Unveiling a Masked Polymer of Dewar Benzene Reveals trans-Poly(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

*S Supporting Information

ABSTRACT: A dibromo derivative of Dewar benzene, trans-5,6-dibromobicyclo[2.2.0]hex-1-ene, was polymerized usingring-opening metathesis polymerization (ROMP). Thereaction proceeded in a controlled manner as changing theinitial monomer-to-catalyst ratio afforded monodispersedpolymers with tunable molecular weights and growingpolymer chains were extended upon subsequent exposure toadditional monomer. Treatment of the halogenated polymerswith an alkyllithium reagent resulted in elimination followedby isomerization to afford trans-poly(acetylene). Based on aseries of mechanistic and spectroscopic studies, the trans-formation was proposed to proceed through a cyclobutenyl intermediate that undergoes rearrangement. The methodology wasfound to be versatile as triblock copolymers containing the halogenated homopolymer were prepared and converted to theirpoly(acetylene)-containing derivatives. The polymers were characterized using gel permeation chromatography as well as arange of spectroscopic (NMR, FT-IR, UV−vis, and Raman) and analytical techniques.

■ INTRODUCTION

Because of their potential uses in contemporary electronicdevices, including light-emitting diodes,1,2 solid-state lasers,3

photovoltaics,4,5 and chemical sensors,6,7 polymers containingπ-conjugated backbones have attracted significant atten-tion.8−13 The prototypical example is poly(acetylene), whichwas synthesized from its constituent monomer by Natta in the1950s.14 The polymer is comprised only of carbon andhydrogen and represents one of the first organic materialsshown by Shirakawa, MacDiarmid, and Heeger to exhibitmarkedly enhanced electrical conductivities upon exposure tohalogen vapor.15−17 Because poly(acetylene) contains alkenesalong the backbone, it can exist in multiple isomeric forms,18

each of which displays a different set of properties. Forexample, cis- and trans-poly(acetylene) exhibit uniquedecomposition temperatures (cf. 467 vs 412 °C, respectively),glass transition temperatures (357 vs 337 °C), oxidationpotentials (−5.49 vs −5.19 eV), and infrared signatures (ν =740 vs 1015 cm−1).19 Moreover, trans-poly(acetylene) hasbeen reported to display a higher electrical conductivity valuethan its cis isomer (10−5 vs 10−9 Ω−1 cm−1)20 as well as ahigher polarizability due to the differential projection lengthsof the corresponding repeating units (2.425 Å vs 2.125 Å).21

As such, trans-poly(acetylene) features characteristics that maybe 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

multifold: (1) poly(acetylene) typically becomes insolublewhen the chain length exceeds 10 repeat units,22 and (2)catalysts that enable the direct polymerization of acetylene in amanner that is controlled and facilitates access to high polymerremain elusive.23−26 Moreover, few methods are available foraccessing stereopure cis- or trans-poly(acetylene), andrelatively sophisticated conditions are often required.20,27 Forexample, Shibahara reported that the latter is obtained bypolymerizing pressurized acetylene (ca. 1.0 kPa) on a liquidcrystalline substrate under a magnetic field.28 Exposure ofstereorandom poly(acetylene) to elevated temperatures orisomerization catalysts over extended periods of time alsofacilitates access to derivatives with high contents of transrepeat units; however, side reactions (e.g., cross-linking) areoften observed.29,30

To address the aforementioned challenges, a number ofindirect methods to access poly(acetylene) have beendeveloped (see Scheme 1). Feast demonstrated that 7,8-bis(trifluoromethyl)tricyclo[4,2,2,02,5]deca-3,7,9-triene, alsoknown as the “Durham precursor”,31−33 can be polymerizedusing ring-opening metathesis polymerization (ROMP) andthen transformed to poly(acetylene) via a retro [4 + 2]cycloaddition.34,35 Grubbs also reported different methods foraccessing poly(acetylene) through the ROMP of cyclo-

Received: January 1, 2019Revised: March 10, 2019

Article

pubs.acs.org/MacromoleculesCite This: Macromolecules XXXX, XXX, XXX−XXX

© XXXX American Chemical Society A DOI: 10.1021/acs.macromol.8b02754Macromolecules XXXX, XXX, XXX−XXX

