All-Conjugated, All-Crystalline DonorâAcceptor Block Copolymers

Article pubs.acs.org/Macromolecules

All-Conjugated, All-Crystalline Donor−Acceptor Block Copolymers P3HT‑b‑PNDIT2 via Direct Arylation Polycondensation Fritz Nübling,†,‡ Hartmut Komber,§ and Michael Sommer*,†,‡,∥ †

Makromolekulare Chemie, Universität Freiburg, Stefan-Meier-Straße 31, 79104 Freiburg, Germany Freiburger Materialforschungszentrum, Stefan-Meier-Straße 21, 79104 Freiburg, Germany § Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany ∥ Freiburger Institut für interaktive Materialien und bioinspirierte Technologien, Georges-Koehler-Allee 105, 79110 Freiburg, Germany ‡

S Supporting Information *

ABSTRACT: The synthesis and characterization of allconjugated, all-crystalline donor−acceptor block copolymers (BCPs) containing poly(3-hexylthiophene) (P3HT) and poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (PNDIT2) is presented. Direct arylation polycondensation (DAP) of dibromonaphthalenediimide and bithiophene is carried out in the presence of P3HT end-cappers to allow the in situ formation of BCPs P3HT-b-PNDIT2. As-prepared, well-defined H-P3HT-Br with hydrogen and bromine chain termini shows nonoptimal reactivity under the DAP conditions used. Therefore, H-P3HT-Br is converted into either H-P3HTTh (thiophene) or H-P3HT-Mes (mesitylene), giving α,ω-hetero-C−H functionalized P3HT with modulated C−H reactivity. The influence of the different C−H chain termini of P3HT on the ability to act as end-capper and the resulting block structures is investigated in detail using wavelength-dependent size exclusion chromatography (SEC) and NMR spectroscopy. Different C−H reactivities of α,ω-hetero-C−H functionalized P3HT cause different contents of multiblocks, which in turn lead to varied degrees of crystallinity. These results show that careful tuning of C−H reactivity is a promising way to obtain well-defined, all-conjugated block copolymers via DAP.

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INTRODUCTION Semiconducting polymers show several interesting properties like high charge carrier mobilities, strong and broad absorption of light in the visible region, and strong light emission, which are tunable by structural design. Good solubility in organic solvents enables cost-efficient manufacturing by well-known printing techniques. This opens the door for optoelectronic thin film devices such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and many others.1−4 Within the past two decades OPV devices have been mostly manufactured by combining semiconducting polymers as electron donors and fullerenes as electron acceptor.5−7 So far, high power conversion efficiencies (PCEs) up to 11.7%8,9 were obtained, which are the highest values for single-layer solar cells containing semiconducting polymers. Besides these results, manufacturing of organic photovoltaics, in which both electron donor and acceptor are polymer compounds, has been presented as an alternative.10,11 Such all-polymer solar cells have several important advantages over polymer−fullerene solar cells and have therefore moved into the center of interest especially during the very last years. In contrast to fullerene derivatives, acceptor polymers offer ample opportunity for © XXXX American Chemical Society

structural design in order to to adjust and combine properties such as light absorption and band gap, frontier molecular orbitals, exciton diffusion, and charge carrier mobility.1,3 Moreover, these systems exhibit improved mechanical properties like flexibility and strength, which is a benefit especially for applications that rely on flexible substrates.12 However, state-of-the-art all-polymer solar cells show lower PCEs compared to polymer fullerene systems because of weak control over morphology, which results in large-scale phase separation. So far, interpenetrated donor−acceptor nanomorphologies are adjusted by trial and error approaches using thermal annealing, solvent vapor annealing or additives, all of which lead to thermodynamically unstable phases and additionally are challenging to reproduce.13−15 Fully conjugated block copolymers (BCPs) for photovoltaic applications are of particular interest because of their unique properties regarding structure formation. Therefore, they can be used to build up defined, thermodynamic stable and nanophase-separated structures highly suitable for exciton diffusion and separaReceived: February 2, 2017 Revised: February 20, 2017

