Controlled Syntheses of Poly(phenylene ethynylene) - ACS Publications

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Controlled Syntheses of Poly(phenylene ethynylene)s with Regiochemically-Tuned Optical Band Gaps and Ordered Morphologies Songsu Kang,†,∥ Alexander D. Todd,‡ Abhijit Paul,§ Stanfield Y. Lee,†,∥ and Christopher W. Bielawski*,†,∥,⊥ †

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States § EEStor Inc., 715 Discovery Boulevard #107, Cedar Park, Texas 78613, United States ∥ Department of Chemistry and ⊥Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

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‡

S Supporting Information *

ABSTRACT: A series of well-defined poly(phenylene ethynylene)s (PPEs) comprised of para- and/or metasubstituted monomers was synthesized and characterized. With use of a modified catalyst transfer method, the requisite polymerization reactions were found to proceed in a chain growth manner which enabled control over polymer molecular weight while maintaining a low polydispersity. In addition, varying the monomer feed ratio afforded copolymers with tunable regiochemistry and consequent electronic properties. The methodology was found to be general and successfully combined with an isocyanide polymerization reaction to prepare the respective diblock copolymers. With use of differential scanning calorimetry in conjunction with atomic force microscopy, the copolymers were found to adopt ordered, phase-separated morphologies in the solid state.

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INTRODUCTION

dialkoxyarene (1) proceeds in a controlled fashion using PhPd(P-tBu3)Br (2) as the catalyst.10 Similar to other catalyst transfer polymerizations (CTPs),11−19 a linear relationship between the initial monomer-to-catalyst ratio and the molecular weight of the polymer produced (3) was observed. It was also demonstrated that polymers synthesized using such methodology could be chain-extended or adapted to facilitate polymer growth from the surfaces of SiO2 nanospheres. Building upon these findings, we envisioned that the scope of the aforementioned methodology could be expanded to include analogous meta-linked monomers and combined with other polymerization techniques. In contrast to the linear morphologies displayed by the poly(para-phenylene ethynylene)s (p-PPEs), the introduction of meta-substituted monomers was hypothesized to alter the intrinsic thermal and electronic characteristics of the resultant polymers. Likewise, integration of the CTP methodology with other polymerization reactions may facilitate access to new classes of block copolymers.20−30 Herein, we show that the aforementioned chain-growth polymerization is general and PPEs comprised of monomers that feature para as well as meta linkages can be prepared in a controlled fashion. In addition, well-defined PPE-block-poly(isocyanide) copolymers with properties that extend beyond

The poly(phenylene ethynylene)s (PPEs) are a class of versatile conjugated polymers that have found utility as molecular wires,1 as the basis of light-emitting diodes,2,3 as sensors for the detection of nitroaromatics and other compounds found in high-energy devices,4,5 and in bioimaging applications.6,7 Historically, PPEs have been synthesized by coupling a 1,4-diethynylphenylene with a 1,4-dihaloarene under Sonogashira-type conditions or by condensing diynes using alkyne metathesis.8,9 The underlying step growth mechanism of such polymerization methods not only intrinsically challenges control over the fundamental properties of the polymers produced, including molecular weight, polydispersity, and end-group functionality, but also limits access to block copolymers and adaption into surface-initiated polymerization reactions. As summarized in Scheme 1, we reported that the polymerization of a bifunctional monomer containing iodo and ethynylstannanyl groups at the para positions of a 1,4Scheme 1. Controlled Synthesis of a PPE Using Chain Transfer Polymerization (CTP) (R = 2-ethylhexyl)

Received: April 11, 2018 Revised: July 15, 2018

© XXXX American Chemical Society

A

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Table 1. Summary of Data Collected for a Series of p-PPEblock-m-PPE Copolymers (7)a

those commonly found in their respective homopolymers can be synthesized in a single-reaction vessel.

