Synthesis and Characterization of a High Temperature Thermoset

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Synthesis and Characterization of a High Temperature Thermoset Network Derived from a Multicyclic Cage Compound Functionalized with Exocyclic Allylidene Groups Kyle E Rosenkoetter, Michael D Garrison, Roxanne L. Quintana, and Benjamin G. Harvey ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00542 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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ACS Applied Polymer Materials

Synthesis and Characterization of a High Temperature Thermoset Network Derived from a Multicyclic Cage Compound Functionalized with Exocyclic Allylidene Groups Kyle E. Rosenkoetter, Michael D. Garrison, Roxanne L. Quintana, and Benjamin G. Harvey* US NAVY, NAWCWD, Research Department, Chemistry Division, China Lake, California 93555 *Corresponding Author (Email: [email protected]) Abstract A heptacyclo[6.6.0.02,6.03,13.04,11.05,9.010,14]tetradecane (HCTD) complex with terminal allylidene groups at the 7- and 12-positions (HCTD-7,12-diallylidene, 2) was generated at the multi-gram scale from norbornadiene via an efficient six-step synthesis. Thermal polymerization of 2 at temperatures ranging from 160 to 240 °C yielded a robust cross-linked material with thermal stability up to 485 °C in air, a glass transition temperature of 377 °C, and a char yield (600 °C) of 56% in air. This degree of thermal stability is remarkable for a non-aromatic hydrocarbon polymer and is likely due to the rigid multicyclic cages that make up the bulk of the material. To elucidate the polymerization mechanism, a model compound, 7-allylidenenorbornane (4), was synthesized and thermally cured. This resulted in the formation of polymeric material, suggesting that the cross-linking reaction of 2 proceeds via a free-radical reaction and not through DielsAlder cycloaddition. Addition of dibutylhydroxytoluene (BHT) to compound 2 delayed the onset of cure, providing further support for a radical mechanism. Based on these results it can be concluded that exocyclic allylidene groups represent a new class of thermosetting endcap capable of generating highly cross-linked materials with thermal stabilities that rival that of high temperature polyimides. Applications include heat resistant composites utilized in the aerospace, electronic, automotive and textile industries.

Keywords: cage complex; heptacyclotetradecane; bisallylidene; high temperature; thermoset network; heat resistant; aerospace 1 ACS Paragon Plus Environment

ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction Heptacyclo[6.6.0.02,6.03,13.04,11.05,9.010,14]tetradecanes (HCTD) represent a unique class of cage molecules with highly symmetrical structures.1 Similar to the multicyclic cubanes (fourmembered rings) and adamantanes (six-membered rings), HCTD species are a distinct repository of fused five-membered carbocyclic rings (Scheme 1).1,2 The rigid, strained, and highly compact scaffold, as well as the unusual overall shape, has interested chemists since the 1960s. Unfortunately for earlier investigators, the isolation of HCTD species through the [4+4] cycloaddition of norbornadiene was hindered by low yields (2-3%) and the use of stoichiometric amounts of metal carbonyl complexes.3-5 Over the last several decades, efforts have been pursued to increase the yield of HCTDs, as well as to functionalize the secondary carbons at the 7- and 12-positions.1,2 In 1999, Mitsudo et al. demonstrated a high yield synthesis (66%) for the formation of the [4+4] cycloaddition product using a ruthenium based catalyst in DMSO.5 Mitsudo attempted the same transformation using THF or DMF and only generated 1-2% yields of HCTD, indicating a solvent effect for the cyclization reaction.5 Building off this result, Dong and coworkers demonstrated the ability to prepare HCTD complexes with various R-group substituents on the apical 7- and 12-positions (R = alkyl, aryl, ether, etc.).1 Although HCTD complexes have been the subject of significant interest, their incorporation into thermoplastic or thermosetting resins has yet to be reported. This is despite the extensively investigated polymerization chemistry of multicyclic compounds such as norbornene, Scheme 1. Multicyclic cage compound containing fused cyclobutanes (cubane), cyclopentanes (HCTD), and cyclohexanes (adamantane) 7

Cubane

12

Heptacyclotetradecane HCTD

Adamantane

2 ACS Paragon Plus Environment

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norbornadiene, and dicyclopentadiene (Scheme 2).6-12 In general, polymers of these simple unsaturated species exhibit high Tgs, good mechanical strength, and high thermal stability, leading to their utility in a variety of aerospace and industrial applications.13 In contrast, polymers based on the adamantane core have only been sparsely investigated since 1960. A notable adamantane monomer, 1,3-diethynyladamantane, undergoes thermal cross-linking through one of the ethynyl groups to form high molecular weight poly(diethynyladamantane) (PDA, Scheme 2).14 PDA exhibits thermal stability up to 450 °C and a Tg > 260 °C.14 Polyacetylene has been reported to exhibit thermal stability up to about 410 °C with no observable glass transition temperature.15 The Scheme 2. Ring-opening metathesis polymerization (ROMP) of norbornene, norbornadiene, dicyclopentadiene, and thermal polymerization of 1,3-diethynyladamantane. ROMP n Poly(norbornene)

Norbornene linear

n

ROMP Norbornadiene

cross-linked

n Poly(norbornadiene)

ROMP n Dicyclopentadiene

Poly(dicyclopentadiene) C 

1,3-Diethynyladamantane

C H n

Poly(diethynyladamantane)

increased thermal stability of PDA compared to polyacetylene can be directly attributed to the presence of the thermodynamically stable adamantane core.14-16 Thermosetting monomers based on multicyclic cage compounds have the potential to generate highly cross-linked networks with 3 ACS Paragon Plus Environment


outstanding resilience towards heat degradation and chemical attack. These properties are of significant interest for fire-retardant coatings, consumer goods, injection molded parts, as well as military, aerospace, and medical applications.17-19 The rigid, strained, and highly compact scaffold of HCTD complexes offers the opportunity to synthesize a unique class of polymers. Herein, we describe a pathway for the synthesis of an HCTD-7,12-diallyldiol (1) species which can then be dehydrated to yield HCTD-7,12diallylidene (2). Both HCTD cage complexes are isolated as analytically pure solids (99%) in excellent yields without the use of column chromatography. Upon melting, HCTD-7,12diallyidene (2) readily cross-links to form a thermally stable polymer network (2a). Elucidation of the polymerization mechanism for the formation of this new thermosetting material is probed through an examination of the model half-cage compound, 7-allylidenenorbornane (4). Experimental General Considerations. The compounds and reactions reported below show various levels of air- and moisture sensitivity. Therefore, all manipulations were carried out using standard Schlenk techniques unless otherwise noted. Norborna-2,5-diene, tert-butylperoxybenzoate, iodotrimethylsilane, allyl bromide, magnesium powder, pyridinium chlorochromate (PCC), pyridine, phosphoryl chloride, dibutylhydroxytoluene (BHT), and CDCl3 were purchased from Millipore Sigma. CD3OD was purchased from Cambridge Labs and all reagents were used as received. 7-tert-butoxynorborna-2,5-diene,3 HCTD-7,12-di-tert-butylether (A),1 HCTD-7,12-diol (B),4 and HCTD-7,12-diketone (C)5 were synthesized using modified literature procedures; further details are included below. 7-tert-butoxynorbornane,20 norbornan-7-ol,21 and norbornan7-one5 were all synthesized using methods similar to those described in the literature.


