Influence of the Carboxylic Acid Additive Structure on the Properties of

Jan 29, 2015 - Carboxylic acids are known to have a profound effect on the rate of direct arylation. Despite this, little attention has been paid to t...
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Influence of the Carboxylic Acid Additive Structure on the Properties of Poly(3-hexylthiophene) Prepared via Direct Arylation Polymerization (DArP) Andrey E. Rudenko and Barry C. Thompson* Department of Chemistry, Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, California 90089-1661, United States S Supporting Information *

ABSTRACT: Carboxylic acids are known to have a profound effect on the rate of direct arylation. Despite this, little attention has been paid to them as additives for direct arylation polymerization (DArP). Here we report the influence of the carboxylic acid chemical structure on the properties of poly(3-hexylthiophene) (P3HT) prepared via DArP. We study the effect that acid pKa, steric bulk, and backbone cyclization have on the reactivity of the DArP catalytic system. We found that pKa values do not correlate with DArP reactivity in the pKa range of 4.76−5.05. The increase of acid size in the classes of linear, secondary, and tertiary acids leads to a continuous increase in polymer yield and molecular weight, whereas for the case of cyclic secondary acids the trend is reversed and a decrease of acid size leads to an increase in polymer yield and molecular weight in an alternating (zigzag) fashion depending on whether the acid contains an even or odd number of carbons. A profound enhancement of reactivity was found for the case of the smallest cyclopropanecarboxylic acid, which yields P3HT with remarkably high molecular weights (Mn 33.4 kDa and Mw 207.1 kDa), which is expected to benefit the development of direct arylation protocols for inherently less reactive substrates.

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INTRODUCTION Since the early synthetic investigations of the aromatic C−H activation reaction, the profound positive effect of the carboxylate additives became apparent. As such, addition of certain carboxylic acids (acetic, pivalic, etc.) into the reaction mixture greatly facilitates reactivity and improves the reaction yield.1,2 This effect was studied in detail in the seminal work of Fagnou et al.,3−5 and it was attributed to the lowering of the activation barrier of the C−H abstraction step through channeling the reaction into the so-called concerted metalation−deprotonation (CMD) pathway, which allows the reaction to give products in high yields under relatively mild conditions. These significant advances in the realm of organic and organometallic chemistry directly influenced the development of the direct arylation polymerization (DArP) for the synthesis of conjugated polymersthe new alternative to traditional methods of cross-coupling polymerizations, such as Stille, Suzuki, etc.6−10 As a C−H activation method, DArP eliminates the need to metalate monomers with, for example, organotin functionalities, and as a result it eliminates the toxic organotin byproducts of polymerization. Therefore, DArP is a substantially simplified, cost-effective, and environmentally friendly polymerization protocol. In the current literature two distinct sets of DArP conditions can be identified: so-called Ozawa and Fagnou conditions.6,11,12 The Ozawa conditions have been developed by Ozawa et al.,12 and they include a complex palladium-based Herrmann’s catalyst, a typically airsensitive phosphine-based ligand (usually tri-o-anisylphos© XXXX American Chemical Society

phine), Cs2CO3 as a base, and often a carboxylate additive (pivalic acid).13 DArP with Ozawa conditions is typically conducted at 110−120 °C in THF or toluene, which requires a pressurized vessel and may impede the development of industrially relevant DArP conditions. The alternative Fagnou conditions appear to be more attractive as they use inexpensive, bench-stable reagents and allow the reaction to be conducted at ambient pressure.6,11 As such, Pd(OAc)2 is used as a catalyst, phosphine ligands may or may not be used, K2CO3 serves as a base, and a carboxylic acid as an additive that plays a critical role in the CMD process. The reaction is conducted typically in N,N-dimethylacetamide (DMA), which has a high boiling point (165 °C) and therefore does not require pressurized vessels. Studies conducted using a limited set of carboxylates (two or three acids) indicate that, together with other important components of the DArP reaction mixture, the carboxylate additives play a major role in DArP, strongly influencing monomer conversion and determining polymer yield, molecular weight, and polydispersity index. As such, small variations in the acid structure could mean the difference between obtaining polymer in high yield and with high molecular weight and obtaining only short oligomers with a negligible yield.14,15 Additionally, it was shown that the structure of the acid also influences the structure of the resulting polymer. As such, Received: October 17, 2014 Revised: January 13, 2015

