Letter pubs.acs.org/macroletters
Synthesis and Characterization of Multicomponent ABC- and ABCDType Miktoarm Star-Branched Polymers Containing a Poly(3hexylthiophene) Segment Tomoya Higashihara,*,† Shotaro Ito,‡ Seijiro Fukuta,† Satoshi Miyane,† Yuto Ochiai,† Takashi Ishizone,‡ Mitsuru Ueda,† and Akira Hirao‡,§,∥ †
Department of Organic Device Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan ‡ Division of Soft Material Chemistry, Polymeric and Organic Materials Department, Graduate School of Science and Engineering, Tokyo Institute of Technology, S1-13, 2-12-1, Ohokayama, Meguro-ku, Tokyo 152-8552, Japan § Department of Chemical Engineering, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan ∥ Department of Applied Chemistry, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan S Supporting Information *
ABSTRACT: The new series of ABC-type miktoarm star polymer (ABC star, A = polyisoprene (PI), B = polystyrene (PS), and C = poly(3-hexylthiophene) (P3HT)) and ABCDtype miktoarm star polymer (ABCD star, A = PI, B = PS, C = poly(α-methylstyrene) (PαMS), and D = P3HT) could be synthesized by the combination of the controlled KCTP, anionic linking reaction, and Click chemistry. By the copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition click reaction of the azido-chain-end-functional P3HT (P3HT-N3) with the alkyne-in-chain-functional AB diblock copolymer (A = PI and B = PS) (AB-alkyne) or alkyne-core-functional ABC miktoarm star polymer (A = PI, B = PS, and C = PαMS) (ABC-alkyne), the target ABC star and ABCD star, respectively, were obtained, as confirmed by size exclusion chromatography (SEC) and proton nuclear magnetic resonance (1H NMR). The thermal and optical properties of these star polymers were examined by thermal gravimetric analysis (TGA) and UV−vis spectroscopy. The dynamic scattering calorimetry (DSC), atomic force micrograph (AFM) image, and grazing incidence smallangle X-ray scattering (GISAXS) results showed that the periodic P3HT fibril nanostructures rather than microphase separation occurred in the ABCD star film. In addition, it was found that highly crystalline P3HT domains aligned in the “edge-on” orientation, as supported by grazing incidence wide-angle X-ray scattering (GIWAXS).
M
effects on the polymer properties, so that the fulfillment for diverse requirements could be easier by modulating the dual structural factors. Among such branched polymers, the synthesis of well-defined miktoarm star polymers is the most difficult due to the following reasons: (1) there are strict requirements for the multistep reactions corresponding to chemically different arms, (2) each polymer−polymer reaction should be nearly quantitative (the successful establishment of miktoarm star polymers is mainly due to the combination of the living/controlled polymerization system and quantitative polymer−polymer reactions, the so-called arm-first method), and (3) isolation and purification of intermediate polymers are often required. Indeed, although there are some examples of regular star polymers which possess the same P3HT-based arm
uch attention has been paid to poly(3-hexylthiophene) (P3HT) and related materials in the field of polymer electronic devices, such as organic field-effect transistors1 and organic photovoltaic cells2 due to their well-balanced properties in terms of solubility, chemical stability, and charge mobility. To address the current diverse requirements in electronic devices, the design of only a single polymer structure has no longer been adopted. The breakthrough of the controlled synthesis of P3HT via catalyst-transfer polymerization systems by Yokozawa’s group,3 shortly followed by McCullough’s group,4 has made it possible to synthesize various architectural polymers with P3HT segments. The block copolymer could be one of the key architectures to combine two or more dissimilar components of polymer segments. In addition, some branched polymers also possess dissimilar components, including graft copolymers, star-block copolymers, and miktoarm (asymmetric) star-branched polymers.5 These branched polymers are quite interesting because heterophase and branching structures would provide double © XXXX American Chemical Society
Received: March 14, 2016 Accepted: May 2, 2016
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DOI: 10.1021/acsmacrolett.6b00207 ACS Macro Lett. 2016, 5, 631−635