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Analytical Methods. NMR spectra were collected on Bruker Avance 300 or 500 MHz spectrometers in CDCl3 or CD3OD. Chemical shifts are reported using the standard δ notation in parts per million. Solid-state NMR data were obtained using a 500 MHz Bruker Avance III HD spectrometer. Cross-polarization magic angle spinning (CPMAS) experiments were conducted with the following parameters: cp pulse program with a 4.5 ms 1H 90 degree pulse, continuous wave 1H decoupling, an acquisition time of 0.048 s, a cross-polarization time of 4000 microseconds, a relaxation delay of 4 s, and 20K scans with a time domain (TD) of 3K. Infrared spectroscopy was performed on a Thermo Nexus spectrometer using an MCTA liquid nitrogen cooled detector. The samples were analyzed using attenuated total reflectance (ATR) with a germanium, single reflection crystal. The FTIR spectra are an average of 16 scans at 4 cm-1 resolution. The samples were placed directly on the crystal with minimal sample preparation. Thermal Gravimetric Analysis-FTIR Spectroscopy (TGA-FTIR) was conducted on a TA instruments Q50-TGA equipped with an interface connected to the Thermo Nexus spectrometer described above. Gas chromatography-mass spectrometry (GC-MS) data were collected on DCM or methanol solutions with an Agilent 6890N GC-MS using a 30 meter, 0.25 mm id, 0.25 µM def column with a temperature profile of three min at 40 °C, a temperature ramp of 20 °C per min until reaching 300 °C, and finally a ten min hold at 300 °C. Direct Insertion Probe Mass Spectrometry (DIP-MS) was performed on a Thermo Scientific DSQII equipped with a direct probe controller using a method starting at 40 °C and a temperature ramp of 5 °C per minute until reaching a maximum temperature of 450 °C. Differential scanning calorimetry (DSC) studies were performed on a TA Instruments Q200 differential scanning calorimeter. Samples were contained in hermetically sealed, aluminium pans under a stream of N2 gas with a flow rate of 50 mL per minute. High purity indium was used to calibrate the calorimeter. DSC scans were



performed on 2 – 10 mg samples. Samples were heated at a rate of 10 °C per min between -80 – 300 °C. Thermogravimetric analysis (TGA) studies were performed on a TA Instruments Q5000 thermogravimetric analyzer in a standard, aluminium pan under a nitrogen or air atmosphere. Sample sizes were between 2 – 10 mg and the samples were heated from 50 – 600 °C at 10 °C per minute. Dynamic-mode thermomechanical analysis (TMA) was performed using a TA Instruments Q400-0537 analyzer under 100 mL per min N2 flow. Sample bars were placed in contact with a flexural probe under a force of 0.20 N. Samples were heated at 5 °C per min from 40 – 400 °C. The force was modulated ±0.08 N at a frequency of 0.10 Hz during this cycle. TMA samples were prepared by pressing 2 under 10,000 psi into a 2 mm thick bar which was then cut into a 10 x 2 x 2 mm3 bar using a razor blade. The bar was placed in an aluminium foil mold of the same size. The monomer was degassed under reduced pressure, and then heated in an oven under a N2 atmosphere to 160 °C to melt the material. The sample was then cured under N2 at 160 °C for two h, 200 °C for an additional two h, and lastly allowed to cure at 240 °C for 16 h. Gel permeation chromatography (GPC) was conducted on a Viscotek TriSEC Model 302 GPC system equipped with a Refractive Index (RI) Detector and two Varian PLgel 5µm MixedD 300 X 7.5mm GPC Columns in series, preceded by a 50 X 7.5mm Guard Column. Tetrahydrofuran (THF) was used as the mobile-phase with a 1.0 mL/min flow rate. The system was calibrated using three different polystyrene samples (Mw = 162 to 364,000 Daltons). Retention times were identified using an RI detector and corrected based upon the elution time of the flow rate marker (toluene). The corrected retention times were then used to calculate the molecular weight based upon the calibration curve previously established. Samples were prepared in approximately 50-200 ppm concentration and filtered through a 0.45 µm filter prior to injection.


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Crystallographic Methods. X-ray diffraction studies were conducted at low temperature (99 K) on a single crystal mounted on a glass fiber using paratone oil. Data were acquired using a Bruker SMART APEX II diffractometer equipped with a CCD detector using Mo Kα radiation (λ

=

0.71073

Å),

which

was

wavelength-selected

with

a

single-crystal

graphite

monochromator.22 The SMART program package was used for determination of the unit-cell parameters and for data collection. The raw frame data were processed using SAINT23 and SADABS24 to yield the reflection data file. Subsequent calculations were carried out with SHELXTL.25 The structure was solved by direct methods and refined on F2 by full-matrix leastsquares techniques using OLEX2.26 Analytical scattering factors for neutral atoms were used throughout the analyses.27 Hydrogen atoms were included using a riding model. The HCTD unit and the butadiene group (–C7=C15–C16=C17) were modeled as multiple components using partial site-occupancy factors (50:50). At convergence, wR2 = 0.1479 and Goodness-of-fit on F2 = 1.003 for 271 variables refined against 3417 data (0.76 Å), R1 = 0.0517 for those 3417 data with I > 2.0σ. An ORTEP diagram showing the disorder of 2 was generated using ORTEP-3 for Windows (Figure S1).28 The crystallographic data are included with the supporting information, and have been uploaded to the Cambridge Structural Database (CCDC 1913481). Synthesis of HCTD-7,12-di-tert-butylether (A).1 A 250 mL Schlenk flask was charged with a magnetic stir bar, Ru[(p-cymene)Cl2]2 (1.77 g, 2.90 mmol, 0.05 eq.), manganese powder (0.956 g, 17.4 mmol, 0.30 eq.), and dimethylfumarate (1.7 g, 10.0 mmol, 0.173 eq.). The contents were put under a positive flow of N2. To the flask was added 7-tert-butoxynorborna-2,5-diene (9.5 g, 57.9 mmol, 1.0 eq.). The contents were then suspended in dry, degassed, DMSO (1 M, 60 mL). The flask was equipped with a reflux condenser with an N2 inlet, and the contents were heated to 120 °C in an oil bath overnight. The contents were then quenched with 100 mL of DI water and



transferred to a 500 mL separatory funnel. The organic matter was extracted with Et2O (5 x 200 mL), and the organic extracts were combined, dried with magnesium sulfate, filtered through a fritted filter, and the volatiles were removed on a rotary evaporator. The dark brown residue was washed with ice cold acetone to yield a white powder (> 99% purity by GC-MS; 6.52 g, 68% yield). The product can be further purified by recrystallization from a concentrated acetone solution at -30 °C. Characterization data obtained for the isolated material were consistent with literature precedent.1 1H-NMR (300 MHz, CDCl3) δ / ppm: 4.28 (br. s, 2H, -C(OtBu)H), 2.75 (br. s, 4H, -CH), 2.34 (br. s, 6H, -CH), 2.17 (br. s, 2H, -CH), 1.18 (br. s, 18H, -C(CH3)3). 13C {1H}NMR (75.5 MHz, CDCl3) δ / ppm: 86.0, 72.8, 55.7, 53.2, 51.8, 51.3, 48.9, 48.3, 28.6. IR (neat) ṽ / cm-1: 2967, 1380, 1196, 1082, 1053. GC-MS (DCM): 15.202 min. (328 m/z = [M], 313 m/z = [M-CH3], 255 m/z = [M-OC(CH3)3]). Synthesis of HCTD-7,12-diol (B). Complex B was obtained using a similar prep to Bird et al.4 To a 100 mL round bottom flask was added HCTD-7,12-di-tert-butylether (A, 8.86 g, 27.0 mmol, 1.00 eq.), a magnetic stir bar, and chloroform (12 mL). The flask was then capped with a rubber septum and put under a positive nitrogen flow. Iodotrimethylsilane (10.0 mL, 70.3 mmol, 2.60 eq.) was then added via a disposable syringe to afford an immediate color change from a clear pale yellow solution to a red/yellow solution. The mixture was stirred for 10 min at ambient temperature and then poured into a 100 mL beaker containing a slurry of NaHCO3 (10 g) in methanol (200 ml). The resulting mixture was swirled for ten additional min, filtered from the residual solid using a Buchner funnel, and the solid was washed with additional methanol (200 mL). The filtrate volatiles were removed under reduced pressure to yield a yellow solid. The solid was suspended and stirred for one h in 250 mL of an aqueous 10% sodium thiosulfate solution. The suspension was then transferred to a fritted filter, washed with additional aqueous


