Highly efficient transformation of linear poly(phenylene ethynylene)s into zigzag-shaped π-conjugated microporous polymers through boron-mediated alkyne benzannulation

Abstract

Porous polymers offer great advantages compared to other microporous materials in terms of solubility and processability. However, the design of porous polymers has suffered from the limited availability of suitable building blocks. Here we propose a conceptually new strategy for the design of porous polymers, which involves the transformation of a rigid linear polymer into a rigid zigzag polymer with a large free volume around the polymer backbone. This strategy relies on a boron-mediated alkyne benzannulation reaction, which was recently developed by our group. When the benzannulation reaction was applied to poly(phenylene ethynylene) (PPE) derivatives, a linear-to-zigzag structural transformation successfully occurred to give the corresponding π-conjugated polymers with a diarylphenanthrene unit in the main chain. As revealed by N₂ adsorption experiments, while the parent PPEs were non-porous, the zigzag polymers in the solid state possessed porosity with a specific surface area of up to 366 m² g⁻¹, where the surface area largely depended on the steric bulkiness of the substituents on the polymer. Considering the fact that a wide variety of PPE derivatives have so far been synthesized, the present strategy may open a new avenue for the development of functional porous polymers.

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Introduction

Microporous materials have attracted considerable attention because they have a wide variety of applications, including gas storage and separation, catalysis, ion transport and drug delivery.¹ As represented by metal–organic frameworks (MOFs),^1a–d,2 covalent-organic frameworks (COFs)^1e,3 and highly crosslinked porous π-conjugated polymers,⁴ many porous materials have been developed, which have three-dimensional robust network structures formed by metal ion-to-ligand coordination and/or covalent linkages to give a particular space in the solid state. However, these porous materials suffer from insolubility and low processability due to their highly crosslinked network structures. In this regard, non-crosslinked porous polymers offer the advantage of high solubility, which is beneficial for film fabrication and thus enables membrane-based applications.

A key for the design of porous polymers is how to make free volume around the polymer backbone in the solid state.^5,6 As represented by poly[1-(trimethylsilyl)-1-propyne], rigid polyacetylenes carrying sterically bulky substituents are known to provide a microporous solid.⁵ Recently, McKeown and co-workers have reported an interesting class of microporous polymers, called polymers of intrinsic microporosity (PIM).^1f,g,6 PIMs are characterized by their rigid and randomly contorted ladder-type backbones, which provide high solubility as well as processability, to allow the fabrication of free-standing microporous membranes.⁶ However, the limited availability of suitable building blocks for the construction of soluble microporous polymers might be an obstacle for the further development of polymer-based porous materials with various functionalities such as redox, catalytic and sensing activities.^5,6 Therefore, a new design concept of porous polymers, which realizes both design flexibility and processability, is highly desired.

We conceived a new design strategy for the construction of porous polymers based on the structural transformation of a rigid linear polymer into a rigid zigzag-shaped polymer (Fig. 1a). As expected from the structural feature of PIMs, zigzag polymers with high rigidity may offer a large free volume around the polymer backbone, thereby leading to a porous solid upon aggregation (Fig. 1a). To embody this strategy, we used a recently developed benzannulation reaction of alkyne derivatives with 9-chloro-9-borafluorene⁷ (1_H), which enables the facile and efficient transformation of various diarylacetylenes into the corresponding 9,10-diarylphenanthrenes by a one-pot reaction protocol (Fig. 1b).⁸ The first step of this reaction is 1,2-carboboration of diarylacetylene with 1_H to form borepin, which has a boron-containing seven-membered ring (Fig. 1b). Upon one-electron oxidation of the resulting borepin, deborylation/C–C coupling takes place to afford 9,10-diarylphenanthrene (Fig. 1b). From a geometrical point of view, a linear alkyne moiety is bent by 120 degrees through the benzannulation reaction (Fig. 1). Here we report that the boron-mediated benzannulation can be successfully applied to linear poly(phenylene ethynylene)s (PPEs)⁹ to give π-conjugated zigzag polymers (Fig. 1 and Scheme 1). N₂ adsorption–desorption experiments revealed that the obtained diarylphenanthrene-based zigzag polymers in the solid state possess a microporous structure with a Brunauer–Emmett–Teller (BET) surface area (S_BET) of up to 366 m² g⁻¹, where the sterically bulky substituents on the diarylphenanthrene are essential for achieving microporosity. Considering the high design flexibility of PPEs as well as the wide substrate scope of the boron-mediated benzannulation,⁸ the present design strategy is promising for the development of porous polymers with various functionalities.

Fig. 1 (a) Schematic illustration of the design strategy for constructing a porous polymer through the structural transformation of linear PPE into a zigzag-shaped diarylphenanthrene-based polymer by benzannulation of the internal alkyne units in the main chain of PPE. (b) Schematic illustration of a borafluorene-mediated alkyne benzannulation reaction that allows the facile transformation of diarylacetylene into 9,10-diarylphenanthrene. Orange arrows indicate the direction of chemical bonding.

