Direct (Hetero)arylation Polymerization: Simplicity for Conjugated

Review pubs.acs.org/CR

Direct (Hetero)arylation Polymerization: Simplicity for Conjugated Polymer Synthesis Jean-Rémi Pouliot,† François Grenier,† J. Terence Blaskovits, Serge Beaupré, and Mario Leclerc* Département de Chimie, Université Laval, Quebec City, Quebec G1V 0A6, Canada ABSTRACT: Direct (hetero)arylation polymerization (DHAP) has recently been established as an environmentally benign method for the preparation of conjugated polymers. This synthetic tool features the formation of C−C bonds between halogenated (hetero)arenes and simple (hetero)arenes with active C−H bonds, thereby circumventing the preparation of organometallic derivatives and decreasing the overall production cost of conjugated polymers. Since its inception, selectivity and reactivity of DHAP procedures have been improved tremendously through the careful scrutinity of polymerization outcomes and the fine-tuning of reaction conditions. A broad range of monomers, from simple arenes to complex functionalized heteroarenes, can now be readily polymerized. The successful application of DHAP now leads to nearly defect-free conjugated polymers possessing comparable, if not slightly better, characteristics than their counterparts prepared using classical cross-coupling methods. This comprehensive review describes the mechanisms involved in this process from experimental and theoretical standpoints, presents an up-to-date compendium of materials obtained by this means, and exposes its current limitations and challenges.

CONTENTS

References

1. Introduction 1.1. Overview of the Synthesis of Conjugated Polymers 1.2. Mechanisms of Direct (Hetero)arylation Reactions 1.2.1. Mechanisms of Palladium-Catalyzed Cross-Coupling Reactions 1.2.2. Mechanisms of Concerted MetalationDeprotonation (CMD) 1.2.3. Concerted Metalation-Deprotonation: Studies of the Active Catalytic Species 1.2.4. Mechanism of a Heck-Type Direct (Hetero)arylation Reaction 2. Direct (Hetero)Arylation Polymerization 2.1. Synthesis of Poly(thiophene) Derivatives 2.1.1. Synthesis of Poly(3-alkylthiophene)s 2.1.2. Synthesis of Other Poly(thiophene) Derivatives 2.2. Arene on Arene Coupling 2.3. Arene on Heteroarene Coupling 2.3.1. Arene on β-Protected Heteroarene 2.3.2. Arene on β-Unprotected Heteroarene 2.3.3. Random terpolymers 2.4. Heteroarene on Heteroarene Coupling 2.4.1. β-Protected Heteroarenes 2.4.2. Nonalkylated Heteroarenes 3. Conclusion and Outlook Author Information Author Contributions Notes Biographies Acknowledgments © 2016 American Chemical Society

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1. INTRODUCTION Conjugated polymers are materials that combine the electronic properties of inorganic semiconductors and the mechanical and physical properties of organic polymers. Owing to their solubility in organic solvents, these macromolecules with extended conjugation length can be easily processed to form thin, uniform, and lightweight films using high-throughput methods such as roll-to-roll printing, inkjet printing, and spray-coating.1−3 These readily accessible processing techniques are undeniable advantages for the development of low-cost organic electronic devices such as solar cells,4,5 transistors,6,7 light-emitting diodes,8,9 memory devices,10,11 and sensing technologies.12−15 To suit the particular needs of such a wide range of applications, tailor-made conjugated polymers must be designed and prepared. For instance, conjugated aromatic backbones are often functionalized with electron-donating or electron-withdrawing moieties in order to fine-tune optical, electronic, and physical properties,16,17 whereas nonconjugated side-chains are used to improve solubility or impart additional functionality.18,19 The impact and the significance of the field of conjugated polymers research was emphasized when the Nobel Prize of Chemistry in 2000 was awarded to Shirakawa, MacDiarmid, and Heeger for their pioneering work on conductive polymers. To obtain such diverse polymeric structures, reliable and versatile formation of C−C bonds between two (hetero)aryl groups is needed. Over the years, palladium- (e.g., Stille, Suzuki) and nickel-catalyzed (e.g., Kumada, Negishi) cross-coupling

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Received: July 28, 2016 Published: November 3, 2016 14225

DOI: 10.1021/acs.chemrev.6b00498 Chem. Rev. 2016, 116, 14225−14274

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Figure 1. Overview of traditional C−C coupling techniques for the preparation of π-conjugated materials.

properties, which in turn impact their performance in organic devices. Robust, reproducible, and reliable polymerization methods are therefore of utmost importance. Unlike small organic molecules, it is not possible to remove side reaction byproducts such as homocoupling or branching, as they become covalently embedded within the polymer chain. According to the Carothers equation (eq 1), the degree of polymerization (xn̅ ) depends on the stoichiometric balance of functional groups r and on the degree of reacted functional groups p.49 In order to achieve high molecular weights, a high p value (approaching 1) is required.

reactions have proven to be robust and useful synthetic methods. These allow couplings of (hetero)aryl halides (-Cl, -Br, and -I) or pseudohalides with organometallic (hetero)aryl derivatives, providing versatile methods to synthesize conjugated homoand copolymers. Recent advances in catalysis research have extended palladium-catalyzed coupling reactions to include C−C bond formation using the sp2 C−H bonds of (hetero)aryl derivatives as coupling partners directly, foregoing the use of organometallic functions.20−25 This promising synthetic procedure results in improved atom economy, fewer synthetic steps, and the generation of only benign byproducts. These key advantages contribute to decreasing the overall production cost of conjugated polymers and could have far-reaching implications regarding their large-scale applicability.26 Direct arylation polymerization (DAP or DArP) has been studied intensively in the past few years and has been highlighted as a promising tool for the synthesis of well-defined conjugated polymers. Its scope now includes both aryl and heteroaryl derivatives, and as such, a more general terminology is direct (hetero)arylation polymerization (DHAP). This research field is relatively young, essentially emerging in 2010. Several short review, viewpoint, and perspective articles have already been published on this topic, though a comprehensive exposition of this rapidly evolving field is yet to be written.27−35 The present review aims to shed light on the synthesis of well-defined πconjugated polymers using DHAP, while providing an extensive and up-to-date summary of the most important aspects of this new and promising polymerization method by evaluating C−H bond reactivity and selectivity, the crux of this type of reaction.

The Carothers equation49 xn̅ =

1+r 1 + r − 2rp

(1)

r defines the reactant ratio and p defines the extent of the reaction at a given time. Accurate measurements of molecular weights and molar-mass dispersity are primordial. By far the most common and convenient way to measure molecular weights is by sizeexclusion chromatography (SEC). This method separates molecules or macromolecules based on their hydrodynamic radius in solution. However, as conjugated polymers can have strong aggregation properties, care must be taken to fully solubilize them, as aggregates will cause gross overestimation of both molecular weights and molar mass dispersity. To achieve more accurate SEC results for these materials, high temperature analysis with solvents such as 1,2-dichlorobenzene and 1,2,4trichlorobenzene are recommended.50 Superheated chloroform has also demonstrated superior performances for breaking conjugated polymer aggregates.51 Molecular weights measured in this way are relative and not absolute, as standards used in SEC are generally monodisperse polystyrene samples. However, conjugated polymers have larger hydrodynamic radii relative to

1.1. Overview of the Synthesis of Conjugated Polymers

A critical aspect of conjugated polymers lies in their preparation as their optical and electronic properties are intrinsically linked to their extended conjugation pathway. Parameters such as molecular weight,36−41 molar-mass dispersity,42,43 and the presence of structural defects44−48 are known to affect these 14226


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Figure 2. Schematic representation of the formation of regioregular homopolymers and alternating copolymers using cross-coupling methods.

reactions have now been transposed into efficient methods to form regioregular homopolymers from monomers bearing two different reactive groups (Figure 2). Another synthetic approach utilizes two bis-functionalized monomers to obtain alternating copolymers (Figure 2). This is by far the most common synthetic strategy since symmetric monomers are generally more easily synthesized and purified. Furthermore, this also allows the efficient synthesis of many structurally similar polymers from a library of prefunctionalized building blocks. Palladium-catalyzed cross-coupling methods have contributed significantly to the advancement of the field of conjugated polymers, but they are not flawless. Indeed, they require the functionalization of monomers with costly organometallic moieties, followed by tedious purification procedures. Stoichiometric quantities of organometallic byproducts are formed, such as toxic trialkylstannanes produced during Stille polycondensation. Palladium impurities, for example nanoparticles, are formed during polymerization and can be challenging to remove, which can be detrimental to electronic applications.74 However, some applications can actually benefit from these impurities.75 The development of novel polymerization methods for producing conjugated materials is an ongoing topic. Prime candidates for improved synthetic methods are cross-coupling reactions which bypass the need for preactivated organometallic arenes and heteroarenes. The most recent example is DHAP, featuring palladium-catalyzed cross-coupling between (hetero)aromatic C−H and C-halogen bonds (Figure 3). DHAP has imposed itself as an attractive method to procure well-defined and high molecular weight conjugated polymers and will be the main focus of this review.

their molecular weight than polystyrene due to their increased rigidity. This often leads to overestimations of molecular weight.52,53 1H NMR spectroscopy can also be used to obtain absolute molecular weights, though analysis of high molecular weight materials and materials with defects is difficult and molarmass dispersity cannot be obtained.54−56 MALDI-TOF spectroscopy can also be used for absolute molecular weight determination and provides information regarding molar-mass distribution.55,57 However, sample preparation and matrices must be optimized to allow for the ionization of polymeric chains, while at the same time avoiding fragmentation of the chains. To obtain high molecular weight polymers with extended πconjugation, multiple C−C bond couplings between two sp2 or sp hybridized carbon atoms are necessary. Initial examples included the Ziegler−Natta polymerization of acetylene58 and oxidative polymerization for the synthesis of poly(phenylene)s, poly(thiophene)s, poly(pyrrole)s, and poly(aniline)s.59−62 Following these early methods, the synthesis of poly([hetero]arene)s has increasingly relied on transition metal-catalyzed cross-couplings63 such as Heck,64 Kumada,65 Negishi,66 Suzuki,67 Stille68 and Sonogashira69 polycondensations (Figure 1). This popularity stems from their ability to form well-defined conjugated polymers from a broad scope of substrates in relatively mild conditions. While usually proceeding in a stepgrowth fashion, some examples of cross-coupling reactions feature chain polymerizations with living characteristics involving catalyst transfer. This was notably observed for the synthesis of highly regioregular poly(3-hexylthiophene) (P3HT) using Negishi and Kumada polycondensations.70,71 The first organometallic cross-coupling reactions used to synthesize conjugated polymers relied on nickel catalysts (Yamamoto, Kumada, and Negishi polymerization methods). However, many functional groups are incompatible with these reactions due to the nucleophilicity of the Grignard and organozinc intermediates involved in the Kumada and Negishi couplings, respectively. Greater compatibility with functional groups came with the advent of palladium-catalyzed crosscoupling reactions, such as the Suzuki and Stille reactions. Discovered in the 1970s and actively developed in the 1980s, these reactions allow for the formation of C−C bonds between aryl halides and more stable organometallic intermediates, such as organoboron and organostannane derivatives. Moreover, palladium catalysts are often more stable, rendering them compatible with a wider array of functional groups (such as nitro functions) and leading to less homocoupling side-reactions.72 However, it should be noted that the field of nickel catalysis is rapidly evolving and user-friendly reaction conditions utilizing air stable catalyst precursors are increasingly reported for crosscoupling reactions.73 It is noteworthy that the development of these reactions was recognized when the 2010 Nobel Prize in Chemistry was awarded to Suzuki, Heck, and Negishi for their contributions to this field. Palladium-catalyzed cross-coupling

Figure 3. Direct (hetero)arylation polymerization reaction.

1.2. Mechanisms of Direct (Hetero)arylation Reactions

As mentioned above, reactions which exhibit both high selectivity and yield are necessary to obtain well-defined, high molecular weight materials. Understanding the mechanism of the direct (hetero)arylation polymerization reaction, as well as potential competitive pathways, are of paramount importance in order to improve its efficiency. Direct (hetero)arylation reactions evolved from simple intramolecular reactions performed at high temperatures into highly selective intermolecular couplings at room temperature.76,77 Part of this work was achieved prior to the development of DHAP and served as the basis for initial polymerization experiments. However, new catalytic systems and mechanistic studies are still frequently reported. Therefore, the development of DHAP is not entirely independent from C−H bond activation reactions on small molecules. Rather, it could be said that both fields of research have become intertwined, with the former usually focusing on improving reactivity and C−H 14227


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or a pseudohalogen (e.g., sulfonates such as triflates). It is worth mentioning that the oxidative addition is often the rate-limiting step in palladium-catalyzed cross-coupling reactions,78 though other steps can be rate limiting. For example, transmetalation is frequently reported as the slowest step for Stille couplings.68,79 The Pd(II) intermediate bearing a Pd−Ar bond and an X-type ligand reacts with a nucleophilic organometallic aryl compound (R2-M) during the transmetalation step, transferring the second aryl group to the catalyst (forming LnR1PdR2) while eliminating M-X. The cycle ends with the regeneration of the active catalyst PdLn via the reductive elimination of the bis-aryl compound R1R2. The transmetalation step is generally what differentiates the various cross-coupling reactions from one another. The mechanism involved during this step varies depending upon the nature of the ligands, precatalyst, solvent, and organometallic compounds utilized.78,80−82 Direct (hetero)arylation reactions proceed using this same general catalytic cycle, with the key difference being that they do not involve a transmetalation step per se but rather a metalation step proceeding via the direct cleavage of a C−H bond. Several mechanisms have been proposed for this process. The most studied are (1) concerted metalation-deprotonation (CMD); (2) aromatic electrophilic substitution (SEAr); and (3) Heck-type arylation (see Figure 5). The elucidation of the reaction mechanisms of direct (hetero)arylation has been undertaken by combining experimental data and theoretical calculations, revealing that CMD is involved in most direct (hetero)arylation processes through the use of carbonate or carboxylate bases.83 Interestingly, recent reports have demonstrated that the competing Heck-type mechanism is at play when using specific reaction conditions, thereby offering alternative reaction selectivity when compared to the CMD process.84−88 1.2.2. Mechanisms of Concerted Metalation-Deprotonation (CMD). We present herein an overview of the discovery of the CMD mechanism and its importance in the understanding of direct (hetero)arylation reactions. This mechanism contributes to explaining the reactivity and selectivity of carbonate and carboxylate-assisted direct (hetero)arylation reactions, and several reviews have been devoted to it.83,89−91 Prior to the discovery of this mechanistic pathway, notable progress was

bond selectivity and the latter providing in-depth study of side reactions embedded within polymer chains. This section covers the mechanistic studies regarding the direct (hetero)arylation reaction of small molecules, a crucial step to pave the way to robust and versatile polymerization methods. We report below the discovery and understanding of the C−H bond cleavage transition state, focusing on the reactivity and selectivity of this process. Studies of the effect of ligands on the reactivity of the active palladium catalyst will also be discussed. Finally, recent developments in direct (hereto)arylation reactions undergoing a Heck-type mechanism will also be presented. 1.2.1. Mechanisms of Palladium-Catalyzed CrossCoupling Reactions. As illustrated in Figure 4, most