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ACS Macro Letters segments,6−10 examples of miktoarm star polymers containing P3HT segments have rarely been reported.11 Higashihara et al. reported a well-defined ABC-type miktoarm star polymer, where A, B, and C are P3HT, polystyrene (PS), and poly(2-vinylpyridine) (P2VP) segments, respectively, by the linking reaction of living P2VP with the inchain-functionalized AB diblock copolymer (A = P3HT and B = PS) containing an α-phenyl acrylate (PA) moiety at the junction.12 The ternary system of the P3HT-based star polymer was synthesized for the first time. Soon after, Swager and coworkers reported the synthesis of more complex ABCBA Hshaped branched polymers containing the P3HT segment by ring-opening metathesis polymerization (ROMP). The dual functionality of the polymerizable olefin moieties at the α,ωchain ends of P3HT made it possible to be linked with living polynorbornene, followed by crossover to another type of norbornene monomer.13 Although the combination of KCTP with other living polymerization systems (i.e., living anionic polymerization and ROMP) is found to be effective, there have still been only two examples of the ternary systems in asymmetrically branched polymers with P3HT segment(s). In this study, not only an ABC-type miktoarm star polymer with a new combination of arm segments (ABC star, A = polyisoprene (PI, newly employed), B = PS, and C = P3HT) but also a structurally new ABCD-type miktoarm star polymer (ABCD star, A = PI, B = PS, C = poly(α-methylstyrene) (PαMS, newly employed), and D = P3HT) could be prepared by the combination of KCTP, anionic linking reaction, and Click chemistry to further extend the approach, combining KCTP and anionic linking reaction, to multicomponent systems. To the best of our knowledge, the tetracomponent system in the P3HT-based block/branched polymers has never been reported. Scheme 1 shows the synthetic routes for the ABC-type miktoarm star polymer. First, the azido-chain-end-functional P3HT (P3HT-N3) was synthesized by the SN2 reaction between trimethylsilyl azido (TMS-N3) and the bromobutylchain-end-functional P3HT (P3HT-C4-Br) which was prepared following a previous report14 (see Scheme 1(a)). The quantitative conversion from the bromobutyl to azido terminal group was confirmed by the 1H NMR spectra of the polymers before and after the SN2 reaction, shifting the triplet signal of − CH2Br (3.45 ppm) to − CH2N3 (3.33 ppm) (Figure S1). On the other hand, 1,1-diphenylethylene-chain-end-functional PI (PI-DPE) was synthesized according to the literature.15 PIDPE was then linked with sBuLi-initiated polystyryllithium to generate the intermediate AB diblock copolymer having a carbanion at the junction point, followed by reacting with 4trimethylsilylethynylbenzyl chloride. After the trimethylsilyl (TMS) groups were deprotected by tetranbutylammonium fluoride (Bu4NF), the alkyne-in-chain-functional AB diblock copolymer (A = PI and B = PS) (AB-alkyne) was formed (see Scheme 1(b)). The 1H NMR spectra of AB-alkyne before and after deprotection showed the quantitative deprotection of the TMS group (Figure S2). Finally, the copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition click reaction of AB-alkyne with P3HT-N3 synthesized the target ABC star (A = PI, B = PS, and C = P3HT) (see Scheme 1(c)). All the SEC curves of P3HT-N3, PI-DPE, AB-alkyne, and ABC star are shown in Figure S3. For P3HT-N3 and PI-DPE, a unimodal and sharp distribution was found in the SEC curves (Figures S3(a) and S3(b)). The SEC curve of AB-alkyne before the SEC fractionation showed a major peak for AB-
Scheme 1. Synthesis of ABC-Type Miktoarm Star Polymer
alkyne with a minor peak for the protonated polystyryllithium used in slight excess in the linking reaction as expected (Figure S3(c)). After the SEC fractionation, the pure AB-alkyne was obtained free of the PS homopolymer (Figure 3(d)). This result indicates that the anionic linking reaction quite effectively occurred. The SEC curve of the crude products after the click reaction showed a top peak shift compared to that of ABalkyne, although the unreacted AB-alkyne and P3HT-N3 can be seen (Figure S3(e)). Such an incomplete click reaction for the synthesis of star polymers has also been previously reported.11b In addition, there is a small shoulder peak for the high-molecular-weight coupled byproducts. There is no clear explanation for generating such byproducts; however, we speculate that the minor cross-linking reaction between PI segments carrying double bonds might be undergone during the click reaction. Nevertheless, after the SEC fractionation, the SEC curve of the ABC star became much narrower (Figure S3(f)). The characterization results of the number molecular weights (Mns), molecular weight distributions (Mw/Mns), and composition for P3HT-N3, PI-DPE, AB-alkyne (after SEC fractionation), and ABC star (after SEC fractionation) are summarized in Table 1. It was found that the observed Mn values agree well with those calculated based on the monomer/ initiator feed ratio. All the polymers possess very low Mw/Mn 632
DOI: 10.1021/acsmacrolett.6b00207 ACS Macro Lett. 2016, 5, 631−635
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ACS Macro Letters Table 1. Characterization Results in Synthesis of ABC-Type and ABCD-Type Miktoarm Star Polymers Mn
Mw/Mn
composition PI/PS/PαMS/P3HT (wt %)
polymer
calcd
SEC-RALLS
SEC
calcd
P3HT-N3 PI-DPE AB-alkyne ABC star AB-DPE ABC-alkyne ABCD star
10.0 9.77 20.0 27.9 20.0 28.8 38.7
8.37 10.5 19.5 30.7 19.5 30.3 44.0