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DOI: 10.1021/acs.macromol.7b00251 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules tion.16−18 Moreover, the morphology of nanophase separated, optoelectronically active BCPs can be controlled by block length and volume fraction.19 Despite their outstanding properties, reports on fully conjugated BCPs are still rare due to challenges in synthetic methologies.16,18,20,21 So far, most polymerization methods used for conjugated polymer synthesis are conventional transition-metal-catalyzed polycondensations such as Stille,22 Suzuki,23 and Kumada24,25 polycondensations. Nevertheless, each of these methods has its own shortcomings. For example, the in situ preparation of monomers without purification (Kumada), the use of organometallic monomers resulting in highly toxic side products26 (Stille), or the need for cryogenic temperatures or catalytic protocols for monomer functionalization (Stille and Suzuki) are environmentally problematic. One of the most promising method to build up conjugated polymers in a more atomeconomic fashion is the direct C−H arylation polycondensation (DAP).27−32 Condensation reactions take place between (hetero)arenes and (hetero)aryl halides to form new C−C bonds by producing only acidic byproducts like hydrogen halides. In this work, poly(3-hexylthiophene) (P3HT) as donor material and an alternating donor−acceptor naphthalenediimide bithiophene copolymer poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt5,5′-(2,2′-bithiophene)} (PNDIT2) as acceptor material have been chosen because of their complementary absorption profiles, energy levels, and excellent charge carrier mobilities. Both polymers have already been successfully combined for manufacturing and characterization of all-polymer bulk heterojunction solar cells.13 Within the past few years several reports dealing with the synthesis of BCPs containing P3HT and PNDIT2 have been published using Yamamoto33 and Stille couplings.34,35 With these methods exhibiting significant disadvantages with respect to atom economy, stoichiometric use of nickel, and toxicity as well as the need for prefunctionalization of monomers, further elaboration of synthetic avenues toward well-defined allconjugated donor−acceptor block copolymers becomes necessary. Here we report the first synthesis of block copolymers P3HT-b-PNDIT2 by direct C−H arylation polycondensation. PNDIT2 is synthesized via DAP, and the evolution of end groups is monitored as a function of time, which is important for chain termination by P3HT. These results further guide the development of different α,ω-hetero-C−H functionalized P3HT end-cappers for use during PNDIT2 synthesis. The influence of the different C−H chain termini of P3HT on the ability to act as end-capper and the resulting block structures is investigated in detail using wavelength-dependent size exclusion chromatography (SEC) and NMR spectroscopy. Different C−H reactivities of α,ω-hetero-C−H functionalized P3HT causes different contents of multiblocks, which in turn leads to varied degrees of crystallinity as deduced from differential scanning calorimetry (DSC). This work illustrates the great potential of synthesizing all-conjugated donor− acceptor block copolymers via DAP.

bithiophene (T2) and subjected them to 1H NMR spectroscopy (Scheme 1). Scheme 1. Copolymerization of NDIBr2 and T2 To Monitor the Evolution of End Groups

The samples withdrawn were extracted with methanol to remove residual monomer, base, and impurities, and the end groups were analyzed by 1H NMR spectroscopy (Figure 1a).37 While in the first 2 h mostly oligomers were formed, the decreasing intensity of end group signals compared to the two backbone signals at 8.90 and 7.42 ppm indicates polymer formation with time. Interestingly, the purified products show an excess of T2-H end groups in comparison to NDI-Br end groups. At the very beginning of the reaction T2-H end groups are found almost exclusively, while the ratio between T2-H and NDI-Br converges to unity with increasing reaction time with the reaction being stopped after 1 day due to gelled and high molecular weight PNDIT2 (Figure SI-1). The obtained end group ratio for long reaction time is the expected result of a polycondensation reaction with perfect stoichiometry and in the absence of side reactions, clearly revealing the highly optimized protocol toward PNDIT2 via DAP (Figure 1b). While for common polycondensations the ratio between end groups is thought to result from the monomer feed ratio in the absence of side reactions, the exceptionally high content of T2H end groups at early stages of the reaction is caused by a distinct mechanism that includes a ring-walking process of Pd(0)L over the NDI skeleton, as proposed for Pd-catalyzed Stille38 and radical anion polymerizations.39 Scheme 2 depicts a mechanism for PNDIT2 synthesis via DAP that explains the unbalanced formation of T2-H end groups at early stages. With the catalytically active species formed from Pd2dba3 being unclear at present, for now we assume that Pd(0)dba is mainly involved. Thus, Pd(0)dba first inserts into the C−Br bond by oxidative addition (OA), followed by concerted metalation deprotonation (CMD) and reductive elimination (RE) steps to form a T2−NDI bond. After RE, the catalyst remains associated with the electrondeficient NDI skeleton and subsequently undergoes intramolecular oxidative addition (im-OA) into the remaining C−Br bond of the same NDI unit. This latter step must proceed by a “ring-walking” mechanism.40,41 This results first in symmetric T2-H end-capped NDI molecules, oligomers and prepolymers, which can finally further react with NDIBr2 to obtain long chains with more asymmetric chain termination as expected for a typical polycondensation. To further corroborate the proposed mechanism, a model reaction of H-P3HT-Mes (2b) and an excess of 10 equiv of NDIBr2 was performed (Scheme 3). The progress of the reaction was monitored by withdrawing aliquots which were purified and characterized by SEC and 1H NMR spectroscopy.

