A Rational Entry to Cyclic Polymers via Selective Cyclization by Self

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A Rational Entry to Cyclic Polymers via Selective Cyclization by Self-Assembly and Topology Transformation of Linear Polymers Daisuke Aoki, Gouta Aibara, Satoshi Uchida, and Toshikazu Takata J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01151 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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Journal of the American Chemical Society

A Rational Entry to Cyclic Polymers via Selective Cyclization by Self-Assembly and Topology Transformation of Linear Polymers Daisuke Aoki,*,† Gouta Aibara,† Satoshi Uchida,† and Toshikazu Takata*,†,‡ †

‡

Department of Chemical Science and Engineering, Tokyo Institute of Technology and JST-CREST, Ookayama, Meguro, Tokyo 152-8552, Japan Supporting Information Placeholder

ABSTRACT: A simple and effective synthetic route to cyclic polymers has been developed based on the following sequence: i) selective cyclization of two self-complementary sec-ammonium-containing crown ether monomers to afford [c2] daisy chain bifunctional initiators, ii) living polymerization to afford the corresponding linear polymers, and iii) a topology transformation of these linear polymers to furnish cyclic polymers. The key step in this sequence is the quantitative cyclization via selfassembly of two crown ether molecules with hydroxyl and sec-ammonium moieties. After the living polymerization, the linear polymers release the daisy-chain assembly to generate a cyclic topology. The specific advantages of the present synthetic protocol, i.e., procedural simplicity and concentration independence, are demonstrated by a gram-scale synthesis.

For a long time, polymers with topologies other than linear have attracted much scientific and industrial interest on account of their unique properties that are based on their structural characteristics.1-9 Although recent advances in polymer chemistry have made it possible to synthesize polymers with various topologies, effective synthetic routes that selectively yield cyclic polymers in high yield and quantity still remain limited. This is mostly due to difficulties associated with purification, and, in particular, the selectivity of macrocyclizations.4-6 Usually, the syntheses of cyclic polymers is accomplished by two main methods: i) the cyclization of linear precursors with terminal homo- or hetero-bifunctional groups,4-6 or ii) ringexpansion polymerization.10-13 Although the cyclization of linear precursors is more commonly used, it requires high dilution conditions to prevent intermolecular reactions, as well as elaborate purification procedures to remove byproducts and/or precursors. Conversely, most ringexpansion polymerization approaches lack control over molecular weight and polydispersity. These shortcomings have so far prevented the development of effective synthetic routes to cyclic polymers and their characterization. Recently, we have established a synthetic

method for the formation of cyclic polymers, in which the polymer topology can be changed from linear to cyclic by exploiting the mobility of the junction point of rotaxane linkers.14-16 These reports have demonstrated that the rotaxane protocol allows the quantitative formation of cyclic polymers from linear precursors without requiring dilute conditions.17 However, in order to connect the linear precursor via the rotaxane structure, a cyclization is still required. Figure 1. Synthetic strategy to cyclic polymers based on a combination of

introducing axle polymer chains into precursors with self-assembled [c2] daisy chain structures and a topology change induced by a positional change of the rotaxane linkers.

As cyclization processes are inevitable in the synthesis of cyclic polymers, and considering the aforementioned difficulties associated with such cyclizations, when and how these cyclizations are carried out in the synthetic sequence should be the key to success. Specifically, cyclizations should be carried out during the most favorable

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step in the entire synthesis, i.e., when the smallest ring can be formed. Herein, we describe an effective and reliable synthetic protocol to cyclic polymers, involving automatic quantitative cyclization via the self-assembly of secammonium-containing crown ethers to afford [c2] daisy chains, followed by introduction of linear polymer chains and a topological change. Scheme 1. Synthetic pathway to cyclic polymer Dimer-PCL_U.

Figure 1 illustrates the synthetic strategy to these cyclic polymers based on self-complementary monomers that contain two mutually recognizing moieties in one molecule. Such monomers have the ability to spontaneously and selectively self-assemble into [c2] daisy chain units. This self-assembly can hence be regarded as the key cyclization step for the synthesis of cyclic polymers. The thus obtained small rings, consisting of [c2] daisy chain dimers, can be expanded into larger circles by introducing polymer chains as axle components, followed by a topological change induced by the rotaxane protocol. As self-complementary monomers that can assemble into [c2] daisy chains, which may act as initiators for the introduction of polymer chains, crown ethers containing a sec-ammonium moiety and a terminal hydroxyl group (1) were designed and synthesized (Scheme 1). In solvents with moderate polarity, e.g.

