Facile Synthesis of an Eight-Armed Star-Shaped Polymer via

Letter Cite This: ACS Macro Lett. 2018, 7, 1308−1311

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Facile Synthesis of an Eight-Armed Star-Shaped Polymer via Coordination-Driven Self-Assembly of a Four-Armed Cavitand Natsumi Nitta,†,‡ Mei Takatsuka,†,‡ Shin-ichi Kihara,§ Ryo Sekiya,† and Takeharu Haino*,† †

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Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526 Japan § Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8527 Japan S Supporting Information *

ABSTRACT: The polystyrene chains were installed at the lower rim of a resorcinarene-based cavitand via reversible addition− fragmentation (RAFT) polymerization to form a four-armed starshaped polymer. A star-shaped polystyrene-functionalized supramolecular capsule was prepared through the coordination-driven self-assembly of the four-armed start-shaped polymer with silver ions. The eight-armed start-shaped supramolecular capsule encapsulated 4,4′-diacetoxybiphenyl as did a cavitand-based selfassembled capsule. A DOSY measurement indicated that the eightarmed star-shaped polymer was twice as large as the four-armed star-shaped polymer. The solution behaviors of these compounds resulted in a difference in their zero-shear viscosities.

S

Johnson et al. successfully developed a coordination-driven self-assembly of ligand-terminated polymer chains that allowed control of the number of chains and the resulting geometries, which determined the properties of the polymer.29 Although these self-assembly strategies for generating star-shaped supramolecular polymers have become powerful tools to fabricate large and elaborate organizations, there are few examples of the self-assembly directed synthesis of star-shaped polymers that result in well-defined star-shaped geometries. Our group has been developing calixarene-based host molecules.30−32 During the course of these studies, a cavitand possessing four bipyridyl groups was assembled to form a dimeric capsule with the assistance of metal ions.33−37 We envisioned utilizing the self-assembled capsule for the synthesis of a star-shaped polymer. Here, a new synthetic approach for the assembly of an eight-armed star-shaped polymer poly-2 via the self-assembly of poly-1 is reported. The synthetic strategy is outlined in Scheme 1. Cavitand 1 possesses the four chain transfer agent that allows the installation of the polymer chains at the lower rim. The subsequent self-assembly should result in eight-armed star-shaped polymer poly-2 in a facile manner. The synthesis of cavitand 1, possessing reversible addition− fragmentation chain transfer (RAFT) agents,38 is outlined in Scheme 2. Sherman’s resorcinarene 339 was employed as a starting compound. The primary hydroxyl groups of 3 were protected by tert-butyldimethylsilyl (TBS) groups to give 4.

tar-shaped polymers are a novel class of macromolecules with more than three linear polymeric chains radiating from a central core. 1−3 They represent higher-order architectures with unique properties4−7 compared to the linear counterparts due to their spatially defined yet compact threedimensional structure. Therefore, star-shaped polymers have attracted significant attention and have been used in drug delivery,8 gene delivery,9 imaging,10−12 catalysis,13,14 and rheology,15,16 as well as for their thermal properties.17 These functions are associated with their star-shaped structures and the properties of their side chains; therefore, establishing the desired functions requires a variation of the side chain and the structures. Although a diverse range of star-shaped polymers have been prepared by various covalent synthetic methods, including arm-first,18 core-first,19 and coupling onto20 methods, controlling the number and structures of the side chains can be difficult, which limits the variability of starshaped polymers.21 Recently, polymer scientists have explored the use of supramolecular chemistry22 for the facile construction of starshaped polymers. Gibson et al. employed molecular recognition with a tris-crown ether and paraquat-terminated polystyrene to provide a triarmed star-shaped supramolecular polymer.23 More complex structures of star-shaped supramolecular polymers have been constructed by using host− guest interactions of cyclodextrins.24−27 Fraser et al. reported that the single point convergence of polymer chains could be achieved using the multiple coordination sites of macroligands around a certain metal center.28 This concept was expanded for controlling the geometries of star-shaped polymers. © XXXX American Chemical Society

Received: September 5, 2018 Accepted: October 5, 2018

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DOI: 10.1021/acsmacrolett.8b00669 ACS Macro Lett. 2018, 7, 1308−1311

Letter

ACS Macro Letters

number-average molecular weight of the polymer was found to be 10100 g mol−1 from the molecular weight distribution curve based on polystyrene standards (Figure 1a). All the isolated polymers showed narrow molecular weight distributions, with Mw/Mn < 1.10, confirming the controlled chaingrowth nature of the polymerization (Figure 1b).

Scheme 1. Schematic Representation of the CoordinationDriven Self-Assembly of Poly-1 and Guest G1

Figure 1. (a) SEC chromatogram of poly-1 (eluent: chloroform) and (b) plot of the molecular weight and polydispersity index vs conversion.