1.08 1.02 1.02 1.06 1.01 1.02 1.05
0/0/0/100 100/0/0/0 51/49/0/0 36/35/0/29 51/49/0/0 35/33/32/0 27/26/25/22
1
H NMR
0/0/0/100 100/0/0/0 50/50/0/0 29/38/0/33 51/49/0/0 33/35/32/0 14/30/28/28
From SEC (Figure S5) and 1H NMR (Figure S6), it is found that the synthesis of the objective ABCD star is also successful. All the results confirmed the successful extension of the methodology, combining KCTP, anionic linking reaction, and Click chemistry, to the complex multicomponent system. Since the iterative methodology for synthesizing core-functional multicomponent star polymers based on the living anionic polymerization has already been established,5o−u further introducing of the fifth, sixth, or more segments might be possible in the future. The thermal stability of ABC star and ABCD star was investigated by thermal gravimetric analysis (TGA), as shown in Figure S7. The onset decomposition temperatures of ABC star and ABCD star for a 5% weight loss (Td,5%) are 316.2 and 297.5 °C, respectively, which are somewhat lower than our previous ABC-type miktoarm star polymer sample (A = P3HT, B = PS, and C = poly(2-vinylpyridine) (P2VP)) (Td,5% = 335− 354 °C).12 This could be due to the existence of the PI arm segment which typically shows a relatively lower thermal stability. Nevertheless, the Td,5% values around 300 °C confirmed that the thermal stability of ABC star and ABCD star is sufficient for future application in optoelectronic devices. The DSC thermogram of the structurally new ABCD star is shown in Figure 1. There is a clear exothermic peak at 128 °C,
values (Mw/Mn < 1.06), indicating the low structural heterogeneity. Based on the 1H NMR spectra of AB-alkyne (Figure S2) and ABC star (Figure S4), the compositions were determined and compared to the calculated values. Although the content of PI is somewhat lower than calculated, the existence of the three different components, PI/PS/P3HT, definitely confirm that the combination of KCTP, anionic linking reaction, and click reaction could afford the multicomponent miktoarm star polymer, ABC star. To further extend this synthetic approach to a more complex four-component system, the synthesis of ABCD star was performed according to Scheme 2. The synthetic strategy is Scheme 2. Synthesis of ABCD-Type Miktoarm Star Polymer
Figure 1. DSC thermogram of ABCD star (at 10 °C/min).
assignable to the Tc of P3HT domains (Tc,P3HT), in the first cooling scan. In addition, an endothermic peak at 227 °C is also found in the second heating scan, corresponding to the Tm of P3HT domains (Tm,P3HT). These results indicate the crystalline domain formation of the P3HT segment linked at the core of the ABCD star. There are no individual glass transitions for each of PI (Tg,PI = −69 °C17), PS (Tg,PS= 101 °C18), and PαMS (Tg,PαMS = 170 °C18) domains; instead, only one fused glass transition temperature (Tg,fusion) is obtained at 81.7 °C, presumably due to the force compatibilization of PI, PS, and
almost the same for ABC star, except for the use of 1,1diphenylethylene-in-chain-functional AB diblock copolymer (AB-DPE)16 and sBuLi-initiated poly(α-methylstyryl)lithium instead of PI-DPE and sBuLi-initiated polystyryllithium, respectively. It should be mentioned that exactly the same P3HT-N3 was employed for the synthesis of ABCD star. 633
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ACS Macro Letters PαMS segments. This unexpected behavior can be explained by the P3HT nanofibril morphology derived from its high crystallinity, as discussed in detail below, in which the driving force for the self-assembly of P3HT fibrils would be very strong so that the microphase separation between B, C, and D domains did not occur well in narrow areas excluded by P3HT fibrils. The optical property of architecturally new ABCD star was also studied by ultraviolet−visible (UV−vis) spectroscopy. The UV−vis spectra of ABCD star films show a shoulder at around 605 nm corresponding to the vibronic absorption from the intermolecular π−π interaction (Figure S8, solid line). They also support the formation of the P3HT crystallines even when the ABC arm segments are covalently linked to the P3HT segment at the sterically hindered core. The UV−vis spectrum of ABCD star in chloroform (Figure S8, dotted line) clearly shows a large blue shift of λmax, indicating an isotropic state of the ABCD star in the solution. In order to investigate the morphology of the ABCD star film in detail, the thin film was investigated by grazing incidence X-ray scattering (GIXS) using synchrotron radiation sources. Figure S9(b) shows the 2D grazing incidence small-angle X-ray scattering (GISAXS) patterns, which were measured using ABCD star film coated on silicon substrates from toluene solutions. Based on this pattern, the out-of-plane and in-plane scattering profiles have been extracted, and the resulting scattering profiles for ABCD star are shown in Figure 2. The
Figure 3. Tapping mode AFM phase image of the ABCD Star film (1 μm × 1 μm).