Scheme 1 Synthesis of zigzag-shaped π-conjugated poly-3 (fully benzannulated) by the boron-mediated benzannulation of poly(phenylene ethynylene) poly-2 with borafluorene 1. TBS = tert-butyldimethylsilyl and ODCB = o-dichlorobenzene.

Results and discussion

Synthesis of rigid zigzag-shaped π-conjugated polymers with a diarylphenanthrene backbone

As precursors for zigzag polymers, we used 2,7-di-tert-butyl-9-chloro-9-borafluorene (1_^tBu) in addition to non-substituted 1_H (Scheme 1). We also chose PPE with bulky tert-butyldimethylsilyl (TBS)-protected phenol groups on the phenylene rings (poly-2_OTBS, M_n = 6400, PDI = 2.02) as well as PPE having flexible hexyloxy side chains (poly-2_OHex, M_n = 10 400, PDI = 4.50) (Scheme 1).¹⁰ The benzannulation reactions of PPEs with these borafluorenes afforded four types of zigzag polymers, each with a different steric environment (Scheme 1).

Typically, poly-2_OTBS was reacted with 1_^tBu in o-dichlorobenzene (ODCB) at 80 °C, and the resulting mixture was treated with 2-azaadamantane-N-oxyl (AZADO) as a one-electron oxidant. By reprecipitation of the reaction mixture in MeOH, fully benzannulated poly-3_OTBS-^tBu₁₀₀ was obtained as an orange powder in 53% yield. As shown in Fig. 2, the FT-IR spectrum of poly-3_OTBS-^tBu₁₀₀ was devoid of a C≡C stretching band at 2205 cm⁻¹, which was observed for poly-2_OTBS. The ¹H NMR spectrum (500 MHz) of poly-3_OTBS-^tBu₁₀₀ in CDCl₃ at 25 °C displayed broad signals in the aromatic region at δ = 8.61 and 8–6 ppm, which coincide with the ¹H NMR signals of 9,10-diarylphenanthrene derivatives.⁸ Thus, the alkyne units of PPE were successfully benzannulated (Fig. 3). The electronic absorption spectrum of poly-3_OTBS-^tBu₁₀₀ in CH₂Cl₂ at 25 °C (Fig. S1, ESI†) showed that the absorption maximum of poly-3_OTBS-^tBu₁₀₀ (λ_max = 264 nm) was significantly blue-shifted relative to that of parent poly-2_OTBS (λ_max = 389 nm), suggesting that the effective π-conjugation length of poly-3_OTBS-^tBu₁₀₀ is shorter than that of poly-2_OTBS due to the zigzag-shaped structure. Interestingly, in the gel permeation chromatography (GPC) analysis, the retention time of benzannulated poly-3_OTBS-^tBu₁₀₀ was longer than that of poly-2_OTBS (Fig. S2, ESI†), suggesting that the hydrodynamic volume of the polymer decreases upon bending.¹¹ In addition to fully benzannulated poly-3_OTBS-^tBu₁₀₀, we also prepared poly-3_OTBS-^tBu₅₀ (Fig. 2, 3, Fig. S15, S16 and Scheme S1, ESI†), in which half of the alkyne units are benzannulated (Fig. 2), so that we could evaluate the effect of the benzannulation ratio on the solid-state properties. In a manner similar to that for poly-3_OTBS-^tBu₁₀₀, fully benzannulated poly-3_OTBS-H₁₀₀, poly-3_OHex-^tBu₁₀₀ and poly-3_OHex-H₁₀₀ were obtained as orange powders in respective yields of 65, 74 and 76%, and unambiguously characterized by FT-IR spectroscopy as well as ¹H and ¹³C NMR spectroscopy (Fig. S17–S21, ESI†). The obtained zigzag polymers were sufficiently soluble in common organic solvents such as chloroform, dichloromethane and toluene and thus allowed for not only the full structural characterization but also solution processing of the material.

Fig. 2 IR spectra (KBr) and magnification of poly-3_OTBS-^tBu₁₀₀, poly-3_OTBS-^tBu₅₀ and poly-2_OTBS at 25 °C. The area ratio of the peak due to the C≡C stretching band (2205 cm⁻¹) is 2.1 : 1.0 (poly-2_OTBS : poly-3_OTBS-^tBu₅₀).

Fig. 3 ¹H NMR spectra (500 MHz) of poly-3_OTBS-^tBu₁₀₀ (red), poly-3_OTBS-^tBu₅₀ (orange) and poly-2_OTBS (black) in CDCl₃ at 25 °C. TMS = tetramethylsilane.

