Recent Developments in Chain-Growth Polymerizations of

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Review

Recent developments in chain-growth polymerizations of conjugated polymers Melissa P. Aplan, and Enrique D Gomez Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01030 • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 18, 2017

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Industrial & Engineering Chemistry Research

Recent developments in chain-growth polymerizations of conjugated polymers Melissa P. Aplan1 and Enrique D. Gomez1,2* 1

Department of Chemical Engineering, The Pennsylvania State University, University Park, PA

16802 2

Materials Research Institute, The Pennsylvania State University, University Park, PA 16802

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

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Finally, to emphasize the potential of controlled chain-growth polymerizations, the synthesis of conjugated block copolymers by sequential chain-growth reactions is briefly discussed.

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Introduction Semiconducting polymers may lead to a new generation of chemically versatile, mechanically flexible, solution-processed electronic materials. Incorporating aromatic repeat units to form fully conjugated backbones allows for extensive delocalization of π-electrons and efficient charge transport. Alkyl or alkoxy sidechains are appended to the backbone, permitting solubility in organic solvents; this introduces the possibility for solution processing techniques including screen printing, inkjet printing, and roll-to-roll casting. Thus, conjugated polymers have potential applications in photovoltaics,1-9 flexible displays,10-14 thermoelectrics,15-20 and transistors21-26. In the laboratory, it has been demonstrated that molecular weight,27-35 dispersity,36,37 and end-groups38-43 of synthesized conjugated polymers can have substantial effects on device performance. For example, in solar cells where the active layer is composed of poly[5-(2hexyldodecyl)-1,3-thieno[3,4-c]pyrrole-4,6-dione-alt-5,5-(2,5-bis(3-dodecylthiophen-2-yl)thiophene)] (PTPD3T) as the donor and poly[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8bis(dicarboximide)- 2,6-diyl]-alt-5,5′-(2,2′-bithiophene)] (P(NDI2OD-T2) as the acceptor, device efficiency increases by nearly a factor of two when Mn values of the donor and acceptor are individually optimized. Highest power conversion efficiencies of 3.2% are realized for intermediary molecular weights (~ 40-50 kg/mol) of both the donor and acceptor polymers.35 In addition, the Yu group has established an inverse relationship between polymer dispersity (Đ) values and organic solar cell efficiencies. Increasing the dispersity (Đ) of poly(4,8-bis[(2ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl) (PTB7) leads to a red shift in absorbance, and,

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when incorporated in the photoactive layer, devices with weaker exciton generation, stronger bimolecular recombination, and lower charge carrier mobilities.37 Functional groups that terminate polymer chains as a result of common synthetic procedures (typically H or Br) can have detrimental effects when the materials are included in electronic devices. Bromine end groups in particular can quench photogenerated excitons, disrupt chain packing, and accelerate degradation.40-42 Alternatively, functional groups that are intentionally appended to the end of polymer chains can lead to devices with enhanced efficiencies. For instance, end-capping a donor polymer with an electron-accepting porphyrin moiety increases photovoltaic power conversion efficiency in devices from 1.5 to 4.5%.43 Creating the alternating double bond-single bond chemical structure of conjugated polymers often relies on cross-coupling reactions. As a consequence, synthesis is more commonly performed through polycondensation reactions that follow an uncontrolled stepgrowth mechanism. Nevertheless, in order to access the full potential of semiconducting polymers, the development of synthetic methods that follow a controlled chain-growth mechanism is necessary. Synthetic techniques that facilitate precise control over molecular weight, dispersity, and end groups will permit the optimization of polymer properties for industrial applications in electronic devices. Living chain-growth synthesis techniques also provide a method for end group functionalization of conjugated polymers.39,44-48 Furthermore, careful control of end groups will facilitate well-defined, fully conjugated block copolymers synthesized by sequential chaingrowth reactions. Driven by chemical incompatibility between the blocks, block copolymers self-assemble into well-defined microstructures.49,50 Taking advantage of this phenomenon, fully