Figure 4. General mechanism of Pd-catalyzed cross-coupling reactions.78

palladium-catalyzed cross-coupling reactions involving (hetero)aryl molecules follow the same general catalytic cycle. This begins with the oxidative addition of a Pd(0) species (PdLn) to an electrophilic aromatic compound (R1-X), forming the intermediate LnR1PdX, where X is generally a halogen (i.e., Cl, Br, I)

Figure 5. Transition states for the carboxylate-assisted concerted metalation-deprotonation (CMD), aromatic electrophilic substitution (SEAr) and Heck-type reaction mechanisms using benzene as a model.88 14228


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with a transition state similar to the one calculated by Davies et al.96 Following Echavarren’s study, Fagnou and colleagues investigated reactions using electron-poor fluorinated benzenes.99 In competitive reactions, the most electron-deficient substrates reacted more readily. The proposed mechanism, corroborated by DFT calculations, also involved a CMD transition state. Since then, several reports were published demonstrating similar results for a wide variety of substrates.89,91 In-depth mechanistic studies generally dismiss the SEAr mechanism as DFT calculations fail to isolate the key cationic Wheland intermediate. Furthermore, high primary kinetic isotope effects are often observed in direct (hetero)arylation reactions, which indicate a rate-determining C−H bond cleavage. This is uncharacteristic of SEAr mechanisms, where deprotonation and re-establishment of aromaticity generally occur rapidly.100 However, some recent examples have demonstrated that direct (hetero)arylation reactions can undergo non ratedetermining CMD transition states with limited to no kinetic isotope effect.101−103 Thus, kinetic isotope effects alone cannot be used to establish (or dismiss) the correct mechanism. In another account, Fagnou and co-workers developed a better understanding of direct (hetero)arylation selectivity.104 Activation-strain analysis was used to deconstruct the CMD enthalpic contributions to the transition state energy into factors associated with (1) bond distortion and (2) the interaction between substrates. The distortion factor stems from the deformation of the palladium-ligand bond (Edist[PdL]), as well as the out-of-plane bending and elongation of the C−H bond (Edist[ArH]). The interaction energy (Eint) is determined by the bond strength between the palladium atom and the π-orbitals of the aromatic compound. Distortion energies account for the high energy transition state, while the Eint counteracts this by stabilizing the transition state (see Figure 7). Edist(PdL) is

made in achieving highly reactive direct (hetero)arylation catalytic systems. However, up until 2005 the vast majority of reactions reported were either intramolecular or intermolecular by way of strongly coordinating directing groups.20 The first successful example of a nondirected intermolecular direct (hetero)arylation was published by Fagnou and co-workers using benzodioxole derivatives.92 In these early studies, several important observations were made which guided future mechanistic investigations. For instance, the utilization of carbonate or carboxylate bases was found to be essential to obtain the desired C−C coupling. Carbonates have also been mixed with catalytic amounts of bulky carboxylic acids to enhance reactivity. This was rationalized by the fact that the carboxylate anion acts as a proton shuttle between the carbonate base and the homogeneous catalytic species.93 Fagnou et al. demonstrated that iodide can be a poison to the catalyst in certain conditions, which explains, for some cases, lower reaction yields associated with using aryl iodides instead of bromides, and highlights the importance of sequestering iodide ions.93 These early reports focused on developing reaction conditions affording high yields and selectivity. Heck-like or SEAr processes were often suggested as reaction mechanisms, although early mechanistic investigations were often incomplete or inconclusive. Moreover, some results were incompatible with these mechanisms, such as high primary kinetic isotope effect and excellent reactivity with electron-poor substrates. The first clues that a different mechanism was at play were revealed with the investigation of stoichiometric palladation of benzene derivatives. As early as 1985, Ryabov and co-workers proposed that the ortho-palladation of N,N-dimethylbenzylamine featured a concerted, base-assisted mechanism.94 Similar conclusions were drawn from theoretical calculations by Sakaki et al. in 2000, with the deprotonation of benzene with formate and subsequent palladation.95 DFT calculations performed in 2005 by Davies and MacGregor demonstrated that C−H bond cleavage of N,N-dimethylbenzylamine did not involve a Wheland (arenium) intermediate with a positive charge on the aromatic ring, as would be expected in a SEAr mechanism. Rather, the most accessible route featured an agostic C−H interaction with only negligible positive charge transferred to the aromatic ring (Figure 6). An intramolecular acetate-assisted deprotonation afforded

Figure 7. Enthalpic contributions to the CMD transition state energy, as divided into the factors of distortion (Edist) and interaction (Eint).104

94

Figure 6. Palladium intermediates, as proposed by (a) Ryabov, Sakaki,95 and (c) Davies.96

generally negligible, which means that the transition state is mainly governed by the antagonist E dist (ArH) and E int contributions. Eint is more important in electron-rich compounds, which explains the higher reactivity of more πnucleophilic substrates. On the other hand, electron-poor compounds are more reactive due to a generally lower Edist(ArH) barrier, which is linked to the acidity of the C−H bond.99 This antagonist effect makes predicting selectivity and reactivity difficult without performing DFT calculations beforehand. Fagnou and Gorelsky made such calculations on a large number of compounds and developed a classification for the reactivity and selectivity of (hetero)arenes in the CMD process (see Figure 8).104 For class A molecules, the Edist(ArH) is the determining factor, and the reaction will typically occur at the most acidic C−H bond position. On the other hand, the

(b)

the palladated product.96 Soon after this computational study, Echavarren and co-workers performed competitive intramolecular reactions using conditions developed by Fagnou and colleagues (i.e., palladium acetate, a bulky phosphine ligand and K2CO3 in DMAc).97,98 Selectivity toward substituted phenyls was observed, rather than toward unsubstituted ones, regardless of the electronic influence of the substituents. This was not expected in a SEAr mechanism, in which more nucleophilic substrates should react more readily. Aided by density functional theory calculations (DFT), they proposed a CMD mechanism, 14229


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Figure 8. Classification of various arenes and heteroarenes based on the factors which govern selectivity, with the most favorable sites for a CMD event (in color).91

steric and electronic influences of ligands within the coordination sphere. Phosphine ligands are popular choices in cross-coupling reactions, with high binding energy to palladium and often significant steric bulkiness. Amine ligands have also been reported in direct (hetero)arylation reactions.24 This helps to control the reactivity and site selectivity of the catalyst while preventing the formation of inactive palladium aggregates, such as palladium black. For example, electron-rich ligands increase oxidative addition kinetics, while reducing the rate of reductive elimination and transmetalation. The opposite trend is observed for electron-poor ligands.82 The electron-donating ability of the ligand can thus be modulated to improve the kinetics of the ratedetermining step. For instance, the C−H bond cleavage is often the rate-determining step in direct (hetero)arylation, and electron-poor phosphines quicken this process.76,97,105 The increased electrophilicity of the Pd center using these ligands increases the Eint which reduces the transition energy for the CMD process.106,107 Hartwig et al. recently isolated and characterized (PtBu3)Pd(2MeC6H4) (OPiv), an organometallic palladium compound with

reactivity of class B compounds is principally determined by the Eint and reactions occur at the most nucleophilic position. Selectivity in class C compounds is determined by a combination of both effects. Thiophene compounds, important building blocks for conjugated polymers, are part of this third category and as such, their reactivity can be difficult to predict. For instance, substituents in the 5 position can either enhance (e.g., N-pyrrolidine, cyano, and fluoride) or decrease (e.g., alkyl and ketone) the reactivity of thiophenes, without a clear correlation with their electronic influence. Fortunately, several heterocycles, notably five-membered heteroarenes (such as thiophenes, pyrroles, and indoles), possess an intrinsic electronic bias favoring α substitution when a CMD mechanism is involved, facilitating their use in polymerization reactions. 1.2.3. Concerted Metalation-Deprotonation: Studies of the Active Catalytic Species. With strong evidence in favor of the CMD transition state, a better understanding of the role of the active catalyst involved in direct (hetero)arylation is also necessary to develop more efficient catalytic systems. Two of the most important aspects of the palladium catalyst activity are the 14230


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a bulky phosphine and a tolyl pendant group.108 This hindered palladium compound was reacted in an equimolar ratio with benzene, but no desired 4-methylbiphenyl was formed. This result is in contrast with the high catalytic activity reported previously for reactions involving Pd(OAc)2 and PtBu3 added separately.109 However, the authors observed that reagents displacing or oxidizing the phosphine ligand significantly enhanced reactivity. Calculated CMD energy barriers were much higher for the Pd ligated by P(tBu)3 than ligated by the N,N-dimethylacetamide solvent, due to the increased steric hindrance of the former. The same study reported that Davephos (2-dicyclohexylphosphino-2′-[N,N-dimethylamino]biphenyl) also slowed the direct (hetero)arylation reaction compared to “‘ligandless”’ conditions. In contrast, Ozawa and colleagues isolated [PdAr(μ-O2CR) (PPh3)]n, a species which reacted readily with 2-methylthiophene to form cross-coupling products.110 They demonstrated that the catalyst formed inactive dimeric or tetrameric species, and that increased steric bulk in the aryl or acetate ligands promoted the formation of the active monomeric species (see Figure 9). Balancing steric influences on the catalyst is therefore critical in order to obtain good reactivity in direct (hetero)arylation reactions.

Figure 10. Equilibrium between the dimeric, inactive species and the monomeric, active species of the catalyst using a P(o-OMeC6H4)3 phosphine ligand.103

meric form of the catalyst in apolar solvents while retaining the catalyst’s ability to form CMD transition states by displacing one of its coordinating atoms from the metal center. In contrast, [PdAr(OAc)(PPh3)]n species do not readily form monomeric species in apolar solvents unless they bear bulky Ar pendant groups. Apolar solvents, due to their inability to stabilize the polar monomeric active catalyst, require ligands which destabilize the polymeric form of the catalyst. P(o-NMe2C6H4)3, a phosphine with a similar bis-ligating ability, has also been used successfully in polymerization reactions. Another example of bidentate ligands used in direct (hetero)arylation reactions are bis(phosphine) ligands. Given the generally accepted CMD transition state with only one phosphine ligand present on the active catalyst, the reactivity of such ligands is quite unexpected. The need to displace one of the phosphines present on these ligands seems highly unfavorable energetically, and one would expect these ligands to be inferior to monodentate phosphine ligands. However, high yields were achieved using these ligands, as demonstrated by Echavarren et al.120 Recent work by Eastgate and Blackmond demonstrated the stoichiometric palladation of ethyl-4-bromo-1ethyl-5-nitro-1H-pyrrole-2-carboxylate with PdCl2(Xantphos) and KOPiv.121 The metalation product obtained was an unexpected compound possessing a mono-oxidated bis(phosphine) ligand. In their study, only the mono-oxidated bis(phosphine) ligand afforded reactive intermediates for C−H activation. Addition of another equivalent of Xantphos displaces the mono-oxide derivative and inhibits reactivity. This basepromoted in situ formed ligand presents a hemilabile phosphine oxide moiety which can be easily displaced to open a coordination site on the palladium to form the CMD transition state (see Figure 11). It is noteworthy that this type of hemilabile phosphine oxide was previously proposed to also stabilize the catalyst following reductive elimination.122 Bis(phosphine) ligands have been scarcely used in DHAP, but this novel insight sheds light on the importance of stoichiometry when using these ligands. As there are few phosphines capable of yielding high molecular weight polymers in apolar conditions, this is an interesting development which opens new possibilities for DHAP. An often overlooked parameter of cross-coupling polymerization is the activation of the catalyst precursor, which can be a critical factor in reducing defects. The active catalyst is a Pd(0) species, but a majority of direct (hetero)arylation reactions use more air-stable Pd(II) precursors, such as palladium acetate.20,23 A common reduction pathway for Pd(OAc)2 in cross-coupling reactions is the intramolecular Pd(II) reduction through the oxidation of the phosphine ligands of a PdL2(OAc)2 compound formed in situ.123 This reaction can be more challenging with

Figure 9. Equilibrium between the inactive polymeric species of the catalyst and the monomeric active species.110

Solvents with coordinating abilities such as DMF and DMAc can serve as ligands in very reactive, phosphine-free conditions.111−114 Regardless of their role as ligand, polar solvents have been favored for direct (hetero)arylation reactions due to high reaction rates. However, such solvents may not be ideal with the synthesis of high molecular weight conjugated polymers flanked with alkyl side chains, due to the limited solubility of growing conjugated polymer chains in such polar conditions. This issue will be discussed in section 2. Less polar solvents such as THF and dioxane isomers have been used successfully in direct (hetero)arylation, and nonpolar solvents such as toluene and xylenes are also often used in polymerization reactions. Bulky phosphines such as PCy3, P(tBu)2Me, and P(tBu)3 have been successfully utilized in such solvents.115−118 The solvent/ precatalyst/ligand combination can also affect regioselectivity of the C−H activation of some molecules. As demonstrated by Lévêque, Leclerc, and co-workers, changes in these parameters led to the activation of thiazoles on either the 2 or 5 positions, resulting in synthetic pathways to small conjugated molecules which did not require the use of any protecting groups.119 More recently, P(o-MeOC6H4)3 has been used as a very efficient ligand in apolar solvents and is often used in polymerization reactions. Isolated complexes of [PdAr(OAc)(P(o-MeOC6H4)3)]n were found to be dimeric or monomeric in the solid state.103 In solution, they readily dissociate into two distinct monomeric species which exist in equilibrium between two forms: one in which acetate acts as the bidentate ligand and the other in which P(o-MeOC6H4)3 serves this purpose (see Figure 10). The [PdAr(OAc)(P(o-MeOC6H4)3)]n species reacted readily to form cross-coupling products, regardless of solvents used or the Ar ligand bulkiness. The bidentate, hemilabile P(o-MeOC6H4)3 greatly increases the active mono14231