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RESULTS AND DISCUSSION Initial efforts were directed toward exploring the ability of the CTP method described above to polymerize a meta-substituted Scheme 2. Synthesis of the meta-Substituted Monomer 4 (R = 2-ethylhexyl)

7

Mn of 6 (kDa)b

Đ of 6b

[4]0/ [6]0c

Mn of 7 (kDa)d

Đ of 7d

p-PPE (mol %)e

yield (%)f

7a 7b 7c 7d 7e

3.8 6.4 8.2 4.2 8.8

1.41 1.49 1.48 1.45 1.41

58 25 16 13 12

11.2 12.0 12.6 5.6 11.0

1.53 1.50 1.56 1.42 1.43

32 56 64 75 83

85 78 77 75 73

The block copolymers 7 were synthesized by first generating macroinitiator 6 followed by the addition of 4 (see Scheme 4). bThe Mn and Đ of 6 was determined by GPC analysis of an aliquot removed from the reaction mixture prior to the addition of 4. cThe feed ratio of 4:6 is expressed in terms of the copolymer repeat units. dThe Mn and Đ values of the p-PPE-block-m-PPE copolymers (7) were determined by GPC. eDetermined using 1H NMR spectroscopy (CDCl3) by comparing a signal assigned to the p-PPE component (δ 7.00 ppm) to a signal assigned to the meta analogue (7.05 ppm). fIsolated yields. The GPC values are reported as their polystyrene equivalents. a

Table 2. Summary of Data Collected for a Series of p-PPEco-m-PPE Copolymers (8)a Scheme 3. Synthesis of a Poly(m-phenylene ethynylene) (R = 2-ethylhexyl)

Scheme 4. Synthesis of a Poly(p-phenylene ethynylene)block-poly(m-phenylene ethynylene) (7) (R = 2-ethylhexyl)

8

1:4b

Mn (kDa)c

p-PPE (mol %)d

Eg (eV)e

λmax (nm)f

λem (nm)g

8a 8b 8c 8d 8e

9:1 7:3 5:5 3:7 1:9

9.8 10.7 8.4 7.4 5.3

88 76 52 25 12

2.57 2.59 2.62 2.71 2.73

441 423 399 383 375

477 472 447 435 411

a

The random copolymers 8 were synthesized as shown in isolated yields of 65−87%. Conditions: 2 mol % PhPd(P-tBu)3Br (2), 20 mol % PPh3, 20 mol % CuI, room temperature, 3 h. bInitial monomer feed ratio. cThe Mn and Đ values (1.49−1.58) were measured using GPC and are reported as their polystyrene equivalents. dDetermined using 1 H NMR spectroscopy (CDCl3) by comparing signals assigned to the para-linked repeat units (δ 7.01 ppm) to their meta analogues (7.05 ppm). eThe optical band gaps were determined in CH2Cl2 from the onsets of the bathochromic absorption signals. fMaximum absorption wavelength as determined using UV−vis spectroscopy in CH 2Cl2 . gMaximum emission wavelength as determined by fluorescence spectroscopy in CH2Cl2.

analogue of 1. As shown in Scheme 2, synthesis of the requisite monomer (4) began with the etherification of 3,5-diiodophenol. With use of 2-ethylhexyl bromide in the presence of potassium carbonate, the expected 1-[(2-ethylhexyl)oxy]-3,5diiodobenzene was obtained in 87% yield. Subsequent

Figure 1. (a) Representative GPC data recorded for macroinitiator 6 (blue) and its respective block copolymer 7 (red); see Scheme 4 for structures and entry 2 in Table 1 for the corresponding Mn and Đ data. (b) Plot of Mn and Đ values measured for 7 as a function of the feed ratio of 4:6 (Mn, initial = 8.2 kDa; Đ = 1.48) as expressed in terms of the copolymer repeat units. B

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Figure 2. (a) UV−vis and (b) emission spectra recorded for a series of random copolymers (8) in CH2Cl2 ([8] = 5 × 10−3 mg mL−1) (see Table 2 for more details).