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conjugated block copolymers have the potential to control the mesoscale morphology and interfacial structure, enhancing performance of organic electronic devices.51-58 In addition to linear homopolymers and block copolymers, controlled chain-growth polymerizations can be used to synthesize non-linear fully conjugated polymers including graft copolymers,59-61 surface-grafted polymers,62-64 and branched polymers65,66. Facile and carefully controlled synthesis of complex structures may extend potential applications of conjugated polymers to biomedical devices,67 water purification,68 and sensors69-71. In this Review, we will first outline the defining characteristics of step-growth, chaingrowth, and living chain-growth polymerizations. When attempting to develop a controlled polymerization reaction, it is necessary to confirm the mechanism using kinetic data. We then highlight recent advances in chain-growth polymerizations of conjugated polymers. The review will emphasize reactions using the catalyst transfer mechanism but also include less common methods, such as gold-activated and monomer deactivation polymerizations. We then move to a discussion of reported limitations in chain-growth polymerizations. Careful analyses of unsuccessful reactions will be instrumental in the rational development of new syntheses. We also highlight the impact of chain-growth polymerizations using the synthesis of fully conjugated block copolymers as a case study and conclude with a brief outlook for conjugated polymers from living polymerizations.

Establishing a chain-growth mechanism A polymerization can be classified into one of two categories based on the underlying reaction mechanism: either step-growth or chain-growth. The defining features that distinguish step-growth and chain-growth polymerizations originate in the reaction kinetics. This review will

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highlight the key differences in the kinetics of step-growth versus chain-growth reactions and how these can be used to experimentally verify a polymerization mechanism. Detailed derivations can be found elsewhere.72-76 In step-growth reactions, polymer chains grow through reactions between any two molecular species. As illustrated in Figure 1a, two monomers can react, a monomer can react with a growing chain, or two growing chains can react. The overall polymerization rate is the sum of rates of all reactions between two molecular species. Assuming reactivity is independent of the size of the reacting molecular species, the number average degree of polymerization (Xn) and dispersity (Đ) are expressed as:

1

X n 1-p

(1)

Đ 1 p

(2)

In equations 1 and 2, p is the extent of reaction, defined as the fraction of functional groups that have reacted at a given time. Defining features of a step-growth mechanism are that monomers are consumed soon after the reaction begins, high molecular weight products are only formed as p approaches 1, and Ɖ approaches 2 as the reaction approaches completion (p = 1).

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

In chain-growth polymerizations, a chain grows only by reaction of a monomer with the active end of a growing chain. The kinetics of chain-growth polymerization are more complex than step-growth. A chain-growth mechanism consists of three individual reactions: initiation, propagation, and termination. The overall polymerization rate can be modeled as the net rate of the three competing reactions.

The explicit rate expression will depend on nature of the

individual reaction as well as the relative rates of initiation, propagation, and termination. If termination takes place without combination of two chains, such as through disproportionation, then Ɖ follows equation 2 as in step-growth polymerizations. If termination reactions require two chains ends to come together, such as through combination, then

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p

Đ 1

(3)

2

and Ɖ approaches 1.5 at high p. Some significant features that distinguish a chain-growth from step-growth mechanism are that monomers are consumed steadily throughout the reaction and high molecular weight polymers can be formed at low monomer conversions. Living polymerization is a sub-set of chain-growth polymerizations. A living polymerization is defined as a chain-growth reaction in which no termination or chain transfer reactions occur during the polymerization (p < 1). For optimal control over molecular weight with a narrow size distribution, there are additional criteria. First, the rate of initiation (ri) must be much greater than the rate of propagation (rp). If ri >> rp, initiation is effectively instantaneous and will be complete before propagation begins. This, combined with no chain termination or transfer reactions, will generate a constant number of polymer chains throughout the reaction. The second requirement is that all chains must be equally likely to react with a monomer throughout the polymerization; this ensures all chains grow at the same rate. When these conditions are satisfied, the resulting polymer molecular weight will follow a Poisson distribution. In this review, we will refer to such a mechanism as a “controlled chain-growth” polymerization (Figure 1b). In a controlled chain-growth polymerization, all chains are initiated before propagation begins and grow at the same rate such that:

X n

M0 I

p

(4)