Scheme 2. Synthesis of poly-1 and poly-2a

The coordination-driven self-assembly of 1 in the absence of a polymer chain was assessed in the presence of two equivalents of silver tetrafluoroborate in chloroform-d1. The changes in the chemical shifts of the 2,2′-bipyridyl protons confirmed the silver coordination to the bipyridyl units (Figure S17a,b).35,36 The formation of 2 was confirmed by a guest encapsulation experiment. When a rigid guest G1 was added into a solution of 2, the acetoxyl protons appeared at shifts higher than −1 ppm, with a large complexation-induced upfield shift (CIS) of −3.7 ppm (Figure 2a−c). Accordingly,

a

Reagents and conditions: (i) DMAP, imidazole, TBSCl, DMF, 65%; (ii) n-BuLi, I2, Et2O, 77%; (iii) 5-(4-(pinacolatoboryl)phenyl)-2,2′bipyridine, Cs2CO3, AsPh3, [Pd(PPh3)2]Cl2, 1,4-dioxane-water (25:1), 81%; (iv) TBAF, THF, 88%; (v) DMAP, HOBt, DCC, 4(butyltrithiocarbonylmethy)benzoic acid, DMF, 80%; vi) AIBN, styrene, 75%, vii) AgBF4, CHCl3, 99%; viii) AgBF4, CHCl3, 99%. Figure 2. Partial 1H NMR spectra of (a) 2 (1.2 mmol L−1), (b) 2 (1.2 mmol L−1) with 20 equiv of G1, (c) G1, (d) poly-2 (1.9 mmol L−1), and (e) poly-2 (1.9 mmol L−1) with 40 equiv of G1 at 293 K in chloforom-d1.41

Subsequent lithiation with n-butyllithium, followed by the addition of iodine, afforded tetraiodocavitand 5. The Pdcatalyzed cross-coupling reaction of 5 with the bipyridyl derivative40 successfully gave cavitand 6. Deprotection of 6, followed by the introduction of the RAFT agents furnished desired cavitand 1 in good yield. The four polymer chains of poly-1 were successfully introduced via RAFT polymerization. The polymerization reaction in a degassed styrene solution of 1 and AIBN at 90 °C afforded poly-1. The polymerization was confirmed by the disappearance of the benzyl protons adjacent to the RAFT agents and the emergence of the broad signals of the polystyrene chains (Figures S4 and S11). To evaluate the RAFT polymerization process of 1 with styrene, the monomer conversion, Mn and Mw/Mn values were monitored using 1H NMR spectroscopy and size exclusion chromatography (SEC). The SEC chromatograms of poly-1 at different conversions showed unimodal elution peaks. The value of Mn increased linearly and in proportion to the conversion of the monomer. When the conversion reached approximately 100%, the

guest G1 was encapsulated within the cavity and surrounded with two cavitands along with the principal axis, and the acetoxy methyl groups experienced the strong shielding effect from the cavity. Therefore, the trithiocarbonate groups of 2 do not interfere with the self-assembly of 1 with silver cations. Then, the self-assembly of poly-1 was examined with 2 equiv of silver tetrafluoroborate in chloroform-d1. The downfield shifts of the bipyridyl protons were similar to what was seen in the assembly of 1 (Figure S17d,e). The encapsulation of guest G1 was confirmed by the appearance of the acetoxy methyl protons at a shift higher than −1 ppm (Figure 2d,e). Therefore, the formation of supramolecular star-shaped polymer poly-2 was confirmed.42 The stability of poly-2 was assessed in the concentration of poly-2 from 2.0 to 0.1 mmol L−1 (Figure S19). The bipyridyl proton signals were not 1309

DOI: 10.1021/acsmacrolett.8b00669 ACS Macro Lett. 2018, 7, 1308−1311

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ACS Macro Letters concentration-dependent, suggesting that poly-2 is quite stable and exists predominantly in solution. DOSY measurements demonstrated the changes in volume upon the self-assembly of poly-1 (Figures 3a). The diffusion

Figure 4. (a) Specific viscosities of poly-1 (open circles) and poly-2 (filled circles) and (b) reduced viscosities of poly-1 and poly-2 at 298 K in 1,2-dichloroethane.

