Rigid-to-Flexible Conformational Transformation: An Efficient Route

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Rigid-to-Flexible Conformational Transformation: An Efficient Route to Ring-Opening of a Trö ger’s Base-Containing Ladder Polymer Fumitaka Ishiwari,* Nobuhiko Takeuchi, Takahiro Sato, Hiroshi Yamazaki, Ryota Osuga, Junko N. Kondo, and Takanori Fukushima* Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan S Supporting Information *

ABSTRACT: The synthesis of ladder polymers is still a big challenge in polymer chemistry, and in particular, there are few examples of conformationally flexible well-defined ladder polymers. Here we report an efficient and convenient route to conformationally flexible ladder polymers, which is based on a postpolymerization reaction of a rigid ladder polymer containing Tröger’s base in its main chain. The postpolymerization reaction involves sequential Nmethylation and hydrolysis for the Tröger’s base unit, resulting in a diazacyclooctane skeleton that can exhibit a ring-flipping motion. Molecular dynamics simulations predicted that this motion provides conformational flexibility with the resultant ladder polymer, which was demonstrated by 1H NMR spectroscopy in solution. The presence of the diazacyclooctane units in the flexible ladder polymer allowed further functionalization through reactions involving its secondary amine moiety. The present synthetic method may lead to the development of a new class of ladder polymers that exhibit both conformational and design flexibility.

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adder polymers, in which the repeating units are connected by more than two bonds,1a are expected to exhibit excellent thermal and mechanical properties due to the multiple bonds in their main chains.1 The current successful approaches to the synthesis of well-defined ladder polymers make use of postpolymerization cyclization,2 Diels−Alder polymerization,3 double aromatic nucleophilic substitution polymerization,4 Tröger’s base-formation polymerization,5 and palladium-catalyzed annulation polymerization.6 However, the previously reported ladder polymers consist of a rigid main chain, and there have been no reports on structurally well-defined ladder polymers with a conformationally flexible main chain, despite their great potential for the development of new polymer materials that exhibit viscoelastic, dynamic, and even stimuliresponsive properties, together with high structural and mechanical robustness. Here we report an efficient and convenient synthetic route to conformationally flexible ladder polymers (poly-3, Figure 1), which relies on a postpolymerization reaction of a Tröger’s base-containing ladder polymer (Scheme 1). Through this postpolymerization, the rigid diazabicyclo skeleton of Tröger’s base can be converted into a conformationally flexible diazacyclooctane skeleton, which displays a ring-flipping motion. Since the secondary amine moiety in the main chain of poly-3 allows the attachment of side chains, various chemical modifications into, for example, poly-4 and poly-5 are possible. By virtue of the Tröger’s base-formation polymerization method established by McKeown et al.,5 a variety of ladder polymers containing Tröger’s base units have become available. © XXXX American Chemical Society

Figure 1. Schematic illustration of the structure and conformational behavior of a ladder polymer with a diazacyclooctane unit formed by the postpolymerization ring-opening reaction of a Tröger’s basecontaining rigid ladder polymer.

Because of the rigid bicyclic structure, the main chains of the ladder polymers adopt highly limited conformations as in poly1 (Figure 1, left), thus, providing intrinsic porosity in the solid state.5 Meanwhile, in 1957, Cooper and Partridge reported that the N-methylation of the nitrogen atom of Tröger’s base and subsequent hydrolysis reaction caused ring opening to give rise to a 1,5-diazacyclooctane skeleton,7 which exhibits a ringflipping motion.8 We expected that this ring-opening reaction Received: May 25, 2017 Accepted: June 27, 2017

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DOI: 10.1021/acsmacrolett.7b00385 ACS Macro Lett. 2017, 6, 775−780