8


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IM p

I

Đ 1 (I M0 p)2 1 M 0

0p

(5)

M0 and I represent the initial amounts of monomer and initiator, respectively. Equation 4 leads to two important conclusions. First, assuming the reaction has gone to completion, the degree of polymerization can be calculated by the molar ratio of monomer to initiator. Second, the molecular weight increases linearly with extent of reaction. Furthermore, according to equation 5, dispersity is ≈ 1 for significant reaction conversion (approximately p ≥ 0.2, but this depends on M0 and I). Thus,

a

controlled

chain-growth

polymerization

mechanism

can

be

proven

experimentally by: 1) completing several reactions and demonstrating that the molecular weight of the product is linearly proportional to the ratio of monomer to initiator; or 2) monitoring the progress of a single reaction and confirming that the molecular weight of the product increases linearly with monomer conversion. An additional, albeit less conclusive, indication that a reaction follows a controlled chain-growth polymerization is that the dispersity value does not change as a function of monomer to initiator ratio or monomer conversion (assuming the conversion is high enough such that Đ ≈ 1 according to equation 5). Another experimental indication of a living chain-growth mechanism (but not necessarily a controlled chain-growth mechanism) is a chain-extension reaction. Once an initial reaction has gone to completion, if molecular weight continues to increase linearly upon a second monomer addition, this indicates no termination has occurred and the reaction is living.

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Once the reaction has gone to completion, active chain ends can be terminated by the addition of an appropriate reagent with no effect on molecular weight. If the rate of termination, (rt) is nonzero during the reaction, but rt > 1. The McNeil group investigated the effect of different external initiators on the polymerization of a para-phenylene monomer by KCTP.102 During the reductive elimination step, the associated πcomplex increases the electron density on the catalyst. DFT calculations reveal that when these charges are able to delocalize onto the Ar2 moiety of the external initiator, the activation free energy associated with the initial reductive elimination step is stabilized and the rate of initiation increases. Furthermore, the enhanced charge delocalization is unique to the Ar2 moiety of the external initiator and thus, the rate of propagation is not effected. By selecting the most appropriate ligand, a phenyl ring functionalized with trifluoroethoxy and dimethylamine substituents, ri/rp is increased. Control over the polymerization of para-phenylene is enhanced and dispersity values of ~ 1.1 are achieved. Note that this does not affect the potential for chain transfer and termination reactions to occur. Thus, although control is enhanced, dispersity values remain higher than those of a truly ideal controlled chain-growth polymerization. Suzuki catalyst transfer polymerization (SCTP)

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Palladium-catalyzed Suzuki polycondensation is one of the most common methods used to synthesize conjugated polymers. In 2007 it was demonstrated that Suzuki coupling can also proceed by a controlled chain-growth mechanism. Yokozowa and coworkers reported the controlled chain-growth synthesis of PF8 from the asymmetric monomer 2-(7-bromo-9,9dioctyl9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane using a palladium catalyst with a

bulky

electron-donating

ligand,

(tert-butylphosphine)phenylpalladium(II)

bromide

(tBu3PPd(Ph)Br).118 Molecular weight increased linearly with conversion as well as the molar ratio of monomer to catalyst. The reactive Pd complex is more likely to undergo oxidative addition than diffuse away from the growing polymer chain, thereby promoting a chain-growth mechanism.119 Unlike KCTP, the monomer does not have to be generated in-situ, eliminating difficulties associated with incomplete metal-halogen exchange and unreacted Grignard reagents. In general, SCTP is relatively less controlled than KCTP and is more successful with monomers that consist only of carbon and hydrogen; the reasons for these trends have yet to be elucidated. Controlled chain-growth SCTP has been reported for fluorene118,120,121, paraphenylene122, and thiophene120,123,124 monomers. Analogous to palladium-catalyzed KCTP, an unconventional nickel-catalyzed SCTP was recently developed by Noonan and coworkers.125 P3HT and poly(3-hexylesterthiophene) (P3HET) were synthesized using Ni(dppp)Cl2 and 1,3-bis[2,6-bis(1-methylethyl)phenyl]-1,3dihydro-2H-imidazol-2-ylidene(triphenylphosphine)nickel(II)

chloride

(Ni(PPh3)IPrCl2),

respectively. A controlled chain-growth mechanism was indicated by molecular weight modulation with catalyst loading and chain extension of either P3HT or P3EHT to form a block copolymer.

