107,108 When the nickel catalyst chelates with the TT monomer, one of the thiophene rings retains complete aromaticity; the authors speculate that this creates the stabilizing effect that is detrimental to polymerization. On the basis of this work, we speculate that other fused ring structures may also present challenges in developing chain-growth polymerization mechanisms.

monomer

KCTP (Ni catalyzed)

electron rich thiophene fluorene furan selenophene tellurophene pyrrole thiazole cyclopentadithiophene para-phenylene electron deficient 3,5-pyridine benzotriazole

KCTP (Pd catalyzed)

electron rich thiophene para-phenylene

SCTP (Pd catalyzed)

electron rich fluorene para-phenylene thiophene electron deficient fluorene-benzothiadiazole

SCTP (Ni catalyzed)

electron rich thiophene

Stille CTP

electron rich thiophene

Negishi CTP

electron rich dithienosilole

Au(I) activated

electron rich thiophene

Zn activated

electron deficient naphthalene diimide perylene diimide

monomer deactivation

electron rich thiophene fluorene selenophene

Poly(p-phenylenevinylene) (PPV) was one of the earliest known semiconducting polymers, yet synthesizing PPV by a catalyst transfer polymerization has proved challenging. Although PPV can be synthesized by other living chain-growth polymerizations, these methods are not amenable to other conjugated polymers.140−142 This precludes the synthesis of welldefined all-conjugated block copolymers that incorporate PPV. Yokozawa and co-workers have shown that neither Kumada nor Suzuki type coupling reactions will yield a controlled chaingrowth mechanism for the synthesis of PPV. Polymerization did not occur when nickel catalysts were used and proceeded in an uncontrolled manner when a palladium catalyst was used.143 It was demonstrated that tBu3PPd(0) readily forms an intermo7894

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Figure 7. Reported limitations of chain-growth syntheses. Although unsuccessful results typically go unreported, these polymers represent examples of unsuccessful attempts at chain-growth polymerization and useful opportunities to deepen our understanding of the field.

Stille coupling reactions. In a recent publication, greater control was demonstrated when BDT was synthesized by Kumada coupling versus a conventional Stille polycondensation reaction. Dispersity values were much lower when Kumada coupling was used (∼1.3 vs 3.7). Nevertheless, a mechanism has yet to be confirmed through a more rigorous kinetic analysis.145 Intriguingly, benzo[2,1-b:3,4-b′]dithiophene (BDP), an isomer of BDT, cannot be polymerized by KCTP.146 Though several different conditions were tested, reactions only afforded low molecular weight oligomers at best. This may be a result of the “non-aromatic double bond” on the benzene ring. Inspired by the conclusions of Yokozawa and co-workers in their attempt to synthesize PPV by CTP, authors suspect the nonaromatic double bond facilitates intermolecular transfer of the catalyst.143,144

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EXAMPLE OF LEVERAGING CONTROLLED POLYMERIZATIONS OF CONJUGATED POLYMERS: BLOCK COPOLYMERS BY SEQUENTIAL CHAIN-GROWTH POLYMERIZATIONS Fully conjugated block copolymers can be used to control the mesoscale morphology in organic electronic devices.51,56,58,147−149 Most fully conjugated block copolymers that have been incorporated into electronic devices were synthesized by KCTP followed by an uncontrolled polycondensation reaction, or successive uncontrolled polycondensation reactions.148,149 To obtain well-defined architectures, it is desirable to synthesize fully conjugated block copolymers by sequential chain growth polymerizations. In principle, this will maximize control over block lengths and minimize homopolymer or multiblock impurities. There are several examples of fully conjugated block copolymers synthesized by sequential chain-growth polymerizations; most were synthesized by successive CTPs (Figure 8). Block copolymers formed by nickel-catalyzed KCTP in a one-pot synthesis include: poly(paraphenylene)-block-poly(pyrrole) (PPP-b-PP),98 poly(3-hexylselenophene)-block-poly(3-hexylthiophene) (P3HS-b-P3HT),150 poly(3-hexylthiophene)-blockpoly(benzotriazole) (P3HT-b-PBTAz) and PBTAz-b-P3HT,108 poly(3-hexylthiophene)-block-poly(cyclopentadithiophene) (P3HT-b-PCPDT),101 P3HT-b-P(3HT-alt-pyridine),106 PF8-b-

Figure 6. Structures of polymers synthesized by the various chaingrowth polymerization methods. As nomenclature can be inconsistent in the literature, structures are labeled according to the monomer used.

