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• W4295678949 abstract "Open AccessCCS ChemistryRESEARCH ARTICLES10 Oct 2022Pressure-Induced Topochemical Polymerization toward Advanced Energetic Materials Guangyu Qi†, Siwei Song†, Zhiying Deng, Dajian Huang, Fang Chen, Bingmin Yan, Huiyang Gou, Qinghua Zhang and Yi Wang Guangyu Qi† Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621999 †G. Qi and S. Song contributed equally to this work.Google Scholar More articles by this author , Siwei Song† Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621999 †G. Qi and S. Song contributed equally to this work.Google Scholar More articles by this author , Zhiying Deng Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621999 Google Scholar More articles by this author , Dajian Huang Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 Google Scholar More articles by this author , Fang Chen Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621999 Google Scholar More articles by this author , Bingmin Yan Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 Google Scholar More articles by this author , Huiyang Gou Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 Google Scholar More articles by this author , Qinghua Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621999 Google Scholar More articles by this author and Yi Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621999 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202249 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Pressure produces closely packed, high-density materials, thereby providing a promising strategy to obtain high-energy-density materials. However, new phases or structures of energetic materials at high pressure are often not quenchable under ambient conditions. In this work, high-pressure topochemical methodology is first introduced for the preparation of stable energetic materials under ambient conditions. A pressure-induced polymerizable energetic material named PIP-1 is designed and prepared. The experimental measurements demonstrate that the polymerization of PIP-1 is caused by the breakage of C≡C bonds and the generation of C=C bonds. In accord with the experimental results, density functional theory calculations further revealed that the monomer PIP-1 is polymerized to generate 1D PIP-1 tape, and the density of polymerized PIP-1 is increased by 4.9% upon decompression. The successful realization of high-energy-density structure using high pressure showcases a new design strategy for advanced polymerizable energetic materials. Download figure Download PowerPoint Introduction Pressure can significantly shorten interatomic distances and alter the geometrical or electronic structures of substances and even change the nature of chemical bonds.1,2 As a result, high-pressure technology is a powerful tool to investigate new phases of substances and prepare advanced functional materials (such as superhard and superconducting materials).3–8 The close packing effect under pressure can endow new materials with high density, which is very suitable for the development of energetic materials (a class of special energy storage materials that pursue high-energy density).9–11 Thus, high-pressure studies on energetic materials have attracted more and more interest in recent years.12,13 For traditional energetic materials that are composed of hydrogen, carbon, nitrogen, and oxygen elements (such as 2,4,6-trinitrotoluene, TNT and hexogen, RDX), most studies focus on their structural evolution under moderate pressure (from ambient pressure to around 40 GPa), and reversible phase transitions are usually observed (Figure 1a).14–16 The highly compressured phases with higher energy density are difficult to achieve with straightforward physical compression. In addition to traditional energetic materials, high pressure has also been used to obtain ultra-high-energy density materials (such as quartzlike carbon dioxide, polymeric nitrogen, and metallic hydrogen).17–22 However, the rigorous synthesis condition (ultra-high pressure and extreme temperature) and instability under ambient conditions significantly limit their practical applications (Figure 1b). Under high pressure of <30 GPa, the polymerization reaction of organic molecules with unsaturated bonds (such as ethylene, acetylene, benzene, and pyridine) can take place.23,24 However, these polymerized materials are usually not energetic, which excludes them as potential high-energy-density materials. On the whole, although high pressure is significantly beneficial for the development of energetic