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10% sodium thiosulfate (150 mL), water (100 mL), acetonitrile (100 mL), and lastly hexanes (100 mL). The resulting white solid was collected and dried under vacuum to yield a white solid (5.72 g, 98%). Characterization data obtained for the isolated material were consistent with literature precedent.4 1H-NMR (300 MHz, CDCl3) δ / ppm: 4.54 (br. s, 2H, -C(OH)H), 2.84 (br. s, 4H, -CH), 2.46 (br. s, 6H, -CH), 2.29 (br. s, 2H, -CH). 1H-NMR (300 MHz, MeOD-d4) δ / ppm: 4.44 (br. s, 2H, -C(OH)H), 2.80 (br. s, 4H, -CH), 2.44 (br. s, 4H, -CH), 2.40 (br. s, 2H, CH), 2.24 (br. s, 2H, -CH). IR (neat) ṽ / cm-1: 3430, 3261, 2952, 1606, 1289, 1197, 1073, 1042. GC-MS (MeOH): 14.217 min. (216 m/z = [M]). Synthesis of HCTD-7,12-diketone (C). Using a preparation similar to that outlined by Marchand and coworkers,29 a 500 mL round bottom flask was charged with PCC (8.7 g, 40.5 mmol, 5.83 eq.) and DCM (200 mL) to give an orange suspension. B (1.50 g, 6.94 mmol, 1.00 eq.) was dissolved in a minimal amount of DMSO (75 mL) and diluted to 100 mL with DCM. This solution was then poured all at once into the PCC suspension to afford an immediate color change to a dark red solution. The reaction mixture was stirred at ambient temperature for three days after which diethyl ether (100 mL) was added to the mixture and the contents stirred for an additional 30 min. The contents were transferred to a 1 L separatory funnel, whereby an additional 100 mL of diethyl ether was added, and the mixture shaken. Upon standing, the DMSO layer (roughly 75 mL) was removed prior to the addition of water. The golden yellow diethyl ether layer was washed with copious amounts of water (500 mL), dried with magnesium sulfate, and the volatiles were then removed under reduced pressure to afford a golden yellow solid. The solid was scraped from the walls of the collection flask, transferred to a glass fritted filter, and washed with hexanes (500 mL) to remove a pale pink impurity. The resulting pale yellow/off-white solid (>97% pure by GC-MS) was collected and dried under reduced pressure



(1.15 g, 80% yield). Characterization data obtained for the isolated material were consistent with literature precedent.29 1H-NMR (300 MHz, CDCl3) δ / ppm: 2.84 (br. s, 8H, -CH), 2.44 (br. s, 4H, -CH). IR (neat) ṽ / cm-1: 2970, 1770, 1750, 1695, 1150, 897. GC-MS (DCM): 14.251 min. (212 m/z = [M]). Synthesis of HCTD-7,12-diallyl-diol (1). A 200 mL three neck round bottom flask was charged with magnesium metal (986 mg, 41.1 mmol, 40-80 mesh, 5.44 eq.), a catalytic amount of I2, diethyl ether (30 mL), and a magnetic stir bar. The flask was equipped with a reflux condenser, a 100 mL addition funnel, and sealed with a rubber septum. The amber suspension was heated to 40 °C in an oil bath under a nitrogen atmosphere. The amber color began to dissipate after about 10 minutes. To the heated suspension was added neat allyl bromide (3.56 mL, 41.1 mmol, 5.44 eq.) dropwise via addition funnel. The addition funnel was rinsed with additional diethyl ether and drained into the reaction flask (10 mL). The contents were allowed to stir at reflux for 90 min to form a cloudy solution of the Grignard reagent. To the heated suspension was added C (1.60 g, 7.55 mmol, 1.00 eq.) as a THF solution (40 mL) dropwise through the addition funnel. The reaction mixture immediately turned clear with the formation of a white precipitate. The reaction mixture was allowed to stir at 40 °C for an additional 20 min and was then removed from heat and cooled to 5 °C in an ice bath. The reaction mixture was then quenched with one mL aliquots of DI H2O (total of 5 mL) and a saturated aqueous NH4Cl solution (total of 5 mL). The rubber septum was then removed and more DI H2O (30 mL) and NH4Cl solution (20 mL) were added. Diethyl ether (50 mL) was then added to the flask and the mixture was stirred until both layers became transparent. The contents were then transferred to a 500 mL separatory funnel and the organic layer was extracted with diethyl ether (2 x 100 mL). The organic layer was washed with DI H2O (3 x 50 mL), dried with MgSO4, filtered through a


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Buchner funnel, and the volatiles were removed under reduced pressure to yield a white residue. The residue was suspended in cold hexanes, collected on a glass fritted filter, and dried to afford complex 1 as a white solid (2.23 g, 99%). 1H-NMR (500 MHz, CDCl3) δ / ppm: 5.96-5.89 (m, 2H, allyl –CH), 5.16-5.13 (m, 4H, allyl –CH2), 2.86 (br. s, 4H, cage –CH), 2.57 (br. s, 4H, cage – CH), 2.46-2.42 (m, 4H, cage –CH), 2.26 (br. s, 2H, cage –CH), 2.17 (br. s, 2H, cage –CH), 1.79 (br. s, 2H, –OH).

13C{1H}-NMR

(125 MHz, CDCl3) δ / ppm: 134.8 (–C(H)=CH2), 118.5 (–

C(H)=CH2), 94.1 (C7,12(OH)–allyl), 57.7, 56.3, 52.7, 52.0, 51.1, 50.4, 41.3. IR (neat) ṽ / cm-1: 3351 (-OH), 2933 (-CH), 1639 (allyl, weak), 994 (allyl), 910 (allyl). GC-MS (DCM): 15.825 min. (255 m/z = [M–allyl]). Anal. Calcd for C20H24O2: C, 81.04; H, 8.16. Found: C, 80.82; H, 8.30. Synthesis of HCTD-7,12-diallylidene (2). A 100 mL round bottom flask was charged with 1 (1.50 g, 5.07 mmol, 1.00 eq.), pyridine (40 mL), and a magnetic stir bar in air. The flask was capped with a rubber septum and placed in an oil bath. To the solution was added phosphoryl chloride (4.0 mL, 43.0 mmol, 8.48 eq.) via a disposable syringe. The reaction mixture remained a pale yellow/colorless solution. The reaction flask was heated to 75 °C (removed from the oil bath immediately upon reaching 75 °C), and then cooled slowly to ambient temperature. The reaction mixture slowly turned slightly pink in color over time. After stirring at ambient temperature for 90 min, the reaction mixture was quenched slowly with sequential one mL aliquots of DI H2O (20 mL). An additional 20 mL of DI H2O was then added, and the contents were transferred to a 500 mL separatory funnel. The organic layer was extracted with diethyl ether (3 x 100 mL (clear colorless organic layer, light pink aqueous layer), and washed with DI H2O (3 x 50 mL). The organic layer was then isolated, dried with MgSO4, filtered using a Buchner funnel, and volatiles were removed using a rotary evaporator to yield a pale yellow solution. Residual pyridine was