lecular CCPdCC complex with the nonaromatic double bond. This facilitates intermolecular transfer from the CC double bond of one chain to another and leads to an uncontrolled step-growth mechanism. Interestingly, experiments using model compounds suggest that formation of this CC PdCC complex can be suppressed if there are substituents at the ortho position of the CC double bond. Assuming an analogous effect will be observed, these substituents may facilitate intramolecular catalyst transfer and permit controlled chain-growth polymerization of polymers containing CC bonds.144 Benzo[1,2-b:,4,5-b′]dithiophene (BDT), a common building block of donor polymers used in high-performing organic photovoltaic devices, can be polymerized by both Kumada and 7895

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Figure 8. Structures of fully conjugated block copolymers synthesized by successive controlled chain-growth polymerizations. Order of monomer addition is usually important and irreversible. Nevertheless, there are some reports of block copolymers that can be synthesized by either sequence of monomer addition.

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P3HT,91 and PPP-b-P3HT.151,152 There is also a report of P3HT-b-PDTS synthesized by KCTP followed by NCTP.128 For the most part, the sequence of monomer addition is critical in forming a block copolymer. The catalyst will preferentially complex with the more electron rich monomer and thus this monomer must be polymerized as the second block to allow for catalyst transfer to the second monomer. Nevertheless, recent developments suggest that this can be circumvented if a more electron donating catalyst is used. This was demonstrated by the carefully designed nickel diimine catalyst for P3HT-b-PBTAz or PBTAz-b-P3HT108 and for palladium catalyzed Kumada coupling of PPP-b-P3HT or P3HT-b-PPP.90 There are also reports of fully conjugated block copolymers synthesized by successive SCTP. PF8-b-PPP can be synthesized by palladium-catalyzed Suzuki reactions.122 P3HT-b-P3HET and P3HET-b-P3HT can be synthesized by Ni(PPh3)IPrCl2 or Ni(dppp)Cl2 catalyzed Suzuki reactions.125 The Koeckelberghs group synthesized fully conjugated triblock copolymers using the monomer deactivation method described earlier. The metal−halogen exchange step, in which an aryl−Br bond is converted to an aryl−ZnBr bond, “deactivates” the monomer to the catalyst. The catalyst can only undergo oxidative addition to the aryl−Br bond at the end of a growing chain, thus enabling a chain-growth mechanism. Because the chain-growth mechanism is independent of the catalyst− monomer association, block copolymers can be synthesized successfully in any sequence of monomer addition. To demonstrate the robustness of this method, the Koeckelberghs group synthesized thiophene−fluorene−selenophene triblock copolymers in every possible sequence.138

CONCLUSIONS AND OUTLOOK In this Review, we discuss recent advances in chain-growth polymerizations of conjugated polymers. The most common technique to develop a chain-growth synthesis is to convert an uncontrolled polycondensation to a controlled chain-growth polymerization. This is accomplished by carefully designing and optimizing system-specific catalyst−monomer interactions, yielding a catalyst-transfer mechanism. The nature of this method necessitates verification of a step-growth, chain-growth, or controlled chain-growth mechanism using kinetic experiments. CTP is not the only way to achieve controlled chain-growth polymerizations for conjugated polymers. New techniques are emerging, including the monomer deactivation method. This method is particularly interesting as its success does not rely on the formation of an associated complex between the catalyst and growing chain. The most exciting recent developments are of chain-growth polymerizations for conjugated polymers that incorporate electron deficient monomers. Controlled chain-growth reactions have been demonstrated for polymers incorporating 3,5-pyridine and benzotriazole, whereas chain-growth reactions with modest control have been demonstrated for naphthalene diimide and perylene diimide. An uncontrolled chain-growth mechanism has been demonstrated for an alternating fluorene−benzothiadiazole polymer. Although the chain-growth polymerizations reported for some electron deficient monomers have not yet been optimized for maximum control, these reports are significant. Incorporation of electron deficient monomers, especially into an alternating electron-rich- electron-deficient architecture, is 7896

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Industrial & Engineering Chemistry Research characteristic of conjugated polymers used as the active material in high-performance electronic devices. The reports of unsuccessful controlled chain-growth syntheses of thienothiophene and benzodithiophene are also significant. These are two very common electron-rich building blocks of conjugated polymers used in organic electronic devices. Insight into why these reactions failed will be indispensable for the development of controlled chain-growth polymerizations for these monomers as well as new systems. Broadening the scope of controlled chain-growth polymerizations to include a wide range of monomers will facilitate the precision synthesis of the polymers most relevant to highperformance devices. We speculate that increased control over polymer molecular weight, dispersity, and end-groups will maximize performance of organic electronics. Controlled chain-growth polymerizations will also enable synthesis of welldefined advanced architectures, including block copolymers. This could extend industrial applications of conjugated polymers to water purification, biomedical devices, and sensors. We envision future studies will broaden the number of monomers capable of being polymerized by a controlled chaingrowth mechanism. This will likely be facilitated by the careful design of new catalysts and a greater understanding of catalystmonomer interactions. Progress in the field will be greatly aided by reports of successes and limitations that deepen the community’s understanding of controlled chain-growth polymerizations for conjugated polymers.