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Corporation) at 70 °C using a Viscotek GPC Max VE 2001 separation module and a Viscotek TDA 305 RI detector. The instrument was calibrated vs polystyrene standards (1050−3 800 000 g/mol), and data were analyzed using OmniSec 4.6.0 software. For thin film measurements solutions were spin-coated onto precleaned glass slides from o-DCB solutions at 4 mg/mL for all polymers. UV−vis absorption spectra were obtained on a PerkinElmer Lambda 950 spectrophotometer. The thickness and roughness of the thin films were determined via X-ray reflectivity (XRR) measurements using a Rigaku diffractometer Ultima IV using Cu Kα radiation source (λ = 1.54 Å) in the reflectivity mode. The grazing-incidence X-ray diffraction (GIXRD) measurements were obtained using the same instrument in a grazing-incidence X-ray diffraction mode. Crystallite size was estimated using Scherrer’s equation:17

utilization of so-called neodecanoic acid (NDA) in place of pivalic acid has been demonstrated to eliminate the branching defects in P3HT prepared by DArP due to the increase of the bulk and selectivity of the catalytic system.16 This clearly illustrates the vast importance of the carboxylate additives for DArP. However, little attention has been paid to this essential variable in the current literature, which primarily focuses on optimizing the structure of phosphine ligands and palladium source rather than carboxylic acids, leaving many important questions regarding the carboxylate structure unanswered. How does the carboxylate structure influence polymer yield and molecular weight? What is the influence of carboxylate additive on polymer structure and chain defects? Is there a correlation between DArP reactivity and carboxylic acid pKa? All these questions are clearly important for the future development of DArP, especially for the case of phosphine-free Fagnou conditions, where the number of variables is limited. Here, in an effort to answer these major questions, we report a comprehensive study of 24 different carboxylic acids as additives for DArP on the example of P3HT prepared via DArP using modified phosphine-free Fagnou conditions. Monocarboxylic acids with purely aliphatic structures were chosen for this study in order to disentangle the effect of the acid structure from the possible interference of other functional groups (such as oxygen and nitrogen heteroatoms as well as πsystems) with the catalytic activity in DArP. The acids were carefully chosen to comprehensively represent the major classes of linear acids, α-secondary acids, α-tertiary acids (or neoacids), secondary cyclic acids, and bicyclic and tricyclic acids, thus making it possible to study the effects of acid pKa, size, and cyclization on the properties of resulting P3HT samples. The primary polymer properties such as number-average molecular weight (Mn), weight-average molecular weight (Mw), polydispersity index (PDI), regioregularity (RR), β-defect content (BDC), and yield were studied before and after the Soxhlet extraction in order to gain deeper insight into the reactivity of the DArP catalytic system based on the entire (nonfractionated) polymer sample. P3HT was chosen as a model polymer due to the simplicity of the monomer structure and the semicrystalline nature of the polymer which allows to study the impact of structural features on the polymer thermal, optical, and solid state chain ordering properties as well as the simplicity of structure analysis through 1H NMR spectroscopy.

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τ=

Kλ β cos θ

(1)

