Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Unveiling a Masked Polymer of Dewar Benzene Reveals transPoly(acetylene) Jinwon Seo,†,‡ Stanfield Y. Lee,† and Christopher W. Bielawski*,†,‡,§ †
Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea Department of Chemistry and §Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
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‡
S Supporting Information *
ABSTRACT: A dibromo derivative of Dewar benzene, trans5,6-dibromobicyclo[2.2.0]hex-1-ene, was polymerized using ring-opening metathesis polymerization (ROMP). The reaction proceeded in a controlled manner as changing the initial monomer-to-catalyst ratio afforded monodispersed polymers with tunable molecular weights and growing polymer chains were extended upon subsequent exposure to additional monomer. Treatment of the halogenated polymers with an alkyllithium reagent resulted in elimination followed by isomerization to afford trans-poly(acetylene). Based on a series of mechanistic and spectroscopic studies, the transformation was proposed to proceed through a cyclobutenyl intermediate that undergoes rearrangement. The methodology was found to be versatile as triblock copolymers containing the halogenated homopolymer were prepared and converted to their poly(acetylene)-containing derivatives. The polymers were characterized using gel permeation chromatography as well as a range of spectroscopic (NMR, FT-IR, UV−vis, and Raman) and analytical techniques.
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INTRODUCTION Because of their potential uses in contemporary electronic devices, including light-emitting diodes,1,2 solid-state lasers,3 photovoltaics,4,5 and chemical sensors,6,7 polymers containing π-conjugated backbones have attracted significant attention.8−13 The prototypical example is poly(acetylene), which was synthesized from its constituent monomer by Natta in the 1950s.14 The polymer is comprised only of carbon and hydrogen and represents one of the first organic materials shown by Shirakawa, MacDiarmid, and Heeger to exhibit markedly enhanced electrical conductivities upon exposure to halogen vapor.15−17 Because poly(acetylene) contains alkenes along the backbone, it can exist in multiple isomeric forms,18 each of which displays a different set of properties. For example, cis- and trans-poly(acetylene) exhibit unique decomposition temperatures (cf. 467 vs 412 °C, respectively), glass transition temperatures (357 vs 337 °C), oxidation potentials (−5.49 vs −5.19 eV), and infrared signatures (ν = 740 vs 1015 cm−1).19 Moreover, trans-poly(acetylene) has been reported to display a higher electrical conductivity value than its cis isomer (10−5 vs 10−9 Ω−1 cm−1)20 as well as a higher polarizability due to the differential projection lengths of the corresponding repeating units (2.425 Å vs 2.125 Å).21 As such, trans-poly(acetylene) features characteristics that may be relatively attractive for use in certain electronic applications. Aside from the intrinsic differences between its geometric isomers, the synthesis of well-defined forms of poly(acetylene) remains challenging. The reasons for the limitation are © XXXX American Chemical Society
multifold: (1) poly(acetylene) typically becomes insoluble when the chain length exceeds 10 repeat units,22 and (2) catalysts that enable the direct polymerization of acetylene in a manner that is controlled and facilitates access to high polymer remain elusive.23−26 Moreover, few methods are available for accessing stereopure cis- or trans-poly(acetylene), and relatively sophisticated conditions are often required.20,27 For example, Shibahara reported that the latter is obtained by polymerizing pressurized acetylene (ca. 1.0 kPa) on a liquid crystalline substrate under a magnetic field.28 Exposure of stereorandom poly(acetylene) to elevated temperatures or isomerization catalysts over extended periods of time also facilitates access to derivatives with high contents of trans repeat units; however, side reactions (e.g., cross-linking) are often observed.29,30 To address the aforementioned challenges, a number of indirect methods to access poly(acetylene) have been developed (see Scheme 1). Feast demonstrated that 7,8bis(trifluoromethyl)tricyclo[4,2,2,02,5]deca-3,7,9-triene, also known as the “Durham precursor”,31−33 can be polymerized using ring-opening metathesis polymerization (ROMP) and then transformed to poly(acetylene) via a retro [4 + 2] cycloaddition.34,35 Grubbs also reported different methods for accessing poly(acetylene) through the ROMP of cycloReceived: January 1, 2019 Revised: March 10, 2019
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DOI: 10.1021/acs.macromol.8b02754 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Selected Examples of Routes That Have Been Used To Synthesize Poly(acetylene)
Scheme 2. Synthesis of trans-5,6-Dibromobicyclo[2.2.0]hex-1-ene (1)
octatetraene (COT)19 or benzvalene.29 Although inclusion of chain transfer agents in the former facilitated access to telechelic derivatives,36 yields of 90%. (b) Representative gel permeation chromatograms recorded for poly(1) (blue) and its chain extended polymer (red). Conditions and data: [1]0/[Ru]0 = 21, CH2Cl2; Mn = 3.0 kDa, Đ = 1.25 (blue); additional 1 was added after 30 min: ([1]0 + [1]1)/[Ru]0 = 42, CH2Cl2, Mn = 7.4 kDa, Đ = 1.13 (red).