Table 3. Summary of Data Collected for a Series of p-PPEblock-PPI Copolymers (9)a

Scheme 5. Synthesis of a Poly(p-phenylene ethynylene)block-poly(arylisocyanide) (9) (R = 2-ethylhexyl)

9

Mn of 6 (kDa)b

Đ of 6b

[10]0/ [6]0c

Mn of 9 (kDa)d

Đ of 9d

p-PPE (mol %)e

yield (%)f

9a 9b 9c 9d 9e

6.4 8.4 6.2 4.2 10.3

1.41 1.49 1.47 1.42 1.49

60 43 14 11 8

13.7 12.5 8.7 5.2 11.3

1.60 1.50 1.47 1.46 1.49

36 57 65 75 88

74 86 68 80 85

a

The block copolymers 9 were synthesized as shown in Scheme 5 by first generating macroinitiator 6 of different Mn’s followed by transfer to a solution of 10. bThe Mn and Đ of 6 were determined by analysis of an aliquot removed from the respective reaction mixture prior to the addition of 10. cThe feed ratio of 10:6 is expressed in terms of the copolymer repeat units. dThe Mn and Đ values were determined by GPC and are reported as their polystyrene equivalents. eDetermined using 1H NMR spectroscopy by comparing a signal assigned to the pPPE (δ 6.99 ppm) to the poly(arylisocyanide) (5.88−6.12 ppm). f Isolated yields over the two steps are indicated.

coupling to trimethylsilylacetylene using Sonogashira-type conditions followed by desilylation with tetrabutylammonium fluoride (TBAF) afforded 1-[(2-ethylhexyl)oxy]-3-ethynyl-5iodobenzene in 52% isolated yield over the two steps. Finally, the terminal alkyne was quantitatively converted to its stannylated derivative 4 upon nucleophilic substitution. With 4 in hand, efforts shifted toward the preparation of poly(m-phenylene ethynylene) (m-PPE) (see Scheme 3). A THF solution of monomer was charged with Pd catalyst 2 ([4]0/[2]0 = 50; [4]0 = 32 mM) in the presence of CuI (20 mol %) and PPh3 (20 mol %). After the resulting reaction mixture was stirred at room temperature for 3 h, it was treated with aqueous HCl (6 N) and then poured into excess methanol. The precipitate that formed was collected by filtration and then washed with cold acetone to afford a paleyellow solid. The 1H NMR spectra recorded for the product

(5) exhibited broadened signals that were similar to those reported for p-PPE 310 and in accord with values expected for a geometric analogue. In addition, the molecular weight and polydispersity index values of the material were measured by gel permeation chromatography (GPC). The experimentally determined number-average molecular weight (Mn,exp = 8.2 kDa) was lower than the theoretical value (Mn,theory = 9.9 kDa,

Figure 3. (a) Representative gel permeation chromatograms of macroinitiator 6 (blue) and its respective block copolymer 9 (red); see entry 2 in Table 3 for the corresponding Mn and Đ data. (b) Plot of Mn and Đ values measured for 9 as a function of the feed ratio of 10:6 (Mn, initial = 8.2 kDa; Đ = 1.49) as expressed in terms of the copolymer repeat units. C

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Figure 4. (Left) DSC thermograms of PPI and p-PPE homopolymers (green and blue lines, respectively) and the corresponding p-PPE-block-PPI copolymer (red). Data were obtained at a heating/cooling rate of 20 °C min−1 under an atmosphere of nitrogen. Note that the x-axes describe different temperature ranges. (Right) Tapping-mode AFM phase image of p-PPE-block-PPI spin-casted from CH2Cl2 onto a Si wafer.