where τ is the mean size of the ordered (crystalline) domains, K is a dimensionless shape factor (K = 0.9), λ is X-ray wavelength, β is the line broadening at half the maximum intensity (fwhm) in radians, and θ is the Bragg angle. DSC profiles were recorded on PerkinElmer DSC 8000 under N2 with a scan rate of 10 °C/min. Sample size was ∼5 mg, and polymers were used as obtained after Soxhlet purification. The second cycle is displayed in the Supporting Information. Synthetic Procedures. The synthetic procedure for the synthesis of 2-bromo-3-hexylthiophene18 was used without modifications as reported in the literature. General Procedure for the Synthesis of DArP P3HT. 247 mg (1 mmol) of 2-bromo-3-hexylthiophene was dissolved in 3 mL of dry DMA. Then 3.75 mol % of carboxylic acid and 207 mg (1.5 mmol) of K2CO3 were added to the reaction mixture. This solution was degassed with nitrogen gas flow for 10 min. Then 0.125 mL of the Pd(OAc)2 (2.5 × 10−3 M) stock solution was added (313 mol ppm, or 0.0313 mol % loading of catalyst; 0.32 M is the final concentration of monomer in DMA), and the reaction mixture was degassed for an additional 20 min. Then the reaction mixture was immersed into a preheated to 160 °C oil bath and stirred at that temperature under a nitrogen atmosphere for 48 h. Then the reaction mixture was cooled, dissolved in hot CHCl3, and extracted with water to remove inorganic residues. The CHCl3 fraction was then precipitated into methanol, filtered, and dried. A portion of this fraction was analyzed as a crude sample, and the rest was purified via Soxhlet extraction with methanol, hexanes, and finally chloroform. The final fraction was concentrated in vacuo and precipitated into methanol. The polymers were filtered and dried in high vacuum overnight.

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RESULTS AND DISCUSSION From a variety of available Fagnou DArP conditions a recently reported protocol employing ultralow loading of Pd(OAc)2, NDA, and DMA was used in this study.19 This method has been shown to produce P3HT in high yield, with high molecular weight, β-defect-free structure, and high regioregularity while utilizing only 313 ppm of Pd(OAc)2 and 3.75% of carboxylic acid, as illustrated in Scheme 1. Such low loadings of acid are particularly suited for this study since some of the acids under investigation are in a limited supply, expensive, or require a lengthy synthesis. For the purposes of this study, all the parameters of this polymerization protocol were fixed, and only the nature carboxylic acid was varied. The acid structures are

EXPERIMENTAL SECTION

Materials and Methods. All reagents were obtained from commercial sources and were used without further purification, unless otherwise noted. All reactions were performed under dry N2, unless otherwise noted. All dry reactions were performed with glassware that was oven-dried and then flamed under high vacuum and backfilled with N2. Flash chromatography was performed using a Teledyne CombiFlash Rf instrument in combination with RediSep Rf normal phase disposable columns. Solvents were purchased from VWR and used without further purification except for THF which was dried over sodium/benzophenone before being distilled. All compounds were characterized by 1H NMR (400 MHz) recorded on a Mercury 400 NMR spectrometer at 25 °C using CDCl3 as a solvent. Polymer 1H NMRs (600 MHz) were obtained on a Varian 600 NMR spectrometer using CDCl3 as a solvent. For polymer molecular weight determination, polymer samples were dissolved in HPLC grade o-dichlorobenzene (o-DCB) at a concentration of 0.5 mg/mL, briefly heated, and then allowed to cool down to room temperature prior to filtering through a 0.2 μm PTFE filter. SEC was performed using HPLC grade o-DCB at a flow rate of 1 mL/min on one 300 × 7.8 mm TSK-Gel GMH H R -H column (Tosoh

Scheme 1. Synthesis of DArP P3HT

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Macromolecules Scheme 2. Chemical Structures of the Aliphatic Carboxylic Acids under Investigationa

a

Tertiary acids marked with asterisk are the neo-acids consisting of mixtures of isomers.20,21