Scheme 3. Synthesis of trans-Poly(acetylene) through a Polymerization−Elimination−Isomerization Sequence
polymer to that of the monomer. For example, the signals attributed to the cyclobutenyl ring of the monomer (δ 6.4 ppm; CDCl3) shifted upfield and became broad (5.3−6.1 ppm; CDCl3) upon polymerization (see Figure S24). To determine whether the polymerization of 1 proceeded via a controlled, chain-growth mechanism, an iterative series of experiments were performed. As summarized in Figure 1a, a linear relationship between the initial monomer-to-catalyst feed ratio and the measured molecular weights of the polymers produced was observed. Moreover, as shown in Figure 1b, increases in molecular weights were seen when additional monomer was introduced to the reaction mixture after consumption of the initial monomer feed, consistent with chain extension. Collectively, these results suggested to us that the polymerization methodology was controlled and afforded well-defined polymers with tunable molecular weights. As summarized in Scheme 3, subsequent efforts were directed toward eliminating the vicinal dibromides from the aforementioned polymers.44−46 Adding methyllithium dissolved in diethyl ether to a THF solution of poly(1) (2.0 equiv of CH3Li per polymer repeat unit; [1]0 = 0.05 M) at −78 °C resulted in a color change from pale brown to red as the mixture warmed to room temperature.47 The color of the solution darkened over time, and an insoluble black material was observed after 30 min. In addition, the formation of CH3Br, an expected elimination byproduct, was observed by mass spectrometry as well as 1H NMR spectroscopy (see Figure S15).48 After quenching the reaction through the dropwise addition of methanol, the precipitate was collected via filtration, washed with methanol, and then dried under vacuum to afford 9 in >99% yield. Conducting the reaction in the dark and under an atmosphere of nitrogen afforded a quantitative yield, presumably due to minimization of photochemical side reactions. The product of the aforementioned reaction (9) was characterized using a range of spectroscopic techniques. For
recrystallization from aqueous sulfuric acid and isolated in 74% yield. Dehydration afforded the corresponding anhydride 4, which was purified by sublimation and collected in 93% yield as a white, crystalline product. Subsequent photochemical isomerization was conducted in diethyl ether at room temperature and monitored over time using 1H NMR spectroscopy. Removal of the residual solvent after the reaction was determined to be complete (ca. 27 h) afforded crude 5. While it was previously reported that the anhydride can be purified via sublimation,40 the technique was inefficient in our hands due to the presence of unidentified byproducts that formed during the photoisomerization reaction. Likewise, other purification techniques, including column chromatography and extraction, were also unsuccessful. To overcome these challenges, 5 was first esterified with methanol in the presence of a catalytic amount of acid to afford 6, which facilitated purification. Subsequent hydrolysis of the purified product resulted in the desired bis(acid) 7. The addition of bromine to a suspension of 7 in CH2Cl2 afforded the vicinal dibromide 8 as a white solid in 97% yield. Finally, electrolytic decarboxylation followed by column chromatography afforded 1 as a colorless liquid.42,43 With monomer 1 in hand, polymerization efforts commenced using the third-generation Grubbs catalyst (G3). A CH2Cl2 solution of the monomer ([1]0 = 0.21 M) was treated with the catalyst ([1]0/[G3]0 = 42) for 30 min at ambient temperature followed by excess ethyl vinyl ether to quench the reaction. The mixture was then poured into cold hexane which resulted in the precipitation of a solid that was collected in 90% yield based on the assumed structure for poly(1). Gel permeation chromatography (GPC) revealed that the material produced was monodispersed (Đ = 1.14) and exhibited a number-average molecular weight (Mn) that was in relatively good agreement (16.4 kDa) with the theoretical value (15.0 kDa). Further support for a successful polymerization was obtained by inspecting the 1H NMR spectrum recorded for the C
DOI: 10.1021/acs.macromol.8b02754 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. Raman spectra recorded for (a) 9 and (b) the polymer obtained from the ROMP of COT. Infrared spectra recorded for (c) 9 and (d) the polymer obtained from the ROMP of COT. (e) A solid-state CP-MAS 13C NMR spectrum recorded for 9. Note: the asterisks (∗) denote spinning side bands. (f) A picture of a film of 9 (1.8 cm × 1.8 cm × 13 μm) as prepared on a glass substrate prior to exposure to iodine.