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One exciting development in the field of SCTP came from the Kiriy group with the successful chain-growth polymerization of an alternating fluorene-benzothiadiazole monomer.126 Kinetic studies confirmed a chain-growth reaction mechanism as monomer concentration decreased gradually throughout the reaction, relatively high molecular weights were produced at low monomer conversions, and dispersity remained relatively low and constant (~1.1-1.3). Nevertheless, a controlled chain-growth mechanism could not be established and only molecular weights up to 10 kg/mol were obtained. These results are especially important as most highperforming conjugated polymers utilize this alternating electron-rich, electron-deficient architecture; they provide a foundation for further experiments toward a living chain-growth mechanism for SCTP of high-performing alternating copolymers.

Stille catalyst transfer polymerization (StCTP) Similar to Suzuki polycondensation, palladium-catalyzed Stille polycondensation is a popular method used to make conjugated polymers. Until recently, this method had only been used to prepare conjugated polymers by an uncontrolled step-growth mechanism. P3HT was prepared from the asymmetric monomer, 2-bromo-5-trimethylstannylthiophene and a controlled chain-growth method confirmed by kinetic studies.127 For two sequential monomer loadings, the molecular weight increased linearly with monomer conversion and dispersity remained fairly constant at ~ 1.2. These results conclusively demonstrate controlled chain-growth behavior with the absence of termination reactions. As with SCTP, StCTP uses asymmetric functionalized monomers and avoids challenges arising with in situ monomer synthesis.

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Negishi catalyst transfer polymerization (NCTP) Negishi

catalyst

transfer

polymerization

(NCTP)

was

used

to

synthesize

poly(dithienosilole) (PDTS) in a quasi-living chain-growth mechanism.128 The molecular weight of the product is dependent on the ratio of monomer to initiator. Nevertheless, the dependence of molecular weight on conversion suggests only modest control, possibly due to chain-termination. Interestingly, PDTS cannot be polymerized in a controlled manner by KCTP due to incomplete metal-halogen exchange of Grignard reagents with the dithienosilole monomer.128,129 Further optimization may increase control over the polymerization and yield a controlled chain-growth mechanism.

Zn activated catalyst transfer polymerization An unusual, but exciting catalyst transfer polymerization method for highly electrondeficient monomers was recently reported by the Kiriy group. In an attempt to synthesize a bithiophene naphthalene diimide (TNDIT) polymer using NCTP, the expected organo-zinc derivative did not form upon the addition of active zinc to the dihalogenated naphthalene diimide monomer. Instead, an unusual radical-anion monomer was produced.130 Upon addition of a PhNi(dppe)-Br catalyst, the radical-anion monomer polymerized rapidly.

A chain-growth

mechanism is apparent, as adjusting the catalyst load results in moderate control over the product molecular weight. Furthermore, high molecular weight polymers are formed at low values of conversion, precluding a step-growth mechanism. A catalyst transfer polymerization mechanism was indicated by

31

P NMR which demonstrated the presence of a Br-TNDIT-Ni(dppe)-Br

complex. Such a complex would be the result of the CTP initiation. In addition, the end-groups

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

Beyond catalyst transfer polymerizations Converting step-growth polycondensation reactions into CTPs is certainly the most common method to develop controlled chain-growth syntheses for conjugated polymers. Nevertheless, there are reports of techniques that do not follow the CTP mechanism. In an effort to minimize processing steps and circumvent the metal-halogen exchange required for many CTP reactions, the Luscombe group developed a method to synthesize P3HT