There are two possible explanations for this discrepancy. First, this apparent absence of correlation may stem from the fact that the literature pKa values were measured at conditions (25 °C in aqueous medium) that are very different from the DArP reaction conditions, and the pKa trends may not be the same if they were measured under the DArP reaction conditions (160 °C in DMA). It is important to mention that such measurement itself would constitue a major research effort due to the absence of straighforward and reliable methods of determining pKa in organic solvents,29 especially at such high temperatures. Second, it may be due to the real absence of any correlation between the reactivity of the DArP catalytic system and the degree of dissociation of carboxylic acid (and therefore basicity of the conjugate carboxylate base). If this turns out to be true, it will necessitate a revision of the direct arylation mechanism and the possible reassignment of the CMD process as the rate-determining step. Therefore, these results illustrate the neccessity for a better understanding of the direct arylation process at the most basic level since the correlation between carboxylic acid pKa and reactivity in direct arylation has not been studied before even for small molecules. Effect of Acid Size on Polymer Properties. The following discussion is focused on the evolution of the polymer yield, molecular weight, structure, and properties with the increase of the acid size (in terms of the number of carbon atoms it contains) from each class of carboxylic acid (Scheme 2). Within the simplest class of linear carboxylic acids (samples 1−6, Scheme 2 and Table 1) only one acid, specifically acetic acid, has been used for DArP.15 Increasing the acid size by using larger homologues results in a steady increase of the polymer yield (after Soxhlet purification) and PDI (Table 1 and

presented in Scheme 2, and the properties of the resulting polymer samples are summarized in Table 1. As an important control experiment, DArP was conducted in the absence of carboxylic acid additive (sample 25, Table 1), and this experiment did not result in formation of any amount of polymer or even oligomer, thereby emphasizing the importance of the carboxylic additives. Influence of Acid pKa on Polymer Properties. Among many properties of carboxylic acids the most central parameter that is frequently studied is the pKa. Generally speaking, the larger is the acid pKa, the lesser tendency it has to dissociate to proton and conjugate base and, therefore, the stronger is the conjugate base. As expected from the mechanism of the CMD step,4,5,26 stronger carboxylate bases should accelerate the direct arylation reaction rate. For the case of DArP, it has been suggested that an increase of the carboxylic acid pKa should favorably influence the C−H activation process as well and improve the polymer molecular weight and reaction yield due to the increase of basicity of the conjugate carboxylate base.27,28 Therefore, it may appear rational to use acid pKa as a guiding principle in the search for more effective acid additives for DArP. However, our findings indicate that using pKa values alone as a guiding principle in the choice of carboxylate additive for DArP may not lead to a successful polymerization, and a more detailed consideration of acid chemical structure is necessary. In order to visually illustrate this lack of correlation between acid pKa and the reactivity of the DArP catalytic system, the polymer yield, molecular weights, and PDI (for soxhleted samples) are plotted agains the pKa values of the corresponding acids (in the range of 4.76−5.05; all data taken from Table 1), as shown in Figure 1. C

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Macromolecules Table 1. Properties of DArP P3HT Samples before and after Soxhlet Extraction polymer = acid sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

acid class linear

pKaa

secondary

4.76 4.87 4.82 4.84 4.86 4.90 4.86

tertiary

5.05

tertiary cyclic

secondary cyclic

4.83 4.99 4.90

secondary bicyclic no acid

yield, % (Soxh)

Mn, kDa (Soxh)

Mw, kDa (Soxh)

PDI (Soxh)

91 (38) 96 (65) 99 (70) 95 (71) 95 (72) 99 (77) 93 (67) 96 (79) 91 (58) 98 (74) 97 (74) 100 (75) 96 (83) 97 (78) 97 (74) 93 (71) 94 (76) 100 (81) 95 (75) 100 (63)d 92 (58) 99 (79) 94 (63) 100 (71)e

7.6 (15.2) 13.4 (24.2) 14.9 (26.4) 13.4 (22.4) 12.2 (21.4) 16.3 (27.7) 10.2 (17.1) 13.2 (23.3) 9.4 (16.6) 13.7 (22.0) 16.3 (22.2) 20.2 (23.5) 18.0 (26.5) 14.9 (27.9) 14.6 (24.8) 15.5 (30.2) 17.7 (28.3) 21.0 (33.4) 13.6 (25.8) 16.0 (27.7) 8.8 (16.0) 17.5 (27.3) 9.2 (16.2) 14.4 (27.3)