materials, high-pressure-induced preparation of stable energetic materials under ambient conditions has remained elusive so far. Figure 1 | (a) Molecular structures of typical energetic materials and illustration of their crystal structure evolution under pressure. (b) Preparation conditions and properties of some ultra-high-energy energetic materials.17,18,21 (c) Principle and characteristics of pressure-induced topochemical polymerization to prepare energetic materials. Download figure Download PowerPoint Topochemical polymerizations are a unique kind of solid-state reaction, which are solvent- and catalyst-free either in response to external stimuli (such as photo, thermal, and pressure) or sometimes occur spontaneously.25 In addition, topochemical polymerizations have some advantages, including high yield, high atomic economy, and access to special products that are not obtainable with conventional organic synthesis in solutions. To date, various topochemical polymerizations have been used to develop organic functional polymers.26 But to date topochemical polymerizations have rarely been applied in the community of energetic materials. On the one hand, the polymerization under high pressure facilitates the capture of the highly compressed phase of materials,27–32 making the density and energy higher than that of the original materials. On the other hand, the polymerized structures may have better mechanical properties than those of molecular crystals, which is in favor of the post processing and applications of energetic materials (Figure 1c bottom). Obviously, pressure-induced topochemical polymerization is a promising potential strategy to develop new advanced energetic materials. For traditional energetic materials (such as TNT and RDX), topochemical polymerization under high pressure is hindered by the absence of polymerizable units.33–37 Here, we propose a general methodology for developing new energetic materials that can achieve pressure-induced topochemical polymerizations (Figure 1c). By covalently linking pressure-induced reactable units to energetic molecules, the polymerizable energetic monomers can be fabricated. After that, crystalline energetic monomers can be formed through molecular self-assembly. Under compression, energetic monomer molecules can be polymerized to generate new energetic materials that are stable under ambient conditions. To validate the effectiveness of this pressure-induced topochemical polymerization, a molecule named PIP-1 with both energetic (3,5-dinitro-1H-pyrazol-4-amine, LLM-116) and polymerizable (propargyl group) units, was designed and synthesized. Monomer PIP-1 was well packed, and the propargyl group was likely to polymerize under high pressure. The experimental measurements gave strong evidence for the occurrence of pressure-induced polymerization. With the aid of density functional theory (DFT) calculations, the mechanism of pressure-induced polymerization was revealed, and the monomer PIP-1 was polymerized to generate 1D polymer tape with a density increase of 4.9%. This work not only successfully obtained the high-pressure structure of energetic materials, but it also demonstrated the great potential for pressure-induced topochemical polymerization for the development of new advanced energetic materials. Experimental Methods Sample preparation 4-Chloro-3,5-dinitro-1H-pyrazole: The 4-chloropyrazole (1 g, 9.8 mmol) was slowly added to concentrated sulfuric acid (6 mL), and the temperature was maintained at 0 °C–5 °C. Fuming nitric acid (2 mL) was slowly dripped into this solution. After dripping, the solution was stirred at 100 °C for 4 h. The reaction mixture was poured into iced water (100 mL) after cooling it to ambient temperature. The aqueous solution was extracted with ethyl acetate (3 × 30 mL). The combined organic layers were dried over anhydrous magnesium sulfate and evaporated in vacuum to yield a white solid (1.46 g, 78%). Ammonium 4-amino-3,5-dinitropyrazolate (LLM-116·NH4+): A solution of 4-chloro-3,5-dinitropyrazole (1 g, 5.2 mmol) and ammonium hydroxide (5 mL) was stirred at 170 °C in a sealed reactor (20 mL) for 8 h. The mixture was then cooled to ambient temperature. The precipitate was filtered as orange solid, yield: 0.82 g, 83%. 1-Propargyl-4-amino-3,5-dinitro-pyrazole (monomer PIP-1): The ammonium 4-amino-3,5-dinitropyrazolate (1 g, 5.3 mmol) was added into the mixture solvent of methanol (5 mL) and water (5 mL). Afterward, the 3-bromo-1-propyne (1.25 g, 10.6 mmol) was added, and the reaction was stirred for another 6 h at 80 °C. The precipitate was filtered as golden yellow crystals, yield: 0.84 g, 76% ( Supporting Information Scheme S1). 