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by C-H functionalization of the thiophene at the 5-position.134 This was done by aurylating 2bromo-3-hexylthiophene with chloro(tri-tert-butylphosphine)gold(I). The resulting monomer can be

polymerized

with

[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene](3-

chloropyridyl)palladium(II) dichloride (Pd-PEPPSI-iPr). A controlled chain-growth mechanism was demonstrated with kinetic experiments including a chain-extension reaction and monitoring a linear increase in molecular weight as a function of monomer conversion. Intriguingly, most of the P3HT product was terminated with H end groups on both ends, suggesting polymerization does not proceed by a catalyst transfer process. The authors note that the mechanism has yet to be elucidated and further investigations are ongoing. A clever method for controlled chain-growth polymerization of conjugated polymers by monomer deactivation was developed by Koeckelberghs and coworkers. This was initially demonstrated for the synthesis of P3HT using a 2-dicyclohexylphosphino-2′,6′-diisopropoxy1,1′-biphenyl)[2-(2-aminoethyl)phenyl]palladium(II) chloride (Pd(RuPhos) catalyst in a Negishitype coupling reaction.135 In the metal-halogen exchange step, an aryl-Br bond is converted to an aryl-ZnBr bond. The ZnBr group deactivates the aromatic monomer to the Pd(RuPhos) catalyst and thus, the catalyst can only undergo oxidative addition to the aryl-Br bond at the end of a growing chain. After reductive elimination, the catalyst will diffuse away and proceed with an intermolecular oxidative addition at the terminal aryl-Br bond of another chain. Nevertheless, because the catalyst can only react with the end of a growing chain, it is a chain-growth mechanism. A controlled chain-growth polymerization mechanism was established for thiophene, fluorene, and selenophene by monitoring Mn as a function of conversion (Figure 5). Because the polymerization does not depend on the affinity between the catalyst and monomer, this method has been used to synthesize P3HT with controlled degrees of regioregularity136,

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gradient copolymers of thiophene and fluorene137, and triblock copolymers of thiophene, fluorene, and selenophene138.

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

Table 1 summarizes all of the monomers discussed in this review that have been successfully polymerized by a chain-growth mechanism; chemical structures are illustrated in Figure 6. Other than KCTP, most methods are limited to only a few different monomers. Future efforts will likely increase the variety of monomers that can be polymerized by a chain-growth mechanism. Rational design of new monomer/catalyst systems will likely be inspired by reports of successful reactions, but also guided by insight into unsuccessful reactions.

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Table 1. Monomers used in chain-growth polymerizations of conjugated polymers. Chain-growth method KCTP (Ni catalyzed)

KCTP (Pd catalyzed)

SCTP (Pd catalyzed)

SCTP (Ni catalyzed) Stille CTP Negishi CTP Au(I) activated Zn activated

Monomer deactivation

Monomer Electron rich Thiophene Fluorene Furan Selenophene Tellurophene Pyrrole Thiazole Cyclopentadithiophene Para-phenylene Electron deficient 3,5-Pyridine Benzotriazole Electron rich Thiophene Para-phenylene Electron rich Fluorene Para-phenylene Thiophene Electron deficient Fluorene-benzothiadiazole Electron rich Thiophene Electron rich Thiophene Electron rich Dithienosilole Electron rich Thiophene Electron deficient Naphthalene diimide Perylene diimide Electron rich Thiophene Fluorene Selenophene

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

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Reported limitations: learning opportunities In this rapidly growing field, failed experiments provide excellent opportunities to deepen our understanding as to why some reactions succeed, while others do not. Although negative results often go unpublished, there are a few examples in the literature (Figure 7) that are reviewed in this section.

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

Thienothiophene (TT) is a popular building block of high-performing conjugated polymers. Thus far, attempts to polymerize an alkylthiolated TT by a chain-growth mechanism have proved unsuccessful.139 Authors demonstrated that in a Kumada-type coupling reaction with several different nickel catalysts, no polymer was formed. They were able to isolate a nickel-TT complex that was highly stable in solution. While it has been shown that increasing the stability of the catalyst-monomer associated pair can facilitate a chain-growth mechanism, these results suggest an upper-limit to the stability of the complex.107,108 When the nickel catalyst 27