20.5 (31.9) 44.2 (70.2) 82.0 (82.0) 69.7 (73.9) 47.6 (66.3) 109.2 (133.0) 35.7 (47.9) 76.56 (81.6) 25.4 (33.2) 52.0 (59.4) 53.8 (59.9) 56.6 (63.5) 95.4 (103.4) 94.4 (103.2) 73.0 (81.8) 102.3 (108.7) 134.5 (134.5) 186.9 (207.1) 93.8 (131.6) 142.4 (188.4) 29.0 (35.2) 113.8 (117.4) 30.4 (37.3) 154.1 (155.6)

2.7 (2.1) 3.3 (2.9) 5.5 (3.1) 5.2 (3.3) 3.9 (3.1) 6.7 (4.8) 3.5 (2.8) 5.8 (3.5) 2.7 (2.0) 3.8 (2.7) 3.3 (2.7) 2.8 (2.7) 5.3 (3.9) 6.4 (3.7) 5.0 (3.3) 6.6 (3.6) 7.6 (4.7) 8.9 (6.2) 6.9 (5.1) 8.9 (6.8) 3.3 (2.2) 6.5 (4.3) 3.3 (2.3) 10.7 (5.7)

RR,b % (Soxh) 90 93 93 94 94 94 94 94 93 95 95 95 96 94 94 94 95 94 94 94 93 95 93 93

(93) (94) (95) (95) (94) (95) (95) (95) (95) (95) (96) (96) (96) (95) (95) (95) (96) (95) (95) (94) (95) (95) (95) (94)

BDC,b % (Soxh) 0.04 0.26 0.34 0.11 0.03 0.07 0.14 0.02 0.03 0.02 0.02 0.00 0.00 0.13 0.15 0.15 0.03 0.03 0.16 0.31 0.04 0.07 0.02 0.22

(0.04) (0.24) (0.12) (0.01) (0.01) (0.04) (0.14) (0.01) (0.01) (0.00) (0.00) (0.00) (0.00) (0.07) (0.11) (0.14) (0.02) (0.02) (0.03) (0.24) (0.04) (0.02) (0.00) (0.07)

Tm, °C (Tc, °C)c 205 212 215 210 207 212 204 215 213 216 219 221 223 212 215 210 220 212 211 197 212 212 213 210

(177) (179) (181) (175) (172) (177) (167) (180) (185) (181) (188) (190) (191) (172) (180) (167) (185) (169) (171) (150) (183) (170) (185) (167)

0

a

The available pKa values are taken from ref 22. bRR was measured using aliphatic region of 1H NMR spectra;23 BDC was measured using aliphatic region of 1H NMR spectra24,25 (full NMR spectra are displayed in the Supporting Information, Figures S2−S25). cDSC was measured using Soxhleted polymer samples. d12% of insoluble fraction was formed. e11% of insoluble fraction was formed.

samples both before and after Soxhlet purification, indicating only 0.01−0.34% of the branching defects (compared to previously reported higher values of 1.4% for P3HT obtained at significantly higher 2 mol % catalyst loading).24 However, generally speaking, as can be seen from Table 1, the increase of the acid bulk appears to decrease the BDC. Comparison of the polymer properties before and after Soxhlet purification indicates that Soxhlet fractionation results in an increase of Mn and Mw, decrease of PDI, and the polymer yield. In terms of polymer chemical structure, it increases the RR and decreases the BDC. These trends hold true for all the samples 1−24. The impact of the linear acid size on the polymer optical and thermal properties is consistent with the polymer chemical structure. As such, the polymer absorption coefficient and vibronic shoulder intensity in the UV−vis spectra (Figure S51) increase consistently with the increase of molecular weight and regioregularity. Similar trends are observed for the polymer thermal properties as determined by DSC (Table 1 and Figures S27−S32), which indicates the increase of the melting and crystallization points consistently with the increase of the molecular weight and regioregularity. Importantly, the solid state properties studied by grazing-incidence X-ray diffraction (GIXRD) and X-ray reflectivity (XRR) methods exhibit similar trends. As such, the (100) diffraction peak intensity varies proportionally with polymer Mn, Mw, and RR while the peak position and crystallite size do not change significantly (Figure S52 and Table S1). The thickness and roughness of thin films spin-coated from o-DCB were studied by XRR (Table S1 and Figure S53), and similar trends were observed: films thickness increases for samples with higher molecular weight while film

Figure 1. Apparent lack of correlation between the carboxylic acid pKa values (in the range of 4.76−5.05) and the reactivity of the DArP catalytic system as represented by the polymer Mn, Mw, PDI, and yield (for Soxhleted samples).