Scheme 4. Proposed Isomerization Pathwaysa
a Starting with 10, the pathway to the left involves intermediates that undergo thermally induced, electrocyclic ring-opening and therefore should afford a mixture of polymers that feature cis- and trans-alkenes. The pathway to the right involves radical intermediates that isomerize to transpoly(acetylene) (9).
comparison purposes, prepared from COT literature.19 As shown spectra recorded for methods were similar
the presence of C−C (ca. 1117 cm−1) and CC (ca. 1507 cm−1) bonds. IR spectroscopy has been used to distinguish cisand trans-poly(acetylene) since the respective isomers exhibit distinctive C−H out-of-plane vibrational modes.28 As shown in Figure 2c, a signal at 999 cm−1, which was attributed to the C−
poly(acetylene) was independently using a method reported in the in Figures 2a and 2b, the Raman the polymers prepared using both and featured signals consistent with D
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Macromolecules Scheme 5. Synthetic Methodology Used To Prepare Different Triblock Copolymers
then warmed to room temperature.58 At the conclusion of the reaction, a black precipitate was collected and found to display spectroscopic signatures that were similar to the transpoly(acetylene) described above (see Figure S3). Moreover, the trap was not detected in the product via elemental analysis or other analytical techniques. Collectively, these results indicated that the isomerization may have proceeded through radical intermediates that rapidly isomerize to the thermodynamically favored, conjugated polymer product before reacting with a trap. Because the poly(acetylene) described above was found to be insoluble, subsequent efforts were directed toward capitalizing on the advantages of the polymerization methodology to access soluble derivatives. On the basis of previous reports,59,60 we hypothesized that connecting a poly(acetylene) block to nonconjugated segments would render the entire copolymer processable. exo,exo-5,6-Bis(methoxymethyl)bicyclo[2.2.1]hept-2-ene (11)61 was chosen as a comonomer due to its high solubility in common organic solvents, lack of acidic functional groups, and high ring strain. The polymerization of 11 was found to proceed in a controlled manner62 and afforded monodispersed products with molecular weights that agreed with their theoretical values (see Figure S9). As summarized in Scheme 5, a triblock copolymer derived from 11 and 1 was prepared and then explored as a precursor to poly(acetylene). Exposure of a CH2Cl2 solution of 11 to G3 ([11]0/[Ru]0 = 63) followed by stirring at −15 °C initiated a polymerization reaction. To monitor the formation of polymer over time, an aliquot was removed from the solution after 15 min and poured into a mixture of CH3OH and ethyl vinyl ether to quench the reaction. The precipitate that formed was collected and analyzed by GPC (Mn = 19.3 kDa, Đ = 1.08). A CH2Cl2 solution of 1 was then introduced to the residual catalyst solution (([11]0 + [1]0)/[Ru]0 = 126). After stirring for an additional 15 min, an aliquot was removed and worked up as described above (Mn = 32.3 kDa, Đ = 1.10). Finally, additional 11 (([11]0 + [1]0 + [11]1)/[Ru]0 = 189) was added. After quenching the reaction, triblock copolymer 12 was isolated in 73% overall yield. GPC analysis revealed that the material exhibited a Mn of 51.4 kDa (Đ = 1.18) (cf. the theoretical Mn value of the copolymer based on quantitative monomer conversions was 38.0 kDa), and the number of
H stretching frequency of trans-poly(acetylene), was measured for 9. For comparison, signals at 746 and 931−992 cm−1 were recorded upon analyzing the poly(acetylene) obtained from the ROMP of COT (see Figure 2d) and assigned to the cis and trans isomers, respectively, in accord with literature reports.19 Likewise, a single signal (δ 135 ppm) was observed upon analyzing 9 using solid-state cross-polarization magic angle spinning (CP-MAS) 13C NMR spectroscopy (see Figure 2e). 