Substituent effect on the porosity of diarylphenanthrene-based zigzag π-conjugated polymers

To evaluate the porosity of the diarylphenanthrene-based zigzag polymers, we conducted N₂ adsorption experiments. Fig. 4 shows the N₂ adsorption–desorption isotherms obtained for powder samples of poly-3_OTBS-^tBu₁₀₀, poly-3_OTBS-^tBu₅₀, poly-3_OTBS-H₁₀₀ and poly-2_OTBS at 77 K. As expected, poly-3_OTBS-^tBu₁₀₀ displayed a hysteretic N₂ adsorption–desorption curve (Fig. 4a), the shape of which can be classified as type I according to the IUPAC classification.¹² The S_BET value of poly-3_OTBS-^tBu₁₀₀ was estimated to be 366 m² g⁻¹. Based on the Barrett–Joyner–Halenda (BJH) analysis (Fig. S2, ESI†), the pore size of poly-3_OTBS-^tBu₁₀₀ was considered to be approximately 2 nm. While the observed S_BET value is not very high compared to those of other microporous polymers,^5,6poly-3_OTBS-^tBu₁₀₀ surely possesses micropores in the solid state. Note that since the parent linear poly-2_OTBS is nonporous (Fig. 4d), the microporosity of poly-3_OTBS-^tBu₁₀₀ should arise from its rigid zigzag-shaped structure, which is brought about by the benzannulation of the linear poly(phenylene ethynylene) backbone (Fig. 1).

Fig. 4 N₂ adsorption–desorption isotherms of powder samples of (a) poly-3_OTBS-^tBu₁₀₀, (b) poly-3_OTBS-^tBu₅₀, (c) poly-3_OTBS-H₁₀₀ and (d) parent poly-2_OTBS (M_n = 6400, PDI = 2.02) at 77 K.

The benzannulation ratio of the polymer backbone greatly affected the porosity of the zigzag polymer. The N₂ adsorption–desorption isotherm showed that a powder sample of poly-3_OTBS-^tBu₅₀ (Scheme S1, ESI†), in which half of the alkyne units are benzannulated, hardly possessed micropores (Fig. 4b). The estimated S_BET value for poly-3_OTBS-^tBu₅₀ (64.9 m² g⁻¹) was significantly smaller than that of poly-3_OTBS-^tBu₁₀₀ (366 m² g⁻¹), and the BJH analysis suggested that the pore-size distribution for poly-3_OTBS-^tBu₅₀, centered at 6.4 nm, was much larger than that for poly-3_OTBS-^tBu₁₀₀ (Fig. S4, ESI†). Thus, formation of a microporous structure is hampered for poly-3_OTBS-^tBu₅₀, most likely because alkyne and benzannulated units are randomly distributed in the polymer backbone (Scheme S1, ESI†).

Since poly-3_OTBS-H₁₀₀, which lacks ^tBu groups at the phenanthrene units, was hardly microporous (Fig. 4c, blue), the sterically bulky ^tBu groups are considered to be essential for endowing the microporous structure with poly-3_OTBS-^tBu₁₀₀. Even in the presence of ^tBu groups at the phenanthrene units, the zigzag polymer loses porosity, when the bulky TBS groups are replaced by flexible hexyl groups (poly-3_OHex-^tBu₁₀₀, Fig. 5, red). As expected, poly-3_OHex-H₁₀₀ was also found to be nonporous (Fig. 5, blue).

Fig. 5 N₂ adsorption isotherms of powder samples of poly-3_OHex-^tBu₁₀₀ (red), poly-3_OHex-H₁₀₀ (blue) and parent poly-2_OHex (black, M_n = 10 400, PDI = 4.50) at 77 K.

Deprotection of the TBS groups of a zigzag polymer (poly-3_OTBS-^tBu₁₀₀)

According to the N₂ adsorption–desorption isotherm profiles in Fig. 4 and 5, we initially thought that both bulky ^tBu and TBS groups are required to ensure the porosity of the zigzag polymers. Unexpectedly, however, removal of the TBS groups of poly-3_OTBS-^tBu₁₀₀ did not significantly affect the microporosity. For example, an ODCB solution of poly-3_OTBS-^tBu₁₀₀, synthesized from poly-2_OTBS (M_n = 5900, PDI = 2.32), was treated with tetrabutylammonium fluoride (Bu₄N⁺F⁻) at 25 °C, affording poly-3_OH-^tBu₁₀₀ as an orange powder in 77% yield (Scheme 1). FT-IR spectroscopy and ¹H and ¹³C NMR spectroscopy of poly-3_OH-^tBu₁₀₀ confirmed the absence of TBS groups (Fig. S5, S22 and S23, ESI†). As shown in Fig. 6, the N₂ adsorption–desorption isotherm of a powder sample of poly-3_OH-^tBu₁₀₀ at 77 K (green) was very similar to that of the precursor poly-3_OTBS-^tBu₁₀₀ under identical measurement conditions (red). The S_BET and BJH pore size for poly-3_OH-^tBu₁₀₀ were estimated to be 283.9 m² g⁻¹ and ∼2 nm, respectively, which were comparable to those obtained for poly-3_OTBS-^tBu₁₀₀ (S_BET: 303.9 m² g⁻¹, BJH pore size: 2.4 nm). Therefore, even after the removal of the large TBS groups, the free volume around the diarylphenanthrene backbone is not impaired, to preserve microporosity in the solid state. The fact that the diarylphenanthrene-based zigzag π-conjugated polymer allows chemical transformation while maintaining microporosity is noteworthy.