Figure 2a). The polymer molecular weight (Mn and Mw) also generally increases; however, there is a slight decrease for the cases of pentanoic and hexanoic acids (Table 1, Figure 2b, and Figure S26). Importantly, the polymer regioregularity also increases with the increase of the number of carbon atoms in the acid from 93% for acetic acid to 95% for octanoic acid (Table 1). The BDC appears to be quite low for the P3HT D

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Figure 2. Dependence of polymer yield, PDI, Mn, and Mw on the number of carbon atoms in carboxylic acid for three different classes of acids (after Soxhlet purification): (a, b) linear acids (samples 1−6); (c, d) tertiary acids (samples 9−13); (e, f) secondary cyclic acids (samples 18−23).

β-defects, while acids 10 and 11 do not give β-defects for the Soxhleted polymer samples while the crude sample exhibit small amount of branching. As found previously,16 pivalic acid (9) does allow formation of β-defects which cannot be completely removed by Soxhlet purification (Table 1). To summarize, in all three classes of linear, α-secondary, and α-tertiary carboxylic acids increase of the acid bulk generally increases the reactivity and selectivity of the DArP catalytic system simultaneously. This result is very significant as it leads to a general design principle for the carboxylate additive tailored for high molecular weight, high RR, β-defect free conjugated polymers, which are highly sought after for multiple optoelectronic applications.24,31 Additionally, it draws a connection between the acid structure and the reactivity as well as selectivity of the palladium−acid complex, highlighting NDA (sample 12, Table 1) as the most attractive, commercially available, and inexpensive acid additive. However, this phenomenon of the reactivity enhancement through the increase of the acid bulk currently cannot be definitively interpreted, and additional studies will be required. Nevertheless, some insight into the enhancement of the selectivity may be provided based on the seminal work of Ozawa et al.32 In this case, it was demonstrated that palladium−carboxylate complexes have lesser tendency to exist as dimers (which are deactivated) in solutions when the substituents around palladium increase in steric size. Importantly, it has also been demonstrated that the ability of arylpalladium species to come in close contact with one another and form a dimer is the

roughness remains relatively low for all polymer samples (∼1− 3 nm, rms typical for high RR P3HT).30 Notably, the properties of DArP P3HT obtained in this regime of low reagent loading are not significantly affected by the presence of vanishingly low quantities of β-defects. The class of α-secondary aliphatic carboxylic acids is represented here by only two acids 7 and 8 for the reasons of availability. Nevertheless, similar trends are observed for this class as well. As such, the increase of the acid size leads to the increase of the polymer yield, Mn, Mw, and PDI as well as to fewer regio- and branching defects in the polymer chemical structure as evidenced by the increase of polymer RR and decrease of BDC (Table 1, samples 7 and 8). These variations in chemical structure translate into corresponding variations of the polymer optical and thermal properties as evidenced by the UV−vis (Figure S51), GIXRD (Figure S52), and XRR (Figure S53) spectra as well as DSC traces (Figures S33 and S34). The class of α-tertiary carboxylic acids follows similar trends. This class is of particular interest since the majority of the acids in that class are in fact mixtures of acid isomers with the same chemical composition (acids 10−13).20,21 Nevertheless, the increase of the acid bulk within this class of acids leads to the same trends as observed for linear and secondary acids (Table 1 and Figure 2c,d). Importantly, preparation of sample 12 with NDA has been reported previously,19 and similar results are obtained in this study as well, thus indicating the overall reliability and reproducibility of the current DArP protocol. Notably, neo-acids 12 and 13 completely suppress formation of E