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octatetraene (COT)19 or benzvalene.29 Although inclusion ofchain transfer agents in the former facilitated access totelechelic derivatives,36 yields of <50% were reported due tobackbiting and competitive formation of benzene as a side-product.19,36 Benzvalene, on the other hand, features the sameempirical formula as acetylene, and the corresponding polymercan be rearranged to poly(acetylene) without significant massloss.29 However, cross-linking (ca. 19%) occurs upon isomer-ization, which was attributed to the electrophilic nature of therequisite catalyst (HgCl2). In all these approaches, thepoly(acetylene) products consisted of mixtures of cis andtrans isomers.To mitigate the issues outlined above, we envisioned a

synthetic route to poly(acetylene) that entailed the ROMP ofDewar benzene followed by isomerization of the correspond-ing intermediate. Although Dewar benzene is strained and thuscould afford well-defined polymers using contemporarycatalysts, the monomer is volatile,37 facilely isomerizes to

benzene (t1/2 = 2 days),37 and features two cyclobutenyl unitswhich may lead to network formation during polymerization.38

To circumvent these fundamental and practical drawbacks,efforts were recentered on a halogenated derivative, trans-5,6-dibromobicyclo[2.2.0]hex-1-ene (1). Compound 1 is aknown39 liquid that features vicinal dihalides capable ofelimination. As will be described below, high molecular weightpolymers were successfully polymerized in a controlled mannerand even incorporated into multiblock derivatives. Dehaloge-nation of these polymers resulted in spontaneous isomerizationto poly(acetylene). Surprisingly, the trans form of poly-(acetylene) was obtained as the exclusive product.

■ RESULTS AND DISCUSSION

The route used to synthesize 1 was modified from previouslyreported procedures40,41 and is summarized in Scheme 2. Afterreducing phthalic acid (2) with a sodium mercury amalgam,the resulting 1,3-cyclohexadiene derivative 3 was purified by

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)

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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% yieldas a white, crystalline product. Subsequent photochemicalisomerization was conducted in diethyl ether at roomtemperature and monitored over time using 1H NMRspectroscopy. Removal of the residual solvent after the reactionwas determined to be complete (ca. 27 h) afforded crude 5.While it was previously reported that the anhydride can bepurified via sublimation,40 the technique was inefficient in ourhands due to the presence of unidentified byproducts thatformed during the photoisomerization reaction. Likewise,other purification techniques, including column chromatog-raphy and extraction, were also unsuccessful. To overcomethese challenges, 5 was first esterified with methanol in thepresence of a catalytic amount of acid to afford 6, whichfacilitated purification. Subsequent hydrolysis of the purifiedproduct resulted in the desired bis(acid) 7. The addition ofbromine to a suspension of 7 in CH2Cl2 afforded the vicinaldibromide 8 as a white solid in 97% yield. Finally, electrolyticdecarboxylation followed by column chromatography afforded1 as a colorless liquid.42,43

With monomer 1 in hand, polymerization efforts com-menced using the third-generation Grubbs catalyst (G3). ACH2Cl2 solution of the monomer ([1]0 = 0.21 M) was treatedwith the catalyst ([1]0/[G3]0 = 42) for 30 min at ambienttemperature followed by excess ethyl vinyl ether to quench thereaction. The mixture was then poured into cold hexane whichresulted in the precipitation of a solid that was collected in 90%yield based on the assumed structure for poly(1). Gelpermeation chromatography (GPC) revealed that the materialproduced was monodispersed (Đ = 1.14) and exhibited anumber-average molecular weight (Mn) that was in relativelygood agreement (16.4 kDa) with the theoretical value (15.0kDa). Further support for a successful polymerization wasobtained by inspecting the 1H NMR spectrum recorded for the

polymer to that of the monomer. For example, the signalsattributed to the cyclobutenyl ring of the monomer (δ 6.4ppm; 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 ofexperiments were performed. As summarized in Figure 1a, alinear relationship between the initial monomer-to-catalystfeed ratio and the measured molecular weights of the polymersproduced was observed. Moreover, as shown in Figure 1b,increases in molecular weights were seen when additionalmonomer was introduced to the reaction mixture afterconsumption of the initial monomer feed, consistent withchain extension. Collectively, these results suggested to us thatthe polymerization methodology was controlled and affordedwell-defined polymers with tunable molecular weights.As summarized in Scheme 3, subsequent efforts were