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Figure 11. Displacement of the phosphine oxide on the oxidized Pd(OPiv)2(Xantphos) via a Berry-like rotation mechanism.121

Figure 12. Palladium reduction via the oxidative C−H/C-H coupling of bithiophene in phosphine-free conditions.133

Several other publications have shown similar dependence with regards to α- vs β-selectivity.85−87

electron-rich ligands, but can be effectively promoted using water123,124 and a base.125,126 Pd(II) species can also undergo reduction using alcohols,127 amines,128 and organometallic compounds such as aryllithiums,129 arylboranes,130 organostannanes,131 and diboronic esters.132 Reduction of Pd via the oxidation of organometallic species generates homocoupling products, as these will transmetalate twice and perform reductive elimination to afford the Pd(0) catalyst. A similar phenomenon has been documented by Kanbara and colleagues in phosphinefree direct (hetero)arylation reactions. Direct metalation of C−H bonds by way of Pd(OAc)2 occurs, followed by homocoupling with another C−H bearing molecule, resulting in the formation of the active Pd(0) species following reductive elimination (Figure 12).133 A reduction in catalyst loading was found to alleviate this homocoupling issue. 1.2.4. Mechanism of a Heck-Type Direct (Hetero)arylation Reaction. As mentioned above, five-membered heteroarenes such as pyrroles, thiophenes, and indoles possess an intrinsic electronic bias which favors α-substitution when a CMD mechanism is involved.91 However, different selectivity can be achieved by favoring another, competing mechanism which is of lower energy than the CMD process. Itami et al. recently showed that it is possible to adapt the direct (hetero)arylation reaction selectivity (>99%) to the β C−H bonds for thiophene, thienothiophene, and benzothiophene derivatives through the use of P(OCH(CF3)2)3 along with PdX2 (X = Cl, Br, or I) or Pd2dba3 as the precatalyst (see Figure 13).84 This switch from α- to β-selectivity is entirely catalyst-driven.

Figure 13. Reaction conditions leading to β-selective direct (hetero)arylation products, as proposed by Itami and colleagues.84

A study by Fu et al. offered an interesting explanation for this. DFT calculations suggested that using P(OCH(CF3)2)3 as the phosphine promotes a Heck-type mechanism.88 Unlike a standard Heck mechanism, this particular case features an antiβ proton elimination following the insertion of thiophene on the catalyst (see Figure 14), due to the inability of the cyclic C−C bonds of thiophene to rotate into a syn configuration. Despite this, the Heck-type pathway presents a lower energy barrier than the CMD process. In-depth DFT experiments failed to isolate electronic or steric influences which could be responsible for this mechanistic change. Rather, it was the mildly acidic nature of the C−H bond present on the phosphine ligand which caused Hbonding of P[OCH(CF3)2]3 with the carbonate base on the catalyst. This bond was strong enough (11.2 kcal·mol−1) to elevate the energy barrier of a CMD transition state over that of a Heck-type mechanism. Larossa et al. recently published reaction conditions showing similar selectivity by using a mildly acidic solvent, 1,1,1,3,3,3-hexafluoro-2-propanol.134 In their study, 14232


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Figure 14. Heck and Heck-type reaction mechanisms described by Fu et al.88

Figure 15. Example of homocoupling defects in Suzuki and Stille polycondensations.47,138

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kinetic isotope effects of both 2H and 13C, corroborated by DFT modeling, provided compelling evidence that a Heck-type pathway was indeed involved. This new selectivity remains largely unexplored in conjugated polymers and could be beneficial for deliberate functionalization of the materials in the β position or for the synthesis of two different polymers from the same precursors.

and mechanical properties. This feature is frequently used to evaluate the selectivity of DHAP conditions, and optimizations are often performed to improve regioregularity. Therefore, molecular weights are not the only figure of merit for polymerizations involving poly(thiophene)s. 2.1.1. Synthesis of Poly(3-alkylthiophene)s. Poly(3hexylthiophene) (P3HT) is by far the most studied semiconducting polymer for plastic electronics applications. This material is regularly utilized as a reference to evaluate new synthetic methods for the preparation of conjugated polymers as many characteristics, such as the regioregularity and crystallinity, can be easily monitored and correlated with regiodefects. P3HT presents an asymmetrical repeat unit, which can lead to head-totail (HT), head-to-head (HH), and tail-to-tail (TT) configurations along the polymer chain (Figure 16). Head-to-head configuration features two adjacent alkyl side-chains sterically repelling each other, causing twisting of the conjugated backbone and thus reducing the effective conjugation between repeat units. 139 Breakthroughs in synthetic methods, reported independently by Rieke140 and McCullough,141 produced the highly regioselective head-to-tail polymerization of P3HT using nickel-catalyzed Negishi and Kumada cross-coupling polymerizations, respectively. Improvements in the crystallinity and the device properties of regioregular P3HT over its regiorandom counterpart were extensively demonstrated. However, small amounts (1−2%) of head-to-head and tail-to-tail defects are still observed in these materials with most optimized methods, though regioregular P3HT without observable defects can be obtained.142 The same trend is observed with other polymerization methods, such as Stille and Suzuki polycondensations.44 The very first example of DHAP was provided by Lemaire et al. in 1999 with the synthesis of poly(3-alkylthiophene)s (P3AT) with different side-chains. These were synthesized directly from 2-iodo-3-alkylthiophenes (Figure 17; monomers 1−4).143 At

2. DIRECT (HETERO)ARYLATION POLYMERIZATION On the basis of all these studies performed on small molecules, DHAP should provide a shorter synthetic pathway and an attractive alternative for the cost-effective, environmentally benign synthesis of conjugated polymers.135,136 For example, a recent study on a naphthalene diimide copolymer showed a decrease of 35% of the cost for the polymerization by DHAP compared to Stille polymerization.137 However, to allow DHAP to become a useful approach, it must be able to produce materials which offer at least comparable or even superior performance to those obtained using the classical polymerization methods. A prime concern for these reactions was the C−H bond selectivity which, if insufficient, could lead to defects in the polymers. For this reason, publications focusing on DHAP often provide an indepth study of polymerization side-reactions. Such thorough studies are seldom performed for Stille and Suzuki polymerizations, as they were generally assumed to deliver defect-free materials. However, defects arising from the homocoupling of two C−Br bonds have been documented in Suzuki polycondensation (see Figure 15).47 Stille-synthesized copolymers can produce similar C−Br/C−Br homocoupling as well as coupling between two organostannane compounds.138 These polymerization methods are thus not always devoid of side reactions but still offer high performance materials. As such, a figure of merit for DHAP is that it leads to a rate of defects which is inferior, or at least comparable, to that which was observed in a given polymer prepared using other cross-coupling techniques. Along these lines, this section summarizes reports found in the literature regarding DHAP, with emphasis on the analysis of defects and on the progress made to avoid side-reactions. A first subsection will be devoted to the synthesis of poly(thiophene)s, notably regioregular poly(3-hexylthiophene), mainly through the homopolymerization of monobrominated thiophene or oligothiophene monomers. Copolymerizations will be divided according to the nature of the monomers used, with arene on arene, arene on heteroarene and heteroarene on heteroarene couplings presented in separate subsections.

Figure 17. First example of polymerization via direct C−H activation, involving Heck-like conditions for the synthesis of P3AT’s.143,145

2.1. Synthesis of Poly(thiophene) Derivatives

that time, the polymerization was thought to be a Heck-type coupling reaction. The polymerization was performed at 80 °C using palladium(II) acetate (Pd(OAc)2) as the precatalyst, potassium carbonate (K2CO3) as a base, and N,N-dimethylformamide (DMF) as the solvent. A phase transfer agent, tetrabutylammonium bromide (Bu4NBr), was used in stoichiometric quantity in order to increase the catalyst reactivity. These reaction conditions are often referred to as Jeffery’s conditions (Figure 17; P1−4) and had been initially developed for highyielding Heck cross-couplings.144 Although the polymerization yields reached 90%, only low molecular weight materials with ∼90% of head-to-tail (HT) couplings were obtained. Replacement of the 2-iodo-3-octylthiophene (4) monomers with 2-bromo-3-octylthiophene (5) did not improve the molecular weights or regioregularity of the polymer (Figure 17; P5). With the brominated derivative, neither different bases (NaOAc or KHCO3), phase transfer agents ((C2H5)4NBr or

This section highlights the work done for the synthesis of poly(thiophene)s using monohalogenated thiophene or oligothiophene monomers. Such homopolymers, obtained from asymmetric thiophene monomers are notable for their regioregularity (dyad structural motifs, as shown in Figure 16), a parameter of the utmost importance for their electronic, optical

Figure 16. Dyad configurations in P3HT due to monomer unit asymmetry. 14234


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with high number-average molecular weight (M̅ n) of 30.6 kg/mol and regioregularity (RR > 98%), a notable improvement from previously published conditions. The regioregularity obtained is comparable to Kumada, Negishi, Suzuki,39 and Stille-prepared P3HT.147 Variations on the previous conditions were investigated by Ma et al. with the utilization of bidentate phosphines (Figure 19) with greater coordinating ability than P(o-NMe2C6H4)3.148 An increased amount of base was also used, from 1 to 2 equiv. These changes enabled the reaction to occur at higher temperatures (e.g., 130 °C). Of the phosphines assessed, only P(oOMeC6H4)3 performed well and yielded a polymer with M̅ n of 37 kg/mol, whereas dppe, dppf, and P(m-MeC6H4)3 led to the formation of oligomers. Even though the conditions using P(oOMeC6H4)3 contributed to an increase in number-average molecular weight, lower regioregularity (86%) and yield (78%) were obtained. Replacing the base (Cs2CO3) with potassium acetate (KOAc), potassium pivalate (KOPiv), cesium fluoride (CsF), or potassium carbonate (K2CO3), or changing THF for toluene, o-xylene, or 1,4-dioxane, resulted in inferior catalytic activity or total inactivity. In another study, Ozawa et al. used a preformed, well-defined palladium complex [PdPh(μ-Br)(P(o-tolyl)3)]2.149 The purpose of this was to allow the first coupling to be the selective formation of a C−C bond between the pendant phenyl group present on the catalyst and 2-bromo-3-hexylthiophene (6), resulting in a polymer chain end-capped with a single aryl group (Figure 20).

(C2H5)3(CH2Ph)NBr), and solvents (toluene, 1,4-dioxane, 1methyl-2-pyrrolidinone, chloroform, and THF) nor an increase in reaction temperature (150 °C) improved the regioregularity or the molecular weights of the resulting polymers.145 The authors proposed two competitive chain termination pathways in order to explain the low reactivity of the catalytic system: (1) the reduction of the C-halogen bond to an unreactive C−H bond and (2) the homocoupling of two C-halogen bonds. Mass spectrometry of both polymers synthesized using iodinated or brominated monomers confirmed the presence of dehalogenation, while homocoupling experiments using Ullman reaction conditions on the iodinated monomer were inconclusive. In 2010, Ozawa et al. revisited the synthesis of poly(3alkylthiophene)s using DHAP and provided the first example of high molecular weight P3HT.146 Superheated tetrahydrofuran (THF) at 120−125 °C was utilized as the polymerization medium rather than polar DMF. The catalytic system consisted of the Herrmann-Beller precatalyst (trans-di(μ-acetato)bis[o(di-o-tolyl-phosphino)benzyl]dipalladium(II), Pd(Herrmann)), a phosphine ligand and cesium carbonate (Cs2CO3) as the base (Figure 18). Among the numerous ligands tested, only P(o-

Figure 18. Ozawa and colleagues’ synthesis of P3HT.146

OMeC6H4)3 and P(o-NMe2C6H4)3 led to polymeric materials along with high regioregularity, with the latter offering the best selectivity. It was proposed that the coordinating ability of ortho functionalities of the ligands, rather than electronic or steric effects, was the determining parameter in their performance. Nonbidentate, electron-rich (P[p-OMeC6H4]3) and electronpoor (P[o-FC6H4]3) ligands led to oligomers. The palladium source was chosen because it features high thermal stability when compared to Pd(OAc)2. Catalyst activity was demonstrated between 110 and 130 °C, after which catalyst degradation occurs. With these reaction conditions, the authors obtained a P3HT

Figure 20. In-situ aryl-bromide end-capping of P3HT.149

However, only 75% of the polymer chains were end-capped despite the fact that additional ArBr was added, which resulted in a decrease in molecular weights. In order to better understand the early stages of this polymerization, they performed timecontrolled experiments and found discrepancies between endcapped ratios at different reaction times. End-capping groups were present in 41% of polymer chains until monomer

Figure 19. Variations of Ozawa’s conditions by Ma and colleagues.148 14235


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Figure 21. Competitive pathways for end-capped polymers, as proposed by Ozawa and colleagues.149