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chloroform and dichloromethane, 1 dimerized spontaneously into [c2]-daisy-chain-type intermediate 2 via a self-assembly process. The self-assembled structure of 2 was confirmed by 1H-NMR spectroscopy in CDCl3. The corresponding spectrum revealed broadened signals, probably due to the restricted motion of the components on account of the interlocked structure (Figure S1). In more polar solvents such as DMSO-d6, sharp signals characteristic for monomer 1 were observed (Figure S2), suggesting that the self-assembly occurs only in moderately polar solvents. MALDI–TOF–MS and ESI-TOF-MS spectra exhibited exclusively the molecular ion peak corresponding to the theoretical value of 2, while peaks attributable to 1 or its higher oligomers were not observed (Figure S3, S4). These results are thus consistent with those previously reported by Stoddart et al and Gibson et al.18-20 The introduction of polymer chains into 2 was carried out according to the “rotaxane–from method”, an effective synthetic method to introduce a rotaxane structure into polymer chains.15,21,22 Accordingly, 2 was employed to initiate the diphenyl phosphate (DPP)-catalyzed living ring–opening polymerization (ROP) of ε-caprolactone (CL) using a [CL]0 / [2]0 / [DPP]0 ratio of 60/1.0/2.2 (CH2Cl2, 24 h, room temperature). The polymerization was quenched by the addition of 3,5-dimethylphenyl isocyanate in order to cap the propagation end with a bulky stopper and to introduce a urethane moiety, which acts as a second recognition site at the end of the polymer chain. The thus obtained polymer was purified by precipitation followed by preparative gel permeation chromatography (GPC). This one-pot reaction furnished poly(ε-caprolactone) (PCL)based polymer Dimer-PCL_A in 61% overall isolated yield relative to 2 (Scheme 1; suffix “A” indicates that the wheel component is fixed on the “sec-ammonium moiety”). In a similar fashion, intermediate polymer Dimer-PCLOH_A was quantitatively isolated by direct precipitation of the polymerization mixture into ethanol/hexane (1/9, v/v) prior to adding the end-capping agent. The isolation of Dimer-PCL-OH_A demonstrates once more the excellent stability of the [c2] daisy chain structure that arises from the strong interactions between the sec-ammonium and the crown ether moieties, even after attaching the polymer. Subsequently, the linear polymer topology in DimerPCL_A, which is due to the stable [c2] daisy chain structure, was changed by releasing the interaction between the sec-ammonium moiety and the crown ether wheel at the center of the polymer chain. For that purpose, the secammonium moiety of Dimer-PCL_A was N-acetylated using acetic anhydride and triethylamine. The subsequent topology change, resulting from the elimination of the secammonium/crown ether interactions under concomitant emergence of urethane/crown ether interactions, afforded cyclic polymer Dimer-PCL_U in 82% isolated yield (suffix “U” indicates that the crown ether wheel component is fixed at the “urethane moiety”).15,16,22-27 The chemical structures of Dimer-PCL_A and DimerPCL_U were characterized by 1H NMR spectroscopy and MALDI-TOF-MS. A degree of polymerization (DP) of ~ 30 (n = 15) was estimated for the PCL moiety on the basis of 1H NMR data.


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Figure 2. MALDI-TOF-MS spectra for (a) Dimer-PCL-OH_A, (b) Dimer-PCL_A, and (c) Dimer-PCL_U.

Figure 2 shows the MALDI-TOF-MS spectra of DimerPCL-OH_A, Dimer-PCL_A, and Dimer-PCL_U. For Dimer-PCL-OH_A and Dimer-PCL_A, two series of peaks with intervals of 114 Da and 144 Da were observed, respectively (Figure 2a and b). The interval of 144 Da corresponds to the molecular weight of [PF6]–, indicating that the two series of peaks originate from two types of molecular ions, i.e., [(Dimer-PCL-OH_A)]+ and [(DimerPCL-OH_A)[PF6]]+ for Dimer-PCL-OH_A, and [(DimerPCL_A)]+ and [(Dimer-PCL_A)[PF6]]+ for DimerPCL_A. In contrast, MALDI-TOF-MS spectra of the neutral compound Dimer-PCL_U showed only one series of peaks with an interval of 114 Da, which corresponds to the molecular weight of CL (Figure 2c). These results clearly demonstrate that the N-acetylation, as well as the DPP-catalyzed ROP and the subsequent end-capping proceeded successfully and selectively to afford DimerPCL_A and Dimer-PCL_U.

