Preparation of Peapod Polymer via the Supramolecular Chain

Jan 6, 2017 - The preparation of peapod polymer was carried out by the olefin metathesis polymerization of spiroborate twin bowls via the formation of...
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Preparation of Peapod Polymer via the Supramolecular Chain Formation by Tris(spiroborate) Twin Bowl Hiroshi Danjo,*,† Toshi Nakagawa,‡ Akio Morii,‡ Yusuke Muraki,‡ and Koichi Sudoh§ †

Department of Chemistry and ‡Graduate School of Natural Science, Konan University, 8-9-1 Okamoto, Higashinada, Kobe 658-8501, Japan § The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan S Supporting Information *

ABSTRACT: Successive guest-containing tubular polymer was prepared by the olefin metathesis polymerization of tris(spiroborate) twin bowl after the formation of supramolecular polymer. The cationic iridium(III) complexes were topologically fixed inside the polymer to form a peapod-like structure. The polymer was evaluated by SEC, ICP-AES, and DLS analyses, and string-like structures were found in the AFM observation of the peapod polymer.

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As the next step, we apply our strategy to the creation of a new class of polymer materials bearing functional guest molecules that are topologically fixed within the main chain. Here, the supramolecular polymer formation is used as the preorganization step, and subsequent covalent bond formation between adjacent twin bowls in the supramolecular polymer leads to the preparation of a peapod-like polymer that contains guest molecules inside the chain (Figure 1).6,7 In this protocol, no chemical transformation of the guest molecules is required in order to fix them within the polymer structure. This would bring us not only the high accessibility but also the structural versatility. Various types of cationic molecules could be employed as the incorporated guest only by mixing with twin bowl in the supramolecular polymer formation step. The preparation of peapod polymer was carried out by the olefin metathesis polymerization of spiroborate twin bowls via the formation of the supramolecular chain structure.8 For this purpose, we employed twin bowl (+)-1·(Me2NH2)3 possessing homoallyl groups at the 6-position of the naphthalene rings (Figure 1). In addition, 4-(n-octyl)benzyl groups were also introduced at the 4-position of the naphthalene rings to increase solubility in various organic solvents. Twin bowl 1· (Me2NH2)3 and [Ir(tpy)2]·(PF6)3 (tpy: 2,2′:6′,2″-terpyridine, 2·(PF6)3) were dissolved in N,N-dimethylformamide (DMF) and then purified by precipitation in methanol to remove Me2NH2PF6, and the supramolecular polymer [1·2]m was obtained as an orange solid (Scheme 1). The supramolecular polymer and Grubbs second catalyst were dissolved in tetrahydrofuran (THF) and heated at 40 °C for 24 h. The reaction mixture was again precipitated in methanol and washed with acetonitrile to give peapod polymer 3 as a dark

or the development of functional polymer compounds, various strategies have been so far developed to introduce functional molecules or substructures into the main chains or side chains. In those strategies, the chemical transformation of the target molecules is often required to fix them within the polymer chain through the formation of covalent bonds. However, such functionalization might restrict the properties and structures of the target molecules. Therefore, the introduction of functional molecules into the polymer chain without covalent bond formation is an attractive idea for chemists who are engaged in the development of functional polymers. Recently, topologically fixed supramolecular compounds, such as catenanes, rotaxanes, or other interlocked structures, have been designed and prepared by various research groups, and characteristic behavior based on their motional freedom has been elucidated.1 Polymer materials possessing those topological structures such as polycatenanes, polyrotaxanes, or daisy chains are well documented and expected to have novel properties and functions.2 Increasing attention has also been paid to insulated molecular wires (IMWs).3,4 In those polymers, rotaxane or a rotaxane-like structure was effectively utilized for the isolation of conjugate polymer chains, such as poly(phenylene ethynylene)s or polythiophenes. Previously, we have demonstrated the construction of supramolecular polymers by the use of twin-bowl-shaped tris(spiroborate) cyclophanes as molecular connecting modules.5 In those polymers, spherical cationic guests are iteratively glued to each other by the spiroborate twin bowl through noncovalent interaction. That strategy has afforded us a convenient way to construct polymer materials containing functional guest molecules within the main chain. In that system, it is possible to introduce the guest molecules into the polymer structure without any chemical modification. © XXXX American Chemical Society