13 C NMR signals assigned to trans-alkenes in poly(acetylene) have been reported (136−139 ppm) and can be distinguished from their corresponding geometric isomers (126−129 ppm).27 Electrical conductivities were measured (10−4 Ω−1 cm−1) by applying contacts to films of 9 (see Figure 2f) and found to be in agreement with values reported for transpoly(acetylene).20 The conductivity values increased (30 Ω−1 cm−1) upon exposure of the polymer films to iodine vapor. Collectively, the spectroscopic data indicated that transpoly(acetylene) was obtained as the only detectable product from the methodology described above, which was a surprising result and prompted us to postulate potential mechanisms (see Scheme 4). Although previous reports have shown that certain cyclobutenes and related polyladderenes can undergo selective electrocyclic ring-opening,49−52 mixtures of cis and trans products may result when the precursors feature assorted stereoisomers.53,54 Because the spectroscopic data recorded for 9 indicated that the material was composed of a single stereoisomer, an alternative route wherein ring-opening proceeds through a radical intermediate that isomerizes to the thermodynamically favored (trans) polymer product was considered. To probe the underlying mechanism, efforts were first directed toward identifying potential intermediates. Precursor poly(1) was subjected to a solution of CH3Li in THF-d8 at −50 °C and then monitored by 1H NMR spectroscopy. Signals assigned to the hydrogens bonded to the brominated carbons (δ 4.4−4.8 ppm) disappeared over time and were accompanied by the formation of new signals that were consistent with those expected from the cyclobutenyl moieties in 10 (6.0−6.2 ppm) (see Figure S13).55,56 Next, the isomerization reaction was conducted in the presence of a radical trap.57 A THF-d8 solution of poly(1) was charged with CH3Li (2.0 equiv per repeat unit) at −50 °C followed by (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) (3.0 equiv per repeat unit) and E
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used as a light source. Electrolysis was conducted using a GW Instek GPR-11H300 instrument and platinum plates (4 cm × 4 cm, 0.2 mm). Solvents were dried and degassed using a Vac Atmospheres solvent purification system. 1H NMR and 13C NMR spectra were recorded in acetone-d6 (1H: 2.05 ppm; 13C: 29.8 ppm), THF-d8 (1H: 3.58 ppm), methylene chloride-d2 (1H: 5.32 ppm; 13C: 54.0 ppm), or chloroform-d1 (1H: 7.26 ppm; 13C: 77.2 ppm) using Bruker 400 and 100 MHz spectrometers, respectively. Solid-state CP-MAS 13C NMR spectra were recorded using a Bruker 125 MHz spectrometer. Coupling constants (J) are expressed in hertz (Hz). Splitting patterns are indicated as follows: br, broad; bd, broad doublet; bt, broad triplet; bm, broad multiplet; s, singlet; d, doublet; t, triplet; m, multiplet. High-resolution mass spectra (HR-MS) were obtained from a JMS-T100LP AccuTOF LC-plus 4G atmospheric pressure ionization high resolution time-of-flight mass spectrometer (Waters Inc.) Gel permeation chromatography (GPC) was performed on a Malvern GPCmax solvent/sample module. THF was used as the eluent at a flow rate of 0.8 mL min−1. Infrared (IR) spectra were recorded on an Agilent Cary-630 FT-IR spectrometer. Raman spectra were recorded on a WITec alpha 300M confocal Raman microscope. Thermogravimetric analyses (TGA) were performed on a Thermal Advantages (TA) Q500 at a heating rate of 10 °C min−1 under an atmosphere of nitrogen. Differential scanning calorimetry (DSC) was performed under an atmosphere of nitrogen on a Thermal Advantages Q2000 at a heating or cooling rate of 20 °C min−1. Electrochemical measurements were conducted using a rotating ring−disk electrode (Pine Research Instrumentation, Durham, NC) that was connected to an Echochemie Inc. PGSTAT302N bipotentiostat running the NOVA version 1.1 software program (Metrohm Autolab, Utrecht, Netherlands). Elemental analyses were performed using a ThermoScientific Flash 2000 organic elemental analyzer that was calibrated with 2,5bis(5-tert-butylbenzoxazol-2-yl)thiophene. Conductivity values were acquired on an Advanced Instrument Technology CMT-SR200N sheet resistance/resistivity measurement system. Melting points were determined with a MPA 100 Optimelt automated melting point system and are uncorrected. Atomic force microscopy measurements were conducted with a Multimode 8 nanoscope V (Bruker). X-ray photoelectron spectroscopy experiments were performed using an Escalab 250Xi (Thermo Fisher Scientific, Waltham, MA) equipped with a monochromated aluminum Kα source (1486.6 eV). UV−vis spectroscopy data were recorded on an Agilent Cary 100 UV−vis spectrometer outfitted with a Peltier multicell temperature controller. Synthesis of Poly(1). 