removed under high vacuum (0.01 mmHg) to give a white solid. The solid was scraped from the sides of the flask, collected in a 20 mL scintillation vial, and further dried under high vacuum (0.01 mmHg) at room temperature to yield complex 2 as a white solid (1.25 g, 95%). X-ray quality crystals were collected by sublimation at 50 °C under static vacuum (0.01 mmHg) for one week. Compound 2 was stable indefinitely in the solid state under ambient conditions. 1HNMR (500 MHz, CDCl3) δ / ppm: 6.50-6.42 (m, 2H, allyl –CH), 5.66-5.64 (m, 2H, allyl –CH), 5.09-5.06 (m, 2H, allyl –CH2), 4.96-4.86 (m, 2H, allyl –CH2), 3.08 (s, 2H, cage-CH), 2.66 (s, 2H, cage –CH), 2.59 (s, 8H, cage –CH).

13C{1H}-NMR

(125 MHz, CDCl3) δ / ppm: 160.8

(C7,12=C(H)–), 134.7 (allyl –C(H)=CH2), 114.12 (C15,18 or C17,20), 114.07 (C15,18 or C17,20), 53.5, 53.1 52.9, 48.4. IR (neat) ṽ / cm-1: 2966 (C-H), 1674 (allylidene), 997 (allylidene), 891 (allylidene). GC-MS (DCM): 14.791 min (260 m/z = [M]). Synthesis of 7-allylnorbornan-7-ol (3). Using the same procedure for isolation of 1. Reagents used: magnesium metal (764 mg, 31.8 mmol, 2.50 eq.), iodine (cat.), allyl bromide (2.75 mL, 31.8 mmol, 2.5 eq.), and norbornan-7-one (1.40 g, 12.7 mmol, 1.0 eq.). A viscous pale yellow oil was collected (crude yield = 1.94 g, 99%). The material was trap-to-trap distilled under vacuum (20 mTorr) with an oil bath temperature incrementally increased from 25 to 100 °C to afford a clear, colorless distillate (873 mg, 45%). 1H-NMR (300 MHz, CDCl3) δ / ppm: 5.89-5.85 (m, 1H, allyl-CH), 5.13-5.09 (m, 2H, allyl=CH2), 2.38-2.36 (m, 2H, allyl-CH2), 1.95-1.91 (m, 3H, -CH and -OH), 1.69-1.63 (m, 4H, -CH), 1.23-1.20 (m, 4H, -CH). 13C{1H}-NMR (75.5 MHz, CDCl3) δ / ppm: 135.0 (-C(H)=CH2), 118.2 (-C(H)=CH2), 85.2 (C(OH)-allyl), 42.0 (-CH2 norbornane), 38.1 (-CH2-allyl), 28.7 (-CH norbornane), 27.7 (-CH2 norbornane). IR (neat) ṽ / cm-1: 3400 (OH), 2956 (-CH), 2878 (-CH), 1639 (allyl, weak), 986 (allyl), 911 (allyl, weak). GC-MS (DCM): 8.85 min (151 m/z = [M-H]), 137 m/z = [M-OH], 111 m/z = [M-allyl].


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Synthesis of 7-allylidenenorbornane (4). Using the same procedure described for 2. Reagents used: 3 (220 mg, 1.44 mmol, 1.00 eq.), POCl3 (400 µL, 4.30 mmol, 2.99 eq.), pyridine (10 ml). The ether extraction was washed with copious amounts of DI water (200 ml), 1M aqueous HCl (200 mL), and an additional water aliquot (100 mL). A clear, pale yellow oil was collected (140 mg, 73%). 1H-NMR (300 MHz, CDCl3) δ / ppm: 6.48-6.42 (m, 1H, allylidene-CH), 5.74-5.71 (m, 1H, allylidene-CH), 5.10-4.92 (m, 2H, allylidene-CH2), 2.75 (br. s, 1H, -CH norbornane), 2.30 (br. s, 1H, -CH norbornane), 1.59-1.57 (m, 4H, -CH2 norbornane), 1.36-1.33 (m, 4H, -CH2 norbornane).

13C{1H}-NMR

(75.5 MHz, CDCl3) δ / ppm: 154.9 (7-position C=C(H)-), 134.9

(allyl -CH=C(H)2), 114.0 (-C=C(H)-), 113.6 (allyl –C=CH2), 39.9 (-CH norbornane), 34.9 (-CH norbornane), 28.8 (-CH2 norbornane). IR (neat) ṽ / cm-1: 2955 (-CH), 2868 (-CH), 1681 (allylidene), 1609 (allylidene), 988 (allylidene), 893 (allylidene). GC-MS (DCM): 7.81 min (134 m/z = [M]). Synthesis of poly(HCTD-7,12-diallylidene) (2a). A 7 mL, three-inch long scintillation vial was charged with HCTD-7,12-diallylidene (2, 30 mg, 0.12 mmol). Nitrogen was flushed over the top of the vial to fill the headspace and the vial was sealed with a Teflon cap. The vial was submerged within a preheated oil bath (200 °C) to cover the glass of the vial. The solid melted within seconds. Five separate vials containing 2 were submerged within the oil bath for 15 min, 30 min, 45 min, 60 min, and 22 hrs. All trials resulted in the isolation of a highly cross-linked, insoluble material in quantitative yield.

13C{1H}-CPMAS

NMR (125 MHz) δ / ppm: 159.4,

154.9, 135.1, 115.3, 67.5, 53.5, 49.4, 36.3. IR (neat) ṽ / cm-1: 2953 (C–H), 1679, 890. Synthesis of poly(7-allylidenenorbornane) (4a). A 100 mL Schlenk tube was charged with a clear, colorless solution of 4 (90 mg). The flask was purged with N2 and then submerged in a preheated oil bath (180 °C) for 16 h. The flask was removed from heat and allowed to cool to