Figure 22. Preparation of P3HT from 2-bromo-3-hexylthiophene (6) and 2-bromo-4-hexylthiophene (7).150

used, 1% of TT homocoupling arising most likely from C−Br/ C−Br homocoupling is observed. No homocoupling or branching reactions occurred from the positions adjacent to the alkyl side-chain. Thompson et al. reported a different route to regioregular P3HT.151 The authors opted for reaction conditions put forward by Fagnou for direct (hetero)arylation reactions, with Pd(OAc)2, K2CO3, a catalytic amount of pivalic acid (PivOH) and the polar aprotic solvent DMAc (Figure 23). Reaction temperature, time

conversion attained 76%. After that, the end-capped ratio increased slightly, up to 46%, for 92% monomer conversion, indicating the presence of competitive reaction pathways. In their report, Ozawa et al. proposed an equilibrium which could lead to the reductive elimination of bromobenzene from [PdPh(μBr)(P(o-tolyl)3)]2, forming a Pd0 complex in situ. The newly formed Pd0 species would then become susceptible to an oxidative addition on 2-bromo-3-hexylthiophene (6), leading to a polymer chain without phenyl end groups (Figure 21). Once all of the monomer is consumed, end-capped and non end-capped ligands polymeric chains may combine in a step-growth fashion, yielding higher end-cap ratios and molecular weights. Nevertheless, polymeric materials displayed a nearly perfect regioregular backbone (98−99% HT couplings), indicating an excellent selectivity in C−C couplings. The importance of the bromine position on 3-hexylthiophene to afford well-defined polymers was investigated by Leclerc, Ozawa, and Li.150 Two regioisomers, namely 2-bromo-3hexylthiophene (6) and 2-bromo-4-hexylthiophene (7), were copolymerized using Ozawa’s conditions (Figure 22). βunprotected, α-halogenated monomer 7 led to a P3HT with lower M̅ n (16 kg/mol, ĐM = 1.3) than the β-protected, αhalogenated monomer 6 (M̅ n of 33 kg/mol, ĐM = 1.7). From the dyad (HT) analysis, regioregularity for both polymers was over 99.5%. However, in-depth analysis of the 1H NMR spectrum allowed the identification of TT homocoupling defects at a level of 0.5%, which arise from C−H/C−H homocoupling of monomer 6. It is noteworthy that such defects are rarely analyzed as they cannot be identified from the dyad peaks, but provide valuable insight regarding this type of defect. When 7 was

Figure 23. Analysis of β-branching of phosphine-free DHAP conditions for P3HT.153

and catalyst concentration were investigated. More structural defects were observed with higher catalytic loads. Decreasing the reaction temperature from 120 to 20 °C improved the regioregularity from 82.6 to 89.3% but decreased the numberaverage molecular weight from 39 to 14 kg/mol. Optimized conditions used a low catalyst load of 0.25 mol %, with reaction time of 48 h at 70 °C to yield a polymer with a higher regioregularity of 93.5%, albeit with limited M̅ n of 16 kg/mol. The addition of neodecanoic acid (NDA) to the reaction increased M̅ n to 20 kg/mol while retaining the same regioregularity.152 In a follow-up paper, Rudenko and Thompson studied the formation of β-branching using these reaction conditions.153 They found that the bulkier additive NDA 14236


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Figure 24. Development of hyper-branched P3HT materials by promoting β-branching.157

Figure 25. Example of an N-heterocyclic carbene catalyst in the DHAP preparation of P3HT.158

hypothesized that the steric influence rendered unfavorable the reaction at the more hindered 4-position of 3-hexylthiophene, leading to limited or total suppression of branching side reactions. In another study, they assessed the role of the amide solvent in the same reaction conditions.156 Increased molecular weights were obtained with N,N-dimethylpropionamide and N,N-diethylacetamide. After Soxhlet extractions, which were not performed in previous studies, M̅ n values of 31.5 and 32.5 kg/mol were obtained with these two solvents respectively, and up to 23.5 kg/mol with DMAc. All formamide solvents tested led to lower molecular weights. Additional steric hindrance equal to or greater than isopropyl functions became detrimental to both M̅ n and yield, whereas regioregularity was not affected. Using an amide bearing an aromatic substituent led to its incorporation in the polymer backbone. A balance between steric hindrance and the ability of the amide function to coordinate the metal center seems necessary to ensure good catalytic activity of phosphinefree polar reaction conditions. In a different approach, Luscombe et al. designed a set of experimental conditions which exploited usually undesirable side-reactions to achieve a hyper-branched P3HT.157 The degree of branching, which quantifies the dendritic nature of the resulting polymeric structure, was estimated from 1H NMR spectroscopy with the aid of model compounds. Accordingly, broad α-methylene signals in the 2.18−2.38 ppm region were identified as β-coupled thiophenic structures. Using Pd(OAc)2/ K2CO3 or PdCl2/KF catalytic systems, both in DMAc, a wide range of the degree of branching (0−40%) could be obtained depending on the supporting ligands utilized (ligandless, PPh3, diphenylphosphinoethane (dppe), Xphos, bpy and TMEDA, see Figure 24 for chemical structures). The highest degrees of branching were achieved using PdCl2 as the precatalyst without the intervention of a ligand. Materials free of β-defects were isolated with TMEDA and bpy ligands with the Pd(OAc)2/

prevented the formation of branched structures. These studies demonstrated that careful optimization of DHAP conditions can yield selective cross-coupling polymerization on α positions of thiophene in polar conditions. To push further the optimization of the DHAP parameters, the same authors studied the effect of the catalyst load.154 A significant drop of both M̅ n and yield were noted when the amount of Pd was reduced from 0.25 to 0.0625%. During these experiments, the effective palladium concentration was maintained by reducing the volume of solvent. This led to a rapid precipitation of the polymer chains out of the solution, preventing high molecular weights to be reached. To overcome this difficulty, the temperature was increased from 70 to 100 °C and the M̅ n increased accordingly from 9.9 to 16.1 kg/mol. Best results were obtained by further reducing the amount of precatalyst to 0.0313 mol %, combined with a higher temperature (160 °C). M̅ n of 24.2 kg/mol with a yield of 91% and regioregularity of 96.5% were obtained. Lowering the concentration of palladium reduced the 1H NMR spectroscopic signal believed to originate from debrominated end groups. Surprisingly, a higher content of debrominated chains had no correlation with lower molecular weights. The authors proposed that the debromination side-reaction occurs at a slower rate than the coupling reaction and therefore may mainly occur once chain growth has ceased. Thompson et al. further investigated their low catalyst load polymerization system with numerous carboxylic acid derivatives.155 Primary, secondary, tertiary, and cyclic carboxylic acids were scrutinized. The bulkiness of the acid additive was found to be a key factor for both molecular weights and yield, with limited influence on regioregularity. Best results were obtained with NDA and neotridecanoic acid (NTDA), with M̅ n of 23.5 kg/mol (Y = 75%) and 26.5 kg/mol (Y = 75%) for NDA and NTDA, respectively, with regioregularity reaching 96%. The authors 14237


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(9 or CNT) units in a HT regioregular arrangement. The incorporation of CNT into the polymer matched the feed ratio, indicating that a cyano-based monomer can be efficiently used in DHAP. 1H NMR spectroscopy confirmed both the regioregular, albeit random, nature of the copolymers and did not reveal any βcoupling defects. DHAP and Stille-prepared polymers displayed subtle differences with regards to the distribution of 3HT and CNT unit within the polymer chain, as evidenced by minor 1H NMR spectroscopic discrepancies. Molecular weights and yields were consistent between the two methods given the same monomer ratios. Another example of a random poly(thiophene) derivative was reported by Coughlin et al. in which a phosphine-based catalytic system in superheated THF was used to incorporate 3-hexyl-4fluorothiophene (10) units in P3HT to modulate its electronic properties (Figure 28).163 Upon increasing the feed ratio of 10 (0−100%), a systematic drop in the regioregularity (from 95 to 78%) was observed, presumably due to a difference in monomer reactivity. Thompson et al. synthesized P6 from monomer 11, bearing a thermo-cleavable ester side chain (Figure 29).164 The ester moiety served to induce an improved reactivity and selectivity by means of the chelation of the palladium species. The regioregularity was increased by 3% (RR = 96%) for P6 compared to P3HT (RR = 93%) synthesized using the same conditions. Oxidative C−H/C−H coupling using nonbrominated 11 was also used to obtain P6, with a low regioregularity of 86%. Leclerc et al. proposed a model system, poly(3,3‴-didodecyl2,2′:5′,2″:5″,2‴-quaterthiophene) (P7), to evaluate the selectivity of α/β couplings without homocoupling interference.165 Two structural isomers with β-unprotected positions leading to P7 were polymerized, with monomer 12 featuring alkyl chains near the terminal β position (Figure 30). In this study, UV−vis spectroscopy and DSC measurements proved to be more useful to indirectly identify minute quantities of defects than NMR spectroscopy. Monomer 12 gave a well-defined material with M̅ n of 43 kg/mol using the Herrmann-Beller precatalyst along with P(o-NMe2C6H4)3 in 1,4-dioxane. Monomer 13 gave slightly lower molecular weights (M̅ n = 29 kg/mol) and required the use of NDA as an additive for optimal results. Thus, steric hindrance in the γ positions (monomer 12) suffices to prevent branching. In the case of monomer 13, NDA prevents β-branching at the expense of reactivity. Optical and thermal properties were found to be in line with P7 synthesized using Stille or chemical oxidative (FeCl3) polymerization.

K 2 CO 3 system, although low molecular weights were obtained.157 N-Heterocyclic carbene-based catalysts were also investigated using polar conditions for the preparation of highly regioregular P3HT and poly(3-hexylselenophene) (P3HS).158 Carbenoid ligands offer high thermal stability due to their strong coordinating ability to transition metals such as palladium. Commercially available bulky NHC-precatalysts were tested for the synthesis of P3HS. The best result for P3HS was obtained using a Pd-IPr/K2CO3/PivOH/DMAc catalytic system (Pd-IPr = [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene](3chloropyridyl)palladium(II) dichloride) with M̅ n of 6.3 kg/mol, Y = 41% and RR = 93% % (Figure 25). By using the same catalytic system, P3HT with M̅ n = 26.9 kg/mol and regioregularity of 94% was obtained in a low yield of 57%. Broader molar-mass dispersity (3.56) was attributed to a slow initiation of the Pd-IPr catalyst system. Limited M̅ n values for P3HS compared to P3HT was assumed to be a result of the low solubility of P3HS in DMAc. Heterogeneous catalysts offer recyclability and facile separation from the reaction medium. Their previous use in direct (hetero)arylation reactions suggested that the active catalyst is still in fact homogeneous in nature,159 but examples of crosscoupling polymerizations using Pd/C have nonetheless shown a reduction of residual metal content in the resulting purified polymers.160 Hayashi, Kojima, and Koizumi reported a catalytic system involving Pd/C for the synthesis of highly regioregular P3HT (Figure 26).161 Using N-methylpyrrolidine at 120 °C (48

Figure 26. Example of DHAP conditions for preparing P3HT using a heterogeneous catalyst.161

h), they isolated P3HT with M̅ n = 16.3 kg/mol (Y = 91%) and regioregularity of 97%. The addition of tricyclohexylphosphonium tetrafluoroborate (PCy3·HBF4) in either DMAc or NMP led to the formation of a large quantity of insoluble material. Pearlman’s catalyst (Pd[OH]2/C) outperformed Pd/C when the reaction temperature was carefully maintained at 100 °C, leading to P3HT with M̅ n = 18.9 kg/mol (Y = 99%) and regioregularity of 96%. 2.1.2. Synthesis of Other Poly(thiophene) Derivatives. As mentioned above, P3HT is unarguably the most studied polymer among poly(thiophene)s derivatives, but several other thiophene-based materials have been synthesized using DHAP. One of the earliest examples was devoted to the synthesis of a random poly(3-hexylthiophene-co-cyanothiophene) (P3HTCNT), as shown in Figure 27.162 This copolymer features a random distribution of 3-hexylthiophene and 3-cyanothiophene

2.2. Arene on Arene Coupling

In parallel, one of the first alternating copolymers prepared by DHAP was reported by Kanbara et al. in 2011.166 Based on DFT calculations and excellent reactivity reported for small molecules,99 1,2,4,5-tetrafluorobenzene (14) was chosen as the C−H monomer with 9,9-dioctyl-2,7-dibromofluorene (15) as the

Figure 27. Synthesis of P3HT-CNT.162 14238


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Figure 28. Synthesis of random derivatives of partially fluorinated P3HT with varying degrees of regioregularity.163

Figure 29. Synthesis of P6.164


Figure 31. Synthesis of tetrafluorophenyl- and octafluorobiphenyl-based copolymers P8−12.166,168

system. The reaction was also only possible in DMAc and DMF, as no reaction occurred in toluene or dichlorobenzene. Moreover, only a slight increase of molecular weights was observed for longer reaction times (over 24 h), which was believed to be due to the fact that either the growing polymer chain becomes insoluble and precipitates out of the reaction medium or the catalyst becomes inactive. 1H, 13C, and 19F NMR spectroscopic experiments confirmed the alternating chemical

dibrominated comonomer (Figure 31; P8). The authors utilized a Pd(OAc)2/PtBu2Me·HBF4/K2CO3 catalytic system in DMAc to obtain P8 (M̅ n = 31.5 kg/mol; Đm = 3.45; Y = 81%). Unlike the synthesis of P3HT in polar conditions, the reaction did not proceed without PtBu2Me·HBF4. Also, no reaction occurred when PtBu3-HBF4 or 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (S-Phos) was used, which indicated that increasing the steric hindrance of the phosphine can hamper the catalytic 14239


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Figure 32. Optimized conditions used to obtain high molecular weights P8, as reported by Ozawa and co-workers.170