5,6-Dibromobicyclo[2.2.0]hex-1-ene (0.10 g, 0.42 mmol) and CH2Cl2 (1.5 mL) were added to an air-free reaction flask (10 mL) at room temperature under a positive flow of nitrogen. A solution of the G3 dissolved in anhydrous CH2Cl2 (4.4 mg in 0.5 mL) was injected, and the mixture was gently stirred for 30 min at room temperature. The reaction was quenched upon the addition of ethyl vinyl ether (0.1 mL), and the resulting mixture was poured into cold hexane (ca. 100 mL). A dark pink precipitate formed and was subsequently collected in 90% yield. 1H NMR (400 MHz, CDCl3): δ 5.30−6.10 (br, 2H), 4.25−4.75 (br, 2H), 3.21−4.0 (br, 2H). Synthesis of Poly(11). exo,exo-5,6-Bis(methoxymethyl)bicyclo[2.2.1]hept-2-ene (0.20 g, 1.09 mmol) and CH2Cl2 (1.6 mL) were added to an air-free reaction flask (10 mL) at room temperature under a positive flow of nitrogen. A solution of G3 dissolved in anhydrous CH2Cl2 (4.4 mg of catalyst in 0.5 mL of solvent) was injected, and the mixture was gently stirred for 30 min at room temperature. The polymerization was then quenched upon the addition of ethyl vinyl ether (0.1 mL), and the resulting mixture was poured into cold methanol (ca. 100 mL) to induce precipitation. The product was collected and dried under vacuum (83% yield). 1H NMR (400 MHz, CDCl3): δ 5.10−5.46 (br, 2H), 3.22−3.65 (br, 10H), 1.78−2.80 (br, 5H), 1.01−1.37 (br, 1H). Preparation of trans-Poly(acetylene) (9). A solution of methyllithium (1.6 M in diethyl ether) was added dropwise (0.53 mL total volume) to a THF solution containing poly(1) (0.10 g polymer/8.3 mL THF) at room temperature. The formation of an insoluble black material was observed after 30 min. The material was
repeat units in the segment of poly(1) was calculated to be 55 from its deduced Mn value (13.0 kDa).63 Adding methyllithium to a THF solution of the triblock copolymer 12 (2 equiv of CH3Li per polymer repeat unit) at room temperature resulted in the formation of a dark red color, consistent with the formation of 13 followed by isomerization to 14.64,65 Pouring the treated solution into cold methanol afforded a precipitate, which was collected and characterized. 1 H NMR spectroscopic analysis of the product revealed that signals assigned to the halogenated methines found in the segment of poly(1) disappeared and a new signal (δ 6.0−7.0 ppm; THF-d8), consistent with the repeat unit of poly(acetylene), had formed. A solution of 14 in THF exhibited a λmax value near 500 nm and an irreversible oxidation at 0.5 V (vs ferrocene in CH3CN), in agreement with values expected for poly(acetylene) (see Figures S1 and S2).66 Data recorded in the solid state were also in agreement with the structural assignment. For example, 14 exhibited Raman signals at 1110 and 1492 cm−1 as well as an IR absorption band at 1012 cm−1. Likewise, a 13C NMR signal (δ 134 ppm) consistent with that expected for trans-poly(acetylene) was measured (see Figure S27).67 Although iodine doping was found to significantly enhance the conductivity displayed by a film of the copolymer by more than 2 orders of magnitude (5 × 10−3 vs 2 × 10−5 Ω−1 cm−1), the change was less significant than that observed with poly(acetylene), presumably due to the presence of the insulating segments of poly(11). Regardless, these data suggested to us that the aforementioned methodology afforded well-defined triblock copolymers wherein poly(norbornene)based homopolymers were connected to a poly(acetylene) core.68
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CONCLUSIONS Ring-opening metathesis polymerization (ROMP) of a dihalogenated derivative of Dewar benzene led to the formation of a poly(acetylene) precursor in a controlled manner. Subjecting the polymer to an alkyllithium agent resulted in elimination to form an intermediate that was consistent with a ring-opened, polymeric form of Dewar benzene. Rapid isomerization ensued and afforded transpoly(acetylene), as determined using a range of spectroscopic and analytical techniques as well as through comparison with independently prepared samples. The methodology was also successfully adapted to prepare triblock copolymers containing segments of poly(acetylene). The copolymers were found to be soluble in common solvents and formed films that exhibited increased electrical conductivities upon doping. The method described herein is an effective tool for preparing poly(acetylene) with a high trans-olefin content and well-defined copolymers thereof and thus can be expected to find utility in the growing number of applications that require the use of conjugated polymeric materials.