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chelates with the TT monomer, one of the thiophene rings retains complete aromaticity; the authors speculate that this creates the stabilizing effect that is detrimental to polymerization. Based on this work, we speculate that other fused ring structures may also present challenges in developing chain-growth polymerization mechanisms. Poly(p-phenylene vinylene) (PPV) was one of the earliest known semiconducting polymers, yet synthesizing PPV by a catalyst transfer polymerization has proved challenging. While PPV can be synthesized by other living chain-growth polymerizations, these methods are not amenable to other conjugated polymers.140-142 This precludes the synthesis of well-defined all-conjugated block copolymers that incorporate PPV. Yokozawa and coworkers have shown that neither Kumada nor Suzuki type coupling reactions will yield a controlled chain-growth mechanism for the synthesis of PPV. Polymerization did not occur when nickel catalysts were used and proceeded in an uncontrolled manner when a palladium catalyst was used.143 It was demonstrated that tBu3PPd(0) readily forms an intermolecular C=C-Pd-C=C complex with the non-aromatic double bond. This facilitates intermolecular transfer from the C=C double bond of one chain to another and leads to an uncontrolled step-growth mechanism. Interestingly, experiments using model compounds suggest that formation of this C=C-Pd-C=C complex can be suppressed if there are substituents at the ortho position of the C=C double bond. Assuming an analogous effect will be observed, these substituents may facilitate intramolecular catalyst transfer and permit controlled chain-growth polymerization of polymers containing C=C bonds.144 Benzo[1,2-b:,4,5-b’]dithiophene (BDT), a common building block of donor polymers used in high-performing organic photovoltaic devices, can be polymerized by both Kumada and Stille coupling reactions. In a recent publication, greater control was demonstrated when BDT

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

An example of leveraging controlled polymerizations of conjugated polymers: block copolymers by sequential chain-growth polymerizations Fully conjugated block copolymers can be used to control the mesoscale morphology in organic electronic devices.51,56,58,147-149 Most fully conjugated block copolymers that have been incorporated into electronic devices were synthesized by KCTP followed by an uncontrolled polycondensation reaction, or successive uncontrolled polycondensation reactions.148,149 In order to obtain well-defined architectures, it is desirable to synthesize fully conjugated block copolymers by sequential chain growth polymerizations. In principle, this will maximize control over block lengths and minimize homopolymer or multi-block impurities. There are several examples of fully conjugated block copolymers synthesized by sequential chain-growth polymerizations; most were synthesized by successive CTPs (Figure 8). Block copolymers formed by nickel-catalyzed KCTP in a one-pot synthesis include: poly(paraphenylene)-block-poly(pyrrole)

(PPP-b-PP)

29

98

,

poly(3-hexylselenophene)-block-



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Page 30 of 51

poly(3-hexylthiophene) (P3HS-b-P3HT)150, poly(3-hexylthiophene)-block-poly(benzotriazole) (P3HT-b-PBTAz)

and

PBTAz-b-P3HT108,

poly(3-hexylthiophene)-block-

poly(cyclopentadithiophene) (P3HT-b-PCPDT)101, P3HT-b-P(3HT-alt-pyridine)106, PF8-b-P3HT 91

, and PPP-b-P3HT

151,152

. There is also a report of P3HT-b-PDTS synthesized by KCTP

followed by NCTP.128

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

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For the most part, the sequence of monomer addition is critical in forming a block copolymer. The catalyst will preferentially complex with the more electron rich monomer and thus this monomer must be polymerized as the second block to allow for catalyst transfer to the second monomer. Nevertheless, recent developments suggest that this can be circumvented if a more electron donating catalyst is used. This was demonstrated by the carefully designed nickel diimine catalyst for P3HT-b-PBTAz or PBTAz-b-P3HT108 and for palladium catalyzed Kumada coupling of PPP-b-P3HT or P3HT-b-PPP90. There are also reports of fully conjugated block copolymers synthesized by successive SCTP. PF8-b-PPP can be synthesized by palladium-catalyzed Suzuki reactions.122 P3HT-bP3HET and P3HET-b-P3HT can be synthesized by Ni(PPh3)IPrCl2 or Ni(dppp)Cl2 catalyzed Suzuki reactions.125 The Koeckelberghs group synthesized fully conjugated triblock copolymers using the monomer deactivation method described earlier. The metal-halogen exchange step, in which an aryl-Br bond is converted to an aryl-ZnBr bond, “deactivates” the monomer to the catalyst. The catalyst can only undergo oxidative addition to the aryl-Br bond at the end of a growing chain, thus enabling a chain-growth mechanism. Because the chain-growth mechanism is independent of the catalyst-monomer association, block copolymers can be synthesized successfully in any sequence of monomer addition. To demonstrate the robustness of this method, the Koeckelberghs group synthesized thiophene-fluorene-selenophene triblock copolymers in every possible sequence.138