Figure 34. Synthesis of fluorinated porous organics copolymers P14−16.172,173

to obtain P12 from 2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-diphenylene (19), a monomer with a higher C−H reactivity. Similarly to experiments comparing thiophene to bithiophene,169 moving from a phenyl derivative (14) to a biphenyl (19) species increased the C−H reactivity and facilitated the direct (hetero)arylation reaction. Thus, polymerization of 19 with 15 led to a polymer with higher M̅ n (43.2 kg/mol) and enhanced yield (95%). However, the 19F NMR spectrum of P12 indicated that a small amount of defect was formed with monomer 19. The synthesis of P8 was revisited by Ozawa et al. in 2013 using apolar conditions, closely related to those used to obtain highly regioregular P3HT (see section 2.1.1).170 Several palladium precatalysts were investigated, and Pd2dba3·CHCl3 gave the best results (Figure 32). This was the first successful utilization of Pd2dba3·CHCl3 in DHAP conditions, and a rare example of superior reactivity from a Pd(0) versus a Pd(II) precatalyst in this type of polymerization. Changing the ligand to PPh(oOMeC6H4)2, P(o-NMe2C6H4)3, P(o-tolyl)3 or PPh3 led to lower molecular weights. By adding pivalic acid, very high M̅ n (347 kg/mol, ĐM = 2.83) was achieved in 96% yield. It is noteworthy that replacing THF by either toluene or cyclopentyl

structure of the copolymer without any evidence of branching. The synthesis of this polymer was later revisited by Koizumi et al. using phosphine-free conditions, which led to M̅ n of 20 kg/mol with a yield of 86%.167 To expand the scope of this reaction, other aryl bromides were polymerized with the previously optimized conditions, namely Pd(OAc)2/PtBu2Me·HBF4/K2CO3 in DMAc (Figure 31; P912).166,168 19F NMR spectroscopic analyses of P8 and P11 showed a single resonance peak, demonstrating the alternating structures of these polymers without noticeable defects. However, P9 was found to be insoluble, presumably due to cross-linking. This was potentially caused by the higher nucleophilicity of the C−H bonds at the 3 and 6 positions of the carbazole derivative. 19F NMR and MALDI-TOF spectra of P10 showed an alternating structure with some structural defects (branching). Reactions using model compounds N-octadecyl3,6-dibromocarbazole and pentafluorobenzene led to the identification of these undesirable side reactions.166 Carbazole C−Br/C−Br homocouplings and oxidative C−H cross-coupling between 14 and 17 were identified as the main source of branching defects. Further investigation was undertaken in order 14240


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Figure 35. Synthesis of P17 and P18.174

Figure 36. Unsuccessful attempt to copolymerize 15 with 26, as reported by Kanbara and colleagues.176

polymers using 1,3,5-tribromobenzene (23) or tetrakis(4bromophenyl)methane (21) with 14 (Figure 34, P14 and P16).172 Although insoluble in common organic solvents, incorporation of each monomer in the polymer structures was confirmed by CP/MAS 13C NMR spectroscopy and X-ray photoelectron spectroscopy analyses. Solid-state structural analyses did not rule out the existence of above-mentioned C− H/C-H or C−Br/C−Br homocoupling side-reactions. A very similar material was reported by Feng using 2,2′,7,7′-tetrabromo9,9′-spirobifluorene (22) with 14 as comonomers (see Figure 34, P15).173 In 2016, another report by Hayashi and Koizumi featured hyper-branched materials based on 2,4,6-tris(4bromophenyl)-1,3,5-triazine (25), copolymerized with either 14 or 1,3,5-trifluorobenzene (24), as shown in Figure 35 (P17 and P18).174

methyl ether led to polymers with similar properties, whereas commonly used polar solvents such as DMF and DMAc completely suppressed the catalytic activity. An analog of P8, in which comonomer 14 is replaced by 5,6difluoro-2,1,3-benzothiadiazole (20), was reported by Zhang et al. (see Figure 33).171 This polymer, P13, was obtained using the same polymerization conditions described by Kanbara for the synthesis of P8. However, only low molecular weights were obtained (M̅ n = 4.8 kg/mol). Using PivOH as an additive and replacing DMAc with toluene increased M̅ n to 41.0 kg/mol, with an 86% yield. These conditions failed to produce polymeric materials when 2,1,3-benzothiadiazole and 5,6-dioctyloxy-2,1,3benzothiadiazole were used instead of 20. It was hypothesized by the authors that only the electron-deficient monomer 20 possessed C−H bonds reactive enough to be activated by this particular catalytic system. This emphasizes the fact that DHAP conditions often need to be tailored to specific substrates. P13 was also synthesized using Suzuki cross-coupling, yielding a polymer with M̅ n of 15.2 kg/mol. Although the Suzuki-prepared P13 displayed lower M̅ n, UV−vis absorption spectra showed a bathochromic shift of 4 nm of the maximum of absorption in chloroform solution compared to the maximum observed for the polymer prepared by DHAP. This discrepancy may indicate the presence of structural defects within the conjugated backbone, affecting the conjugation length. Branching side-reactions were identified as minor signals by 19F NMR spectroscopy. Also, 1H NMR spectroscopic analyses revealed a high content of debromination of 15, indicating that dehalogenation may possibly be a molecular weight-limiting side-reaction. In 2013, Chen, Qi, and Han used Pd(OAc)2/PtBu2Me·HBF4/ K2CO3 in DMAc to synthesize two rigid fluorinated microporous

2.3. Arene on Heteroarene Coupling

In recent years, low bandgap π-conjugated materials have become staple materials for plastic electronics applications.175 A useful approach for their synthesis consists of the copolymerization of an electron-rich, or “push” unit, with an electron-deficient, or “pull” unit.16,17 This design allows for highly hybridized molecular orbitals with good π-orbital overlap between repeating units, leading to narrow energy bandgaps. To promote long effective conjugation length, steric hindrance at the junction of two (hetero)arenes must be minimized. The utilization of arenes alternated with heteroarenes such as thiophene-based units is a good strategy to minimize steric hindrance and promote efficient conjugation channels along the polymer chain. For these reasons, many studies reported in the DHAP literature feature arene on heteroarene couplings, both using β-protected and unprotected heteroarenes. 14241


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Figure 37. Model reaction of 2,2′-bithiophene with 1-bromo-4-methylbenzene.176

Figure 38. Synthesis of P19−26.178

2.3.1. Arene on β-Protected Heteroarene. The first example demonstrating the use of a DHAP protocol to copolymerize an arene with a heteroarene was published by Kanbara et al. in early 2012, with the polycondensation of 2,7dibromo-9,9′-dioctylfluorene (15) with 2,2′-bithiophene (26; Figure 36). Reaction conditions used were the same as the one they utilized for the synthesis of P8, namely Pd(OAc)2, PivOH, and K2CO3 in DMAc at 100 °C.176 However, this led to an insoluble, presumably cross-linked material. Studies on model compounds (molecules 26 and 27) revealed that unsubstituted bithiophene promoted unspecified (trace) amounts of branching structures in those polymerization conditions (Figure 37). However, homocoupling byproducts, another requirement for cross-linking to occur, were not analyzed. To circumvent the problem of branching at the β-position of bithiophene, 3,3′,4,4′tetramethyl-2,2′-bithiophene (28) was employed (Figure 38). A high number-average molecular weight and processable material (P19) was obtained in this way (M̅ n = 35.4 kg/mol, ĐM = 5.30) in 87% yield. Adding PCy3·HBF4 to the reaction allowed a significant reduction of ĐM to 2.76 with minimal effect on M̅ n (36.1 kg/mol) and yield (82%). However, the presence of this ligand slowed the polymerization rate significantly. Using 3,3′-

dialkylbithiophene or 4,4′-dialkylbithiophene instead of 28 also gave rise to α-selective couplings with 15.177 The same authors synthesized other copolymers from various dibromoarenes and 28 using Pd(OAc)2/PivOH/K2CO3/DMAc as the reaction conditions (Figure 38).176,178 As is the case with arene on arene polymerizations, a nonplanar backbone would be expected of these copolymers due to steric hindrance between βmethyl groups on 28 and C−H bonds on the arene unit. The authors found a correlation between the electronic affinity of the halogenated arenes used and polymerization times. Indeed, a longer time (from 1.5 to 6 h) was needed to complete the reaction going from electron-rich monomer 15 to electrondeficient monomer 32. A comparison between 2,7-dibromocarbazole (16) and 3,6-dibromocarbazole (17) provided insight regarding the formation of branching defects. From MALDITOF-MS analyses, P20 (from monomer 16) displayed branching, whereas P21 (from 17) did not, which was consistent with their previous report using 14 (see previous section).168 Branched structures were also found in P26. The isoindigo monomer 32 also has C−H bonds ortho and para to a nitrogen atom, similar to 16. The defects associated with the increased reactivity of these positions could be controlled to a certain extent by monitoring reaction time. Other copolymers 14242


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Figure 39. Michinobu and colleagues’ investigation of the impact of substitution pattern on carbazole units.182

Figure 40. Synthesis of P30 with various catalytic systems.167,183,184

incorporating β-protected 2,2′-bithiazole and thieno[3,2-b]thiophene units were also obtained using the same optimized catalytic system.179−181 Similar conclusions related to the branching of 2,7dibromocarbazole derivatives were made by Michinobu and co-workers in early 2016.182 Microwave-assisted copolymerization between 3,4-ethylenedioxythiophene (EDOT; 36) and three dibromo-carbazole derivatives (Figure 39; monomers 33− 35) confirmed the importance of the halogen position on the reaction outcome. In agreement with Kanbara’s report, a welldefined copolymer was isolated in good yield (M̅ n = 5.6 kg/mol, Y = 87%) using the 3,6-dibromocarbazole derivative (monomer 33). In the same conditions, an insoluble material was obtained from 34. Branching reactions were stated to be the problem because experiments with longer alkyl chains, which would otherwise counteract issues with polymer solubility, did not prevent the formation of a strongly cohesive gel. Low reactivity was noted for 35, possibly due to steric hindrance of the adjacent bulky side-chains. As a promising substrate for DHAP due to its lack of β protons, EDOT (36) was extensively studied by several research groups. Kanbara et al. first looked at the polycondensation of 36 with 15 using conditions similar to those developed in their previous studies (Figure 40). Following optimization, a linear and defect-

free sample P30 was isolated in 87% yield with M̅ n of 42.7 kg/ mol.183 A key factor was the use of a bulkier additive, 1adamantanecarboxylic acid (1-AdCOOH), instead of the commonly used PivOH. Although its specific role was not determined with certainty, the authors hypothesized that steric stabilization prevents aggregation of Pd into an inactive species. A second account from this group addressed the synthesis of P30 using microwave-assisted heating.184 The first tests displayed no significant differences between the PivOH and 1-AdCOOH additives. However, KOPiv proved to be an efficient base as the M̅ n increased to 56 kg/mol. Further optimizations, mainly on monomer concentration and reaction time, resulted in a defectfree copolymer with M̅ n of 147 kg/mol and yield of 89%. Using the same microwave-assisted conditions to polymerize diketopyrrolopyrrole (DPP) derivatives instead of 15 also afforded a high molecular weight material.185 A subsequent report related the influence of the molecular weights and the synthetic method of P30 on OFET and organic photovoltaic (OPV) performance.186 Later, Koizumi et al. studied the effect of precatalysts PdCl2 and Pd(OAc)2 on the polymerization kinetics of P30.167 Higher M̅ n (80 kg/mol) from PdCl2 was obtained, with longer polymerization times compared to Pd(OAc)2. The authors noticed that the limitation with Pd(OAc)2 can be somewhat 14243


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Figure 41. Synthesis of P31 and P32.187

Figure 42. Lee’s and Leclerc’s approach for the polycondensation of P33 and P34.188,189


which led to P31 and P32, respectively (Figure 41).187 Ligand and base were optimized for each pair of monomers, thus highlighting differences in reactivity even among monomers with similar chemical structures. Attempts to produce these materials using Suzuki polymerization were not successful. This was mainly due to monomer degradation in the Suzuki reaction

circumvented by the addition of stoichiometric amounts of tetrabutylammonium chloride to the reaction. Adaptation of a DHAP protocol for an isoindigo derivative (37) was proposed by Leclerc et al. in 2013. In this study, 37 was copolymerized with two electron-deficient monomers, 5octylthieno[3,4-c]pyrrole-4,6-dione (38) and its dimer (39), 14244


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Figure 44. “Push/pull/push/pull” approach enabled by a DHAP protocol.192

conditions, demonstrating the ability of DHAP to provide an alternative route to materials which may be otherwise difficult to synthesize from established methods. Lee and Leclerc groups both reported the polycondensation of 37 and 36 to obtain P33 and P34 (Figure 42).188,189 Lee used Jeffery’s conditions (Pd(OAc)2/KOAc/(Bu)4NBr in DMF) to obtain P34 (M̅ n of 43 kg/mol, Y = 80%) while Leclerc and coworkers used the Herrmann-Beller precatalyst (5 mol %) with P(o-OMeC6H4)3 (20 mol %), Cs2CO3 and PivOH in toluene to obtain P33 (M̅ n = 93 kg/mol; Y = 95%). It was observed that the copolymer formed cohesive gels which were insoluble in hot chloroform. However, these gels could be readily solubilized in odichlorobenzene at high temperatures. Once precipitated from this solution, the material was soluble in hot chloroform, indicating that the solubility of P33 is limited in chloroform due to high molecular weight and not due to the presence of crosslinked structures. In the same study, Leclerc et al. used continuous flow chemistry to address the issue of batch-tobatch variation. Despite lower M̅ n (42 kg/mol), transfer to a continuous flow system demonstrated good reproducibility on a 0.2 mmol scale (triplicate) and scale-up potential from a 0.2 to 0.8 mmol scale. Kanbara and co-workers synthesized a novel copolymer using a EDOT-flanked naphthalene diimide derivative (41) and 15, as shown in Figure 43.190,191 Using a Pd(OAc)2/1-AdCOOH/ K2CO3/DMAc system, defect-free P35 was isolated in high yield. P35 could thus be obtained after two subsequent direct (hetero)arylation steps, bypassing several synthetic intermediates in order to obtain the final material. Subsequent reports took advantage of this two-step direct (hetero)arylation approach for the preparation of conjugated polymers. Lee et al. exploited this concept to achieve regioregular “push/pull1/push/pull2” compositions (Figure 44).192 Jefferytype conditions proved to be efficient for the copolymerization of an EDOT-flanked isoindigo derivative (42) with 4,7dibromobenzo[c]-1,2,5-X-diazole (X = oxa, thia, seleno) derivatives (43−45). Wang et al. attempted a similar synthesis with an EDOT-flanked benzo[c]-1,2,5-thiadiazole derivative.193 Unfortunately, the resulting copolymer was found to be insoluble in chloroform or THF. In 2015, Kanbara et al. presented the synthesis of P39 through sequential bromination/DHAP steps.194 This methodology allows for the synthesis of a conjugated copolymer from two nonfunctionalized monomers without purifying the intermediates. To test this concept, the authors synthesized a copolymer from 38 and 46 (see Figure 45). Benzyltrimethylammonium