Scheme 6. Synthesis of (Poly(p-phenylene ethynylene)-co-poly(m-phenylene ethynylene))-block-poly(arylisocyanide) ((pPPE-co-m-PPE)-block-PPI)a

a

(inset) Gel permeation chromatograms recorded for macroinitiator 11 (blue) and its respective block copolymer 12 (red).

nearly twice that of the latter (Mn = 12 kDa, Đ = 1.50 vs Mn = 6.4 kDa, Đ = 1.49), and in agreement with the values expected from the initial monomer feeds and a yield of 78%. Residual pPPE homopolymer was not observed in the copolymer and 1H NMR spectroscopic analysis (CDCl3) of the isolated product exhibited signals that were assigned to segments of p-PPE (δ 7.00 ppm) as well as m-PPE (7.29 and 7.05 ppm; see Figure S20). Collectively, these data suggested to us that the block copolymer, p-PPE-block-m-PPE (7), was successfully prepared. To gain additional insight into the aforementioned copolymerization reaction, a series of chain extension experiments were performed. First, a stock solution of a p-PPE macroinitiator 6 was prepared using the procedure that is summarized in Scheme 4. Briefly, a THF solution of monomer 1 was treated with Pd catalyst 2 ([1]0/[2]0 = 40; [6]0 = 32 mM) in the presence of CuI (20 mol %) and PPh3 (20 mol %), and the resulting mixture was stirred at room temperature for 3 h. An aliquot was then removed and analyzed by GPC (Mn = 8.2 kDa, Đ = 1.48). Next, the stock solution of 6 was split and different quantities of monomer 4 ([4]0 = 32 mM in THF) were independently added to each fraction. After 3 h, each

based on the yield of the reaction; 87%) and the polydispersity index of the polymer was relatively broad (Đ = 2.5). Plotting the Mn of the polymers produced against their respective monomer conversions indicated that the homopolymerization of 4 was not well-controlled (see Figure S1, Supporting Information). We reasoned that control may be enhanced by using an in situ generated p-PPE as a macroinitiator (6). To test this hypothesis, the polymerization of 1 was initiated by adding the Pd catalyst 2 to a THF solution of monomer 1 ([1]0/[2]0 = 25; [1]0 = 32 mM) in the presence of CuI (20 mol %) and PPh3 (20 mol %) (see Scheme 4). After the resulting reaction mixture was stirred at room temperature for 3 h, an aliquot was removed for comparative analysis. A solution of monomer 4 in THF ([4]0/ [Pd]0 = 25; [4]0 = 32 mM) was then added, and the resulting mixture was stirred for another 3 h at room temperature. A yellow solid was obtained from the reaction mixture after treatment with aqueous HCl (6 N) and excess methanol. As depicted by the GPC data shown in Figure 1a, the isolated product exhibited a lower retention volume than the macroinitiator. Indeed, the molecular weight of the former was D