Fig. 6 N₂ adsorption isotherms at 77 K of powder samples of poly-3_OTBS-^tBu₁₀₀ (red) and poly-3_OH-^tBu₁₀₀ (green), both of which were converted from poly-2_OTBS (M_n = 5900, PDI = 2.31).

Structural requirements for microporosity in diarylphenanthrene-based zigzag polymers

Table 1 summarizes the results of the N₂ adsorption experiments in this study. As mentioned above, the zigzag polymers have micropores with a size of approximately 2 nm. Obviously, not only the rigid zigzag-shaped backbone but also sterically bulky ^tBu groups on the phenanthrene units are needed to achieve microporosity (entries 1 and 3). The comparison between entries 1 and 6 indicates that the molecular weight and PDI may affect the porosity to some extent. Meanwhile, the microporosity of the zigzag polymers is impaired when the alkyne units of PPEs are insufficiently benzannulated (entries 1 and 2). Flexible chains such as a hexyloxy group may also hamper the formation of micropores, most likely through filling voids (entries 4 and 5). If the above-listed structural requirements are satisfied, further chemical modification of the zigzag polymers may become possible while maintaining porosity (entries 6 and 7).

Table 1 Specific surface areas (S_BET) and BJH pore sizes of poly-3

Entry	Polymer	S BET (m^2 g^−1)	Pore size (nm)
a Synthesized from poly-2OTBS (Mn = 6400, PDI = 2.02). b Synthesized from poly-2OTBS (Mn = 5900, PDI = 2.32).
1	Poly-3OTBS-^tBu100	365.5	∼2
2	Poly-3OTBS-^tBu50	64.9	6.4
3	Poly-3OTBS-H100	Nonporous	—
4	Poly-3OHex-^tBu100	Nonporous	—
5	Poly-3OHex-H100	Nonporous	—
6	Poly-3OTBS-^tBu100	303.9	2.4
7	Poly-3OH-^tBu100	283.9	∼2

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Conclusions

We have demonstrated the successful synthesis of a new type of π-conjugated zigzag polymer by the benzannulation reaction of PPEs with borafluorene derivatives. Importantly, the boron-mediated benzannulation⁸ was highly effective even for polymeric substances with multiple reaction sites, thus enabling quantitative conversion of the internal alkynes in the main chain of PPEs into diarylphenanthrene units. The resulting polymers possess a large free volume around the main chain to exhibit microporosity. Basically, the formation of the microporous structure in the rigid zigzag-shaped polymers can be rationalized in light of the general guideline for the design of organic and polymeric microporous materials.¹³ However, the present system is remarkable in that it utilizes the structural transformation of inherently nonporous linear PPEs, and the π-conjugated systems can be maintained after the polymer reaction. Furthermore, owing to the wide substrate scope of the benzannulation reaction,⁸ various functional PPEs can be used as precursors for constructing zigzag polymers,⁹ which would be beneficial for the design of microporous materials with synergistic properties (e.g., redox, catalytic and sensing activities) in combination with gas adsorption/permeation activity. In this context, poly-3_OH-^tBu₁₀₀ with a redox active hydroquinone backbone may be interesting. Taking all the above features into account, the present design strategy for porous, zigzag, and π-conjugated polymers may open a new avenue for the development of functional porous polymeric materials. Further studies along these lines are now underway.

Experimental section

Materials

Air- and/or moisture-sensitive compounds were handled in a glove box under argon. Unless otherwise noted, all commercial reagents were used as received. o-Dichlorobenzene (ODCB) was dried over CaH₂ and freshly distilled prior to use. 9-Chloro-9-borafluorene (1_H) was prepared according to a previously reported procedure.⁷