Figure 45. Synthesis of P39 by Kanbara et al.194

tribromide and ZnCl2 were used to brominate 46. The byproduct of this reaction is a commonly used phase transfer agent, benzyltrimethylammonium bromide. Polymerization was initiated in situ by adding toluene, monomer 38, Cs2CO3 and a catalyst solution containing Pd(OAc)2 and the ligand PCy3. A defect-free copolymer with M̅ n of 34.5 kg/mol was isolated in 80% yield. In comparison, the corresponding copolymer obtained from 38 and dibrominated 46 (monomer 15) achieved higher molecular weights (M̅ n = 129.2 kg/mol; Y = 96%). Adding benzyltrimethylammonium tribromide (M̅ n = 89.5 kg/mol; Y = 95%) or benzyltrimethylammonium tribromide combined with ZnCl2 (M̅ n = 50.5 kg/mol; Y = 95%) affects the polymerization reaction. However, succinimide, a byproduct of the commonly used brominating agent N-bromosuccinimide, completely suppresses polymerization. With careful optimization, this type of protocol can allow the one-pot synthesis of copolymers from nonfunctionalized monomers, thus further reducing the number of synthetic steps. The same group also investigated the correlation of reactivity and electron affinity of the thiophene monomers. In their study, monomer 15 was coupled with various 3,4-disubstituted thiophenes (Figure 46; monomers 36 and 47−51) through two sets of experimental conditions which used the same catalyst.195 Catalytic system A consisted of phosphine-free polar conditions, whereas catalytic system B involved phosphineassisted apolar conditions. Electron-rich monomers, such as 36, reacted more readily in polar conditions, whereas electrondeficient compounds, such as 49, could be polymerized efficiently in toluene (system B). Only 3,4-dichlorothiophene (50) reacted in both sets of conditions, though only low molecular weight copolymers were obtained. In parallel to this work, a series of copolymers was prepared by Ling and coworkers using β-protected thiophenes (Figure 47; monomers 48 14245


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Figure 46. Copolymerization of 15 with various 3,4-substituted thiophenes.195

Figure 47. Synthesis of P41,P44−46, as presented by Ling and co-workers.196

Figure 48. Synthesis of P47−50.193,197,199 14246


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Figure 49. Contribution of Scherf et al. with regards to identifying DHAP defects in P51-like model polymers.200,201

Figure 50. Synthesis of P51 and P56−58, as studied by Horie.202

2.3.2. Arene on β-Unprotected Heteroarene. The first demonstration of arenes coupled with heteroarenes without β substituents was reported by Scherf et al., in which P51 was synthesized from 4,7-dibromo-2,1,3-benzothiadiazole (44) and a β-unprotected polycyclic monomer, 4,4-dialkyl-cyclopenta-[2,1b:3,4-b′]dithiophene (61), as shown in Figure 49.200 Excellent reactivity was achieved using a phosphine-free system consisting of Pd(OAc)2/K2CO3/DMAc (M̅ n = 40.3 kg/mol, Y = 70%). However, a 7 nm hypsochromic shift of the UV−vis absorption maximum (λmax) was observed for the DHAP-prepared copolymer when compared to a Stille-synthesized reference sample. Several additional peaks were found in 1H NMR spectra and were first attributed to homocoupling defects, along with multiple substituents found on compound 61 (branching). A follow-up study evaluated the nature of these defects using methyl substitution patterns on the β-positions of monomers 44 and 61. A homopolymer of 61 was also prepared to aid in identifying signals resulting from homocoupling.201 Peaks originally thought to be related to β-branching were in fact found to be related to homocoupling defects, as they were not eliminated when the β-methylated compound 62 was used. The presence of methyl groups on the monomer 44 (to become monomer 59) or on both monomers confirmed the nature of the defects as being both C−H/C−H and C−Br/C−Br homocouplings. The addition of PCy3·HBF4 to the polymerization reduced the amount of defects but did not entirely eliminate them. Replacing Pd(OAc)2 with the Herrmann-Beller precatalyst in DMAc led to P51 (M̅ n = 38 kg/mol; Y = 40%) without C−Br/ C−Br homocoupling and with a lower content of C−H/C−H homocoupling. Polymerization of 60 and 61 in these conditions only afforded oligomers. In parallel, Horie and colleagues reported a series of similar copolymers prepared from 44 and bridged bithiophene derivatives (Figure 50; monomers 61, 63 and 64).202 P51 was synthesized via the copolymerization of 44 and 61, as well as via

and 51−53). Copolymerization of 15 in DMAc revealed a decrease in reactivity from electron-rich (52) to electrondeficient (51) monomers when using the Herrmann-Beller precatalyst.196 An opposite trend was observed with a Pd(OAc)2/phosphine system. The impact of a Pd(OAc)2-based catalytic system on undesired side-reactions was thoroughly investigated by Sommer et al. using P47 as a model compound (Figure 48).197 To overcome the low solubility of this copolymer in pure DMAc, the authors employed a mixture of DMAc:toluene (1:1 v/v). Using KOPiv as the base in phosphine-free conditions, a soluble sample with M̅ n of 24 kg/mol was obtained in almost quantitative yields. However, a relatively high degree of carbazole homocoupling (54) was identified using 1H NMR spectroscopy. Such defects could be significantly reduced by using a combination of PivOH, K2CO3, and PCy3. Utilizing PivOH and K2CO3 without PCy3 did not suppress homocoupling, nor did using PCy3 with PivOK. A mechanism was proposed where less-hindered acetate ligands can bridge two Pd centers, which then could exchange ligands, creating a Pd species bearing two 54 units. Reductive elimination from this intermediate then causes homocoupling. This form of metathesis has already been reported in the literature198 to explain homocoupling of 54, in which a phosphine ligand could prevent or reduce the formation of acetate bridges by sterically hindering the Pd coordination sphere. It is worth noting that reducing 54 homocoupling reactions did not improve molecular weights. Moreover, homocoupling and dehalogenation were also identified for monomer 57, limiting number-average molecular weight to 10 kg/mol. This study demonstrated that C−Br/C−Br homocoupling was promoted by high temperatures and was still present upon reduction of the temperature from 120 to 90 °C. It is worth noting that no branching was observed. Following this work, two independent studies exploited the reactivity of the monomers 57 and 58 in various conditions to yield other copolymers (Figure 48; P48-50).193,199 14247


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Figure 51. Synthesis of P59 as presented by Wang et al.203

Figure 52. Synthesis of P60 and P61 as presented by Wang et al.204

Figure 53. Synthesis of P62−P67 as presented by Wang et al.205

the homopolymerization of a monobrominated repeating unit (65) prepared by the direct (hetero)arylation of 44 and 61. The best copolymerization conditions for P51 were a phosphine-free Pd(OAc)2/K2CO3/PivOH catalytic system in 1-methyl-2pyrrolidinone at 80 °C. The presence of phosphine-based ligands was detrimental to the catalyst activity for the DHAP

protocol. It was proposed that NMP offered superior performance compared to DMAc and DMF due to a better solubility of the resulting copolymer. A copolymer was obtained in a 76% yield after 20 h of reaction time with M̅ n of 71.7 kg/mol following Soxhlet extractions. Interestingly, the above-mentioned homopolymerization of P51 in the same conditions afforded M̅ n of 14248


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Figure 54. Synthesis of tropone-based copolymers by Swager and co-workers.206

Figure 55. Two sets of optimized conditions for DHAP with a thienyl-diketopyrrolopyrrole derivative, as developed by You and Coughlin.207,208

only 27.4 kg/mol. Additional peaks in the 1H NMR spectra (5− 10% intensity compared to the main peaks) combined with MALDI-TOF analyses suggested branching or C−H/C−H homocoupling defects. UV−vis spectra of P51 synthesized by DHAP showed a 15 nm bathochromic shift in λmax compared to the Suzuki-prepared analog. As shown in Figure 50, when these optimized reaction conditions were transposed to the preparation of three other polymers (P56-58), only low molecular weight materials were obtained. Wang and colleagues reported the copolymerization of unprotected fused-thiophene monomers. Their initial report focused on a systematic investigation of reaction parameters (catalyst, ligand, solvent, base, additive, and concentration) on the resulting M̅ n, yield and optical properties of 44 copolymerized with a benzodithiophene derivative (see Figure 51; monomer 67).203 P59 (M̅ n = 24.5 kg/mol; Y = 98%) was obtained from a Pd2dba3/P(o-OMeC6H4)3/K2CO3 catalytic system in o-xylene. Although a comparable yield was obtained for the polymerization in DMAc, only a low molecular weight material could be isolated due to rapid precipitation of the copolymer. This particular system proved to be versatile, as

another series of oligomers was obtained using a thiophene-fused naphthalene diimide derivative (68) copolymerized with brominated 44 or 15, to afford P60 (Y = 45%) and P61 (Y = 38%), respectively (see Figure 52).204 Copolymerization of an unprotected thienoisoindigo (69) unit with brominated aryl monomers was also investigated.205 Pd(Herrmann) was found to be most efficient when using electron-rich aryl bromides such as 15 and 70, whereas Pd(OAc)2 offered better results with electron-deficient monomers 14, 44, 60, and 71 (Figure 53). The reaction temperature was also optimized for each polymer individually. Hightemperature 1H NMR spectroscopic analyses did not reveal any branching or homocoupling defects, indicating good selectivity toward the α C−H bonds for the monomer 69. Swager and co-workers used a DHAP procedure to produce copolymers from thiophene-fused tropones bearing unprotected aromatic C−H bonds.206 The copolymerization of 15 with dialkyloxy-substituted dithienobenzotropone units was achieved using a Pd(OAc)2/PCy3·HBF4/K2CO3/PivOH catalytic system with DMAc as the solvent (Figure 54). Long linear or branched aliphatic side-chains (monomers 73 and 74) were necessary to 14249


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Figure 56. Comparison of the impact of the choice of brominated compound for diketopyrrolopyrrole-based copolymers.209−211

Figure 57. Synthesis of P84 and P85 by Sommer et al.213

obtain soluble copolymers P69 and P70 with M̅ n values of 6.2 and 7.6 kg/mol, respectively. Monomer 72 yielded oligomers with molecular weights inferior to 2.5 kg/mol (P68). It is noteworthy that DHAP was the only viable method for the preparation of P70, as the bromination or iodation of the monomers 73 and 74 resulted in low yields (58%). Structural analyses suggested the formation of irregular C−C 14253


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Figure 65. Preparation of P98 and P99, as presented by Thompson et al.222

Figure 66. Farinola et al. offered this synthesis of the random terpolymer P100.223

Figure 67. Synthesis of P101 and P102 by Jacob and co-workers.199

of P98 and P99. NMR and UV−vis spectroscopic analyses indicated that the polymers prepared using these two methods were structurally different, and GIXRD, DSC, and TGA analyses revealed that P98 and P99 were less crystalline than their analogs obtained by Stille polymerization. Another recent example of a terpolymer was reported by Farinola et al. (Figure 66).223 Two acceptor units (44 and 109) were copolymerized with a donor unit (108) using a Pd(OAc)2/ PivOH/K2CO3 catalytic system in DMAc with a 108:44:109 monomer ratio of 2:1:1. The reaction only proceeded when brominated electron-withdrawing monomers were used with nonbrominated 108. The optimized conditions provided P100 with M̅ n = 10.4 kg/mol (Y = 70%). The use of PCy3·HBF4 as ligand had no impact on either molecular weight or yield. It is noteworthy that only oligomers were obtained when NMP, Nbenzylpyrrolidinone or a mixture of NMP and toluene were used

dispersed polymer corresponded with peripheral end groups. UV−vis absorption spectroscopy of the dispersed polymer displayed a significant red shift of the absorption spectrum of P97 compared to the monomer 105, which served as a strong indication of extended conjugation. 2.3.3. Random terpolymers. DHAP methodologies have been successfully used to synthesize random terpolymers. An early attempt was presented by Thompson et al. in which P98 and P99 were prepared from 2-bromo-3-hexylthiophene (6), thiophene (106), and either 4,7-dibromo-2,1,3-benzothiadiazole (44) or a diketopyrrolopyrrole derivative (107; Figure 65).222 P98 (M̅ n = 19.2 kg/mol) and P99 (M̅ n = 6.8 kg/mol) were obtained in 47 and 53% yields, respectively. The authors attributed the low yields and low M̅ n, compared to those obtained by Stille cross-coupling polymerization, to cross-linked products since insoluble fractions were recovered after Soxhlet extraction 14254