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Macromolecules necessary prerequisite for the homocoupling side reaction.6,33 Thus, the increase of the steric bulk of carboxylic ligands is expected to decrease the degree of homocoupling and therefore increase the polymer RR. As for the branching defects, it has been previously postulated that increase of the steric bulk for carboxylates will increase the bulk of the entire catalytic system at the moment of C−H activation16 since, according to the work of Fagnou et al.,3−5 the process of CMD is concerted. This means that more sterically congested β-positions of 3hexylthiophene ring will experience a decrease in reactivity in the step of CMD, thus leading to fewer β-defects. This simplicity and the straightforward dependence of polymer properties on the size of the carboxylate additive ends when acids with aliphatic cyclic structure come under investigation. Specifically, secondary monocyclic acids 18−24 (Scheme 2 and Table 1) exhibit a behavior that is entirely opposite from that of linear, secondary, and tertiary acids. In this case, the increase of the catalytic activity, polymer molecular weight, PDI, and yield comes with a decrease of the acid size, as illustrated in Table 1 and Figure 2e,f. Even more intriguing is that this increase in reactivity is not continuous as is the case with the previously described classes of acids, but it is alternating depending on whether the cycle contains an even or odd number of carbon atoms (Figure 2e,f). Importantly, the magnitude of the reactivity increase with decreasing cyclic acid size is so profound that for the case of a very simple, stable, and inexpensive cyclopropanecarboxylic acid 18 the value of Mw reaches 207.1 kDa (Table 1)an unprecedentedly high molecular weight for DArP P3HT. This exceptionally reactive catalytic system significantly surpasses that based on commonly used pivalic acid (sample 9) in terms of the polymer yield and molecular weight. From the standpoint of structural defects, both acids 9 and 18 give similar values of RR and BDC. This observation is significant as it suggests a method to increase the rate of the C−H activation process, which could be important for inherently less reactive substrates. Interestingly, the odd−even effect has been previously observed for cyclic alkanes and their derivatives, and it has manifested itself in many properties such as melting points,34 heats of fusion,34 refraction,35 and even 1H and 13C NMR chemical shifts.36 It has been explained in terms of how well molecules can pack with one another depending on the most thermodynamically stable ring conformation. Such packing phenomenon may also be relevant for the closely packed acid− palladium−arene associated system, where one type of conformation could lead to a decrease of the activation barrier of CMD rate-limiting step while other could lead to an increase. The apparent dichotomy in the reactivity trends between cyclic and noncyclic acids is significantly more challenging to explain. Many parameters may be responsible for the increase of the reactivity with the decrease of the acid ring size. One parameter, the pKa, however, does not have a very good correlation. In principle, as the ring size of cycloalkanecarboxylic acid decrases, the sp3 bond angles between cycle and the exocyclic bond approach those of sp2 bonds, thus engendering the sp2 character and enhancing electronegativity of carbon atom connected to carboxyl group and thus decreasing the pKa values of the acid. Indeed, the pKa of cyclopropanecarboxylic acid 18 is the smallest in this class of cyclic acids (Table 1), yet the acid reactivity is the highest, which additionally supports the apparent lack of correlation between acid pKa and DArP reactivity as discussed earlier.