1H NMR (400 MHz, dimethyl sulfoxide, DMSO-d6): δ ppm: 7.36 (2H), 5.47 (2H), 3.67 (1H). 13C NMR (100 MHz, DMSO-d6): δ ppm: 140.70, 131.40, 131.02, 78.72, 76.43, 45.38 ( Supporting Information Figure S1). Infrared (IR; KBr, cm−1): 3469, 3357, 3290, 3013, 2133, 1641, 1513, 1483, 1470, 1424, 1409, 1372, 1353, 1318, 1234, 1190, 1064, 973, 951, 888, 832, 788, 760, 749, 705, 651. High-pressure measurement The symmetric diamond anvil cell (DAC) with type-II diamonds was applied to generate high pressure. The culet size of DAC was about 400 μm. The T301 stainless steel gasket was compressed to about 50 μm, and the sample chamber diameter was about 150 μm. Pressure was recorded by ruby luminescence technique. High-pressure ultravisible absorption measurements were performed by a UV–vis absorption system with a xenon light source from 320 to 1600 nm, and silicone oil was used as the pressure-transmitting medium (PTM). A camera equipped with a microscope was utilized to get the micrographs of the sample in DAC at various pressures. Several small pieces of monomer PIP-1 crystal together with a ruby ball were placed in the sample chamber. In situ high-pressure angle-dispersive X-ray diffraction (ADXRD) experiments with a wavelength of 0.6199 Å were performed at 15U1 beamline, Shanghai Synchrotron Radiation Facility (SSRF). The XRD patterns were collected using a Mar-345 charge-coupled device (CCD) detector and then were integrated using the DIOPTAS program. In situ high-pressure Raman measurements were collected using a Horiba LabRAM HR Evolution (HORIBA France SAS, Palaiseau France) with a laser of 633 nm as the excitation source (output power 17 mW). The Raman spectroscopic measurements were carried out with silicon oil with and without PTM. The scattering signals were recorded using a liquid nitrogen-cooled CCD camera. IR microspectroscopy of PIP-1 in the crystalline state was carried out with a Bruker Vertex 70v FT-IR spectrometer (Bruker Scientific LLC, Billerica, MA, USA). KBr was used as PTM. All the experiments were performed at room temperature. High-pressure simulation Structure optimizations (at ambient pressure and under hydrostatic externally applied step-wise pressure conditions) and the band structure calculation (monomer and polymerized structure at ambient pressure) were carried out using DFT within the Perdew–Burke–Ernzerhof parametrization38 of generalized gradient approximation and DFT-D3 (BJ) van der Waals corrections39 as implemented in the Vienna Ab Initio Simulation Package (VASP) code.40 The optimized structure was used as the input geometry for the next optimization step. The all-electron projector augmented wave pseudopotentials was adopted. The energy cutoff 900 eV was chosen. Brillouin zone sampling was obtained using constant Monkhorst–Pack grids across the pressure series of 2 × 3 × 3. All structures were relaxed with the force convergence tolerance set to 0.01 eV/Å. Using these relaxed structures, density-functional perturbation theory calculations were performed with VASP to determine the IR frequencies and associated intensities. Results and Discussion Design, preparation, and crystal analysis of polymerizable monomer PIP-1 Topochemical polymerization is a typical solid-state reaction that heavily depends on the distance and direction of polymerizable units. Only when the polymerizable units were well preorganized at suitable proximity and in the required geometry could the expected topochemical polymerizations occur.36 Thus, the energetic monomer molecule should be finely designed at both the molecular and crystal levels. In general, the polymerizable units consist of some molecular fragments with unsaturated double or triple bonds, which are usually not energetic. In this case, however, their introduction will unavoidably lower the energy of energetic monomers. To obtain a relatively higher-energy monomer, the precursor molecule should better have a high-energy density. Here, we selected 3,5-dinitro-1H-pyrazol-4-amine (LLM-116)41 as energetic precursor mainly based on two reasons: (1) LLM-116 has a high density of 1.90 g cm−3 at 298 K and good detonation performances (detonation velocity is 8470 m s−1 and detonation pressure is 32.8 GPa, respectively); (2) the alternating nitro-amino-nitro arrangement can form intra/intermolecular hydrogen bonds, which facilitate the self-assembly of energetic moiety in the crystals of energetic monomer. The propargyl groups can be readily polymerized under pressure.28,32 Meanwhile, it is not easy for propargyl