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RESULTS AND DISCUSSION Mechanistic Insight into PNDIT2 Synthesis via DAP. As the nature of end groups of PNDIT236,37 made by DAP is important for the in situ reaction with P3HT, we took aliquots during DAP of 2,7-dibromonaphthalenediimide (NDIBr2) and B


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Figure 1. (a) 1H NMR spectra (aromatic protons region) of PNDIT2 samples withdrawn at different reaction times (C2D2Cl4 at 120 °C). Atom numbering corresponds to the chemical structure of PNDIT2 with observed NDI-Br and T2-H end groups. (b) Ratio of end groups estimated from signal intensities of H2 (T2-H) and H7 (NDI-Br).

The 1H NMR spectra were analyzed and the variation in composition was calculated, showing full conversion of the C− H end group of H-P3HT-Mes and coupling to NDI between 60 and 120 min (Figure SI-2). Despite the large excess of NDIBr2, the major product is the symmetric Mes-P3HT-NDIP3HT-Mes as deduced from SEC (Figure 2). From a statistical point of view, such a product distribution can only be explained by the aforementioned ring-walking process, which allows imOA of Pd(0)dba into the second NDI−Br bond to proceed much faster than the first OA step. Because of the small amount of H-P3HT-Mes and its fast consumption, the remaining MesP3HT-NDI-Pd(dba)-Br species do not find coupling partners, and hence end group degradation occurs leading to NDI-OPiv, NDI-OH, and also NDI-Br end groups (see Figures SI-2−SI4).36,37,42 End Group Functionalization of P3HT. End group functionalized P3HTs were synthesized by polymer analogous Suzuki coupling reactions to establish P3HT with modulated C−H reactivities in BCP formations (Scheme 4). Despite the proved reactivity of the unprotected side of H-P3HT-Mes, access to this C−H bond is sterically hindered by the hexyl group in the ortho-position. Thus, we envisioned that an unsubstituted thienyl end group should exhibit reduced sterical hindrance and hence enhanced reactivity compared to P3HTH. Therefore, H-P3HT-Br was equipped with a mesityl end group (H-P3HT-Mes 2b) or a thienyl end group (H-P3HT-Th 2c). The mesityl end group is unreactive under the DAP conditions used,36 thus leaving one C−H end group in HP3HT-Mes for covalent attachment to PNDIT2. The precursor H-P3HT-Br 2a was synthesized by KCTP, giving narrow distributed polymers with H/Br end groups.43,44 Complete conversion is proven by MALDI-ToF mass spectrometric analysis (Figure 3) and 1H NMR spectroscopy (Figure SI-5). Block Copolymer Synthesis. The effect of the modulated C−H reactivity of the differently functionalized P3HTs 2a−c on block copolymer synthesis and the resulting polymeric topologies was investigated next. As illustrated in Scheme 5, the P3HTs can be reactive at either side except 2b having an