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ACS Macro Letters

2a), while the those for the model oligomer of poly-3 showed a wide distribution between 10 and 36 Å (Figure 2b). The computational simulation suggested that poly-3 would adopt a large number of conformations, which are interconvertible with one another. As outlined in Scheme 1, we investigated the ring-opening reaction of poly-1 into poly-3. Poly-1 was prepared by previously reported procedures, and its chemical structure was fully characterized by FT-IR and 1H NMR spectroscopy (Figure 3a).5a By means of size exclusion chromatographymulti angle light scattering (SEC-MALS), we determined the absolute number-averaged molecular weight (M n) and polydispersity index (PDI) of poly-1 to be 3.45 × 105 Da and 1.84, respectively (Figure 4a, blue curve and blue filled circles). We recently reported a successful solid-state quaternization reaction of a rigid ladder polymer.9 Similarly, a powder sample of poly-1 was immersed in a mixture of diethyl ether (Et2O) and dimethyl sulfate (Me2SO4), and then heated under reflux for 12 h.10,11 During the reaction, the powdery substance in the mixture remained undissolved. The resultant solid (poly-2) isolated by filtration was not soluble in CHCl3, but was soluble in polar solvents such as methanol (MeOH) and dimethyl sulfoxide (DMSO). This preference in solubility was opposite to that observed for poly-1, suggesting that the Tröger’s base units in the main chain were quaternized. The 1H NMR spectral peaks of the product (poly-2) were significantly broadened, which hampered complete characterization (Figure 3c).12 Thus, we synthesized the corresponding model compound (model-2) from Tröger’s base (model-1)10 and compared its 1H NMR spectrum (Figure 3d) to those of poly-2 and Tröger’s base (Figure 3b). The 1H NMR spectrum of model-213 in DMSO-d6 showed signals at 3.66 and 7.97 ppm arising from protons of the N-methyl group (j) and a proton on the aromatic ring (a) in the ortho-position relative to quaternized nitrogen, respectively. Likewise, poly-2 containing quaternized nitrogen also showed characteristics signals around 3.65 and 7.85 ppm, which could be assigned to the protons at (j) and (a) positions, respectively. Furthermore, the chemical shifts of the signals assignable to the N-methylene protons (g, h, and i) in model-2 and poly-2 were almost identical to one another (Figure 3c,d). Judging from the fact that the integral ratio of peak (a) to other aromatic peaks (b−d) was 1:3, the Nmethylation reaction proceeded quantitatively (Figure S5).14 Obviously, this highly efficient conversion of poly-1 into poly-2 is important for the subsequent ring-opening reaction that leads to poly-3 (Scheme 1). We found that the ring-opening reaction also occurred quantitatively when poly-2 was treated simply with an aqueous solution of NaOH in the solid state. Thus, a powder sample of poly-2 was immersed in an aqueous solution of NaOH (1.0 M), and the mixture was stirred for 2 h at 25 °C.10 The resultant solid, obtained after being washed with water,10 was soluble in CHCl3, in contrast to poly-2. The product afforded a welldefined 1H NMR spectrum in CDCl3 (Figure 3e), which agreed well with that observed for model-3 (Figure 3f).10 For example, the 1H NMR spectrum of model-315 showed characteristic signals arising from protons of the N-methyl group (j) at 2.91 ppm and an aromatic proton (c) in the ortho-position relative to the N-H group at 6.55 ppm, along with the N-methylene protons (g) and (h) at 4.39 and 4.21 ppm, respectively. The corresponding signals were clearly observed in the 1H NMR spectrum of the polymeric product at 2.85 (j), 6.55 (c), 4.34 (g), and 4.21 (h) ppm. Moreover, the spectral patterns of other

Scheme 1. Synthesis of Poly-3 and Its Chemical Modifications

would likewise be applicable to Tröger’s base-containing ladder polymers, resulting in the formation of a new class of conformationally flexible ladder polymers (Figure 1, right). To examine the postpolymerization ring-opening reaction, we chose the most representative ladder polymer (poly-1, Scheme 1),5a the main chain of which contains a Tröger’s base unit hybridized with 9,10-dimethyl-ethanoanthracene. Prior to the experiments, we evaluated the conformational behaviors of poly-1 and its ring-opened product (poly-3, Scheme 1) based on molecular dynamics simulations for the model oligomers of poly-1 and poly-3 with four repeating units (Figure S1). Each model oligomer was simulated at 300 K in vacuum with a timestep interval of 2.0 fs, and the end-to-end distances obtained every 100 steps were summarized as histograms (Figure 2). The distribution of the end-to-end distances for the model oligomer of poly-1 fell into a narrow range between 17 and 31 Å (Figure

Figure 2. Histograms of end-to-end distances for the model oligomers of (a) poly-1 and (b) poly-3 obtained by molecular dynamics simulations (Figure S1). Insets illustrate typical conformations for the model oligomers of poly-1 and poly-3. 776


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Figure 3. 1H NMR spectra of (a) poly-1 (500 MHz, in CDCl3, at 25 °C), (b) model-1 (400 MHz, in CDCl3, at 25 °C), (c) poly-2 (500 MHz, in DMSO-d6, at 25 °C), (d) model-2 (400 MHz, in DMSO-d6, at 25 °C), (e) poly-3 (500 MHz, in CDCl3, at 60 °C), (f) model-3 (400 MHz, in CDCl3, at 25 °C), (g) poly-4 (500 MHz, in CDCl3, at 25 °C), (h) model-4 (400 MHz, in CDCl3, at 25 °C), (i) poly-5 (500 MHz, in CDCl3, at 25 °C), and (j) model-5 (400 MHz, in CDCl3, at 25 °C).

freedom and structural symmetry. The protons on the same Nmethylene carbons (g and h) of Tröger’s base derivatives (poly1, poly-2, model-1, and model-2) appeared to be nonequivalent; protons (g) were observed separately as (ga) and (gb), and protons (h) were observed separately as (ha) and (hb). This feature originates from the conformational restriction in the bicyclic structure. In contrast, thecorresponding protons (g and h) in model-315 and poly-3 were observed equivalently, meaning that the diazacyclooctane unit undergoes a rapid ringflipping motion. As expected, the 1H NMR signal of the Nmethylene protons of poly-3 significantly broadened at low

aromatic protons (a, b, and d) for the polymeric product and model-3 were similar to each other (Figure 3e,f). These results indicate the successful formation of poly-3 with a diazacyclooctane skeleton in the main chain.16 Based on SEC-MALS measurement,17 the absolute Mn of poly-3 was determined to be 3.58 × 105 Da (Figure 4b, green curve and green filled circles), which was comparable to that of poly-1. Therefore, side reactions that could result in breaking of the main chain scarcely occurred during the alkaline hydrolysis. Importantly, 1H NMR spectroscopy of model-3 and poly-3 provided information about the degree of conformational 777