Methods

Analytical GPC was performed at 40 °C regulated using a column oven on a TOSOH GPC-8020 system equipped with a refraction index (RI) detector and a UV detector (UV-8020), using chloroform (CHCl₃) as an eluent at a flow rate of 0.40 mL min⁻¹ on a column (Shodex LF-804). The molecular weight calibration curve was obtained by using standard polystyrenes (TSKstandard polystyrene, TOSOH). Size-exclusion chromatography–multi-angle laser light scattering (SEC–MALS) measurements were performed on a TOSOH GPC-8020 system, equipped with a Shodex KF-805L column, a Wyatt Technology DAWN HELEOS multi-angle laser light scattering photometer with a He–Ne laser (λ₀ = 658 nm) and a Wyatt Technology Optilab rEX differential refractometer, using chloroform (CHCl₃) as an eluent at a flow rate of 0.40 mL min⁻¹. The SEC-MALS profiles were recorded and analyzed using ASTRA software (ver.6.1.1, Wyatt Technology). Differential refractive index increments (∂n/∂c) for the polymers in CHCl₃ were determined based on the peak area of the refractive index chromatograms and the polymer mass concentration (c) of injected solutions assuming that all the injected polymers were fully recovered. The ∂n/∂c values of poly-3_OTBS-^tBu₁₀₀, poly-3_OTBS-^tBu₅₀, poly-3_OTBS-H₁₀₀, poly-3_OHex-^tBu₁₀₀, poly-3_OHex-H₁₀₀ and poly-3_OH-^tBu₁₀₀ were determined to be 0.110, 0.152, 0.135, 0.138, 0.164 and 0.072 cm³ g⁻¹, respectively, at λ₀ = 658 nm. Angular dependences of the scattering intensity were analyzed using the Debye plot in a scattering angle range from 45.8° to 138.8° to estimate the weight-averaged molecular weight M_w and the z-averaged mean-square radius of gyration 〈S²〉_z of each fraction. Infrared (IR) spectra were recorded at 25 °C on a JASCO FT/IR-660_Plus Fourier-transform infrared spectrometer. NMR spectroscopy measurements were carried out at 25 °C on a Bruker AVANCE-500 spectrometer (500 MHz for ¹H, 125 MHz for ¹³C and 128 MHz for ¹¹B). Chemical shifts (δ) are expressed relative to the resonances of the residual non-deuterated solvent for ¹H [CDCl₃: ¹H(δ) = 7.26 ppm] and ¹³C [CDCl₃: ¹³C(δ) = 78.16 ppm] and external BF₃OEt₂ in CDCl₃ for ¹¹B (¹¹B(δ) = 0.0 ppm). Absolute values of the coupling constants are given in Hertz (Hz), regardless of their sign. Multiplicities are abbreviated as singlet (s), doublet (d), doublet of doublets (dd), triplet (t), quartet (q), multiplet (m), and broad (br). N₂ adsorption measurements were performed at 77 K after pretreatment at 150 °C under a reduced pressure for 3 h on a BEL Japan Belsorp-mini II apparatus. The Brunauer–Emmett–Teller (BET) surface areas were estimated over a relative pressure (P/P₀) range of 0.05–0.20. The pore size distribution was obtained from the analysis of the adsorption branch of the isotherms using the BJH (Barrett–Joyner–Halenda) method.

Synthesis

2,7-Di-tert-butyl-9-chloro-9-borafluorene (1_^tBu). Under argon at 25 °C, a hexane solution (2.69 M) of n-BuLi (17.5 mL, 4.45 mmol) was added dropwise to a dry benzene solution (20 mL) of 2,2′-dibromo-4,4′-di-tert-butyl-1,1′-biphenyl¹⁴ (7.00 g, 165 mmol), and the resulting mixture was stirred at 80 °C for 4 days, forming a white suspension. After the reaction mixture was allowed to cool to 25 °C, a hexane solution (1.0 M) of BCl₃ (17.3 mL, 173 mmol) was slowly added to the resulting suspension, where the color of the reaction mixture turned from white to yellow. The reaction mixture was stirred at 25 °C for 1 day and evaporated under reduced pressure. To the residue was added dry hexane (100 mL), and the resulting suspension was filtered through a glass filter. The filtrate was collected, evaporated under reduced pressure and recrystallized to give 1_^tBu (2.47 g, 18.7 mmol) as a yellow powder in 48% yield: ¹H NMR (500 MHz, CDCl₃, 25 °C): δ (ppm) 7.56 (s, 2H), 7.35 (d, J = 8.0 Hz, 2H), 7.25–7.18 (m, 2H), 1.32 (s, 18H). ¹¹B NMR (128 MHz, CDCl₃, 25 °C): δ (ppm) 64.2. ¹³C NMR (125 MHz, CDCl₃, 25 °C): δ (ppm) 151.2, 150.5, 138.7 (br), 131.9, 129.8, 119.2, 34.7, 31.2. ¹H, ¹¹B and ¹³C NMR spectra of 1_^tBu are shown in Fig. S4, S5 and S6 (ESI†), respectively.