Page 14 of 35

room temperature. The contents in the flask had changed from a clear, colorless liquid to a vibrant orange sticky residue. 1H-NMR (500 MHz, CDCl3) δ / ppm: 6.49 (m), 5.76 (m), 5.114.93 (m), 2.44 (br. s), 1.62 (br. s), 1.44-1.26 (m), 0.90 (m). IR (neat) ṽ / cm-1: 2951 (C–H), 2868, 1684, 734. Polymerization of 2 in the presence of a radical inhibitor. In a typical experiment, 2 (23-25 mg) was measured into a 20-mL scintillation vial and dissolved with 5 mL of dry, inhibitor-free THF. To the solution was added 1 mol% or 5 mol% of BHT from a freshly prepared solution (18-90 µL) using a 100 µL syringe. The vial was mixed thoroughly for five min, and the volatiles were then carefully removed on a rotary evaporator. The resulting solid was further dried under high vacuum (0.01 mmHg) for an additional five min. The solids were then scraped from the walls using a spatula, packed together, and transferred to an aluminium pan which was hermetically sealed for DSC analysis. Results and Discussion Monomer Synthesis. HCTD cage complexes with pendent allyl- and allylidene functional groups in the 7- and 12-positions were synthesized through a five-step process starting from 7tert-butoxynorborna-2,5-diene (tBuONBD, Scheme 3). This starting material is commercially available, however, due to its cost it was synthesized in large quantities using a literature procedure.18 Unfortunately, the literature reported modest yields of 15-20% and similar results were obtained in our laboratory. Seeking to improve on this synthesis, the use of di-tertbutylperoxide in place of tert-butylperoxybenzoate was attempted. However, despite running the reaction at elevated temperatures (120 °C), we were unable to generate any tBuONBD using this method. We also attempted to install the allyl functional group at the apical position (C7) of tBuONBD

prior to [4+4] cyclization by reaction of

tBuONBD

with allylmagnesium bromide, but

GC-MS data revealed a mixture of products. Of these products, the three most abundant signals 14 ACS Paragon Plus Environment

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were consistent with the formation of 7-allylnorborna-2,5-diene, 6-allyltricyclohept-3-ene, and 6cyclopropyltricyclohept-3-ene (Scheme S1). This result, albeit interesting, was previously described by Baxter et al. in 1986, whereby nucleophilic attack may occur at a double bond of norborna-2,5-diene when using a Grignard reagent.30 This attack leads to the formation of a new carbon–carbon linkage with the apical carbon, resulting in the loss of a leaving group to generate a series of structurally rearranged products.30 The above results indicated that the [4+4] cyclization of 7-tert-butoxynorborna-2,5-diene (tBuONBD) was necessary prior to alkylation. To this end, HCTD-7,12-di-tert-butylether (A) was prepared using similar methods as described by Dong.1 We then attempted the direct allylation of A through addition of an ethereal solution of freshly generated allylmagnesium bromide. The Grignard reagent was introduced to a benzene solution of A, followed by removal of the diethyl ether under a stream of N2, and subsequent refluxing of the benzene solution overnight. Unfortunately, the reaction was unsuccessful in yielding HCTD-7,12-bis-allyl. Therefore, diketone C was synthesized using established methods from the literature.29,31 Complexes A-C were isolated as pure intermediates (>97%) without the need for further purification via column chromatography (Scheme 3). With significant quantities of C available, alkylation methods were then explored. Initially, C was reacted with freshly prepared solid allyllithium (generated from the reaction of tetraallyltin and n-BuLi),32 however, the product was contaminated with HCTD-7-allyl-12-butyl-7,12-diol and HCTD-7,12-dibutyldiol based on GC-MS and 1H NMR results (Scheme S2). This finding indicated that residual n-BuLi was trapped in the solid allyllithium, despite copious washes with hexane. Abandoning the use of allyllithium, C was treated with an excess of freshly synthesized allylmagnesium bromide in an ethereal solution at 40 °C. This method resulted in the isolation of



the desired HCTD-7,12-diallyldiol (1) product in high yields (99%) as a white powder. Removal of the tertiary hydroxyl groups was then attempted using literature methods.33-38 Dehydration of the hydroxyl groups failed under Dean Stark conditions in the presence of p-toluenesulfonic acid,33Amberlyst-15,34 and oxalic acid.35 Furthermore, treatment of 1 with a concentrated acid source such as phosphoric acid36 or sulfuric acid,37 resulted in no formation of dehydration products. Dehydration of 1 was finally accomplished through treatment with phosphoryl chloride38 in neat pyridine to give the desired HCTD-7,12-diallylidene (2) in excellent yields (95%) as a white solid (Scheme 3). Scheme 3. Synthesis of HCTD complexes A-C, 1 and 2.

OtBu 2

OtBu

[Ru(p-cymene)Cl2]2 Mn Powder Dimethylfumarate DMSO N2 / 120 °C

7-tert-butoxynorborna-2,5-diene tBu ONBD

OtBu HCTD-7,12-di-tert-butylether (A, 68%)

i.) TMS-I/CHCl3 ii.) NaHCO3/MeOH O

N2 / RT

OH

PCC DCM / DMSO air / RT OH

O HCTD-7,12-diketone (C, 80%)

HCTD-7,12-diol (B, 98%)

Et2O:THF N2 / 40 °C

BrMg

17

17 16

6

2 1 14

7 5

9

8 3

4 11

HO

19

6

2

POCl3

10

13

16

15

HO 7 15

Pyridine air / RT

1 14

5 9

8 3

4 10

13

11

12

12

18

19

20

18

20

HCTD-7,12-diallyldiol

HCTD-7,12-diallylidene (2, 95%)

(1, 99%)


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A half-cage model complex was also synthesized to allow for studies of the reactivity of the allylidene functional group. 7-tert-butoxynorborna-2,5-diene (tBuONBD) was first hydrogenated with Pd/C under modest hydrogen pressure (50 psi) using a Parr hydrogenator. The hydrogenation was extremely facile, with hydrogen uptake ceasing within ten min. The contents were filtered through Celite and washed with ethanol. The ethanol was carefully removed using a rotary evaporator at 40 °C to afford 7-tert-butoxynorbornane as a pale yellow oil in 70% yield.20 From here, adopting the same reaction methods outlined above for the HCTD molecules, 7allylnorbornan-7-ol (3) and 7-allylidenenorbornane (4) were successfully isolated as viscous, colorless liquids in good yields (Scheme 4). 1H

nuclear magnetic resonance (NMR) spectroscopy unequivocally confirmed the structures of

HCTD cage complexes A-C, 1 and 2, in addition to the norbornane species 3 and 4 (Figures S2S8). The observance of the 7- and 12-position methine proton resonances were observed between 4.0 and 6.0 ppm for A and B. The C(sp3)-H proton resonances of A-B, 1 and 2 were observed between 2.0 and 3.0 ppm. The spectrum for C exhibited a simplified pattern with two broad Scheme 4. Synthesis of 7-allylnorbornan-7-ol (3) and 7-allylidenenorborane (4). OtBu

OtBu

Pd/C H2 EtOH

7-tert-butoxynorborna-2,5-diene tBu ONBD

7-tert-butoxynorbornane pale yellow oil (70%)

i.) TMS-I/CHCl3 ii.) NaHCO3/MeOH O

N2 / RT

OH PCC DCM / DMSO air / RT

norbornan-7-one viscous oil (75%)

norbornan-7-ol waxy white solid (65%)

Et2O:THF N2 / 40 °C

BrMg

10 10

9

HO 7 3

2 1

6

9

8

8 4 5

7-allylnorbornan-7-ol viscous oil (3, 68%)

7

POCl3 Pyridine air / RT

17

3

2 1

6

4 5

7-allylidenenorbornane pale yellow oil (4, 73%)