exhibit strong hydrogen bonding between the crown ether and sec-ammonium or urethane moieties.15,18,22,23 In accordance with this hypothesis, the GPC peak of DimerPCL_U appeared at higher elution time than that of DimerPCL_A. For Dimer-PCL_U and Dimer-PCL_A, an intrinsic viscosity ratio ([η]Dimer-PCL_U / [η]Dimer-PCL_A) was calculated from the peak top retention times. The obtained value agrees well with previously reported values (0.80 estimated from Figure data) of a series of covalently linked cyclic and linear polymers.8 Thus, the GPC results support the formation of cyclic polymers. Subsequently, Dimer-PCL_A and Dimer-PCL_U were subjected to diffusion NMR experiments in CDCl3 in order to compare their solution behavior. Given that their attenuation curves exhibited a linear correlation (Figure S11), these polymers should be considered as pure polymers, but not the mixtures as these have different diffusion constants Dh. For Dimer-PCL_A and DimerPCL_U, Dh values of 2.56 × 10–10 m2/s and 3.10 × 10–10 m2/s were calculated, respectively. The Dh value ratio between Dimer-PCL_U and Dimer-PCL_A (Dh Dimer-PCL_U / Dh Dimer-PCL_A) of 0.83 is in good agreement with previously reported28 and theoretical data.29 These results thus also corroborate the topology change from linear to cyclic.

Figure 4. Huggins plot for Dimer-PCL_A and Dimer-PCL_U.

Figure 3. GPC profiles for (a) Dimer-PCL_A and (b) DimerPCL_U.

Cyclic polymers usually display smaller hydrodynamic radii relative to linear polymers of the same molecular weight.5 To obtain evidence for the cyclic structure of Dimer-PCL_U, GPC profiles of Dimer-PCL_A and Dimer-PCL_U in CHCl3 were compared (Figure 3). In order to maintain their structures, these polymers should

To demonstrate the efficiency and utility of the present method for the preparation of such cyclic polymers beyond the realm of the research laboratory, a gram-scale synthesis of these cyclic polymers with longer polymer chains was carried out. The synthetic protocol is based on the aforementioned procedures, albeit without the preparative GPC purification, which is a common purification technique, but ill-suited to mass production. The living ROP of CL was initiated with 2 using a [CL]0 / [2]0 / [DPP]0 ratio of 50/1.0/1.0 (CH2Cl2, 6 h, room temperature) in relatively high concentration ([CL]0 = 3 M). The thus obtained polymer was purified by the reprecipitation and fractionation technique to afford pure Dimer-PCLn=20_A (yield: 74%; DP = ~40, n = 20, Mw/Mn = 1.1). The topology transformation from liner to cyclic in the aforementioned manner yielded 7.3 g of Dimer-PCLn=20_U. Since an intrinsic viscosity ratio ([η]Dimer-PCLn=20_U / [η]DimerPCLn=20_A) of 0.76 was measured for Dimer-PCLn=20_U and


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Dimer-PCLn=20_A (Figure 4), this scaled-up synthesis, which affords the polymeric target in high yield and purity, clearly demonstrates the efficiency and utility of this synthetic method for the formation of cyclic polymers. Scheme 2. Decomposition of dimer structure to monomer structure.

To confirm the rotaxane-linked dimer structure, the positive decomposition of the dimer structure to the monomer structure was carried out by the N-acetylation of the ammonium moiety of Dimer-PCL-OHn=20_A having no bulky end-cap group (Scheme 2). The N-acetylation afforded Monomer-PCLn=20 possessing the crown ether wheel at the α-end and O-acetyl group at the ω-end of the PVL chain (Figure S12). The GPC peak of MonomerPCLn=20 clearly appeared at later elution time than those of Dimer-PCLn=20_A and Dimer-PCLn=20_U, clearly proving the dimer structures of Dimer-PCLn=20_A and DimerPCLn=20_U along with their high purity (Figure S14). Although the obtained polymers showed different thermal properties (Figure S17-S20), further study should be needed, since we have previously reported that the existence of wheel component fixed by the hydrogen bond with the axle polymer affects the thermal property of the axle polymer.30 In summary, we have successfully developed a highly effective synthetic route to cyclic polymers, in which the immediate and quantitative self-assembly-induced cyclization of sec-ammonium moiety-containing crown ethers to [c2] daisy chain dimers is followed by an attachment of polymer chains with bulky end groups onto the daisy chain. A release of the hydrogen bonding interactions between the sec-ammonium and crown ether moieties induces a topology change from linear to cyclic. This synthetic protocol requires neither high dilution nor special procedures such as preparative GPC, which are common shortcomings of conventional synthetic protocols for the formation of cyclic polymers. Accordingly, we believe that this synthetic protocol may promote the use and characterization of cyclic polymers due to its procedural simplicity and selectivity towards cyclic polymers.

ASSOCIATED CONTENT Supporting Information. 1H-NMR data and synthetic procedures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

ACKNOWLEDGMENT This research was financially supported by a Core Research for Evolutional Science and Technology (CREST) project from the

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Japan Science and Technology Agency (JST) and two JSPS KAKENHI grants (JP16K17910 and JP16H00754). We thank Dr. S. Kuroki and Dr. Y. Sei for help with the diffusion NMR experiments and for fruitful discussions.

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A Rational Entry to Cyclic Polymers via Selective Cyclization by Self

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