groups to generate hydrogen bonds with LLM-116 groups. Therefore, the propargyl group may possibly be packed together via van der Waals forces. The propargyl group was selected at the as polymerizable precursor and introduced by the substitution reaction between LLM-116 and propargyl bromide (detailed synthetic procedure in Supporting Information). Accordingly, the prototype of a polymerizable monomer (PIP-1) was prepared ( Supporting Information Table S1 and Figure S2). As expected, the monomer PIP-1 exhibited relatively high energy, whose room-temperature density (1.64 g cm−3) was comparable to that (1.65 g cm−3) of TNT, and its detonation performance (Dv: 7638 m s−1 and P: 22.36 GPa) was even better than that (Dv: 7303 m s−1 and P: 21.3 GPa) of TNT (Figure 2a). Figure 2 | (a) Molecular structure, synthetic route, and main properties of polymerizable monomer PIP-1, (b) molecular configuration, (c) 2D intralayer molecular arrangement of polymerizable monomer PIP-1 in crystal, and (d) 3D crystal packing of polymerizable monomer PIP-1. Download figure Download PowerPoint After slow solvent evaporation from a saturated ethyl acetate solution, we obtained the single crystal of energetic monomer PIP-1. The energetic LLM-116 fragment had a planar configuration, and the amino interacted with two adjacent nitro groups by intramolecular hydrogen bonds. The three carbon atoms in propargyl groups were linearly arranged, and the angle between the LLM-116 fragment and the propargyl group was 111.8° (Figure 2b). In the intralayer, two rows of LLM-116 fragments further assembled to a supramolecular plane by intermolecular hydrogen bonds. Meanwhile, the propargyl fragments were regularly arranged in an antiparallel and alternate mode by the C–H⋯O hydrogen bonds between propargyl and nitro groups (Figure 2c). Moreover, the van der Waals distance (Rv) of possible reactive carbon atoms between two propargyl groups was 4.065 Å (yellow arrow in Figure 2c), and the tilt angle (θ) of propargyl monomer rods was 66.9° (blue angle in Figure 2c). In the 3D crystal structure of Figure 2d, the monomer PIP-1 had a wavelike layered packing, with interlayer spacings of 3.100 Å (between LLM-116 fragment) and 2.999 Å (between propargyl fragment). By analyzing the crystal structure of PIP-1, we speculated that a topochemical polymerization would probably take place between propargyl groups to form the polymerized PIP-1 under high-pressure conditions. Polymerization of PIP-1 at high pressure On the molecular level, we combined energetic LLM-116 with polymerizable units to synthesize the monomer PIP-1. On the crystal level, PIP-1 was well packed so that its propargyl group could possibly be polymerized under compression. Once the polymerization occurred, the C≡C bond would break to generate the C=C bond. The changes of chemical bond would result in some significant variations of conjugated structure, and in that case the color of PIP-1 would also change. In situ high-pressure photographs were taken to record the color changes of monomer PIP-1 under compression, as depicted in Figure 3a. At ambient pressure, monomer PIP-1 presented as yellow transparent crystals. With the increase of pressure, the color changed from yellow to orange, then to red and finally to dark red. A gradual red-shift of the absorption edge was found from 1 atm to 15.9 GPa in the absorption measurement of PIP-1 as shown in Figure 3c. After total release of pressure, the absorption edge blue-shifted42–45 but did not return to the initial position (Figure 3d). PIP-1 still appeared as a dark red sample similar to that at 11.5 GPa. Considering the unsaturated chemical bonds in LLM-116 skeletons, which may also polymerize under high pressure, a comparison experiment was designed and carried out. We synthesized a new LLM-116 derivative (namely 3,5-dinitro-1-propyl-1H-pyrazol-4-amine, Supporting Information Table S2 and Figure S3), in which the propargyl group in PIP-1 was replaced by a propyl group (Figure 3b left). The color of propyl-substituted LLM-116 changed from light yellow to red from ambient pressure to 17.1 GPa (Figure 3b) and recovered to light yellow after pressure release. The optical photographs and in situ high-pressure Raman spectra ( Supporting Information Figure S4) indicated that propyl-substituted LLM-116 was not reactive upon compression. Therefore, the pressure-induced polymerization of PIP-1 was not caused by LLM-116 groups. From the color and absorption spectra variation of PIP-1, we speculated that monomer PIP-1 polymerized via the C≡C bond during compression, and the polymerized