unreactive mesityl chain end. First, the H-P3HT-Br 2a was used in BCP synthesis under DAP conditions. Both end groups of 2a are moderately reactive, resulting in a mixture of homopolymers H-P3HT-Br (2a) and PNDIT2 and BCP (3a). The mesityl end group of H-P3HT-Mes 2b is unreactive that only the P3HT-H end group of H-P3HT-Mes is available for reaction. The third material, H-P3HT-Th 2c, carries the unsubstituted thienyl group as well as the moderately reactive 3-hexylthiophene end group. Thus, the main difference between 2c and 2a/2b is the absence of the hexyl side chain next to the desired reactive end group. This structural difference is important, given the recently observed large differences in C−H reactivity of benzene-derived solvents under these conditions as a function of the numbers of methyl substituents.36 BCP synthesis was carried out by performing PNDIT2 synthesis under DAP conditions in the presence of 2a−c. The raw products were worked up by Soxhlet extraction with methanol, acetone, ethyl acetate, i-hexanes, dichloromethane, and chloroform. The chloroform fraction was subjected to wavelength-dependent SEC analysis, NMR spectroscopy, and optical and thermal characterization, the results of which are compiled in Table 1. In order to obtain evidence for covalent incorporation of P3HT into PNDIT2-b-P3HT, wavelength-dependent SEC was carried out as shown in Figure 4. From the UV−vis absorption spectra (Figure 4a), it is seen that both segments P3HT and PNDIT2 can be detected independently at 450 and 600 nm, respectively. Figures 4b−d show the wavelength-dependent SEC traces for the purified BCPs 3a−c, respectively. Detection at 254 nm probes all species independently of their nature. In case any homopolymer is absent, the three SEC traces should exhibit minimal variation. Thus, overlaying the SEC traces taken at 254, 450, and 600 nm easily provides useful information about homopolymer content and thus about the degree of covalent incorporation of P3HT into P3HT-bPNDIT2 BCP. Comparing Figures 4b−d, it is clear that only H-P3HT-Th allows for complete reaction and incorporation into BCP. The C


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Scheme 2. Proposed “Ring-Walking”-like Mechanism for the Copolymerization of NDIBr2 and T2 with Reductive Elimination as the Crucial Stepa

a

L2 depicts an unspecified ligand, which most likely Is dba.

Scheme 3. Polymer Analogous Reaction of H-P3HT-Mes (2b) and 10 equiv of NDIBr2 under Direct Arylation Conditionsa

a

Product mixture contains mono- and disubstituted NDI.

very similar SEC traces in Figure 4d show that both homopolymers are mostly absent. For H-P3HT-Br 3a and HP3HT-Mes 3b, the SEC curves indicate both residual P3HT and free PNDIT2, which can be ascribed to the lower reactivity of C−H chain ends having hexyl chains in the ortho-position.

While this result is useful and interesting in that it shows how to minimize homopolymer content, it brings up the question as to what extent the two chain termini of different reactivity of 2a and 2c lead to the formation of multiblock copolymers. To this end, NMR spectroscopy is employed, including end group and D


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Figure 2. SEC analysis of the polymer analogous reaction between HP3HT-Mes (2b) and a 10-fold excess of NDIBr2.

Figure 3. MALDI-ToF mass spectra of polymer analogous end group functionalized P3HT by Suzuki coupling reaction.

block junction group analysis (Figure 5). The 1H NMR spectrum of 3a proves that Br and H end groups are similarly reactive but do not react quantitative (Figure SI-6a). For 3b we observed the predicted stability of the mesityl end group, with the other C−H end group being of comparably weak reactivity (Figure SI-6b). In contrast, 2c having an unsubstituted thienyl end group (Figure 5a) showed quantitative BCP formation as proved by the vanishing signal group at 7.25−7.15 ppm of the thiophene end group of 2c (Figure 5b and Figure SI-6c). Unfortunately, the new signals of the linking 2,5-disubstituted thiophene group are overlapped by the signals of the -NDI-T2H end group and could not be quantified. The signal at 7.18 ppm indicates reaction also of the P3HT-H end group (∼20%). Thus, also the formation of P3HT-centered tri- and/or multiblocks is possible to low extent. The formation of tri- and multiblock copolymers during the synthesis of all-conjugated block copolymers is a general problem. Despite the diblock structure being the most probable, several possibilities exist that lead to tri- or multiblock structures. First of all, 2a and 2c can react at either side, leading to P3HT middle blocks. Also, the PNDIT2 segment can be terminated on either side by -Th-P3HT-H. Only in the case of 3b, the unreactive mesityl end group prevents multiblock formation, and besides the diblock, only Mes-P3HT-PNDIT2P3HT-Mes is possible in principle. Regarding H-P3HT-Br 2a and H-P3HT-Th 2c, both end groups are reactive under DAP conditions. Hence, the formation of multiblock copolymers is possible in both cases, but precise quantification is a challenge. What can be probed by NMR end group analysis is the amount of P3HT end groups that have reacted compared to the pristine P3HT. Table 1 collects the results of covalent incorporation of P3HT into block structures. As can be seen, the conversion of P3HT end groups increases from 2a to 2b to 2c, showing increasing covalent incorporation of P3HT into block structures. With increasing conversion, the probability for triand multiblock formation also increases. Hence, we conclude that for low conversion and block content, such as the reaction