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of new bottlebrush polymers with a ladder structure in the main chain. In summary, we have demonstrated an efficient and convenient route to the synthesis of well-defined conformationally flexible ladder polymers. This achievement was brought about by a successful two-step ring-opening reaction of a Tröger’s base-containing rigid ladder polymer. As revealed by 1 H NMR spectroscopy, conversion of the bicyclic Tröger’s base unit into a diazacyclooctane skeleton results in a rigid-to-flexible transformation in conformational behavior. To the best of our knowledge, the resultant ladder polymer (poly-3) is the first example of structurally well-defined ladder polymers featuring a large degree of freedom for the main-chain conformation. The presence of the secondary amine moiety in poly-3 allows for further functionalization of the polymer structure. Thus, the diazacyclooctane-containing ladder polymer with flexibility for both conformation and molecular design may lead to the development of new polymers that are mechanical robust and exhibit dynamic and viscoelastic properties. Studies along these lines are currently in progress in our group.

Figure 4. SEC-MALS profiles of (a) poly-1 (Mn = 3.45 × 105 Da, PDI = 1.84), (b) poly-3 (Mw = 3.58 × 105 Da, PDI = 2.27), and (c) poly-4 (Mw = 4.12 × 105 Da, PDI = 1.96).

temperatures, since the ring flipping becomes sluggish (Figures S1 and S2). It should be noted that the sequential Nmethylation and hydrolysis reactions can also be achieved using free-standing membranes of poly-1 without any dissolution or mechanical fracture (Figure S3).10 This method would be beneficial for the processing of poly-3 into a film sample. As confirmed by N2 adsorption experiments, solid-state poly1 features intrinsic porosity,5 while poly-3 without bicyclic Tröger’s base units is no longer porous in the solid state. Figure S5 shows the N2 adsorption isotherms of poly-1 and poly-3 at 77 K. Consistent with previous report,5 the isotherm of poly-1 was identified as type I in microporous materials (Figure S5a).18 In contract, the isotherm of poly-3 was characteristic of type III in nonporous materials (Figure S5b).18 This observation indicates that a small modification of the main chain of poly-1, in which only a methylene bridge is removed from the main chain, greatly enhances the degree of freedom of the polymer chains and consequently leads to effective packing in the solid state. Since the conformationally flexible ladder polymer (poly-3) possesses a secondary amine moiety, it is possible to attach pendant groups or side chains to the main chain. Typically, when poly-3 was reacted with an acylating agent such as acyl halide, poly-4 was obtained (Scheme 1).10 To identify the structure of poly-4, we also prepared a model compound (model-4). In the 1H NMR spectrum of model-4, signals arising from protons on the N-methylene carbon (g and h) appeared nonequivalently at 5.11, 4.01, 3.99, and 3.65 ppm (Figure 3h). This nonequivalency of the N-methylene protons is most likely due to slow rotation of the N-Ac bond relative to the 1H NMR time scale. The N-methylene protons (g and h) of poly-4 were observed at 5.09, 4.00, 3.91, and 3.59 ppm (Figure 3g), similar to those observed for model-4. Based on SECMALS measurement, the absolute Mn of poly-4 was determined to be 4.12 × 105 Da (Figure 4c, red curve and red filled circles).17 This value is slightly higher than that of poly-3 without an acetyl group, indicating that breaking of the main chain to decrease Mn did not occur upon N-acylation. Such successful N-acylation to poly-3 would expand the degree of freedom for polymer design. For instance, we have obtained a ladder polymer (poly-5) carrying an initiating group for atom transfer radical polymerization (ATRP),19 which was unambiguously characterized by comparing its 1H NMR spectrum (Figure 3i) to that of a corresponding model compound (model-5, Figure 3j). Poly-5 may be useful for the development

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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.7b00385. Synthesis, 1H NMR, FT-IR spectra, photographs, and N2 adsorption isotherms of the ladder polymers, and synthesis, 1H NMR, 13C NMR, NOE differential spectra, HH−COSY, HMQC and HMBC correlations, FT-IR spectra, and high-resolution MS spectra of the model compounds (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Takanori Fukushima: 0000-0001-5586-9238 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by KAKENHI (Grant-in-Aid for Young Scientists B No. 26810067, Grant-in-Aid for Young Scientists A No. 17H04879) of the Japan Society for the Promotion of Science (JSPS), and Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). We thank Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of Technology, for their support with the NMR and SEC-MALS measurements.

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REFERENCES

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Rigid-to-Flexible Conformational Transformation: An Efficient Route

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