linear chains. Poly-2 showed a further transition at 61 g L−1 with an exponent of 3.9, which implies that the eight-armed star-shaped geometry most likely results in significant overlap, and the molecules become entangled similar to linear chains in the semiconcentrated regime in a good solvent. The reduced viscosities of poly-1 and poly-2 gave rise to obvious differences in their solution behaviors. In the dilute regime, the Huggins plots for poly-1 and poly-2 resulted in good straight-line correlations with the same intrinsic viscosity ([η]) of 0.013 L g−1 for both poly-1 and poly-2 (Figure 4b). The consistent intrinsic viscosity implies that self-assembled capsule poly-2 is completely dissociated due to the labile coordination bond and exists as poly-1 in the dilute concentration regime. Above 20 g L−1, the self-assembly of poly-1 with silver ions becomes obvious, leading to the curved plot of the reduced viscosity; accordingly, concentrating the solution facilitated the self-assembly and self-assembled eightarmed star-shaped polymer poly-2 became dominant, which enhanced the hydrodynamic interactions due to the increase in the molecular dimensions through the association. The reduced viscosity dramatically increased above the second transition at 61 g L−1 (Figure 4b), which most likely implies that poly-2 is entangled through elongating associations along the arm side chains. It is known that a silver cation can coordinate to a sulfur atom; in fact, the solution viscosity of simple polystyrene chains prepared by RAFT polymerization result in a transition from the dilute regime to the semidilute regime at 56 g L−1, which can be shifted to 39 g L−1 by the presence of silver cations (Figure S21). Accordingly, the supramolecular interactions between the trithiocarbonate group and a free silver cation might lead to the elongation of poly-2 along the cavitand in the semiconcentrated regime. In conclusion, we have developed four-armed star-shaped polymer poly-1 and eight-armed star-shaped supramolecular polymer poly-2. The installation of the four polymer chains at the cavitand lower rim via RAFT polymerization gave rise to four-armed star-shaped polymer poly-1 with a narrow poly dispersity. The self-assembly of poly-1 resulted in eight-armed star-shaped supramolecular polymer poly-2. The self-assembled supramolecular polymer had significantly different solution behavior than its precursor. The supramolecular assembly of the polymer-functionalized cavitand is a unique synthetic approach for the preparation of star-shaped polymers, which paves the way for the construction of multiarmed star-shaped polymers with precisely controlled geometries.

Figure 3. (a) DOSY spectra of poly-1 (1.0 mmol L−1) and poly-2 (0.5 mmol L−1) at 293 K in chloroform-d1 and (b) volume-average size distribution measured by dynamic light scattering of poly-1 (2.4 mmol L−1) and poly-2 (0.93 mmol L−1) at 293 K in chloroform.

coefficient of poly-1 (6.23 × 10−11 m2 s−1) was significantly greater than that of poly-2 (5.14 × 10−11 m2 s−1). The StocksEinstein equation gave hydrodynamic radii (r) of 61.4 and 74.5 Å for poly-1 and poly-2, respectively. The resultant volume ratio (rpoly‑2/rpoly‑1)3 of 1.78 confirms that poly-2 is approximately twice as large as poly-1. The size distributions of the four- and eight-armed star-shaped polymers were determined using dynamic light scattering (DLS) analysis. The hydrodynamic diameters which are volume-average were estimated to be 5.5 nm for poly-1 and 8.5 nm for poly-2 (Figure 3b). A volume ratio of 3.7 is fairly consistent with the results of DOSY analysis even in considering the size distribution; therefore, the size of poly-2 was more than double that of poly-1. Star-shaped polymer poly-2 was stimuli-responsive to pH. The addition of excess trifluoroacetic acid (TFA) reduced the hydrodynamic diameter of poly-2 by 15%, which was not recovered after the neutralization with the N,N-dimethylaminopyridine (DMAP; Figure S20). DMAP also disrupted the poly-2. Accordingly, the pH-responsive behaviors of poly-2 were not reversible. Solution viscosities elucidated the differences in the structures of four- and eight-armed star-shaped polymers poly-1 and poly-2 (Figure 4a). The change in the specific viscosity (ηsp) of poly-2 was more apparent than that of poly-1. The log−log plots gave rise to overlap concentrations of 52 and 20 g L−1 for poly-1 and poly-2, respectively, indicating a transition from the dilute to the semidilute concentration regimes. In the dilute regime, the low viscosities of the solutions of poly-1 and poly-2 resulted in exponents of 0.94 and 0.96, respectively, which imply that both polymers are isolated chains below the overlap concentration. Above the overlap concentrations, the exponents of 1.72 and 1.92 for poly-1 and poly-2, respectively, suggest that these solutions become unentangled polymeric solutions similar to solutions of short 1310

DOI: 10.1021/acsmacrolett.8b00669 ACS Macro Lett. 2018, 7, 1308−1311

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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.8b00669.

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Experimental details, 1H and 13C NMR, DQF COSY, and NOESY spectra of all new compounds, DOSY spectra of poly-1 and poly-2, solution viscosities of polystyrene with and without silver cations, critical gelation concentrations of poly-1 and poly-2, and photographs of the gels of poly-1 and poly-2 (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Takeharu Haino: 0000-0002-0945-2893 Author Contributions ‡

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research (B, C), JSPS KAKENHI Grant Nos. JP15H03817 and JP15KT0145 from the Japan Society for the Promotion of Science (JSPS), and by Grants-in-Aid for Scientific Research on Innovative Areas, JSPS KAKENHI Grant Nos. JP17H05375 (Coordination Asymmetry) and JP17H05159 (π-Figuration). Funding from The Futaba Electronics Memorial Foundation, Nippon Sheet Glass Foundation, and Fukuoka Naohiko Memorial Foundation is gratefully acknowledged.

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DOI: 10.1021/acsmacrolett.8b00669 ACS Macro Lett. 2018, 7, 1308−1311