with 2a or 2b, the products are mainly ternary blends comprising P3HT and PNDIT2 homopolymers and BCPs as also proved by SEC analysis. 3c contains block structures only, but these include several species due to the α,ω-hetero-C−H functionalization as present in H-P3HT-Th. Clearly, usage of Mes-P3HT-Th would be most ideal as only one chain end is highly reactive with the other one being inert, leading to welldefined diblock copolymers without homopolymer impurities. Mes-P3HT-Th appears to be accessible through a one-pot procedure via the sequence external initiation, polymerization, and quenching with an excess of thienylmagnesium chloride.45,46 However, both initiation and quenching were not quantitative in our hands, making this procedure not as straightforward as anticipated. Further Fine-Tuning of BCP Synthesis. In order to provide room for tuning morphology of block copolymers and compatibilized blends, further modification of the synthetic protocol with respect to stoichiometry and catalyst loading to vary composition and PNDIT2 block length of P3HT-bPNDIT2 was attempted (Table 2) for the most promising 2c. Starting from entry 3c, the catalyst loading was increased to increase PNDIT2 block length and overall BCP molecular weight (entry 4). Considering that 40 mol % of the H-P3HTTh chains have reacted at both sides, a significant amount of triand multiblocks can be assumed. Interestingly, using an excess of NDIBr2, molecular weight of PNDIT2 and the corresponding BCP can be drastically increased (entry 5). This can be ascribed to the mechanism of polycondensation, by which T2terminated prepolymers form first. These can further react fast in the presence of an excess of NDIBr2 to higher molecular weight chains, however, resulting in 5 to an increase of NDI-Br end groups and hence to higher content of multiblocks by conversion of P3HT-H end groups up to 70%. In the last reaction 6 the equivalents of NDIBr2 and T2 compared to H-P3HT-Th were halved to increase the weight fraction of P3HT. The change in ratio causes only slight increase of P3HT content in BCP compared to 3c and results

Scheme 4. Polymer Analogous End Group Functionalization of H-P3HT-Br 2a via Suzuki Reaction

E


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Macromolecules Scheme 5. P3HT-b-PNDIT2 Synthesis Using End Group Functionalized P3HTs 2a, 2b, and 2ca

a

Reaction conditions: (i) Pd2dba3 (1 mol %), PivOH (1 equiv), K2CO3 (3 equiv), solvent mesitylene (0.1 M).

Table 1. Molecular Weights, Weight Fractions, and Contents of Blocks of Synthesized BCPs entry 3a (X = Br) 3b (X = mes) 3c (X = Th)

Mn,NMR P3HT/kDaa 7.3 7.0 8.9

Mn,SEC P3HT/kDab

Mn,SEC BCP/kDa

11 12 12

b

20 23b 33c

Đ BCP

wt % P3HTd

conv end groups H-P3HT/P3HT-X/%e

1.9 2.1 1.4

27 25 33

20/20e 50/0e 20/100

a

From 1H NMR spectroscopy. bFrom SEC in CHCl3 after Soxhlet extraction with methanol, acetone, and i-hexanes. cFrom SEC in CHCl3 after Soxhlet extraction with methanol, acetone, ethyl acetate, i-hexanes, and dichloromethane. dEstimated from 1H NMR integrals of side chain signals at 4.19 ppm (NCH2 of NDI) and 2.86 ppm (α-CH2 of P3HT). eEstimated from 1H NMR integrals: P3HT-H from linkage signal at 7.18 ppm and P3HT-H end group signal at 6.94 ppm. Conversion of P3HT-X from comparison with the signal integrals of the pristine H-P3HT-X. Estimated error: ±5%.