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EXPERIMENTAL SECTION
General Methodology. Unless otherwise noted, all manipulations were performed under an atmosphere of nitrogen using standard Schlenk techniques. Phthalic acid, sodium acetate, and acetic anhydride were purchased from Sigma-Aldrich. Bromine was purchased from Junsei Chemical. Hydrochloric acid (35 wt %) was purchased from Daejung Chemicals & Metals. cis-5-Norbornene-exo2,3-dicarboxylic anhydride was purchased from Alfa Aesar. A sodium mercury amalgam was prepared according to literature procedures.69 An Ace photochemical reactor (medium pressure Hg, 450 W) was F
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Macromolecules subsequently collected and dried under reduced pressure (quantitative yield). Film Preparation. A solution of poly(1) (prepared by mixing 5.0 mg of polymer and 1 mL of THF) was drop-casted on a glass substrate (1.8 cm × 1.8 cm). A solution of CH3Li (1.6 M in diethyl ether) was added dropwise (26 μL total volume), and the resulting mixture was agitated for 10 s using a glass rod. The resulting film was washed with absolute ethanol and then dried under nitrogen for 24 h at room temperature. Iodine Doping. Iodine (10 g) was placed at the bottom of a chamber subjacent to a glass substrate supporting a polymer film. Evacuation of the chamber (0.05 Torr) facilitated iodine sublimation at room temperature. After 17 h, the chamber was entombed in a glovebag filled with nitrogen, and the conductivity of the sample was measured. Triblock Copolymer Preparation. Monomer 11 was dissolved in anhydrous CH2Cl2 (0.8 mL). Subsequent injection of predetermined quantities of G3 dissolved in 0.6 mL of anhydrous CH2Cl2 followed by stirring at −15 °C for 15 min facilitated the consumption of the monomer. A solution of monomer 1 dissolved in 0.8 mL of anhydrous CH2Cl2 was then injected, and the mixture was stirred for an additional 15 min. Similarly, a solution of monomer 11 dissolved in 0.8 mL of anhydrous CH2Cl2 was added, and the resulting solution was stirred for 15 min. Excess ethyl vinyl ether was added to quench the reaction, and then the resulting mixture was poured into cold methanol to induce precipitation. The product was collected via filtration and dried under reduced pressure.
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(2) Dai, L. M.; Winkler, B.; Dong, L. M.; Tong, L.; Mau, A. W. H. Conjugated Polymers for Light-Emitting Applications. Adv. Mater. 2001, 13, 915−925. (3) McGehee, M. D.; Heeger, A. J. Semiconducting (Conjugated) Polymers as Materials for Solid State Lasers. Adv. Mater. 2000, 12, 1655−1668. (4) Coakley, K. M.; McGehee, M. D. Conjugated Polymer Photovoltaic Cells. Chem. Mater. 2004, 16, 4533−4542. (5) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258, 1474−1476. (6) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Conjugated Polymer-Based Chemical Sensors. Chem. Rev. 2000, 100, 2537−2574. (7) Janata, J.; Josowicz, M. Conducting Polymers in Electronic Chemical Sensors. Nat. Mater. 2003, 2, 19−24. (8) Smela, E. Conjugated Polymer Actuators for Biomedical Applications. Adv. Mater. 2003, 15, 481−494. (9) Petit, P.; Bernard, M.; Andre, J. J.; Reibel, D.; Mathis, C. Sample Dependence of the Magnetic Behavior of Polyacetylene. A Pulsed ESR Comparative Study. Synth. Met. 1993, 54, 67−72. (10) Fu, D.-K.; Xu, B.; Swager, T. M. Alternating Poly(pyridyl vinylene phenylene vinylene)s: Synthesis and Solid State Organizations. Tetrahedron 1997, 53, 15487−15494. (11) Lin, J. W. P.; Dudek, L. P. Synthesis and Properties of Poly(2,5thienylene). J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 2869−2873. (12) Yamamoto, T.; Sanechika, K.; Yamamoto, A. Preparation of Thermostable and Electric-Conducting Poly(2,5-thienylene). J. Polym. Sci., Polym. Lett. Ed. 1980, 18, 9−12. (13) Walatka, V. V.; Labes, M. M.; Perlstein, J. H. Polysulfur NitrideA One-Dimensional Chain with a Metallic Ground State. Phys. Rev. Lett. 1973, 31, 1139−1142. (14) Natta, G.; Mazzanti, G.; Corradini, P. Polimerizzazione Stereospecifica Dell’ Acetilene. Atti. Accad. Naz. Lin. 1958, 25, 3. (15) McNeill, R.; Siudak, R.; Wardlaw, J. H.; Weiss, D. E. Electronic Conduction in Polymers. I. The Chemical Structure of Polypyrrole. Aust. J. Chem. 1963, 16, 1056−1075. (16) Diaz, A. F.; Kanazawa, K. K.; Gardini, G. P. Electrochemical Polymerization of Pyrrole. J. Chem. Soc., Chem. Commun. 