Poly-2_OTBS . Poly-2_OTBS was synthesized according to a slightly modified procedure reported by Lei et al.^10b Under argon, bis(triphenylphosphine)palladium(II) dichloride (59.4 mg, 0.0845 mmol) was added to a dry toluene suspension (50 mL) of a mixture of 1,4-bis(tert-butyldimethylsilyloxyl)-2,5-diiodobenzene¹⁵ (1.00 g, 1.69 mmol), 1,4-diethynylbenzene (214 mg, 1.69 mmol), triphenylphosphine (66.6 mg, 0.254 mmol), copper(I) iodide (32.3 mg, 0.170 mmol) and dry trimethylamine (3.53 mL, 25.4 mmol), and the reaction mixture was stirred at 80 °C for 3 h. To the mixture was added iodobenzene (0.0346 g, 0.170 mmol), and the resulting mixture was stirred at 80 °C for 1 h, allowed to cool to 25 °C, poured into water (50 mL) and extracted with toluene. The organic layer was washed with water, partially evaporated under reduced pressure until the volume of the solution was reduced to ∼25 mL. The resulting concentrated solution was poured into vigorously stirred MeOH (300 mL). The resulting orange precipitate was collected by filtration, washed with MeOH (10 mL × 3 times) and dried under reduced pressure. Repeated reprecipitation of the resultant solid, using toluene (good solvent) and MeOH (poor solvent), afforded poly-2_OTBS (0.749 g) as a yellow powder in 83% yield: FT-IR (KBr): ν (cm⁻¹) 3075, 3047, 2952, 2205 (v_C≡C), 1906, 1792, 1684, 1599, 1515, 1472, 1434, 1402, 1362, 1277, 1255, 1207, 1096, 999, 936, 881, 838, 803, 781, 754, 743, 691, 626, 541, 519. ¹H NMR (500 MHz, CDCl₃, 25 °C): δ (ppm) 7.49 (br, 6H), 1.06–1.03 (m, 18H), 0.29–0.24 (m, 12H). ¹³C NMR (125 MHz, CDCl₃, 25 °C): δ (ppm) 150.9, 150.5, 150.4, 149.6, 135.2, 134.3, 132.6, 131.6, 130.4, 129.8, 129.2, 128.5, 128.4, 127.9, 124.5, 123.8, 123.3, 121.4, 116.9, 116.1, 94.2, 91.4, 88.8, 82.3, 25.9, 18.5, 18.4, −3.9, −4.1. M_n = 6400; M_w = 12 900; PDI = 2.02. Similarly, poly-2_OTBS (M_n = 5900, M_w = 13 600, PDI = 2.32) was obtained as an orange powder in 80% yield. ¹H and ¹³C NMR spectra of poly-2_OTBS are shown in Fig. S9 and S10 (ESI†), respectively.

Poly-2_OHex . Poly-2_OHex was synthesized according to a slightly modified procedure reported by Lei et al.^10b Under argon, bis(triphenylphosphine)palladium(II) dichloride (351 mg, 0.500 mmol) was added to a dry toluene suspension (200 mL) of a mixture of 1,4-bis(hexyloxy)-2,5-diiodobenzene³ (5.00 g, 9.43 mmol), 1,4-diethynylbenzene (1.19 g, 9.43 mmol), triphenylphosphine (371 mg, 1.41 mmol), copper(I) iodide (180 mg, 0.943 mmol) and dry trimethylamine (12.0 mL, 86.1 mmol), and the reaction mixture was stirred at 80 °C for 3 h. To the mixture was added iodobenzene (1.92 g, 9.43 mmol), and the resulting mixture was stirred at 80 °C for 1 h, allowed to cool to 25 °C, poured into water (200 mL) and extracted with toluene. The organic layer was washed with water, partially evaporated under reduced pressure until the volume of the solution was reduced to ∼100 mL. The resulting concentrated solution was poured into vigorously stirred MeOH (1.0 L). The resulting orange precipitate was collected by filtration, washed with MeOH (50 mL × 3 times) and dried under reduced pressure. Repeated reprecipitation of the resultant solid, using toluene (good solvent) and MeOH (poor solvent), afforded poly-2_Hex as a yellow powder (4.05 g) in 81% yield: FT-IR (KBr): ν (cm⁻¹) 3076, 3038, 2952, 2931, 2869, 2859, 2204 (v_C≡C), 1905, 1789, 1664, 1597, 1516, 1595, 1464, 1415, 1380, 1280, 1215, 1122, 1098, 1017, 943, 886, 857, 835, 741, 720, 692, 542, 520, 510, 489. ¹H NMR (500 MHz, CDCl₃, 25 °C): δ (ppm) 7.51 (br, 4H), 7.35 (br, 2H), 4.10 (t, J = 6.0 Hz, 4H), 1.85 (br, 4H), 1.37 (br, 8H), 0.91 (t, J = 6.6 Hz, 6H). ¹³C NMR (125 MHz, CDCl₃, 25 °C): δ (ppm) 153.8, 135.0, 134.3, 132.6, 132.2, 131.8, 130.0, 129.2, 128.5, 128.4, 127.7, 124.0, 123.4, 116.1, 114.1, 69.8, 31.8, 31.7, 31.6, 29.5, 29.4, 29.3, 25.9, 25.8, 22.8, 22.7, 14.2. M_n = 10 400; M_w = 46 600; PDI = 4.50. ¹H and ¹³C NMR spectra of poly-2_Hex are shown in Fig. S11 and S12 (ESI†), respectively.

General procedure A: benzannulation of poly-2_OTBS

Under argon, an ODCB solution (2.2 mL) of a mixture of poly-2_OTBS (100 mg) and borafluorene (1_^tBu or 1_H) was stirred at 80 °C for 72 h and allowed to cool to 25 °C. After the addition of 2-azaadamantane-N-oxyl (AZADO), the resulting mixture was stirred at 25 °C for 30 min under argon and then poured into vigorously stirred MeOH (150 mL). The resulting orange precipitate was collected by filtration, washed with MeOH (10 mL × 3 times) and dried under reduced pressure. Repeated reprecipitation of the resultant solid, using CHCl₃ (good solvent) and MeOH (poor solvent), afforded a benzannulated polymer.