Received: December 27, 2016 Accepted: January 3, 2017

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DOI: 10.1021/acsmacrolett.6b00972 ACS Macro Lett. 2017, 6, 62−65

Letter

ACS Macro Letters

Scheme 1. Preparation of Supramolecular Polymer [1·2]m and Peapod Polymer 3

Figure 1. Chemical structures of twin bowl (+)-1·(Me2NH2)3 and 2· (PF6)3 (top) and schematic representation of the preparation of peapod polymer (bottom).

brown solid. The metathesis polymerization was also tried without purification of the supramolecular polymer. In this case, 1·(Me2NH2)3 and 2·(PF6)3 were once dissolved in DMF and concentrated immediately. The residual solid was again dissolved in THF in the presence of catalyst and heated at 40 °C. The formation and olefin metathesis polymerization of [1· 2]m were monitored by 1H NMR measurement. The proton signals of 23+ were broadened and shifted to upfield in the presence of twin bowl 13−, indicating the formation of the supramolecular polymer (Figure 2c).5a After the treatment of [1·2]m with Grubbs second catalyst, all the signals were strongly broadened, and only the broad signal at 5.4 ppm was observed at the region of olefin protons, probably due to the conversion of the terminal olefins into the internal ones (Figure 2d). Olefin metathesis polymerization would proceed to afford peapod polymer 3. The relatively weak signal broadening was observed when the polymerization was carried out with the nonpurified supramolecular polymer (Figure 2e). The polymerization would proceed insufficiently because of the presence of Me2NH2PF6, which prevent the supramolecular polymerization of 13− and 23+. The similar spectra was also observed for the olefin metathesis polymerization of 1·(Me2NH2)3 in the absence of 23+ (Figure 2f). In this case, low degree of polymerization was expected due to lack of the supramolecular polymer formation as the preorganization step. According to the UV−vis spectra, the hyperchromism was observed after the formation of supramolecular polymer [1·2]m and peapod polymer 3 (Figure S1). Especially at visible region, the absorbance of 3 was increased to more than 3-fold of the linear combination of the absorbance of 1·(Me2NH2)3 and 2· (PF6)3.

Figure 2. Partial 1H NMR spectra (500 MHz, 25 °C in DMF-d7) of (a) 2·(PF6)3 (2.0 mM), (b) 1·(Me2NH2)3 (2.0 mM), (c) [1·2]m (2.0 mM), (d) 3 (4.0 mg/mL), (e) a mixture of 1·(Me2NH2)3 and 2·(PF6)3 (4.0 mg/mL) after treatment with Grubbs second cat. in THF at 40 °C for 24 h, and (f) 1·(Me2NH2)3 (4.0 mg/mL) after treatment with Grubbs second cat. in THF at 40 °C for 24 h. The signals marked by red asterisks are assigned to 23+.

The structural evaluation of peapod polymer 3 was also carried out by size-exclusion chromatography (SEC) analysis (Figure S2). Twin bowl 1·(Me2NH2)3 and supramolecular polymer [1·2]m gave almost the same peak at 12 kDa together 63