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Penn State, Dr. Gomez has won multiple awards, including the Oak Ridge Associated Universities Ralph E. Powe Junior Faculty Enhancement Award, the National Science Foundation CAREER Award, the Rustum and Della Roy Innovation in Materials Science Award, and the Penn State Engineering Alumni Society Outstanding Research Award.

Melissa P. Aplan received a Bachelor’s degree in Chemistry from the University of Maryland, College Park in 2012. From 2012 to 2013, she completed the Intramural Research Training Award program at the National Institutes of Health in Bethesda, Maryland. She is currently a Chemical Engineering Ph.D. student in Dr. Enrique D. Gomez’s laboratory at The Pennsylvania State University. Her research interests include the design, synthesis, and characterization of fully conjugated block copolymers for applications in organic photovoltaics.

AUTHOR INFORMATION

Corresponding Author

*E. D. Gomez. E-mail: [email protected].

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ACKNOWLEDGMENTS

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REFERENCES

Financial support from the Office of Naval Research under Grant N000141410532 is gratefully acknowledged.

ORCID

Enrique D. Gomez: 0000-0001-8942-4480 Notes

The authors declare no competing financial interest.

(1) Bin, H.; Zhang, Z.-G.; Gao, L.; Chen, S.; Zhong, L.; Xue, L.; Yang, C.; Li, Y. Non-Fullerene Polymer Solar Cells Based on Alkylthio and Fluorine Substituted 2D-Conjugated Polymers Reach 9.5% Efficiency. J. Am. Chem. Soc. 2016, 138, 4657. (2) Holliday, S.; Ashraf, R. S.; Wadsworth, A.; Baran, D.; Yousaf, S. A.; Nielsen, C. B.; Tan, C.-H.; Dimitrov, S. D.; Shang, Z.; Gasparini, N.; Alamoudi, M.; Laquai, F.; Brabec, C. J.; Salleo, A.; Durrant, J. R.; McCulloch, I. High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor. Nat. Commun. 2016, 7, 11585. (3) Hwang, Y.-J.; Li, H.; Courtright, B. A. E.; Subramaniyan, S.; Jenekhe, S. A. Nonfullerene Polymer Solar Cells with 8.5% Efficiency Enabled by a New Highly Twisted Electron Acceptor Dimer. Adv. Mater. 2016, 28, 124. (4) Jagadamma, L. K.; Al-Senani, M.; El-Labban, A.; Gereige, I.; Ngongang Ndjawa, G. O.; Faria, J. C. D.; Kim, T.; Zhao, K.; Cruciani, F.; Anjum, D. H.; McLachlan, M. A.; Beaujuge, P. M.; Amassian, A. Polymer Solar Cells with Efficiency > 10% Enabled via a Facile SolutionProcessed Al-Doped ZnO Electron Transporting Layer. Adv. Energy Mater. 2015, 5, 1500204. (5) Jung, J. W.; Jo, J. W.; Jung, E. H.; Jo, W. H. Recent progress in high efficiency polymer solar cells by rational design and energy level tuning of low bandgap copolymers with various electron-withdrawing units. Org. Electron. 2016, 31, 149. (6) Li, N.; Brabec, C. J. Air-processed polymer tandem solar cells with power conversion efficiency exceeding 10%. Energy Environ. Sci. 2015, 8, 2902. (7) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666.

Biographies

Enrique D. Gomez received his B.S. in Chemical Engineering from the University of Florida and his Ph.D. in Chemical Engineering from the University of California, Berkeley. After a little over a year as a postdoctoral research associate at Princeton University, he joined the faculty in the Chemical Engineering Department of the Pennsylvania State University in August of 2009 and is currently an associate professor. His research activities focus on understanding how structure at various length scales affects macroscopic properties of soft condensed matter. Currently, the work in the Gomez group examines the relationship between chemical structure, microstructure, and optoelectronic properties of conjugated organic molecules. During his time at 7897

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DOI: 10.1021/acs.iecr.7b01030 Ind. Eng. Chem. Res. 2017, 56, 7888−7901