General procedure B: benzannulation of poly-2_OHex

Under argon, an ODCB solution (2.5 mL) of a mixture of poly-2_OHex (100 mg, M_n = 6400, PDI = 2.02) and borafluorene (1_^tBu or 1_H) was stirred at 80 °C for 20 h and allowed to cool to 25 °C. After the addition of MnO₂ under argon, the resulting suspension was stirred at 80 °C for 1 h and then allowed to cool to 25 °C. The resulting orange gel was dissolved by the addition of THF (5.0 mL), and insoluble materials in the resulting suspension were removed by centrifugation (15 000 rpm, 15 min). The supernatant solution was collected and poured into vigorously stirred MeOH (150 mL). The resulting orange precipitate was collected by filtration, washed with MeOH (10 mL × 3 times) and dried under reduced pressure. Repeated reprecipitation of the resultant solid, using CHCl₃ (good solvent) and MeOH (poor solvent), afforded a benzannulated polymer.

Poly-3_OTBS-^tBu₁₀₀ . According to general procedure A, poly-3_OTBS-^tBu₁₀₀ was obtained as an orange powder (94 mg) in 53% yield using poly-2_OTBS (100 mg, M_n = 6400, PDI = 2.02), 1_^tBu (162 mg, 0.521 mmol) and AZADO (87 mg, 0.573 mmol): FT-IR (KBr): ν (cm⁻¹) 3078, 3033, 2961, 2929, 2904, 2863, 1614, 1483, 1392, 1362, 1320, 1258, 1206, 1156, 1119, 1101, 1024, 1000, 952, 928, 892, 840, 813, 782, 734, 692, 653, 629. ¹H NMR (500 MHz, CDCl₃, 25 °C): δ (ppm) 8.61 (s, br, 4H), 8.0–6.0 (m, br, 14H), 1.6–1.0 (m, br, 66H). ¹³C NMR (125 MHz, CDCl₃, 25 °C): δ (ppm) 148.9, 138.9, 131.3, 128.6, 124.5, 122.5, 34.9, 31.5, 26.2, 25.1, 18.5, 17.8, −4.00. ¹H and ¹³C NMR spectra of poly-3_OTBS-^tBu₁₀₀ are shown in Fig. S13 and S14 (ESI†), respectively.

Poly-3_OTBS-^tBu₅₀ . According to general procedure A, poly-3_OTBS-^tBu₅₀ was obtained as an orange powder (98 mg) in 65% yield using poly-2_OTBS (100 mg, M_n = 6400, PDI = 2.02), 1_^tBu (67 mg, 0.217 mmol) and AZADO (37 mg, 0.240 mmol): FT-IR (KBr): ν (cm⁻¹) 3078, 3033, 2961, 2929, 2904, 2863, 2205 (v_C≡C), 1906, 1792, 1684, 1615, 1506, 1485, 1473, 1443, 1395, 1363, 1335, 1258, 1204, 1117, 1102, 1004, 928, 890, 838, 815, 782, 735, 690, 620, 542. ¹H NMR (500 MHz, CDCl₃, 25 °C): δ (ppm) 8.70 (s, br, 4H), 7.70–6.58 (m, br, 22H), 1.56–0.42 (m, br, 92H). ¹³C NMR (125 MHz, CDCl₃, 25 °C): δ (ppm) 150.0, 148.9, 138.3, 131.2, 127.8, 123.8, 122.8, 45.4, 37.5, 34.9, 31.4, 29.8, 27.5, 25.9, 25.1, 22.9, 21.4, 20.7, 19.9, 18.4, 17.7, 14.3, −4.2. ¹H and ¹³C NMR spectra of poly-3_OTBS-^tBu₅₀ are shown in Fig. S15 and S16 (ESI†), respectively.

Poly-3_OTBS-H₁₀₀ . According to general procedure A, poly-3_OTBS-H₁₀₀ was obtained as an orange powder (95 mg) in 65% yield using poly-2_OTBS (100 mg, M_n = 6400, PDI = 2.02), 1_H (103 mg, 0.521 mmol) and AZADO (87 mg, 0.573 mmol): FT-IR (KBr): ν (cm⁻¹) 3069, 3030, 2953, 2928, 2894, 2887, 2857, 1602, 1527, 1472, 1447, 1392, 1362, 1331, 1255, 1209, 1101, 1046, 1007, 981, 934, 884, 838, 804, 780, 758, 726, 700, 616, 658, 533. ¹H NMR (500 MHz, CDCl₃, 25 °C): δ (ppm) 8.80 (s, br, 4H), 7.69–6.65 (m, br, 4.6H), 1.71–0.41 (m, br, 8H). The solubility of the product was too low to record a reasonable ¹³C NMR spectrum. ¹H NMR spectrum of poly-3_OTBS-H₁₀₀ is shown in Fig. S17 (ESI†).