DOI: 10.1021/acsmacrolett.6b00972 ACS Macro Lett. 2017, 6, 62−65

Letter

ACS Macro Letters

The detailed structure of the peapod polymer was directly confirmed by atomic force microscopic (AFM) observation. The chloroform solution of the peapod polymer was spincoated on a mica substrate. Isolated string-like structures were found on the substrate, and the thickness of the strings was estimated to be about 1.5 nm from the cross-sectional height profile (Figure 3d,e). According to the crystal structure of the spiroborate twin bowl previously determined, the width of 3 was considered to be about 1.7 nm, in good agreement with the thickness of the strings on the substrate.5a In conclusion, we have demonstrated the preparation of peapod polymer via the formation of supramolecular polymer composed of spiroborate twin bowl bearing homoallyl side chains and [Ir(tpy)2]3+ as the preorganization step. The adjacent spiroborate twin bowls in the supramolecular polymer were covalently bound by olefin metathesis polymerization, and [Ir(tpy)2]3+ was topologically fixed within the main chain of the polymer. In the SEC analysis, the polymerization product was detected at Mw = ∼200 kDa (vs polystyrene standard). The ratio of boron and iridium atoms in the polymer was determined to be 3:1 by the ICP-AES analysis, implying that the one-dimensional array structure of the supramolecular polymer was preserved after the polymerization reaction. The AFM observation of the peapod polymer elucidated the stringlike structure having the similar thickness to that of the twinbowl-containing supramolecular polymer previously reported.

with the small peak at 24 kDa (vs polystyrene standard), corresponding to a monomer and probably some dimeric component. This indicated that the supramolecular chain structure was almost completely dissociated because of the high-dilution condition during SEC analysis (Figure S2a,b). Peapod polymer 3, however, retained the polymer structure even under the SEC conditions, and was detected at Mw = 200 kDa, showing that the olefin metathesis polymerization successfully occurred to form a covalently bound polymer structure from a noncovalently bound supramolecular polymer (Figure S2c). Relatively large polydispersity index (PDI = 2.94) might indicate the existence of nonspecific interchain crosslinking. The small oligomeric peaks were observed for the product obtained from the olefin metathesis reaction of the nonpurified supramolecular polymer or only 1·(Me2NH2)3, indicating the insufficient degree of polymerization as expected above (Figure S2d,e). By the inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis of 3, the ratio of boron and iridium atoms was determined to be about 3:1, and the molar amount of iridium atom was estimated to be equal to that of the repeating unit of the polymer (Figure S3a,b). No loss of iridium(III) guest (23+) took place during the construction of the peapod polymer. The atomic ratio was preserved even after treating 3 with a DMF solution of Me2NH2Cl at 80 °C for 24 h (Figure S3c). In peapod polymer 3, 23+ was topologically fixed, which would suppress the exchange of guest cations. The size evaluation of the polymers was carried out by dynamic light scattering (DLS) analysis. In 1 mg/mL THF solution, twin bowl 1·(Me2NH2)3 showed an average hydrodynamic diameter (DH) of 2.2 nm, and [1·2]m and 3 had DH values of 10.1 and 15.6 nm, respectively (Figure 3a). A

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00972. Detailed experimental procedures for synthesis and characterization of all new compounds, NMR data, SEC, ICP-AES, and DLS data of the main compounds (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hiroshi Danjo: 0000-0001-9850-8301 Funding

JSPS KAKENHI Grant Number 26410103. Notes

The authors declare no competing financial interest.

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Figure 3. Size characterization (average hydrodynamic diameter, DH in THF, 20 °C) by dynamic light scattering (DLS) measurement. (a) 1· (Me2NH2)3 (green), [1·2]m (blue), and 3 (red) at 1.0 mg/mL; (b) [1· 2]m at 1.0 mg/mL (red), 0.4 mg/mL (blue), and 0.05 mg/mL (green); and (c) 3 at 1.0 mg/mL (red), 0.4 mg/mL (blue), and 0.2 mg/mL (green). (d) Tapping-mode AFM image (400 × 400 nm2 on a mica substrate) of 3, and (e) height profile of 3 on a cross-section of the white line.

ACKNOWLEDGMENTS We thank Drs. S. Iwatsuki and K. Chayama (Konan University) for their technical guidance in the ICP-AES measurement. We also thank Dr. M. Naito (Konan University) for invaluable help in the AFM observation.

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remarkable difference was observed between [1·2]m and 3 in terms of the concentration dependence of DH. The DH value of [1·2]m decreased with decreasing concentration (Figure 3b), whereas that of 3 hardly changed, regardless of concentration (Figure 3c). Moreover, the DH value of [1·2]m was substantially decreased by the addition of Ba(OTf)2 (Figure S4).5b In the case of 3, Ba(OTf)2 had no effect on DH. These data again support the covalently polymerized structure of 3.

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