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mol %) and PPh3 (20 mol %). After the mixture was stirred at room temperature for 3 h, an aliquot was removed and examined by GPC (Mn = 8.4 kDa, Đ = 1.50). The residual solution was then charged with isocyanide 10 in THF ([10]0/ [Pd]0 = 16). After the mixture was stirred at 50 °C for 2 h, the resulting mixture was treated with aqueous HCl (6 N) and then poured into excess methanol. The precipitated solids were collected by filtration and washed with methanol as well as cold acetone. The product, which displayed 1H NMR signals consistent with those expected from block copolymer 9, was isolated in 86% yield as a yellow solid. The GPC data recorded for the product exhibited a shift toward higher molecular weight values (Mn = 12.5 kDa) when compared with the p-PPE macroinitiator (see Figure 3a). Furthermore, the gel permeation chromatogram of the product was unimodal and displayed a polydispersity index (Đ = 1.50) similar to that of 6. Collectively, these results indicated that a diblock copolymer was successfully synthesized. A series of chain extension experiments were performed next to determine if the block copolymerization occurred via a controlled, chain-growth process. A stock solution of p-PPE was first prepared by polymerizing 1 using Pd catalyst 2 ([1]0/ [2]0 = 40; [6]0 = 32 mM) in the presence of CuI (20 mol %) and PPh3 (20 mol %) as described above. GPC analysis of an aliquot removed from the reaction mixture after 3 h revealed that a p-PPE macroinitiator had formed (Mn = 8.2 kDa, Đ = 1.49). The residual stock solution was divided and independently transferred to separate vials containing different quantities of monomer 10 ([10]0 = 32 mM in THF). Each reaction mixture was stirred at 50 °C for 2 h, treated with aqueous solution of HCl (6 N), and then poured into excess methanol. Pale yellow solids were isolated by filtration and then washed with methanol as well as cold acetone. As shown in Figure 3, the Mn’s of the polymeric products were proportional to the initial monomer feed ratios and displayed relatively low polydispersities, consistent with a chain-growth mechanism that is under control. The aforementioned copolymerization methodology was adapted to synthesize diblock copolymers with varying segment lengths of p-PPE and PPI. As summarized in Table 3, all of the copolymers were isolated in high yields and displayed compositions that were dependent on the amount of added monomer, as determined by 1H NMR spectroscopy. GPC analysis revealed that each copolymer was unimodal with number-average molecular weights in agreement with their theoretical values. Collectively, these results suggested to us that the chain extension of 6 facilitated the formation of welldefined diblock copolymers 9 with good control over molecular weight and composition. DSC was used to examine the thermal properties displayed by the block copolymers. As shown in Figure 4 (left panel), a p-PPE-block-PPI copolymer (Table 3, entry 10) exhibited glass transitions at 36 and 95 °C, which were assigned to the PPE and PPI segments, respectively, and suggested to us that the copolymer may undergo phase separation. To elucidate the morphology of the diblock copolymer in the solid state, a film was spin-coated onto a silicon wafer and then analyzed via tapping-mode atomic force microscopy (AFM) after vapor annealing. As shown in Figure 4 (right panel), the AFM image recorded revealed that the polymeric material self-assembled into a well-ordered fibrillar structure. Such morphologies are commonly observed in samples that contain p-PPE, which

reaction mixture was treated with an aqueous solution of HCl (6 N) and then poured into an excess quantity of methanol. The resulting precipitates were collected by filtration and analyzed. As summarized in Figure 1b, a linear correlation between the Mn of the block copolymer produced and the initial feed ratio of 4:6 (in terms of the copolymer repeat units) was observed. The polydispersity indices measured for each copolymer were also found to be relatively low and in agreement with controlled, chain-growth processes. Subsequent attention was directed toward refining the methodology to access diblock copolymers with varying molar masses of p-PPE and m-PPE. As summarized in Table 1, the yields of the copolymers were high and their respective compositions, as determined by 1H NMR spectroscopy, were in agreement with the quantity of added monomer. GPC analysis also revealed that the copolymers produced were unimodal and possessed molecular weights that correlated with their theoretical values. Collectively, these results indicated that well-defined diblock copolymers with tunable structures and sizes were successfully prepared. The electronic structures of the block copolymers in dichloromethane were analyzed using UV−vis spectroscopy, which revealed the presence of two distinct absorption bands centered at approximately 300 and 450 nm (see Figure S6). On the basis of the UV−vis absorption data recorded for the constituent homopolymers, the higher energy signal was assigned to the meta-linked repeat units whereas the lower energy absorption was attributed to segments that were connected via the para-substituted analogues. In addition, the intensities of the signals were in accord with the contents of the respective homopolymer segments in the block copolymers. To determine if regiochemistry can be used to tune the electronic properties of the PPEs, the synthesis of copolymers of 1 and 4 was explored. As summarized in Table 2, random copolymers with different compositions were prepared by varying the initial ratio of 1:4. After the monomers were consumed, as determined by analyzing aliquots removed over time from the mixture using 1H NMR spectroscopy, the polymerization reactions were quenched with aqueous HCl (6 N) and the resulting solutions were poured into excess methanol. The resulting precipitated solids were isolated by filtration, washed with cold acetone, and then dried under vacuum. 1H NMR spectroscopic analysis showed that the compositions of the copolymers were well correlated with their respective initial monomer feed ratios. While the NMR spectra recorded for the random copolymers were similar to those collected for the block copolymers, UV−vis absorption spectroscopy revealed that the λmax values for the random copolymers shifted toward shorter wavelengths with increasing amounts of monomer 4 (see Figure 2a). Likewise, the optical band gap (Eg), as calculated from the UV−vis data, was found to be dependent on the monomer feed ratio. A similar trend was observed upon inspection of fluorescence data collected for the copolymers (see Table 2 and Figure 2b). Since the macroinitiators described above featured Pd species at their termini, we reasoned that they may be used to polymerize arylisocyanides.20−30 As summarized in Scheme 5, we targeted the synthesis of poly(p-phenylene ethynylene)block-poly(arylisocyanide) (p-PPE-block-PPI) (9) via chain extension polymerization. The macroinitiator 6 was first prepared by polymerizing monomer 1 with Pd catalyst 2 ([1]0/[2]0 = 43; [1]0 = 32 mM) in the presence of CuI (20 E