PIP-1 is stable at ambient pressure. Figure 3 | (a) Photographs of PIP-1 at elevated pressures, (b) photographs of 3,5-dinitro-1-propyl-1H-pyrazol-4-amine at elevated pressures, (c) absorption spectra with increased pressure, and (d) absorption spectra of PIP-1 before and after compression. Download figure Download PowerPoint In situ high-pressure ADXRD provided direct evidence for the structural variations, which also gave more information about the topochemical polymerization of PIP-1. ADXRD experiments were performed on PIP-1 in the pressure range from 0.5 to 16.6 GPa as depicted in Figure 4a. PIP-1 remained the initial structure with the space group of P21/c until 11.3 GPa. When the pressure reached 11.3 GPa, a shoulder peak (marked with an asterisk) next to the (11-1) peak appeared at 2θ = 7.45°, and its intensity gradually increased up to 16.6 GPa, indicating that the polymerization of monomer PIP had occurred. Upon further compression, broad diffraction peaks were detected, indicating that the crystallinity of PIP-1 was becoming worse.46,47 Upon total release of pressure, the newly generated diffraction patterns with a d-spacing of 5.2 Å still remained. The released ADXRD patterns demonstrated that the polymerization of PIP-1 was irreversible. The a-, b-, and c-axes were compressed, and the angle of β decreased as shown in Figure 4b. The relative lattice parameters of monomer PIP-1 are summarized in Figure 4c, in which the compressibility of a-, b-, and c-axes only exhibited a little difference. Indexing the ADXRD patterns, the unit cell volume and lattice parameters can be obtained. The P–V relationship of PIP-1 was then fitted by the 3rd-order Birch-Murnaghan equation of state with V0 = 869.7(0) Å3 and B0 = 12.2(5) GPa (Figure 4d, details can be found in Supporting Information). Figure 4 | (a) ADXRD patterns of PIP-1 upon compression and decompression. The peak from polymerized PIP-1 is marked with asterisk. (b) Corresponding lattice parameters of monomer PIP-1 from 0.5 to 11.3 GPa. (c) Relative lattice parameters of PIP-1 from 0.5 to 11.3 GPa. (d) Unit cell volume from 0.5 to 11.3 GPa fitted by 3rd-order Birch-Murnaghan equation of state. Download figure Download PowerPoint In situ high-pressure Raman and IR spectroscopy were carried out to reveal the polymerization of propargyl fragments (assignments can be found in Supporting Information Figure S5 and Table S3). With the increase of pressure, all of the vibration modes blue-shifted, and no new peaks appeared (Figure 5a), indicating that the covalent bond shrank, and the phase transition did not occur during our compression to 10.1 GPa. In addition, the background intensity gradually increased, and the peaks broadened. At 11.1 GPa, all the peaks (including the C≡C stretching mode at 2134 cm−1) disappeared, which was caused by the polymerization of monomer PIP-1. Released to ambient pressure, only the strong peaks at 790 and 831 cm−1 were observed. The C≡C stretching mode of propargyl groups disappeared (Figure 5b). In situ IR spectra of PIP-1 were measured up to 19 GPa, as depicted in Figure 5c. Like the Raman spectra, the IR spectra did not exhibit apparent changes below 11.4 GPa. The peaks broadened, and a new broadband, which can be assigned to the CH stretching mode, appeared at 3277 cm−1 and gradually increased in intensity. Comparing the IR spectra of monomer PIP-1 at ambient pressure and recovered sample from 19.9 GPa, the bands at 1282 and 1687 cm−1, assigned to C–C stretching mode and C=C stretching mode respectively, existed in the released IR spectra as depicted in Figure 5d.28,32 And other bands did not exhibit obvious variation, indicating that other covalent varied little in the compression. The Raman and IR spectra revealed that the C≡C bonds of monomer PIP-1 broke, and new C=C bonds formed, thereby generating the polymerized PIP-1. Figure 5 | (a) In situ high-pressure Raman spectra of PIP-1 in the pressure range of 0.3 to 15.8 GPa. (b) In situ Raman spectra of PIP-1 before and after compression to 15.8 GPa. (c) In situ high-pressure IR spectra of PIP-1 in the pressure range of 0.8 to 19.0 GPa. (d) In situ IR spectra of PIP-1 before and after compression to 19.0 GPa. Download figure Download PowerPoint In situ high-pressure absorption spectra, synchrotron X-ray diffraction, Raman spectra, and infrared spectra were carried out to measure the polymerization of PIP-1. Absorption measurements and optical photographs recorded the color variation under pressure, which suggested that monomer PIP-1 polymerized and the polymerized structure remained stable upon