1979, 635− 636. (17) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. Synthesis of Electrically Conducting Organic Polymers: Halogen Derivatives of Polyacetylene, (CH)x. J. Chem. Soc., Chem. Commun. 1977, 578−580. (18) Lopyrev, V. A.; Myachina, G. F.; Shevaleyevskii, O. I.; Khidekel, M. L. Polyacetylene. Review. Polym. Sci. U. S. S. R. 1988, 30, 2151− 2173. (19) Klavetter, F. L.; Grubbs, R. H. Polycyclooctatetraene (Polyacetylene): Synthesis and Properties. J. Am. Chem. Soc. 1988, 110, 7807−7813. (20) MacDiarmid, A. G.; Heeger, A. J. Organic Metals and Semiconductors: The Chemistry of Polyacetylene, (CH)x and its Derivatives. Synth. Met. 1980, 1, 101−118. (21) Huang, Z. H.; Jelski, D. A.; Wang, R. S.; Xie, D. M.; Zhao, C. D.; Xia, X. F.; George, T. F. Polarizabilities of trans and cis Polyacetylene and Interactions Among Chains in Crystalline Polyacetylene. Can. J. Chem. 1992, 70, 372−376. (22) Knoll, K.; Schrock, R. R. Preparation of tert-Butyl-Capped Polyenes Containing up to 15 Double Bonds. J. Am. Chem. Soc. 1989, 111, 7989−8004. (23) Saxman, A. M.; Liepins, R.; Aldissi, M. Polyacetylene: Its Synthesis, Doping and Structure. Prog. Polym. Sci. 1985, 11, 57−89. (24) Schuehler, D. E.; Williams, J. E.; Sponsler, M. B. Polymerization of Acetylene with a Ruthenium Olefin Metathesis Catalyst. Macromolecules 2004, 37, 6255−6257. (25) Munardi, A.; Aznar, R.; Theophilou, N.; Sledz, J.; Schue, F.; Naarmann, H. Morphology of Polyacetylene Produced in the Presence of the Soluble Catalyst Ti(OnBu)4-n-BuLi. Eur. Polym. J. 1987, 23, 11−14.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02754. Additional synthetic details, additional electrochemical measurement details and data, UV−vis spectra, infrared and Raman spectra, an XPS survey spectrum, elemental analysis data, AFM data, DSC and TGA data, GPC data, isomerization kinetics details and data, and 1H and 13C NMR spectra (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]; Tel +82-52-217-2952. ORCID
Stanfield Y. Lee: 0000-0001-6955-2573 Christopher W. Bielawski: 0000-0002-0520-1982 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Institute for Basic Science (IBS-R019) and the BK21 Plus Program as funded by the Ministry of Education and the National Research Foundation of Korea are acknowledged for support. We thank Dr. Karel Goossens and Dr. Sibanarayan Tripathy for their assistance with the solid-state NMR measurements. We are grateful to Ms. Younghye Park for creating the graphical abstract.
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REFERENCES
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DOI: 10.1021/acs.macromol.8b02754 Macromolecules XXXX, XXX, XXX−XXX
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(49) Silva Lopez, C.; Nieto Faza, O.; de Lera, A. R. Electrocyclic Ring Opening of cis-Bicyclo[m.n.0]alkenes: The Anti-WoodwardHoffmann Quest. Chem. - Eur. J. 2007, 13, 5009−5017. (50) Wang, J.; Kouznetsova, T. B.; Niu, Z.; Rheingold, A. L.; Craig, S. L. Accelerating a Mechanically Driven anti-Woodward−Hoffmann Ring Opening with a Polymer Lever Arm Effect. J. Org. Chem. 2015, 80, 11895−11898. (51) Baldwin, J. E.; Gallagher, S. S.; Leber, P. A.; Raghavan, A. Thermal Disrotatory Electrocyclic Isomerization of cis-Bicyclo[4.2.0]oct-7-ene to cis,cis-1,3-Cyclooctadiene. Org. Lett. 2004, 6, 1457−1460. (52) Brauman, J. I.; Archie, W. C. Synthesis and Thermal Isomerization of cis-3,4-Diphenylcyclobutene. Tetrahedron 1971, 27, 1275−1280. (53) Winter, R. E. K. The Preparation and Isomerization of cis- and trans-3,4-Dimethylcyclobutene. Tetrahedron Lett. 1965, 6, 1207− 1212. (54) Dolbier, W. R.; Koroniak, H.; Houk, K. N.; Sheu, C. M. Electronic Control of Stereoselectivities of Electrocyclic Reactions of Cyclobutenes: A