ACS Paragon Plus Environment


singlets at 2.84 and 2.44 ppm, consistent with a highly symmetric HCTD cage capped by two carbonyl functional groups. Reduction of C in the presence of allylmagnesium bromide to form 1 was confirmed through the growth of two multiplet resonances between 5.93 – 5.13 ppm, consistent with the C(sp2)-H proton resonances of the terminal allyl functional groups. The spectrum of 1 revealed three resonances within the alkyl region that were consistent with the HCTD core. Two broad singlets at 2.26 and 2.17 ppm were assigned to the methylene (–CH2–) protons of the allyl groups, while the broad singlet at 1.79 ppm was assigned to the –OH groups. The 1H NMR spectrum collected on 2 (Figure 1, top) exhibited characteristic allylidene resonances between 7.0 – 4.8 ppm, shifted from those observed for 1, with the growth of a third multiplet within this region. A total of three resonances were observed within the alkyl region for 2 (3.08–2.59 ppm), consistent with the C(sp3)-H hydrogens of the HCTD core. These resonances mimic those observed for C, indicating a fully symmetric cage structure. Half-cage complexes 3 and 4 demonstrated very similar proton resonances to those described above for HCTD cages 1 and 2. 7-allylnorbornan-7-ol (3) yielded characteristic resonances in the alkyl region (3.0 – 1.0 ppm) consistent with the C(sp3)-H hydrogens (Figure S7). Similar to 1, the most diagnostic resonances exhibited by 3 were the two multiplets between 6.0 – 5.0 ppm, consistent with the installation of the allyl functional group at the 7-position of the norbornane half-cage. Dehydration of 3 to generate 4 was confirmed through the loss of the –OH resonance at 1.89 ppm and the growth of a third multiplet between 6.0 and 5.5 ppm, consistent with the formation of the allylidene functional group (Figure S8). 13C{1H}

NMR spectroscopy also proved useful in confirming the successful dehydrations of 1

and 3 to form the allylidene containing products 2 and 4, respectively (Figures S9-S12). The spectrum of HCTD 1 exhibited ten carbon resonances between 134.8 – 41.3 ppm (Figure S9),


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with the most diagnostic signals stemming from the allyl-functional groups. The resonance observed at 94.1 ppm for 1 was consistent with the bridgehead carbons coordinated to both alcohol and allyl functional groups, while the terminal alkene carbons were observed at 118.5 and 134.8 ppm for C17/C20 and C16/C19, respectively. Similarly, the resonance observed at 41.3 ppm was attributed to C15/C18, the sp3 hybridized carbons of the allyl groups. The spectrum of 2 was absent the resonance at 94.1 ppm, confirming the loss of water through dehydration (Figure S29, bottom). Similar to 1, the carbon resonances of the terminal olefin were observed at 114.1 and 134.7 ppm. The two overlapping signals at 114.1 ppm were consistent with the sp2 carbon resonances of C15/18 and C17/20. Lastly, the singlet at 160.8 ppm was assigned to the sp2 bridgehead carbons (C7/12). The spectra of the half-cage norbornane species 3 and 4 exhibited similar carbon resonances to those observed in HCTD 1 and 2. Six total carbon resonances were observed for 3, with alkyl-carbon resonances appearing between 30 – 45 ppm. The most unique signals were associated with the three resonances at 85.1 (C7), 118.1 (C10), and 134.6 (C9) ppm (Figure S11). Allylidene containing half-cage 4 demonstrated five total carbon resonances, consistent with the loss of an sp3 carbon resonance at 37.9 ppm (–CH2–) and the formation of an allylidene group. Resonances at 113.6/114.0 (C10,C8) and 134.9 (C9) ppm were consistent with the allylidene functional group, while the bridgehead carbon (C7) was observed at 155.0 ppm (Figure S12). To further characterize 2, crystals were grown via sublimation under static high vacuum (0.01 mmHg) at a temperature of 50 °C over the course of one week. X-ray diffraction studies on a single crystal of 2 confirmed the structure suggested by NMR spectroscopy. Figure 1 illustrates the solid-state structure of 2 as a POV-Ray diagram with selected bond lengths provided in Table S2. The clear colorless crystals diffracted in the triclinic P-1 space group, and



exhibited a density of 1.22 g/cm3. The crystal structure was disordered, however, after careful examination of the data, the disorder was successfully modeled as a stereoisomer within the crystal lattice (Figure S1). Inspection of the HCTD core revealed an average bond distance of 1.54 Å, consistent with sp3 carbon–carbon single bonds.39,40 Further inspection of the cage revealed two elongated carbon–carbon bond distances (1.65 Å) between C1–C14 and C9–C10. This elongation indicates a significant amount of ring strain within the core, however, it has recently been shown that long carbon–carbon single bonds are not uncommon for multicyclic diamondoid structures.41 These long bonds seem to be prevalent in structures where intermolecular van der Waals attractions can occur between neighboring H-terminated species within the crystal lattice.41 Short contacts between neighboring molecules within the crystal lattice are also observed for 2 (see SI). These sorts of attractions in turn lead to stabilization of these long carbon–carbon bonds. Further investigation into the apical allylidene groups revealed four carbon–carbon bonds (C7–C15, C16–C17, C12–C18, and C19–C20) averaging 1.32 Å, consistent with four sp2 carbon–carbon double bonds.40,42 The carbon–carbon bonds at the center of the allylidene groups (C15–C16 and C18–C19) were slightly longer (1.46 Å), consistent with an sp2 carbon–carbon single bond.39,40

Figure 1. POV-Ray diagram rendered in Mercury for HCTD-7,12-diallylidene (2) with thermal ellipsoids shown at the 50% probability level. The disorder has been omitted from this figure for clarity, but is shown in the supplemental information (Figure S1).


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Infrared spectroscopy was used to identify key absorption frequencies associated with complexes A-C and 1-4 (Figures S13-S19). All of these compounds exhibit C–H stretching frequencies between 2933 – 2970 cm-1. The IR spectrum of A revealed three key frequencies at 1360, 1196, and 1081 cm-1, consistent with tert-butylether functional groups at the 7- and 12positions of the HCTD core.43,44 The IR spectrum of B includes a characteristic –OH stretching frequency between 3400 – 3300 cm-1, which is completely lost upon oxidation to C.43,44 The spectrum of diketone C exhibits an intense frequency at 1750 cm-1, consistent with apical carbonyl groups.43,44 Reduction of C to 1 gave rise to frequencies at 3351 (–OH), 1639 (alkene), 994 (alkene), and 910 (alkene) cm-1, consistent with the reduction of the carbonyl groups and the installation of terminal alkenes.43,44 Dehydration of 1, forming diallylidene species 2, resulted in the disappearance of the broad stretching frequency associated with the –OH functional groups. More importantly, frequency shifts to 1674, 994, and 891 cm-1 are consistent with the formation of the unsaturated carbon–carbon double bonds of the allylidene functional group (Figure S30, red). The IR spectrum of half-cage 3 contained an –OH stretching frequency at 3400 cm-1, consistent with the presence of a hydroxyl functional group at the 7-position of the norbornane unit. Similar to HCTD 1, 3 also exhibited frequencies at 1639, 986 and 911 cm-1, consistent with the vibrations of a terminal alkene. Formation of 4 lead to the disappearance of the –OH stretch, providing further evidence for the dehydration reaction, and a shift in the key alkene frequencies to 1681, 988, and 893 cm-1. Important frequencies to note for complexes 2 and 4 are those from 800 – 1000 and at ~1700 cm-1. Vibrations observed from 800 – 1000 cm-1 are consistent with C=CH2 and CH=CH2 bending modes of the terminal alkene.43,44 Vibrations at ~1700 cm-1 are consistent with C=C stretching.43,44