Conclusions and outlook

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In this review, we discuss recent advances in chain-growth polymerizations of conjugated polymers. The most common technique to develop a chain-growth synthesis is to convert an uncontrolled polycondensation to a controlled chain-growth polymerization. This is accomplished by carefully designing and optimizing system-specific catalyst-monomer interactions, yielding a catalyst-transfer mechanism. The nature of this method necessitates verification of a step-growth, chain-growth, or controlled chain-growth mechanism using kinetic experiments. CTP is not the only way to achieve controlled chain-growth polymerizations for conjugated polymers. New techniques are emerging, including the monomer deactivation method. This method is particularly interesting as its success does not rely on the formation of an associated complex between the catalyst and growing chain. The most exciting recent developments are of chain-growth polymerizations for conjugated polymers that incorporate electron deficient monomers. Controlled chain-growth reactions have been demonstrated for polymers incorporating 3,5-pyridine and benzotriazole, while chain-growth reactions with modest control have been demonstrated for naphthalene diimide and perylene diimide. An uncontrolled chain-growth mechanism has been demonstrated for

an

alternating

fluorene-benzothiadiazole

polymer.

Although

the

chain-growth

polymerizations reported for some electron deficient monomers have not yet been optimized for maximum control, these reports are significant. Incorporation of electron deficient monomers, especially into an alternating electron-rich- electron-deficient architecture, is characteristic of conjugated polymers used as the active material in high-performance electronic devices. The reports of unsuccessful controlled chain-growth syntheses of thienothiophene and benzodithiophene are also significant. These are two very common electron-rich building blocks

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of conjugated polymers used in organic electronic devices. Insight into why these reactions failed will be indispensable for the development of controlled chain-growth polymerizations for these monomers as well as new systems. Broadening the scope of controlled chain-growth polymerizations to include a wide range of monomers will facilitate the precision synthesis of the polymers most relevant to highperformance devices. We speculate that increased control over polymer molecular weight, dispersity, and end-groups will maximize performance of organic electronics. Controlled chaingrowth polymerizations will also enable synthesis of well-defined advanced architectures, including block copolymers. This could extend industrial applications of conjugated polymers to water purification, biomedical devices, and sensors. We envision future studies will broaden the number of monomers capable of being polymerized by a controlled chain-growth mechanism. This will likely be aided by the careful design of new catalysts and a greater understanding of catalyst-monomer interactions. Progress in the field will be greatly aided by reports of successes and limitations that deepen the community’s understanding of controlled chain-growth polymerizations for conjugated polymers.

Acknowledgements Financial support from the Office of Naval Research under Grant N000141410532 is gratefully acknowledged. Biographies and pictures: Enrique D. Gomez received his BS in Chemical Engineering from the University of Florida and his PhD in Chemical Engineering from the University of California, Berkeley. After a little over a year as a postdoctoral research associate at Princeton University, he joined the faculty in the 33



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Chemical Engineering Department of the Pennsylvania State University in August of 2009 and is currently an Associate Professor. His research activities focus on understanding how structure at various length scales affects macroscopic properties of soft condensed matter. Currently, the work in the Gomez group examines the relationship between chemical structure, microstructure, and optoelectronic properties of conjugated organic molecules. During his time at Penn State, Dr Gomez has won multiple awards, including the Oak Ridge Associated Universities Ralph E. Powe Junior Faculty Enhancement Award, the National Science Foundation CAREER Award, the Rustum and Della Roy Innovation in Materials Science Award, and the Penn State Engineering Alumni Society Outstanding Research Award.

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

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