Poly-3_OHex-^tBu₁₀₀ . According to general procedure B, poly-3_OHex-^tBu₁₀₀ was obtained as an orange powder (150 mg) in 74% yield using poly-2_OHex (100 mg, M_n = 10 400, PDI = 4.50), 1_^tBu (186 mg, 0.600 mmol) and MnO₂ (57 mg, 0.66 mmol): FT-IR (KBr): ν (cm⁻¹) 3075, 3028, 2966, 2906, 2868, 1656, 1604, 1486, 1394, 1363, 1261, 1201, 1119, 1021, 1000, 894, 814, 735, 680, 612, 530. ¹H NMR (500 MHz, CDCl₃, 25 °C): δ (ppm) 8.67 (s, br, 4H), 7.71–6.55 (m, br, 14H), 3.60 (s, br, 4H), 1.59–0.24 (m, br, 58H). ¹³C NMR (125 MHz, CDCl₃, 25 °C): δ (ppm) 147.6, 135.9, 127.5, 121.8, 66.7, 43.4, 35.5, 32.8, 29.4, 27.8, 25.4, 23.6, 20.7, 19.3, 18.6, 17.9, 12.1. ¹H and ¹³C NMR spectra of poly-3_OHex-^tBu₁₀₀ are shown in Fig. S18 and S19 (ESI†), respectively.

Poly-3_OHex-H₁₀₀ . According to general procedure B, poly-3_OHex-H₁₀₀ was obtained as an orange powder (115 mg) in 76% yield using poly-2_OHex (100 mg, M_n = 10 400, PDI = 4.50), 1_H (123 mg, 0.620 mmol) and MnO₂ (56 mg, 0.64 mmol): FT-IR (KBr): ν (cm⁻¹) 3061, 3021, 2954, 2926, 2868, 2857, 1921, 1657, 1597, 1490, 1469, 1446, 1434, 1413, 1352, 1262, 1200, 1086, 1045, 1020, 951, 930, 861, 800, 760, 726, 700, 651, 618, 530. ¹H NMR (125 MHz, CDCl₃, 25 °C): δ (ppm) 8.80 (s, br, 4H), 7.69–6.41 (m, br, 19H), 3.49 (s, br, 4H), 1.65–0.47 (m, br, 24H). ¹³C NMR (500 MHz, CDCl₃, 25 °C): δ (ppm) 130.1, 127.3, 122.6, 68.9, 32.1, 31.7, 29.8, 29.5, 29.2, 25.9, 25.6, 22.7, 14.2. ¹H and ¹³C NMR spectra of poly-3_OHex-H₁₀₀ are shown in Fig. S20 and S21 (ESI†), respectively.

Poly-3_OH-^tBu₁₀₀ . Under argon at 25 °C, a THF solution of tetrabutylammonium fluoride (1.0 M, 0.31 mL, 0.31 mmol) was added to an ODCB solution (2.5 mL) of poly-3_OTBS-^tBu₁₀₀ (100 mg), which was synthesized from poly-2_OTBS (M_n = 5900, PDI = 2.32), and the reaction mixture was stirred for 2 h at 25 °C, poured into water (10 mL) and extracted with CHCl₃. The organic layer was washed with water, and partially evaporated under reduced pressure until the volume of the solution was reduced to ∼2.5 mL. The resulting concentrated solution was poured into vigorously stirred MeOH (50 mL). The resulting orange precipitate was collected by filtration, washed with MeOH (5 mL × 3 times) and dried under reduced pressure. Repeated reprecipitation of the resultant solid, using CHCl₃ (good solvent) and MeOH (poor solvent), afforded poly-3_OH-^tBu₁₀₀ (54.5 mg) as an orange powder in 77% yield: FT-IR (KBr): ν (cm⁻¹) 3551, 3076, 3030, 2962, 2903, 2867, 1907, 1776, 1720, 1661, 1615, 1574, 1486, 1461, 1403, 1395, 1363, 1324, 1290, 1261, 1198, 1116, 1018, 989, 918, 891, 838, 813, 783, 749, 733, 651, 635, 543, 523. ¹H NMR (500 MHz, CDCl₃, 25 °C): δ (ppm) 8.63 (br, 4H), 7.71–6.44 (br, 14H), 1.55–0.02 (br, 36H). ¹³C NMR (125 MHz, CDCl₃, 25 °C): δ (ppm) 149.7, 139.0, 131.3, 128.5, 125.6, 122.7, 35.0, 31.4, 29.9. ¹H and ¹³C NMR spectra of poly-3_OH-^tBu₁₀₀ are shown in Fig. S22 and S23 (ESI†), respectively.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “π-Figuration: Control of Electron and Structural Dynamism for Innovative Functions” (JP26102008 for T. F.) and KAKENHI (17H03020 for Y. S.) from the Japan Society for the Promotion of Science (JSPS). This work was also supported in part by the Cooperative Research Program “CORE Lab” of “Network Joint Research Center for Materials and Devices: Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” from MEXT, Japan. Y. S. also acknowledges support from the Asahi Glass Foundation. The authors would like to thank Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of Technology, for their support with the NMR measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Supplementary scheme, table, figures and analytical data. See DOI: 10.1039/c7qm00582b

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