Wavelength-dependent SEC traces of BCPs 4, 5, and 6 are illustrated in Figure 6, showing good overlap of the three curves in all cases and indicating quantitative linkage between both polymer blocks and thus BCP formation. Thus, for a given molecular weight of the P3HT end-capper, the weight fraction of PNDIT2 and overall BCP molecular weight can be adjusted, and the requirements for successful P3HT-b-PNDIT2 synthesis with tailored molecular weight and composition can be summarized as follows: (1) Molecular weight of P3HT is controlled by standard Kumada synthesis, which is described elsewhere.44 Important is that H-P3HT-Br has 100% H/Br end group fidelity needed for subsequent Suzuki coupling. (2) HP3HT-Th obtained via polymer analogous Suzuki coupling is needed to maximize C−H reactivity of the P3HT end-capper. In other words, the nature of the P3HT end group is to be as similar as the comonomer T2 to be incorporated efficiently. (3) The molecular weight of PNDIT2 is most efficiently controlled by stoichiometry of NDIBr2 and T2. Usage of an excess of NDIBr2 leads to longer PNDIT2 chains, which can be explained by the mechanism of polymerization (T2-terminated prepolymers form first). The slight excess couples symmetrically T2-terminated PNDIT2 prepolymers to get longer chains faster, i.e., before end group degradation of NDI-Br chain ends

Figure 4. (a) UV−vis spectra of P3HT (red) and PNDIT2 (green). (b−d) Wavelength-dependent SEC spectra of BCPs 3a−c.

in a higher amount of P3HT incorporated to PNDIT2-P3HTTh-PNDIT2. F


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Figure 5. 1H NMR spectra (aromatic region) of (a) H-P3HT-Th 2c and (b) BCP 3c (C2D2Cl4 at 120 °C).

Table 2. BCP Samples with Different Conditions in Stoichiometry stochiometry entry

NDIBr2/ equiv

T2/ equiv

P3HT/ equiv

Pd2dba3/ mol %

Mn,SEC BCP/ kDa

Đ BCP

Mn,SEC P3HT/ kDac

P3HT/ wt %d

conv P3HT-H end group/%e

3c 4 5 6

10.0 10.0 10.0 10.0

9.5 9.5 9.0 9.0

1.0 1.0 1.0 2.0

1 9 1 5

33a 46a 124a 30b

1.4 1.6 2.6 1.8

11.9 11.3 11.3 11.3

33 27 25 37

20 40 70 35

a

From SEC in CHCl3 after Soxhlet extraction with methanol, acetone, ethyl acetate, i-hexanes, and dichloromethane. bSoxhlet purification without dichloromethane. cFrom SEC in THF after Soxhlet extraction with methanol, acetone, and i-hexanes. dEstimated from 1H NMR integrals of side chain signals at 4.19 ppm (NCH2 of NDI) and 2.86 ppm (α-CH2 of P3HT). eEstimated from 1H NMR integrals of linkage signal at 7.18 ppm and P3HT-H end group signal at 6.94 ppm.

Figure 6. Wavelength-dependent SEC curves of BCP samples 4 (a), 5 (b), and 6 (c).

sets in. (4) Obviously, the chain length of PNDIT2 is coupled to composition. Roughly, compositions of 20−50 wt % P3HT are possible, whereby the longer PNDIT2 chains result in a decreased P3HT weight fraction. Optical Properties of P3HT-b-PNDIT2. The optical properties were exemplarily investigated by steady-state UV− vis and photoluminescence spectroscopy of 3c and compared with the individual spectra of both homopolymers (Figure SI7). The BCP shows all features of both homopolymers combined and therefore a broad light absorption up to 850 nm. The absorption peaks (π−π* transition) at 387 and 453 nm can be assigned to PNDIT2 and P3HT blocks, respectively. Also, the CT band of PNDIT2 at 620 nm can be observed in

the spectrum of 3c. Photoluminescence (PL) spectra were recorded to identify the PL quenching in the BCP. The covalent linkage between P3HT and PNDIT2 is expected to lead to smaller domains and thus to more efficient charge transfer compared to the physical blend of donor and acceptor polymer. Therefore, we excited the H-P3HT-Th 2c at its absorption maximum at 450 nm and measured the luminescence intensity perpendicular to the light beam. To prove our presumption, we did the same measurement with the physical blend from P3HT and PNDIT2 and compared it to our BCP 3c (Figure SI-7). It can be clearly seen that the PNDIT2 in the blend already quenches some of the P3HT luminescence. As we expected, the P3HT PL quenching is even G