Effect of Acid Cyclization on Polymer Properties. The cyclic motif in the carboxylic acid additives clearly has a significant effect on DArP reactivity. Therefore, it is also important to discuss the effect of cyclization in the class of tertiary acids on the properties of DArP P3HT. As can be seen from pairs of samples 9 and 14 and as well as 12 and 15, cyclization does result in the increase of polymer Mn, Mw, and PDI; however, the selectivity of the catalytic system suffers as evidenced by the increase of BDC and supported by the decrease of the melting and crystallization point (Table 1). This decrease in selectivity is consistent with the decrease of the acid effective volume upon cyclization, thus corroborating the hypothesis of improved α/β selectivity with increased acid bulk. Importantly, 1-adamantanoic acid 16, which is a common and relatively cheap reagent and which has been previously used for DArP, yields P3HT with a high Mn and Mw and a high yield; however, the α/β selectivity for P3HT is not 100% as is the case with NDA, and some β-defects can be detected as supported by the by 1H NMR and DSC data. Finally, tricyclohexylacetic acid 17 containing 20 carbon atoms is the bulkiest acid reported in this study and possibly the bulkiest aliphatic acid that could be prepared with a reasonable amount of effort. As expected, this acid gives higher Mn, Mw, and PDI of P3HT compared to all neo-acids; however, it is does not provide a 100% α/β selectivity. It is also a point of interest that the values of PDI reported for P3HT in this study are some of the highest reported in the literature reaching values of 10.7 for crude P3HT sample prepared with bicyclic acid 24. This may originate from the fact that polymer precipitates from the DMA reaction mixture early on, but the molecular weight continues to increase with time.16 This may result in heterogeneous polymerization in which some chains continue to grow while others are physically isolated from the reaction mixture and do not increase in length, thus contributing to broad PDI values. However, the effect of large PDI on the thermal and optoelectronic polymer properties has not been explored primarily due to the absence of synthetic methods that would allow to prepare a batch of polymer with a high PDI (∼7−10). The current DArP protocol thus offers an advantage to generate polymer samples with PDI and other properties that may be tuned through a rational choice of a suitable carboxylate additive.

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SUMMARY AND OUTLOOK In conclusion, this work represents a comprehensive study of the effect that carboxylic acid additives on the properties and structure of DArP P3HT. This work encompasses 24 aliphatic carboxylic acids, the majority of which have never been studied as additives for direct arylation. As such, the entire class of secondary aliphatic cyclic acids has previously not been considered, and to the best of our knowledge this is the first report of their utilization for direct arylation. We have demonstrated that acid pKa is a poor guide toward effective carboxylic acids for DArP as we found no correlation between acid dissociation constant (in the pKa range of 4.76−5.05) and polymer molecular weight or monomer conversion. However, the effect of acid size on the polymer properties and structure is most pronounced as the increase of the number of carbon atoms in linear, secondary, and tertiary acid classes leads to increase in reactivity and selectivity of the DArP catalytic system. The best carboxylic acid for high molecular weight branching-free P3HT with high RR was determined to be the NDA as it provides an optimal balance between polymer yield, F

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Article

Macromolecules

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molecular weight, and controlled chemical structure and due to its commercial availability and cost-effectiveness. Secondary cyclic acids, on the other hand, demonstrate entirely different behavior. The decrease in the number of carbon atoms leads to an increase of polymer molecular weight and yield in an alternating manner consistent with the odd−even effect frequently associated with cycloalkane derivatives. Remarkably high values of molecular weight are observed for P3HT samples prepared with the smallest cyclic acidthe cyclopropanecarboxylic acid. Its reactivity in DArP substantially surpasses that of commonly used pivalic acid, making this result a significant step forward in an effort to find more reactive catalytic systems for C−H activation. These findings are very significant since such an impressive enhancement in reactivity through simple tuning of the carboxylate additive may be useful in functionalization of substrates that are inherently less reactive toward direct arylation, thus opening the door to new polymers and polymer architectures that may be synthesized via DArP. The pronounced effect of cyclization is also observed in tertiary acids, where cyclization generally increases reactivity but reduces selectivity. We explored bicyclic and tricyclic acids and found that they too effectively produce P3HT. However, in all cases of cyclic acids the α/β-selectivity did not reach 100%, producing small amounts of β-defects in P3HT. We additionally note extremely high PDI values (10.7) in the case of bicyclic acids. A rational approach to minimize the PDI values is to conduct DArP in a solvent that would ensure solubility of P3HT throughout the course of polymerization and yet not impede the reactivity of the catalytic system. Our laboratory is currently involved in an effort to identify such solvent media.

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ASSOCIATED CONTENT

S Supporting Information *

Full 1H NMR spectra, GPC traces, DSC traces, UV−vis data, GIXRD, and XRR data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.C.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported as part of the Center for Energy Nanoscience, an Energy Frontier Research Center funded by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DESC0001013.

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REFERENCES

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DOI: 10.1021/ma502131k Macromolecules XXXX, XXX, XXX−XXX