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gratefully acknowledged for support. We thank Dr. Karel Goossens for valuable feedback.

indicates that the conjugated segment of the diblock copolymer may govern its solid-state structure. Finally, copolymers with relatively sophisticated structures were synthesized to test the limits of the methodology. As shown in Scheme 6, macroinitiator 11, which was prepared from the random copolymerization of monomers 1 and 4, successfully initiated the polymerization of monomer 10.31 For example, GPC (see inset of Scheme 6) revealed that the product exhibited a higher molecular weight (Mn = 12.7 kDa, Đ = 1.61) than its macroinitiator (Mn = 7.0 kDa, Đ = 1.69) and was in agreement with the theoretical value (Mn, theory = 14.6 kDa). A 1H NMR spectrum recorded for the copolymer was consistent with the structure expected for (poly(p-phenylene ethynylene)-co-poly(m-phenylene ethynylene))-block-poly(arylisocyanide) ((p-PPE-co-m-PPE)-block-PPI) (12) and comprised of the respective monomers in a nearly equimolar ratio (see Figure S23).

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CONCLUSIONS In summary, well-defined p-PPE-block-m-PPE block copolymers of controlled molecular weights and low polydispersities were prepared via sequential monomer addition. A series of random copolymers of para-substituted and metasubstituted monomers were also synthesized using a similar approach. UV−vis spectroscopic analysis of the copolymers indicated that their intrinsic electronic properties were influenced by regiochemistry and effectively tuned by varying the initial monomer feed ratios.32 Finally, the copolymerization methodology was shown to be robust and integrated with isocyanide polymerization chemistry to access new classes of block copolymers with good control over molecular weight and polydispersity. Thermal analysis revealed that the block copolymers underwent phase separation and self-assembled into structures that were well-ordered in the solid state. The methodology described herein provides a versatile method for tuning the electronic and physical properties displayed by the PPEs and is expected to facilitate the development of conjugated polymers that may find utility in a growing number of contemporary applications.

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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.8b00728. Synthetic procedures, GPC data, UV−vis spectra, and NMR spectra (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-52-217-2952. E-mail: [email protected]. ORCID

Stanfield Y. Lee: 0000-0001-6955-2573 Christopher W. Bielawski: 0000-0002-0520-1982 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Institute for Basic Science (Grant IBS-R019-D1) as well as the BK21 Plus Program as funded by the Ministry of Education and the National Research Foundation of Korea are F

DOI: 10.1021/acs.macromol.8b00728 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b00728 Macromolecules XXXX, XXX, XXX−XXX