directed toward eliminating the vicinal dibromides from theaforementioned polymers.44−46 Adding methyllithium dis-solved in diethyl ether to a THF solution of poly(1) (2.0equiv of CH3Li per polymer repeat unit; [1]0 = 0.05 M) at−78 °C resulted in a color change from pale brown to red asthe mixture warmed to room temperature.47 The color of thesolution darkened over time, and an insoluble black materialwas observed after 30 min. In addition, the formation ofCH3Br, an expected elimination byproduct, was observed bymass spectrometry as well as 1H NMR spectroscopy (seeFigure S15).48 After quenching the reaction through thedropwise addition of methanol, the precipitate was collectedvia filtration, washed with methanol, and then dried undervacuum to afford 9 in >99% yield. Conducting the reaction inthe dark and under an atmosphere of nitrogen afforded aquantitative yield, presumably due to minimization of photo-chemical side reactions.The product of the aforementioned reaction (9) was

characterized using a range of spectroscopic techniques. For

Figure 1. (a) Plot of Mn (red) and Đ (blue) values versus the [1]0 to [G3]0 feed ratio. Conditions: [1]0 = 0.21 M, CH2Cl2, 30 min. Isolated yieldswere typically >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

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comparison purposes, poly(acetylene) was independentlyprepared from COT using a method reported in theliterature.19 As shown in Figures 2a and 2b, the Ramanspectra recorded for the polymers prepared using bothmethods were similar and featured signals consistent with

the presence of C−C (ca. 1117 cm−1) and CC (ca. 1507cm−1) bonds. IR spectroscopy has been used to distinguish cis-and trans-poly(acetylene) since the respective isomers exhibitdistinctive C−H out-of-plane vibrational modes.28 As shown inFigure 2c, a signal at 999 cm−1, which was attributed to the C−

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) thepolymer obtained from the ROMP of COT. (e) A solid-state CP-MAS 13C NMR spectrum recorded for 9. Note: the asterisks (∗) denote spinningside 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

aStarting with 10, the pathway to the left involves intermediates that undergo thermally induced, electrocyclic ring-opening and therefore shouldafford a mixture of polymers that feature cis- and trans-alkenes. The pathway to the right involves radical intermediates that isomerize to trans-poly(acetylene) (9).

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H stretching frequency of trans-poly(acetylene), was measuredfor 9. For comparison, signals at 746 and 931−992 cm−1 wererecorded upon analyzing the poly(acetylene) obtained fromthe ROMP of COT (see Figure 2d) and assigned to the cis andtrans isomers, respectively, in accord with literature reports.19

Likewise, a single signal (δ 135 ppm) was observed uponanalyzing 9 using solid-state cross-polarization magic anglespinning (CP-MAS) 13C NMR spectroscopy (see Figure 2e).13C NMR signals assigned to trans-alkenes in poly(acetylene)have been reported (136−139 ppm) and can be distinguishedfrom their corresponding geometric isomers (126−129ppm).27 Electrical conductivities were measured (10−4 Ω−1

cm−1) by applying contacts to films of 9 (see Figure 2f) andfound to be in agreement with values reported for trans-poly(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 trans-

poly(acetylene) was obtained as the only detectable productfrom the methodology described above, which was a surprisingresult and prompted us to postulate potential mechanisms (seeScheme 4). Although previous reports have shown that certaincyclobutenes and related polyladderenes can undergo selectiveelectrocyclic ring-opening,49−52 mixtures of cis and transproducts may result when the precursors feature assortedstereoisomers.53,54 Because the spectroscopic data recorded for9 indicated that the material was composed of a singlestereoisomer, an alternative route wherein ring-openingproceeds through a radical intermediate that isomerizes tothe thermodynamically favored (trans) polymer product wasconsidered.To probe the underlying mechanism, efforts were first