Triumph of Theory in the Prediction of Organic Reactions. Acc. Chem. Res. 1996, 29, 471−477. (55) Adding substoichiometric quantities of CH3Li (e.g., 10−20 mol % with respect to the polymer repeat unit) resulted in an increased absorption between 300 and 400 nm without a significant change in molecular weight. The result suggested to us that the elimination reaction proceeded without significant chain cleavage or cross-linking. (56) Although the 1H NMR signals assigned to the cis- and transalkenes in 10 overlapped, the two stereoisomers were determined to be present in nearly equal quantities through visual inspection. A cisto-trans ratio of 1.2 was calculated by deconvoluting the data. (57) Ingold, K. U.; Pratt, D. A. Advances in Radical-Trapping Antioxidant Chemistry in the 21st Century: A Kinetics and Mechanisms Perspective. Chem. Rev. 2014, 114, 9022−9046. (58) Adding a large excess of TEMPO (6.0 equiv/polymer repeat unit) to the reaction mixture afforded a minor product that gave signals consistent with cis-poly(acetylene). Using lower quantities of TEMPO (e.g., 1.5 or 3.0 equiv/polymer repeat unit) afforded transpoly(acetylene) as the only observable product. (59) Su, J. K.; Feist, J. D.; Yang, J.; Mercer, J. A. M.; Romaniuk, J. A. H.; Chen, Z.; Cegelski, L.; Burns, N. Z.; Xia, Y. Synthesis and Mechanochemical Activation of Ladderene-Norbornene Block Copolymers. J. Am. Chem. Soc. 2018, 140, 12388−12391. (60) Yoon, K. Y.; Lee, I. H.; Kim, K. O.; Jang, J.; Lee, E.; Choi, T. L. One-Pot in Situ Fabrication of Stable Nanocaterpillars Directly from Polyacetylene Diblock Copolymers Synthesized by Mild RingOpening Metathesis Polymerization. J. Am. Chem. Soc. 2012, 134, 14291−14294. (61) Goll, J. M.; Fillion, E. Tuning the Reactivity of Palladium Carbenes Derived from Diphenylketene. Organometallics 2008, 27, 3622−3625. (62) For an example of polymerizing endo-11 see: Lynn, D. M.; Kanaoka, S.; Grubbs, R. H. Living Ring-Opening Metathesis Polymerization in Aqueous Media Catalyzed by Well-Defined Ruthenium Carbene Complexes. J. Am. Chem. Soc. 1996, 118, 784− 790. (63) The triblock copolymers were also analyzed by AFM and DSC (see Figures S6 and S7). (64) Olefin formation was found to be in accord with the quantity of CH3Li added. For example, the absorption became more intense and the λmax value bathochromically shifted toward 500 nm as the quantity of CH3Li added to 12 increased (see Figure S2). Similar results were obtained from an 1H NMR experiment where the formation of polyacetylene was observed as a function of added reductant (see Figure S14). (65) The enhanced solubility of the block copolymers enabled an independent probe of the isomerization mechanisms proposed in Scheme 4. Brauman reported that the concerted ring-opening of cis3,4-diphenylcyclobutene requires an activation energy of 24.5 kcal/ mol (see ref 52 and references therein). Using similar methodology (see the Supporting Information), the activation energy for the H
DOI: 10.1021/acs.macromol.8b02754 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules isomerization of 13 to 14 was measured to be 20.4 ± 1.0 kcal/mol. The relatively low barrier may reflect access to radical pathways during the reaction. (66) Schlenoff, J. B. The Cyclic Voltammetry of Pristine, Isomerized, Deuterated, and Partially Hydrogenated “P-Type” Polyacetylene. J. Electrochem. Soc. 1987, 134, 2179−2187. (67) Chen, Z.; Mercer, J. A. M.; Zhu, X.; Romaniuk, J. A. H.; Pfattner, R.; Cegelski, L.; Martinez, T. J.; Burns, N. Z.; Xia, Y. Mechanochemical Unzipping of Insulating Polyladderene to Semiconducting Polyacetylene. Science 2017, 357, 475−479. (68) The Mn of 14 was measured by GPC and found to be larger than the value expected based on complete monomer conversion (127 vs 40.7 kDa), which may be due to aggregation of the poly(acetylene) segments (see refs 59 and 60). (69) McDonald, R. N.; Reineke, C. E. trans-1,2-Dihydrophthalic Acid. Org. Synth. Coll. 1988, 6, 461.
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DOI: 10.1021/acs.macromol.8b02754 Macromolecules XXXX, XXX, XXX−XXX