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2.4.1. β-Protected Heteroarenes. As mentioned in previous sections, DHAP on thiophene substrates can lead to undesirable defects, and therefore polymerization conditions must be controlled in order to avoid β-branching and crosslinking. A decade after the work published by Lemaire and colleagues, Kumar and Kumar reported the first synthesis of an alternating thiophene-based copolymer by DHAP.224 By using the following conditions: Pd(OAc)2/NaOAc/(Bu)4NBr in DMF; two sets of copolymers were prepared from dihexyl-3,4propylenedioxythiophene and 2,5-dibromoethylenedioxythiophene (Figure 69; monomers 111 and 112), as well as from functionalized ProDOT units and 2,5-dibromo-dihexyl-ProDOT (Figure 69; monomers 113−117 and 118). Low molecular weight materials (M̅ n = 1.3−13.7 kg/mol) were isolated in moderate yields of 50−60%. In 2012, Leclerc et al. reported the successful polycondensation of 5,5′-dibromo-4,4′-dioctyl-2,2′-bithiophene (120), a βalkylated, α-halogenated bithiophene, with an electron-deficient alkylated thieno[3,4-c]pyrrole-4,6-dione derivative (TPD, 119; Figure 70).225 This was the first example of a “push−pull” copolymer synthesized by DHAP. The catalytic system featured a combination of the Herrmann-Beller catalyst and P(oOMeC6H4)3 with Cs2CO3 in superheated THF at 120 °C. P109 prepared using DHAP had higher molecular weights and yield (M̅ n = 56 kg/mol; Y = 96%) compared to the same polymer obtained by Stille cross-coupling polymerization (M̅ n = 9 kg/ mol; Y = 71%). The alternating chemical structure of the conjugated skeleton was confirmed by 1H NMR spectroscopy, but small quantities of C−Br/C−Br homocoupling were identified. Two years later, Ozawa et al. revisited the synthesis of P109 by testing other precatalysts, ligands, additives, and solvents.226 The PdCl2(MeCN)2/P(o-OMeC6H4)3/Cs2CO3/ PivOH catalytic system in THF at 100 °C led to M̅ n of 36.8 kg/mol with a reaction yield of 99%. Thermal analyses revealed higher melting and crystallization temperatures than those reported by Leclerc and co-workers, leading one to believe that this catalytic system and temperature lead to fewer homocoupling defects. Following their initial report, the synthesis of other thieno[3,4c]pyrrole-4,6-dione-based copolymers was undertaken by Leclerc and colleagues using a catalytic systems based on Pd(Herrmann) combined with P(o-OMeC6H4)3.227−232 Electron-rich halogenated β-alkylated monomers derived from 2,2′bithiophene (Figure 71; 128), 2,2′:5′,2″-terthiophene (129) and dithieno[3,2-b:2′,3′-d]thiophene (126 and 127) were used with various 5-alkyl-thieno[3,4-c]pyrrole-4,6-diones or their furan counterparts (Figure 71; monomers 38, 49, and 121−125). DHAP led to better materials when compared to Stille crosscoupling polycondensation products, with higher M̅ n and yield, as well as better conjugation length according to the red-shifted λmax observed in the UV−vis absorption spectra. The same authors studied the synthesis of numerous 5-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD) homopolymers and pseudohomopolymers.230,231 Polymerization from either monohalogenated monomers (Figure 72; monomers 130−132) or copolymerization of dihalogenated derivatives (Figure 73; 133− 135) with nonhalogenated derivatives (Figure 73; 124, 125, and 136) led to polymers with relatively high M̅ n (8−25 kg/mol) in good yields of 55−85% (Figure 72 and Figure 73; P121−127). While Pd(Herrmann) and P(o-OMeC6H4) in superheated THF were used for each cases, the bases were changed depending on the nature of the halogen (iodide or bromide). With brominated monomers, KOAc led to the most reactive catalytic system,

as solvents. The utilization of Pd2dba3 as the precatalyst was detrimental for both M̅ n and yield, but generated less structural defects compared to those obtained when using Pd(OAc)2. By performing NMR spectroscopic analyses on samples obtained after short reaction times, it was determined that the early coupling reactions were highly α-selective. However, homocoupling defects, and possibly β-branching, were observed in the final polymer. The authors hypothesized that as the molecular weight increases, the ratio of β/α C−H bonds also increases, rendering β activation of the C−H bond a more likely event. Both of these factors may have contributed to the hypsochromic shift observed in the UV−vis absorption spectra of the DHAP polymer when compared to its Stille analog. Jacob et al. demonstrated the polymerization of 44 and 110 with either 55 or 56 to yield the terpolymers P101 and P102 (see Figure 67).199 Both polymers were prepared with Pd(OAc)2/ PivOH/K2CO3/DMAc at 80 °C. 2.4. Heteroarene on Heteroarene Coupling

Five-membered heterocycle-based materials are among the most studied classes of π-conjugated systems. These heterocycles are often fused with arenes, as in the case of benzo[1,2-b:4,5b′]dithiophene and cyclopenta[2,3-b:4,5-b′]dithiophene, or other heteroarenes, such as thieno[3,2-b]thiophene, thieno-

Figure 68. Common fused-thiophene cores used as monomers for conjugated polymers.

[3,4-d]thiazole, dithieno[2,3-b:4,5-b′]silole, and thieno[3,4-c]pyrrole-4,6-dione (Figure 68). Formation of C−C bonds between such derivatives generally leads to a coplanar structure, even in the presence of β-substituents. This is one of the crucial parameters which must be taken into account when optimizing the electro-optical properties of conjugated polymers. As discussed earlier, thiophene derivatives are widely used building blocks for the synthesis of conjugated polymers (see also section 2.1). However, heteroatom-containing aromatic rings tend to be more reactive than arenes, and achieving selectivity between available C−H bonds for direct (hetero)arylation reactions becomes more challenging. Development of selective methodologies for such substrates is at the forefront of current studies. Several reports have demonstrated well-defined materials involving five-membered heterocycles with and without protection of the β-positions. These topics will be discussed in details in sections 2.4.1 and 2.4.2. 14255


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Figure 69. Kumar and Kumar’s report of the first alternating thiophene-based copolymer.224

Figure 70. Two different approaches for the synthesis of P109.225,226

when either a monobrominated monomer (147) was homopolymerized or a dibrominated (148) and nonbrominated derivative (149) were copolymerized. These authors also demonstrated that polymerization occurred in the first few minutes of the reaction, with a nonlinear relationship between monomer consumption and observed M̅ n. Following this work, they synthesized several pseudohomopolymers and copolymers using the same reaction conditions with ProDOT, EDOT, and acyclic 3,4-dialkoxythiophene (AcDOT) derivatives (Figure 76).237−239 Kanbara and colleagues also exploited the reactivity of EDOT and ProDOT derivatives by using a similar phosphine-free system for the synthesis of “push−pull” copolymers with dibromo-4,4′-dinonyl-2,2′-bithiazole (Figure 77; monomer 161).240 Using a Pd(OAc)2/K2CO3/1-AdCOOH catalytic system in DMAc, P147−150 were obtained in low M̅ n and yields, except for P149. This was mainly attributed to the limited solubility of the growing polymer chains in DMAc at 100 °C. For P150, increasing the reaction temperature to 120 °C and decreasing the monomer concentration to 0.06 M slightly improved M̅ n from 8.7 to 10.4 kg/mol. The alternating structure of these materials was confirmed by 1H NMR and MALDI-TOF spectroscopic analyses. Nakabayashi et al. reported the polycondensation of a thiophene-substituted naphthalene diimide derivative with 3,4dimethylthiophene (Figure 78; monomers 48 and 162).241 Using Pd(OAc)2/PCy3·HBF4/K2CO3/PivOH conditions in DMAc, defect-free P151 was isolated in good yield (76%) after 24 h of reaction. Later, Wang and co-workers (Figure 78; monomers 36 and 163) optimized the copolymerization of a similar naphthalene diimide derivative with EDOT instead of

whereas for iodinated monomers a combination of Cs2CO3 and an iodide scavenger (AgOAc) was used to prevent the poisoning of the Pd catalyst. Following optimization, it was found that the nature of the halogen moiety did not influence the degree of polymerization. In the past decade, homopolymers based on 3,4-dialkoxythiophenes have shown great promise as hole transport materials for printed electronics, with poly(3,4-ethylenedioxythiophene) (PEDOT) being mass-produced.233,234 Two years after the work of Kumar and Kumar, Yu and co-workers, prepared five homopolymers consisting of 3,4-ethylenedioxythiophene (EDOT; Figure 74) in which monomers 137−144 afforded polymers P128−131 and 3,4-propylenedioxythiophene derivatives (ProDOT; Figure 75) in which monomers 145 and 146 afforded polymers P132.235 In this particular case, polycondensation using Pd(OAc)2 and Cs2CO3 in DMF for ProDOT units (P132) required the use of PCy3·HBF4, whereas for EDOT units (Bu)4NBr (P129−131) was required instead of the phosphine ligand. Interestingly, apolar conditions consisting of Pd(Herrmann)/P(o-OMeC6H4)3/Cs2CO3/THF gave comparable M̅ n and yields for all materials. The expected structures were confirmed by 1H NMR and MALDI-TOF spectroscopic analyses. Good compatibility with commonly used protecting groups (−OTBDMS, −COOtBu, and −NHBoc) was demonstrated using various functionalized monomers. In a similar approach, Reynolds and co-workers synthesized homopolymers based on ProDOT derivatives using Pd(OAc)2/ K2CO3/PivOH/DMAc conditions (Figure 75; P133). The reaction temperature (140 °C) was identified as the key factor in maximizing both the molecular weights and yields for these derivatives.236 A similar degree of polymerization was achieved 14256


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Figure 71. Synthesis of P110−120 by Leclerc and co-workers.227−232

Figure 72. Homopolymers of thieno[3,4-c]pyrrole-4,6-dione derivatives.231

48.242 They chose an apolar approach, using Pd2dba3/P(oOMeC6H4)3 in o-xylene as the catalytic system. After a reaction time of 24 h, defect-free P152 was isolated in 77% yield with M̅ n of 16.4 kg/mol.

2.4.2. Nonalkylated Heteroarenes. In the early stages of development of DHAP, Leclerc et al. explored the reactivity and selectivity of 2,6-dihalogeno-4,4-bis(alkyl)-dithieno[3,2-b:2′,3′d]silole (164) copolymerized with monomer 38 to obtain P153, 14257


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Figure 73. Pseudohomopolymers of thieno[3,4-c]pyrrole-4,6-dione derivatives.230,231

Figure 74. Synthesis of homopolymers of PEDOT derivatives.235

Figure 75. Synthesis of homopolymers and pseudohomopolymers of ProDOT derivatives.235,236

a high performance copolymer for organic solar cells (Figure 79).243 For the synthesis of P153, Pd(Herrmann), P(oOMeC6H4)3, Cs2CO3, and PivOH were employed in toluene, affording a material with a M̅ n of 20 kg/mol. Poor reproducibility, low yields and a large amount of insoluble material after usual extraction procedures suggested cross-linking defects. Replacing 164 by a β-protected derivative (165) prevented the formation of branched structures without any impact on the resulting molecular weights and yields (Figure 79; P154). Similar results were obtained by using a germanium analog to 165 (Figure 79; monomer 166).

Two years later, Ozawa and co-workers revisited the synthesis of P153 with another catalytic system.244 Using the same brominated monomer 164, they reached similar conclusions regarding side reactions and obtained a material with a M̅ n of 14.9 kg/mol (Y = 18%). Through careful NMR spectroscopic analyses, C−H/C−H homocoupling (3.7%) and dehalogenation of 164 were identified. The introduction of a coligand, P(oNMe2C6H4)3, known to produce highly regioregular P3HT, and the replacement of 164 with an iodinated derivative (Figure 80; monomer 167) contributed to reducing homocoupling to 1.9%. This mixed-ligand approach led to an even lower rate of defects 14258


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Figure 76. Synthesis of EDOT-, ProDOT-, and AcDOT-based derivatives by Reynolds et al.237−239

Figure 77. Synthesis of P147−150 as presented by Kanbara et al.240

(1.0%) and a complete suppression of unwanted side reactions on 167 when P(o-OMeC6H4)3 and N,N,N′,N′-tetramethylethylenediamine (TMEDA) were used together.245 In this case, P153 (M̅ n = 20 kg/mol; Y = 88%) was obtained with similar optical properties to its Stille counterpart. 1H NMR spectroscopic analysis revealed a recurrent and precise 1:1 ratio between C−H/ C−H homocoupling and the dehalogenation of the 167 unit; a process in which oxidative homocoupling is balanced by reductive dehalogenation was hypothesized. In parallel to their investigation on β-protected dithienosiloles, Leclerc et al. reported the polymerization of another well-

established electron-rich unit, a benzodithiophene derivative bearing alkoxy side-chains (168). Apolar polymerization conditions were used to afford P156-158, using Pd(Herrmann)/P(o-MeOC6H4)3/PivOH/Cs2CO3 catalytic systems in either toluene or THF (Figure 81).227,246 It was found that these conditions produced equivalent or higher molecular weight materials compared to those obtained by Stille crosscoupling, albeit in lower yields. Polymers based on 38 and 121 showed a slight hypsochromic shift of the maximum of absorption from solid-state UV−vis spectroscopic measurements compared to the Stille analog. In 2015, Lin and co-workers used 14259


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Figure 78. Synthesis of P151 and P152.241,242

Figure 79. Synthesis of P153−155 as presented by Leclerc and co-workers.243

Figure 80. Mixed ligand approach to the synthesis of P153, as proposed by Ozawa and colleagues.244,245

Time-controlled synthesis of P163 and P164, featuring fusedthiophene comonomers 175−178, was realized in phosphinefree, polar conditions and resulted in materials with M̅ n of 27.3 and 9 kg/mol in 71% and 50% yields for P163 and P164, respectively (Figure 83).249,250 Upon longer reaction time, cohesive gels were formed, limiting solubility in chloroform and lowering the overall polymerization yields. Microwave-assisted DHAP was also investigated for the synthesis of P163. In this particular case, rapid formation of defects, attributed to branching, led to an insoluble material. It was supposed that microwave-assisted DHAP, while accelerating reaction kinetics, can also activate β C−H bonds to rapidly create undesired branched structures. A recent contribution by Kim and co-workers utilized DHAP as a tool to obtain copolymers based on an electron-deficient thiophene-phenylene-thiophene fused bislactam unit (monomers 179 and 180).251 A phosphine-free system was used to synthesize copolymers P165−P167 from β-protected thiophene

the same experimental conditions for the synthesis of P158 and obtained lower molecular weights (M̅ n = 20.5 kg/mol). They also polymerized 38 with an analog of 168, dibromo-4,8-di(2ethylhexyloxy)benzo[1,2-d:4,5-d′]bisthiazole.247 However, M̅ n was limited to 5.8 kg/mol despite an excellent yield (96%). Coughlin et al. investigated the copolymerization of a benzodithiophene derivative featuring alkylthiophene sidechains (170) with thieno[3,4-b]thiophene units (monomers 171−174).248 Optimized reaction conditions, consisting of Pd2dba3·CHCl3/P(o-OMeC6H4)3/Cs2CO3/PivOH in THF at 80 °C were applied to all monomer sets (Figure 82). Molecular weights ranging from 18 to 77 kg/mol were obtained after 1 h of reaction and the copolymer structures were confirmed by 1H NMR and MALDI-TOF spectroscopic analyses. Upon longer reaction times, the authors suggested the possible formation of branched or cross-linked structures, arising from either 170 βprotons or the 3-position on monomers 171−174. 14260