directed toward identifying potential intermediates. Precursorpoly(1) was subjected to a solution of CH3Li in THF-d8 at−50 °C and then monitored by 1H NMR spectroscopy. Signalsassigned to the hydrogens bonded to the brominated carbons(δ 4.4−4.8 ppm) disappeared over time and were accompaniedby the formation of new signals that were consistent with thoseexpected from the cyclobutenyl moieties in 10 (6.0−6.2 ppm)(see Figure S13).55,56 Next, the isomerization reaction wasconducted in the presence of a radical trap.57 A THF-d8solution of poly(1) was charged with CH3Li (2.0 equiv perrepeat unit) at −50 °C followed by (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO) (3.0 equiv per repeat unit) and

then warmed to room temperature.58 At the conclusion of thereaction, a black precipitate was collected and found to displayspectroscopic signatures that were similar to the trans-poly(acetylene) described above (see Figure S3). Moreover,the trap was not detected in the product via elemental analysisor other analytical techniques. Collectively, these resultsindicated that the isomerization may have proceeded throughradical intermediates that rapidly isomerize to the thermody-namically favored, conjugated polymer product before reactingwith a trap.Because the poly(acetylene) described above was found to

be insoluble, subsequent efforts were directed toward capital-izing on the advantages of the polymerization methodology toaccess soluble derivatives. On the basis of previous reports,59,60

we hypothesized that connecting a poly(acetylene) block tononconjugated segments would render the entire copolymerprocessable. exo,exo-5,6-Bis(methoxymethyl)bicyclo[2.2.1]-hept-2-ene (11)61 was chosen as a comonomer due to itshigh solubility in common organic solvents, lack of acidicfunctional groups, and high ring strain. The polymerization of11 was found to proceed in a controlled manner62 and affordedmonodispersed products with molecular weights that agreedwith 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 precursorto poly(acetylene). Exposure of a CH2Cl2 solution of 11 to G3([11]0/[Ru]0 = 63) followed by stirring at −15 °C initiated apolymerization reaction. To monitor the formation of polymerover time, an aliquot was removed from the solution after 15min and poured into a mixture of CH3OH and ethyl vinylether to quench the reaction. The precipitate that formed wascollected and analyzed by GPC (Mn = 19.3 kDa, Đ = 1.08). ACH2Cl2 solution of 1 was then introduced to the residualcatalyst solution (([11]0 + [1]0)/[Ru]0 = 126). After stirringfor an additional 15 min, an aliquot was removed and workedup as described above (Mn = 32.3 kDa, Đ = 1.10). Finally,additional 11 (([11]0 + [1]0 + [11]1)/[Ru]0 = 189) wasadded. After quenching the reaction, triblock copolymer 12was isolated in 73% overall yield. GPC analysis revealed thatthe material exhibited a Mn of 51.4 kDa (Đ = 1.18) (cf. thetheoretical Mn value of the copolymer based on quantitativemonomer conversions was 38.0 kDa), and the number of

Scheme 5. Synthetic Methodology Used To Prepare Different Triblock Copolymers

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repeat units in the segment of poly(1) was calculated to be 55from its deduced Mn value (13.0 kDa).63

Adding methyllithium to a THF solution of the triblockcopolymer 12 (2 equiv of CH3Li per polymer repeat unit) atroom temperature resulted in the formation of a dark red color,consistent with the formation of 13 followed by isomerizationto 14.64,65 Pouring the treated solution into cold methanolafforded a precipitate, which was collected and characterized.1H NMR spectroscopic analysis of the product revealed thatsignals assigned to the halogenated methines found in thesegment of poly(1) disappeared and a new signal (δ 6.0−7.0ppm; THF-d8), consistent with the repeat unit of poly-(acetylene), had formed. A solution of 14 in THF exhibited aλmax value near 500 nm and an irreversible oxidation at 0.5 V(vs ferrocene in CH3CN), in agreement with values expectedfor poly(acetylene) (see Figures S1 and S2).66 Data recordedin the solid state were also in agreement with the structuralassignment. For example, 14 exhibited Raman signals at 1110and 1492 cm−1 as well as an IR absorption band at 1012 cm−1.Likewise, a 13C NMR signal (δ 134 ppm) consistent with thatexpected for trans-poly(acetylene) was measured (see FigureS27).67 Although iodine doping was found to significantlyenhance the conductivity displayed by a film of the copolymerby 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 withpoly(acetylene), presumably due to the presence of theinsulating segments of poly(11). Regardless, these datasuggested to us that the aforementioned methodology affordedwell-defined triblock copolymers wherein poly(norbornene)-based homopolymers were connected to a poly(acetylene)core.68