Differential Scanning Calorimetry (DSC). To evaluate the thermal properties of 2 and 4, DSC was performed. HCTD 2 demonstrated a sharp melting point onset at 118.9 °C (peak min at 121.4 °C, Figure 2, red). Quickly following the melting transition, an exothermic curing reaction was observed at an onset temperature of 127.4 °C (peak max at 151 °C; Figure 2, red). An additional exothermic event was also observed at 249 °C, consistent with a secondary cure reaction. A second heating cycle (Figure 2, red dash) revealed no additional curing events, suggesting that all of the accessible alkenes had reacted. A similar DSC experiment performed on 4 (Figure 2, blue) revealed a melting point onset at -33.8 °C (peak min at -24.2 °C) with a slight exothermic curing onset observed at 142.7 °C (peak max at 159.7 °C), consistent with the scan of 2. A second heating cycle (Figure 2, blue dash) showed no evidence of a melting point, consistent with the complex undergoing a similar thermal polymerization reaction to that observed with HCTD-diallylidene 2. Polymer Synthesis. Based on the aforementioned DSC results, thermal polymerization of 2 and 4 was attempted. Facile polymerization of 2 was achieved by heating the neat solid to 200 °C under inert conditions for two h. A sample of 2 immediately melted upon placement in a preheated oil bath, producing an insoluble, thermoset material (2a) in quantitative yield. 2a was isolated as a hard, transparent, pale yellow solid. The density of the polymer (1.18 g/cm3) was measured using a pycnometer, which revealed a similar density to that of the parent hydrocarbon, HCTD-7,12-diallylidene 2 (1.22 g/cm3). The relatively high density of the polymer, as compared to conventional aliphatic hydrocarbon polymers (ρ < 1.0 g/cm3), suggested that the HCTD core remained intact during polymerization.14-16 Similarly, 7-allylidenenorbornane 4 was heated at 180 °C in a Teflon sealed Schlenk tube under nitrogen for 16 h. The contents of the flask changed from a clear, colorless liquid to a vibrant orange sticky residue (4a).


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Figure 2. DSC traces for (solid red) 1st heating cycle of 2; (red dash) 2nd heating cycle of 2; (solid blue) heating cycle of 4; and (blue dash) 2nd heating cycle of 4. The scan rate for these experiments was 10 °C/min

NMR spectroscopy was utilized to evaluate the isolated materials collected after thermal polymerization. Unfortunately, 2a proved insoluble in all solvents and did not swell in CDCl3, DMSO-d6, DMF-d7, or NMP-d9. Therefore, solid-state carbon NMR was performed on a powdered form of the material. The spectrum exhibited a low S/N ratio, however, several key resonances were still observed (Figure S24). A broad peak at 53.5 ppm was observed, consistent with the sp3 carbon resonances corresponding to the HCTD cage. Further downfield, four broad resonances were observed between 110 – 160 ppm. These shifts are analogous to monomer 2, however, resonances observed at 115.3 and 154.9 ppm suggest formation of a new C=C linkage. 1H

NMR data gathered on a CDCl3 solution of 4a indicated the growth of broad resonances

within the alkyl region (3.0 – 1.0 ppm) of the spectrum, while only a minor amount of the residual allylidene functional group was observed between 5.0 – 7.0 ppm (Figure S25). To evaluate the structures of the thermally cured materials, 2a and 4a were subjected to FTIR analysis. The IR spectrum of 2a illustrated key stretching frequencies at 890, 1674, and 2966 cm23 ACS Paragon Plus Environment


1.

Page 24 of 35

The spectrum of 4a demonstrated similar frequencies at 734, 1684, and 2951 cm-1 (Figure

S31). Frequencies observed at 2966 cm-1 and 2951 cm-1 were consistent with sp3 C–H stretches, while those at 1674 cm-1 and 1684 cm-1 were consistent with C=C stretching vibrations. The relative absorbance associated with the C=C frequency at 1684 cm-1 in 4a suggested a significant number of carbon-carbon double bonds remained within the polymer material. The frequency observed at 734 cm-1 in 4a suggested that an isomerization may have occurred within the allylidene functional group during polymerization.43,44 The degree of cure for thermoset 2a was investigated with DSC (Figure S26). No melting point was observed, however, a minor exothermic event was evident near 150 °C. This feature was consistent with the reaction of a trace of residual alkenes. A second heating cycle revealed no further curing events. To evaluate the thermal stability of 2a, fully cured material was subjected to thermogravimetric analysis (TGA) at temperatures up to 600 °C in both air and nitrogen atmospheres (Figure S32). 2a displayed an onset of decomposition near 450 °C with exceptional char yields of 56 % in both air and nitrogen, 5 % weight loss (Td5) at 488 °C, and 20% weight loss (Td20) at 550 °C. A TGA experiment conducted under isothermal conditions in nitrogen at 400 °C revealed a weight loss of only 5% after two hours. This unusually high thermal stability for a non-aromatic hydrocarbon polymer is consistent with the rigidity of the HCTD core.14 The thermal stability of 2a exceeds that of the adamantane-based polymer illustrated in Figure 1; fully cured poly(1,3-diethynyladamantane) exhibits a Td20 at 476 °C in both air and helium.14 TGA-FTIR verified that thermoset 2a does not begin to degrade and produce any gaseous products until temperatures greater than 450 °C are reached. Above that temperature, pyrolysis products included methane and molecules with intact HCTD cages.


These products were

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identified by their characteristic C-H stretching frequencies (Figure S27). The TGA-FTIR results were further supported by a DIP-MS experiment, during which degradation products were not observed until temperatures >450 °C under vacuum (41 mTorr). At these temperatures, analysis of the complex mass spectra suggested pyrolysis products containing the HCTD cage. Further investigation of the sample holder after the experiment indicated that the material was still largely unchanged, demonstrating the stability of 2a. The dimensional stability, or how well a material maintains its shape and mechanical properties at increased temperature, is an important criterion for high temperature thermosetting resins. In order to probe the dimensional stability of 2a, thermomechanical analysis (TMA) was performed. To this end, monomer 2 was compressed and packed into a mold that was subsequently heated under N2 to 160 °C for two h to melt the monomer and induce the crosslinking reaction. The temperature was then increased to 200 °C for an additional two h, followed by 16 h at 240 °C to yield a TMA bar that was pale yellow in color (Figure 3, top). A DSC scan taken from a small portion of the resulting material (2a) indicated that the sample was fully cured, with no curing events observed between 10 – 300 °C. TMA analysis indicated a Tg at 377 °C as measured by the storage modulus (Figure 4, black trace). Similarly, the tan δ trace had an

Figure 3. TMA bar of thermoset 2a (top) pre-analysis and (bottom) post-analysis.



increasing slope, but a maximum was not observed prior to reaching the limits of the instrument (400 °C). The monolith retrieved after TMA analysis was amber in color (Figure 3, bottom), still transparent, and showed no signs of dimensional change, charring, bubbling or cracking, further demonstrating the robust nature of 2a. Similarly, poly(1,3-diethnyladamantane) was reported to have no observable Tg below 400 °C.14 Polymerization Mechanism. We considered two polymerization mechanisms for compound 2; Diels-Alder cycloaddition, and free radical polymerization. Given the intractable nature of 2a, we first considered the polymerization of the model compound 4. Scheme S3 illustrates the expected products obtained through the thermal polymerization of half-cage 4. A Diels-Alder cycloaddition would lead to a dimeric species, while radical polymerization would result in the formation of polymeric material. Radical polymerization could proceed through three different pathways: 1,2-addition, 1,4-addition or 3,4-addition.

Figure 4. TMA data collected on 2a.