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terminated chains as a result of a “ring-walking”-like mechanism in which Pd(0)dba intramolecularly undergoes oxidative addition into the second NDI−Br bond of the same molecule. α,ω-End group functionalized P3HTs were synthesized by polymer analogous Suzuki coupling and were used as terminating reagents in PNDIT2 synthesis under optimized conditions. For H-P3HT-Br and H-P3HT-Mes, we observed mixtures of homopolymers and block copolymers because of the steric hindrance of the hexyl side chain next to the reactive end groups which lowers reactivity. Complete covalent linkage of both blocks was observed for H-P3HT-Th due to absence of the hexyl side chain in the terminal thienyl group. The crystalline structure of P3HT and PNDIT2 within the block copolymer was investigated by DSC measurements, and the results were correlated with different amounts of multiblock copolymers, whereby an increasing amount of multiblock copolymers decreased the tendency of both PNDIT2 and P3HT to crystallize. Compared with existing methods, the herein protocol allows for significantly simplified block copolymer preparation also with other blocks and opens up many possibilities due to the presence of ubiquitous C−H bonds. To the best of our knowledge, this is the first example of P3HT-b-PNDIT2 block copolymers prepared by direct C−H arylation polymerization.

more effective in the BCP. The reason for this is the fact that in 3c the donor and acceptor polymer are covalently linked and fully conjugated over the junction. This enables a fast and effective charge transport from the excited P3HT to the PNDIT2 and leads to a clearly stronger PL quenching compared to the blend. Thermal Properties. The thermal properties of all homo and block copolymers were analyzed by differential scanning calorimetry (DSC) as shown in Figure 7.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00251. Details of synthesis, measurements, and experimental procedures; additional NMR, GPC, MALDI-ToF MS, and DSC data (PDF)

Figure 7. DSC curves of H-P3HT-Th 2c and PNDIT2 in comparison with BCPs 3c, 4, 5, and 6 (from top to bottom). In all cases heating (red upper curve) and cooling (blue lower curve) were measured with 10 K/min.

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The estimated melting and crystallization enthalpies are summarized in Table SI-1. Obviously, the BCPs do not always show melting and crystallization of both polymer blocks, and the content of P3HT in the BCPs is not always related to the observed enthalpies. The content of multiblock copolymers increases in sequence of materials from 3c to 4/6 and to 5 and manifests in a stronger suppression of melting and crystallization signals. As the internal blocks are pinned at two interfaces resulting in reduced segmental mobility of the chains, slower crystallization and broader transitions result.47 For 3c it can be concluded that the amount of multiblock copolymers is lowest among all BCPs made with 2c, which is consistent with the observation of melting and crystallization events being most defined and exhibiting the highest enthalpy values. Thus, it is anticipated that in order to observe well-defined thermal transitions of all-conjugated block copolymers, well-defined structures must be present which ideally have the topology of a diblock copolymer.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (M.S.). ORCID

Michael Sommer: 0000-0002-2377-5998 Funding

The Baden-Württemberg Stiftung (research program Clean Tech) and the Research Innovation Fund of the University of Freiburg is gratefully acknowledged for funding. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank M. Hagios (University of Freiburg) for SEC measurements, A. Warmbold (University of Freiburg) for DSC measurements, and A. Korwitz (IPF Dresden) for performing high-temperature NMR measurements.

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CONCLUSION We have developed a new and straightforward synthetic route toward fully conjugated, all-crystalline donor−acceptor block copolymers of type P3HT-b-PNDIT2 via direct arylation polycondensation (DAP). Using α,ω-hetero-C−H functionalized P3HT with modulated C−H end group reactivity, different yields of P3HT incorporation and block copolymer topologies were obtained. Kinetic resolution of PNDIT2 end groups during DAP revealed symmetrically bithiophene-

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