total release of pressure. Synchrotron X-ray diffraction, the Raman spectra, and the infrared spectra further revealed the polymerization of PIP-1. When the pressure reached 11.3 GPa, polymerization of PIP-1 occurred and generated new structure. The polymerization was caused by the breakage of C≡C bonds and the formation of C=C bonds. High-pressure polymerization simulation of PIP-1 It was extremely hard to perform further structural characterizations on PIP-1 to limit the sample amount synthesized with DAC.48,49 High-pressure DFT calculations were employed to obtain more information about structural variations and the polymerization reaction. Without considering the effect of temperature and time, this calculation could not provide the exact reaction pressure but did estimate how the polymerization of monomer PIP-1 happened.50 With a step of 10 GPa, we calculated the unpolymerized and polymerized structures of PIP-1 from 0 to 100 GPa during compression and decompression optimization procedures. Starting from the crystal structure at 0 GPa, the compression optimizations only yielded unpolymerized structures at each step until 90 GPa. When the pressure reached 100 GPa, a polymerized structure generated spontaneously. The polymerized crystalline structure fit our experimental measurements well. The C≡C bond of propargyl groups broke, and monomer PIP-1 were connected via the new generated C=C bonds to form a polymer tape of PIP-1 as depicted in Figure 6a. In addition, the polymerized PIP-1 tapes were packed via weak intermolecular interactions. There were no covalent bonds between different tapes. Figure 6 | (a) The enthalpy of the monomer and polymerized structures, (b) crystal structure of PIP-1 optimized by DFT calculation at threshold pressure (11.3 GPa), (c) calculated band structure of monomer PIP-1 at ambient pressure, (d) DOS of monomer PIP-1, (e) calculated band structure of polymerized PIP-1 released to ambient pressure, and (f) DOS of polymerized PIP-1. Download figure Download PowerPoint After the compression calculations, this polymeric structure was used as the starting point for the decompression optimization run. From 100 down to 0 GPa, the polymeric structure did not recover to the initial monomer but remained stable under ambient conditions. Tracking the enthalpy with variable pressures (Figure 6a), enthalpy values of the unpolymerized and polymerized structures were both below zero. In addition, the enthalpy of the polymerized product was a little lower than PIP-1 monomer, indicating that the polymerized PIP-1 was more stable when the pressure totally released. The unit cell volume of unpolymerized structure optimized at 0 GPa was 852.52 Å3, very close to the experimental value of 853.42 Å3. The unit cell volume of polymerized PIP-1 was 819.65 Å3 (ρ: 1.72 g cm−3) as shown in Supporting Information Table S4. Comparing the unit cell volume of two structures, polymerized PIP-1 exhibited a density increase of 4.9% over the monomer PIP-1. The detonation properties were also increased to some extent ( Supporting Information Table S5). These DFT calculations strongly suggest that monomer PIP-1 polymerizes at high pressure. Once the polymerization occurs, the polymerized structure will be dynamically stable and will not revert to the original phase upon total release of pressure. Meanwhile, the polymerized PIP-1 has a higher density, so it was expected to have better detonation performance. To gain valuable insight into the reactive bond at threshold pressure, we also optimized the crystal structure of PIP-1 at 11.3 GPa by DFT calculations. Fixed by the ADXRD experiment, the lattice parameters at 11.3 GPa are a = 14.25 Å, b = 6.84 Å, c = 6.71 Å, and β = 100.23°. The nearest C⋯C distance between the polymerizable propargyl groups in a 1D PIP-1 tape (shown in Figure 6b) is 3.305 Å, a little larger than the previously reported polymerization of 1,4-diphenylbutadiyne32 or acetylene28 by Li et al. Besides, other C⋯C distance beyond the 1D PIP-1 tape was smaller than 3.305 Å, as depicted in Supporting Information Figure S6. However, with respect to the influence of hydrogen bonds, the propargyl group of different 1D PIP-1 tape could not polymerize in the DFT calculations (depicted in Supporting Information Table S6). Therefore, it was the most suitable reaction route for the monomer PIP-1 to polymerize to generate 1D PIP-1 tapes. Considering the experimental results of other pressure-induced polymerization studies, the 1D PIP-1 tapes may be oligomers.30 During the p" @default.
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