■ CONCLUSIONS

Ring-opening metathesis polymerization (ROMP) of adihalogenated derivative of Dewar benzene led to theformation of a poly(acetylene) precursor in a controlledmanner. Subjecting the polymer to an alkyllithium agentresulted in elimination to form an intermediate that wasconsistent with a ring-opened, polymeric form of Dewarbenzene. Rapid isomerization ensued and afforded trans-poly(acetylene), as determined using a range of spectroscopicand analytical techniques as well as through comparison withindependently prepared samples. The methodology was alsosuccessfully adapted to prepare triblock copolymers containingsegments of poly(acetylene). The copolymers were found to besoluble in common solvents and formed films that exhibitedincreased electrical conductivities upon doping. The methoddescribed herein is an effective tool for preparing poly-(acetylene) with a high trans-olefin content and well-definedcopolymers thereof and thus can be expected to find utility inthe growing number of applications that require the use ofconjugated polymeric materials.

■ EXPERIMENTAL SECTIONGeneral Methodology. Unless otherwise noted, all manipu-

lations were performed under an atmosphere of nitrogen usingstandard Schlenk techniques. Phthalic acid, sodium acetate, and aceticanhydride were purchased from Sigma-Aldrich. Bromine waspurchased from Junsei Chemical. Hydrochloric acid (35 wt %) waspurchased from Daejung Chemicals & Metals. cis-5-Norbornene-exo-2,3-dicarboxylic anhydride was purchased from Alfa Aesar. A sodiummercury amalgam was prepared according to literature procedures.69

An Ace photochemical reactor (medium pressure Hg, 450 W) was

used as a light source. Electrolysis was conducted using a GW InstekGPR-11H300 instrument and platinum plates (4 cm × 4 cm, 0.2mm). Solvents were dried and degassed using a Vac Atmospheressolvent purification system. 1H NMR and 13C NMR spectra wererecorded 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 NMRspectra were recorded using a Bruker 125 MHz spectrometer.Coupling constants (J) are expressed in hertz (Hz). Splitting patternsare indicated as follows: br, broad; bd, broad doublet; bt, broadtriplet; bm, broad multiplet; s, singlet; d, doublet; t, triplet; m,multiplet. High-resolution mass spectra (HR-MS) were obtained froma JMS-T100LP AccuTOF LC-plus 4G atmospheric pressureionization high resolution time-of-flight mass spectrometer (WatersInc.) Gel permeation chromatography (GPC) was performed on aMalvern GPCmax solvent/sample module. THF was used as theeluent at a flow rate of 0.8 mL min−1. Infrared (IR) spectra wererecorded on an Agilent Cary-630 FT-IR spectrometer. Raman spectrawere recorded on a WITec alpha 300M confocal Raman microscope.Thermogravimetric analyses (TGA) were performed on a ThermalAdvantages (TA) Q500 at a heating rate of 10 °C min−1 under anatmosphere of nitrogen. Differential scanning calorimetry (DSC) wasperformed under an atmosphere of nitrogen on a Thermal AdvantagesQ2000 at a heating or cooling rate of 20 °C min−1. Electrochemicalmeasurements were conducted using a rotating ring−disk electrode(Pine Research Instrumentation, Durham, NC) that was connected toan Echochemie Inc. PGSTAT302N bipotentiostat running the NOVAversion 1.1 software program (Metrohm Autolab, Utrecht, Nether-lands). Elemental analyses were performed using a ThermoScientificFlash 2000 organic elemental analyzer that was calibrated with 2,5-bis(5-tert-butylbenzoxazol-2-yl)thiophene. Conductivity values wereacquired on an Advanced Instrument Technology CMT-SR200Nsheet resistance/resistivity measurement system. Melting points weredetermined with a MPA 100 Optimelt automated melting pointsystem and are uncorrected. Atomic force microscopy measurementswere conducted with a Multimode 8 nanoscope V (Bruker). X-rayphotoelectron spectroscopy experiments were performed usingan Escalab 250Xi (Thermo Fisher Scientific, Waltham, MA) equippedwith a monochromated aluminum Kα source (1486.6 eV). UV−visspectroscopy data were recorded on an Agilent Cary 100 UV−visspectrometer outfitted with a Peltier multicell temperature controller.