assigned to a reflection from the π−π stacked thiophene backbones in the phase-separated P3HT domains.19 It is quite interesting that the complex multicomponent miktoarm star polymer, such as ABCD Star, still shows a highly crystalline P3HT (=D) domain segregated from the A, B, and C domains in the film state. Taking all the results into consideration, there is a hierarchical periodical fibril morphology, in which P3HT crystallines align in the “edge-on” orientation and B, C, and D segments are excluded from those crystallines to form miscible monodomains, as illustrated in Figure 4.
Figure 4. Illustration of the model of hierarchical morphology of the ABCD star thin film. Figure 2. (a) In-plane and (b) out-of-plane GISAXS profiles extracted along qy/qz direction at qz/qy = qz°/qy° from the 2D image measured for the ABCD star film. Here, qz° and qy° are defined as 0.384 and 0.100 nm−1, respectively.
In conclusion, the new ABC-type miktoarm star polymer (A = PI, B = PS, and C = P3HT) and ABCD-type miktoarm star polymer (A = PI, B = PS, C = PαMS, and D = P3HT) could be synthesized by combining the controlled KCTP, anionic linking reaction, and Click chemistry. The tetracomponent system in the P3HT-based block/branched polymers was synthesized for the first time. The ABCD star film shows a periodic P3HT nanofibril morphology with a high crystallinity in the “edge-on” orientation, excluding the monodomains of B, C, and D segments.
ABCD star film shows arc and anisotropic ring scatterings that correspond to the mean P3HT interfibrillar distance of 22.4 nm (in-plane) and 23.4 nm (out-of-plane). The ring scattering was assigned as the first-order reflection of the periodical nanofibril morphology formed in the film, although the second or higher order reflection could not be observed. The tapping mode AFM phase image (Figure 3) of the surface of the ABCD star thin film further supports the periodic P3HT nanofibril structures in the film state. The crystalline structures of the ABCD star film are further confirmed by the 2D grazing incidence wide-angle X-ray scattering (GIWAXS) pattern (Figures S10(a)). The ABCD star film shows three distinct arcs with a higher intensity along the qz direction. These scattering features are indicative of the well-ordered multilayer structure of P3HT with an “edge-on” orientation. The first-order peak for ABCD star appears at qz = 3.90 nm−1 (out-of-plane), having a d-spacing (d(100)) value of 1.61 nm, corresponding to the lamellar spacing of P3HT crystallines (Figure S10(b)). The scattering peak qy = 16.7 nm−1 (in-plane), corresponding to d(010) = 0.376 nm, can be
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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/acsmacrolett.6b00207. Detailed experimental procedures, characterization of compounds for model reactions and polymerization, 1H NMR spectra, SEC UV curves, TG themograms, UV−vis spectra, 2D GISAXS patterns, 2D GIWAXS patterns, DSC themogram, in-plane/out-of-plane GISAXS profiles, and tapping mode AFM phase image (PDF) 634
DOI: 10.1021/acsmacrolett.6b00207 ACS Macro Lett. 2016, 5, 631−635
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ACS Macro Letters
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by Japan Society for the Promotion of Science (JSPS) (KAKENHI: Proposal No. 26620172). Shotaro Ito appreciates the support by Grant-in-Aid for Japan Society for the Promotion of Science JSPS fellows. S. Fukuta and S. Miyane thank Innovative Flex Course for Frontier Organic Material Systems (iFront) at Yamagata University for their financial supports. GIWAXS and GISAXS experiments were performed at the BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2015B1633). We thank Dr. Tomoyuki Koganezawa (JASRI) for operating the GIWAXS and GISAXS experiments.
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