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Figure 81. Synthesis of P156−158 by Leclerc and co-workers.227,246

Figure 82. Synthesis of P159−162, as presented by Coughlin et al.248

Figure 83. Synthesis of P163 and P164.249,250

class of compounds is suitable for DHAP.252 Furthermore, high molecular weight copolymers were obtained in nearly quantitative yields. Using nonbrominated monomer 120 with monomer 183 yielded no polymerization product, probably due to steric hindrance. By transposing the conditions to the brominated 120 and nonbrominated 184 couple, P172 with M̅ n of 51.9 kg/mol was isolated in quantitative yield, highlighting once again the importance of the choice of the unit which bears the halogen.

derivatives (Figure 84) and low molecular weights polymers were isolated in good yields. Due to strong aggregation in solution, no structural analyses could be undertaken. Figure 85 presents the copolymerization of γ-substituted monomers 183 and 184 with either 120 or thiophene derivatives bearing electron-deficient and electron-rich β-protecting sidechains (monomers 136, 160, 185, and 186). Upon optimization, the utilization of Pd2dba3·CHCl3/P(o-OMeC6H4)/Cs2CO3/ PivOH in THF led to linear structures, indicating that this 14261


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Figure 84. Synthesis of P165−P167 as presented by Kim and co-workers.251

Figure 85. Synthesis of P168−172 by Ozawa et al.252

Further investigations on γ-substituted monomers was undertaken by El-Shehawy and Lee with a thieno[3,4-b]pyrazine derivative as the comomomer.253 They demonstrated that thieno[3,4-b]pyrazine must be substituted at the 2- and 3positions, in this case by methyl groups (Figure 86; monomer 187), to avoid unwanted reactions at these positions. Using an optimized protocol consisting of phosphine free conditions, P173 (M̅ n = 22.8 kg/mol) and P174 (M̅ n = 17.7 kg/mol) were isolated in 84% and 81% yield, respectively (Figure 86). Another example of successful polymerization on a γ-substituted thiophene was provided by Leclerc et al. with the polycondensation of 124 with an PEG-substituted thiophene (190).230 Several research groups have published DHAP protocols in order to obtain thienyl-diketopyrrolopyrrole-based (DPP) copolymers with varying degrees of success. Leclerc et al. have reported successful polymerization of 38, 39, 51,254 and 169246 with a DPP moiety (85) that led to P176 to P179 (Figure 87).

Reaction conditions featured toluene or THF as the solvent with Pd(Herrmann)/P(o-MeOC6H4)3/PivOH/Cs2CO3 as the catalytic system. In 2015, Shirai and Han copolymerized dithienonaphthothiadiazole (194) with 85 or 192 using a Pd(OAc)2/K2CO3/1-AdCOOH reaction system in DMAc.255 Due to the low solubility of 194, monomer 191 featuring short 2ethylhexyl side-chains led to oligomers (M̅ n = 1.2 kg/mol). The use of longer 2-octyldodecyl alkyl side chains on the thienyldiketopyrrolopyrrole slightly improved the molecular weights (M̅ n up to 5.5 kg/mol). Wang and co-workers assessed the reactivity of a novel compound, (E)-1,2-bis(3,4-difluorothien-2yl)ethene (193) with 192.256 After optimization, the utilization of Pd(Herrmann)/P(o-MeOC6H4)3/PivOH/Cs2CO3 in toluene led to P180 with a M̅ n = 59.9 kg/mol and 93% yield. It is worth noting that only the fluorinated thienylethene moiety was successfully polymerized by DHAP. The nonfluorinated analog compound failed to produce polymeric material. 14262


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Figure 86. Polymerization of γ-substituted thiophenes to afford P173−175.230,253

Figure 87. Synthesis of P176−181.246,254−256

side chains. Pd2dba3·CHCl3/P(o-MeOC6H4)3/PivOH/Cs2CO3 in a 10:1 mixture of toluene and DMAc was used by Russel and Coughlin to polymerize 75 with 2,5-dibromothiophene (199) and 2,5-dibromo-3,4-difluorothiophene (200).208 Moreover, an important quantity of insoluble material was recovered after Soxhlet extraction for both copolymers, thus explaining the low yields obtained. Leclerc et al. studied the influence of alkyl substitution on the dithienyl-diketopyrrolopyrrole moiety through the synthesis of model homopolymers.258 They used a Pd(Herrmann)/P(oOMeC6H4)3/Cs2CO3/PivOH catalytic system in toluene to obtain P187−190 (Figure 89). From the molecular weights obtained, the authors rationalized that α-brominated thiophenes bearing a β-methyl group, such as the monomer 203, is beneficial for the overall reactivity of this system. On the other hand, copolymerizations involving monomer 201 led to lower molecular weights and possible structural defects.

Several polymerizations were performed with thiophene units flanking the dithienyl-diketopyrrolopyrrole core (Figure 88; monomers 75 and 195). In this regard, Wang reported the polymerization of 195 with a β-alkylated thiophene-substituted naphthalene diimide monomer (196), yielding a well-defined copolymer with M̅ n = 27.4 kg/mol by using Pd2dba3·CHCl3/P(oMeOC6H4)3/PivOH/K2CO3 in o-xylene.242 1H NMR spectroscopic analyses confirmed the alternating chemical structure of P182 with minor peaks associated with the expected C−H and C−Br end-groups. Wang and Ling reported an efficient catalytic system consisting of PdCl2(PPh3)2/PCy3·HBF4/PivOH/K2CO3 in a 1:1 mixture of o-xylene and DMAc for the synthesis of copolymers from 75 with 197 or 198 (Figure 88; P183 and P184).257 Despite good reactivity, the lack of solubilizing sidechains on 197 led only to the formation of a low molecular weight copolymer (M̅ n = 9.9 kg/mol), whereas a copolymer with M̅ n = 86.1 kg/mol was obtained using 198, which bears dodecyl 14263


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Figure 88. Polymerization of P182−186.208,242,257

Figure 89. Leclerc and colleagues’ investigation of the β-substitution pattern on the DPP unit.258

alytic system in refluxing toluene, insoluble material was recovered after Soxhlet extraction of each copolymer regardless of the monomer couples used (192 with 205 and 204 with 206). Based on model compounds and 1H NMR spectroscopic

In 2016, Li and co-workers proposed two DHAP-based synthetic routes to afford a copolymer of dithienyl-diketopyrrolopyrrole and unsubstituted 2,2′-bithiazole (Figure 90; P191).259 Using a Pd(Herrmann)/P(o-OMeC6H4)3/Cs2CO3/PivOH cat14264


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Figure 90. Synthesis of P191, as reported by Li and co-workers.259

of palladium.154 Mixed solvent phosphine-assisted conditions have been recently explored and may be an avenue to balance reactivity, solubility, and selectivity, thereby retaining some of the characteristics of both methods.197 The future of DHAP now lies in the development of novel reaction conditions which could produce conjugated polymers devoid of defects from a broad scope of monomers. Moreover, moving from pressurized conditions, like superheated THF, to ambient conditions could be more appealing for industrial applications. Protocols in nonpolar solvents are currently the most promising, though new catalytic systems overcoming some substrate limitations are sorely needed as very few ligands can currently generate active catalytic species under these conditions. For this reason, DHAP should be studied from an organometallic perspective to overcome present challenges. Similarly to the research done on direct arylation reactions for small molecules, DFT calculations would be a powerful asset to screen and anticipate the reactivity and selectivity of monomers. In parallel, developing a chain growth polymerization (or living polymerization) could bring this field to controlled molecular weights and molar-mass dispersity. A promising pathway to achieve this may lie in the development of dual metal catalysis possessing orthogonal reactivity. A recent report by Luscombe et al. presented the living polymerization of P3HT using a two-step process of direct aurylation of 2-bromo-3hexylthiophene followed by palladium-mediated polymerization.260 This would also enable the preparation of block copolymers, a subject as of yet unexplored with DHAP. Polymerizations completely foregoing the use of preactivated monomers (e.g., C−H on C−H oxidative cross-coupling) is another avenue to further reduce synthetic steps.164 Furthermore, it should not be forgotten that direct (hetero)arylation is not restricted to polymerization methods. Molecular functional materials with extended conjugation length can now be easily prepared using this method.115,261−279 Substrates previously difficult to couple using traditional methods, such as boroncontaining heterocycles,280−282 can now be incorporated more easily in molecular materials. Such molecules could now be incorporated in polymers. Importantly for conjugated polymers, complex monomers can be prepared from direct (hetero)arylation, eliminating reaction, and purification steps.190,283−286

analyses, the presence of branched structures was suggested. However, UV−vis spectroscopy, XRD analyses and performance in OFET hinted that the reaction involving brominated monomer 192 with nonbrominated 2,2′-bithiazole (205) produces a well-defined copolymer.

3. CONCLUSION AND OUTLOOK In the span of a few years, the field of direct (hetero)arylation polymerization (DHAP) has blossomed from a concept to a versatile synthetic tool for the preparation of a wide range of structurally complex conjugated polymers. In many instances, DHAP represents a competitive alternative to traditional organometallic polycondensation reactions when one considers the reduction of synthetic steps and the absence of stoichiometric quantities of organometallic byproducts. Many well-defined, nearly defect-free polymeric materials137,146,150,165,170,178,216,218,242,245,253,256 can now be obtained. For instance, highly regioregular P3HTs have been obtained (over 99% HT-HT coupling) via a DHAP protocol.149,150 Analysis of side-reactions has helped to identify the reaction conditions best suited to given families of monomers. In the vast majority of studies, C−H/C−H and C−Br/C−Br homocouplings have been found to be the major source of chain alternation defects, and during the optimization of reaction conditions, this issue must be addressed. A clear proof of branched and/or cross-linked materials is still missing for most polymerizations, and very few reports have identified such defects. In fact, P3HT is the only polymer for which this phenomenon has been proven.153,157 Decreased polymer yield or insoluble material can be indicative of cross-linking arising from unselective arylation, though this does not constitute definitive proof. Two distinct approaches have emerged for efficient and selective C−C bond formation by DHAP: (1) phosphineassisted conditions in nonpolar solvents (e.g., toluene and THF)125,203 and (2) phosphine-assisted or phosphine-free conditions in aprotic polar solvents (e.g., DMAc and DMF).151,186,201 Nonpolar approaches, once optimized, offer both greater selectivity and high molecular weights.217 Polar systems, on the other hand, use inexpensive reagents and can give rise to higher catalyst activity even at low (ppm) concentrations 14265


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Reactions for molecular materials can be monofunctionalized, and sequential direct arylation steps can yield complex and/or asymmetric conjugated molecules.267,272,278,287,288 Finally, a short-term landmark for this field would be the application of DHAP to synthesize the best performing polymers for optoelectronic applications.

and polymers for applications in micro- and nanoelectronics, electrooptics, and genomics.

ACKNOWLEDGMENTS This work was supported by NSERC grants. M.L. thanks the Killam foundation for a fellowship. J.R.P., F.G., and J.T.B. would like to acknowledge the financial support of the NSERC Postgraduate Scholarship program.

AUTHOR INFORMATION Author Contributions †

J.-R.P. and F.G. contributed equally to this work.

REFERENCES

Notes

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The authors declare no competing financial interest. Biographies Jean-Rémi Pouliot was born in Quebec City, Canada, and received his B.Sc. in Chemistry from Université Laval in 2011. He was awarded a NSERC Graham-Bell scholarship while doing his Ph.D. under the supervision of Professor Mario Leclerc. His research focuses on development and adaptation of direct (hetero)arylation polymerization to thiophene-based compounds, with an emphasis on high-performance materials for plastic electronics. In 2016, he cofounded Brilliant Matters, which is specializes in the fabrication of conjugated materials designed for applications in organic electronics. François Grenier received his B.Sc. degree in Chemistry from Université Laval in Quebec City in 2011. He completed his M.Sc. degree in Chemistry under the supervision of Professor Mario Leclerc in 2013 and was awarded Master’s NSERC Graham-Bell and FRQNT scholarships. He is currently pursuing a Ph.D. degree under the same supervisor, during the course of which he was awarded a NSERC Graham-Bell scholarship. The main subject of his thesis is the polymerization of isoindigo derivatives using direct (hetero)arylation. In 2013, he was a visiting student in Andrew B. Holmes’ group at the University of Melbourne, Australia, where he contributed to develop continuous flow methods for direct (hetero)arylation polymerization. J. Terence Blaskovits grew up in the Peace River region of northern British Columbia, Canada. He completed his B.Sc. in Chemistry at Université Laval in 2016 as a Schulich Leader scholar, and is currently a NSERC Master’s student in Professor Mario Leclerc’s group. His research interests include combining experimental and computational evidence in order to elucidate the scope and limitations of the DHAP reaction. When he’s not in the lab, he’s either playing the violin or piano, running, or climbing mountains. Serge Beaupré was awarded a Ph.D. in Chemistry from Université Laval, Quebec City, Canada, in 2004, under the guidance of Prof. Mario Leclerc. The same year, he accepted a position of research associate and project leader at the Laboratory of Electroactive and Photoactive Polymers at Université Laval. His research interests include the synthesis and characterization of new conjugated polymers for organic electronics, especially for plastic solar cells and organic field effect-transistors. Mario Leclerc was awarded a Ph.D. in Chemistry from Université Laval, Quebec City, Canada, in 1987, under the guidance of Prof. R. E. Prud’homme. After a short postdoctoral stay at INRS-Energie et Matériaux near Montréal with Prof. L. H. Dao, he joined the MaxPlanck-Institute for Polymer Research, in Mainz, Germany, as a postdoctoral fellow in the research group of Prof. G. Wegner. In 1989, he accepted a position of Professor in the Department of Chemistry at the Université de Montréal. He returned to Université Laval in 1998. Since 2001, he has held a Canada Research Chair on Electroactive and Photoactive Polymers. He is on the Thomson-Reuters 2014 and 2015 lists of the most influential chemists. His current research activities include the synthesis and characterization of new conjugated oligomers 14266


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