Synthesis of Poly(1). 5,6-Dibromobicyclo[2.2.0]hex-1-ene (0.10g, 0.42 mmol) and CH2Cl2 (1.5 mL) were added to an air-freereaction flask (10 mL) at room temperature under a positive flow ofnitrogen. A solution of the G3 dissolved in anhydrous CH2Cl2 (4.4mg in 0.5 mL) was injected, and the mixture was gently stirred for 30min at room temperature. The reaction was quenched upon theaddition of ethyl vinyl ether (0.1 mL), and the resulting mixture waspoured into cold hexane (ca. 100 mL). A dark pink precipitate formedand 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) wereadded to an air-free reaction flask (10 mL) at room temperatureunder a positive flow of nitrogen. A solution of G3 dissolved inanhydrous CH2Cl2 (4.4 mg of catalyst in 0.5 mL of solvent) wasinjected, and the mixture was gently stirred for 30 min at roomtemperature. The polymerization was then quenched upon theaddition of ethyl vinyl ether (0.1 mL), and the resulting mixture waspoured into cold methanol (ca. 100 mL) to induce precipitation. Theproduct 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 ofmethyllithium (1.6 M in diethyl ether) was added dropwise (0.53mL total volume) to a THF solution containing poly(1) (0.10 gpolymer/8.3 mL THF) at room temperature. The formation of aninsoluble black material was observed after 30 min. The material was

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subsequently collected and dried under reduced pressure (quantita-tive 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 glasssubstrate (1.8 cm × 1.8 cm). A solution of CH3Li (1.6 M in diethylether) was added dropwise (26 μL total volume), and the resultingmixture was agitated for 10 s using a glass rod. The resulting film waswashed with absolute ethanol and then dried under nitrogen for 24 hat 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 sublimationat room temperature. After 17 h, the chamber was entombed in aglovebag filled with nitrogen, and the conductivity of the sample wasmeasured.Triblock Copolymer Preparation. Monomer 11 was dissolved

in anhydrous CH2Cl2 (0.8 mL). Subsequent injection of predeter-mined quantities of G3 dissolved in 0.6 mL of anhydrous CH2Cl2followed by stirring at −15 °C for 15 min facilitated the consumptionof the monomer. A solution of monomer 1 dissolved in 0.8 mL ofanhydrous CH2Cl2 was then injected, and the mixture was stirred foran additional 15 min. Similarly, a solution of monomer 11 dissolved in0.8 mL of anhydrous CH2Cl2 was added, and the resulting solutionwas stirred for 15 min. Excess ethyl vinyl ether was added to quenchthe reaction, and then the resulting mixture was poured into coldmethanol to induce precipitation. The product was collected viafiltration and dried under reduced pressure.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.macro-mol.8b02754.

Additional synthetic details, additional electrochemicalmeasurement details and data, UV−vis spectra, infraredand Raman spectra, an XPS survey spectrum, elementalanalysis data, AFM data, DSC and TGA data, GPC data,isomerization kinetics details and data, and 1H and 13CNMR spectra (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail bielawski@unist.ac.kr; Tel +82-52-217-2952.

ORCIDStanfield Y. Lee: 0000-0001-6955-2573Christopher W. Bielawski: 0000-0002-0520-1982NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The Institute for Basic Science (IBS-R019) and the BK21 PlusProgram as funded by the Ministry of Education and theNational Research Foundation of Korea are acknowledged forsupport. We thank Dr. Karel Goossens and Dr. SibanarayanTripathy for their assistance with the solid-state NMRmeasurements. We are grateful to Ms. Younghye Park forcreating the graphical abstract.

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isomerization of 13 to 14 was measured to be 20.4 ± 1.0 kcal/mol.The relatively low barrier may reflect access to radical pathwaysduring 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 Semi-conducting Polyacetylene. Science 2017, 357, 475−479.(68) The Mn of 14 was measured by GPC and found to be largerthan the value expected based on complete monomer conversion(127 vs 40.7 kDa), which may be due to aggregation of thepoly(acetylene) segments (see refs 59 and 60).(69) McDonald, R. N.; Reineke, C. E. trans-1,2-DihydrophthalicAcid. Org. Synth. Coll. 1988, 6, 461.

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