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Variable temperature NMR studies were conducted to provide mechanistic insight into the polymerization of both 2 and 4. These experiments were conducted in 5 mm Teflon sealed NMR tubes on solutions in tetrachloroethane-d2. Solutions of 4 were held at temperatures of 140, 146, or 150 °C for one h in an oil bath, and then transferred to an NMR spectrometer for data collection at ambient temperature. At 140 °C, no significant change was observed (Figure S33; bottom). At a temperature of 146 °C, new broad resonances at 2.50 and 1.30 ppm began to form with a clear decrease in signal intensity for the allylidene resonances between 4.5 – 7.0 ppm (Figure S33; middle). Increasing the temperature to 150 °C (Figure S33; top) resulted in almost complete loss of the resonances attributable to the allylidene functional group, with a concomitant increase in the intensity of the broad alkyl resonances, consistent with the spectrum collected after curing 4 overnight (Figure S25). Gel permeation chromatography (GPC) was utilized to determine the molecular weight of 4a polymerized at 180 °C for 16 h.

A multimodal distribution was observed, revealing the

formation of both oligomeric and polymeric material (Figure S28, Table S3). Analysis of the first broad peak yielded a number-average molecular weight (Mn) of 3961, a weight-average molecular weight (Mw) of 12704, and a polydispersity index (PDI) of 3.21. The relatively large PDI and multimodal distribution suggests a free radical mechanism in which chain branching is an important component. The PDI of 4a can be compared to recently described adamantanebased acrylate polymers with PDIs of up to 20.14,45 Further support for the polymerization mechanism of the allylidene capped materials was obtained from FTIR analysis. Svatos and Attygalle have described typical stretching frequencies for allylidene containing species.44 Weak absorbances between 1700 – 1600 cm-1 can be attributed to C=C stretches, while vibrations between ~1000 – 800 cm-1 can be assigned to 27 ACS Paragon Plus Environment


Page 28 of 35

alkene bending modes.43,44 The spectra of both 2a and 4a exhibited almost complete loss of the terminal alkene signals at 890 and 1000 cm-1 (Figure S30 and Figure S31). This indicated that thermoset 2a and polymer 4a are likely formed though the radical polymerization of the primary alkenes of the allylidene functional groups.46 Additionally, in the case of 4a, the growth of a frequency at 734 cm-1 is consistent with isomerization of an unreacted alkene to an internal cisconfiguration.43,44 This result could also support a 1,4-polymerization mechanism which would directly generate an internal alkene (Scheme S3). A few important differences were observed in the cure chemistry and IR spectra of 4a and 2a. The IR spectrum of thermoset 2a exhibited a low relative absorbance at 1673 cm-1. Considering this in light of the DSC data suggests that free radical polymerization occurs immediately upon melting and locks the HCTD cores in place to yield a rigid polymer network (Scheme 5). Further heating of the material results in a secondary cure reaction, which can be attributed to crosslinking of residual alkenes. These curing events result in significant reductions in the intensity of Scheme 5. Putative polymerization mechanism to form Poly(HCTD-7,12-diallylidene) 2a

n



HCTD

Initial Cure

HCTD

m Poly(HCTD-7,12-diallylidene) (2a)

HCTD-7,12-diallylidene (2)

Secondary Cure

n

HCTD

HCTD

m

o


o

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the peak at 1673 cm-1 (Figure S31). In contrast, the IR spectrum of 4a (Figure S31) exhibits a pronounced peak at 1684 cm-1 suggesting the presence of unreacted internal alkenes. This difference could be attributed to the lower polymerization temperature employed for 4 [180 °C for 16 h (4a) compared to 240 °C for 16 h (2a)]. Based on the results described above, it appears that both monomers (2 and 4) undergo a thermal polymerization through a free-radical mechanism. To further test this theory, an experiment was devised using dibutylhydroxytoluene (BHT) as a radical scavenger. DSC analysis of pure 2 revealed a sharp melting point at 121 °C, followed by an immediate curing event. If the polymerization proceeded through a radical mechanism, the addition of a radical scavenger would be expected to delay the onset of curing. To this end, 2 was treated with either 1 or 5 mol% BHT and subjected to DSC analysis. This resulted in a shift of the onset curing temperature from 127 °C to 151 °C and 166 °C, respectively (Table S4), with the latter representing a processing window of more than 40 °C (Figure 5). This finding provided additional evidence that the mechanism proceeds through a free-radical pathway. Additionally, when BHT was present, there was no longer an observable secondary cure at 249 °C. This suggests that when no antioxidant is present, 2 immediately polymerizes upon melting, resulting in rapid vitrification and trapping of unreacted monomers and oligomers within the network. Increasing the temperature above 200 °C then allows further cross-linking of the monomers and oligomers to yield a fully cured thermoset. In contrast, the addition of 1% BHT impedes the cross-linking reaction, allowing time for the monomer to fully melt. The homogenous liquid sample can then cross-link more efficiently, resulting in complete cure prior to reaching 200 °C. Addition of 5% BHT further increases the cure onset, but also reduces ΔHcure, suggesting that high levels of antioxidant reduce the ultimate cross-link density. The observed shift in the curing



Figure 5. 1st heating cycles for DSC cured samples of 2 in the presence of (red) no BHT, (black) 1 mol% BHT, and (blue) 5 mol% BHT. The scan rate for these experiments was 10 °C/min

temperature demonstrates the ability to enhance the processability of this material, potentially allowing for its incorporation into high-temperature composite materials. Conclusions A unique thermosetting monomer combining a rigid multicyclic core structure and apical allylidene groups has been prepared from norbornadiene. This monomer thermally cures at modest temperatures to generate a highly cross-linked material with no release of volatiles. DSC, IR, solid state NMR, and inhibitor studies suggest that the allylidene groups primarily cross-link through free radical polymerization of the terminal alkenes. The resulting material (2a) exhibits remarkable thermal stability, including a Tg of 377 °C, Td5 at 485 °C, and a char yield of 56% in air at 600 °C. These properties rival those of high temperature polyimides and suggest that this new polymer may have utility for aerospace and defense applications. To our knowledge, this monomer is the first of a new class of thermosetting resins with cross-linking exocyclic allylidene groups. Given the relative ease of preparing molecules with this


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functionality, a new chapter in polymer science can now be explored through the incorporation of this endcap into a variety of monomers and oligomers. To realize the promise of these materials it will be critical to study their mechanical properties and explore their use in carbon fiber composites. In addition, studies focused on enhancing the processability of neat resins or blends with co-monomers will accelerate the transition of this technology for both commercial and defense applications.

Acknowledgements The authors gratefully acknowledge the Office of Naval Research (ONR) Independent Laboratory Internal Research (ILIR) program for financial support of this work. The authors thank Alicia Hughes for collecting select GC-MS and DIP-MS data; Drs. Lee Cambrea and Jessica Cash for collecting FTIR and TGA-IR data; Dr. Tom Groshens for collecting the X-Ray diffraction data for 2; Dr. Jason R. Jones for solving the disordered crystal structure of 2; and Dr. Lawrence Baldwin for performing solid state 13C{1H} NMR on 2a.

Supporting Information Reaction schemes, X-ray data for compound 2, characterization data (NMR, IR, GC-MS, GPC, TGA) for compounds A-C and 1-4.



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TOC Graphic

6-steps



Char yield: Air/N2 = 55% Thermal Stability: Air/N2 = >488 °C

Norbornadiene


Synthesis and Characterization of a High Temperature Thermoset

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