Transforming Polybutadiene with Tetrazine Click Chemistry into

Aug 30, 2017 - International Business Machines Corporation, Tucson, Arizona 85744, United States. §International Business Machines Corporation, Austi...
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Transforming Polybutadiene with Tetrazine Click Chemistry into Antioxidant Foams That Fluoresce with Oxidation Robb E. Bagge,† Timothy C. Mauldin,‡ Dylan J. Boday,§ Brandon M. Kobilka,‡ and Douglas A. Loy*,† †

Department of Chemistry & Biochemistry, University of Arizona, Tucson, Arizona 85721, United States International Business Machines Corporation, Tucson, Arizona 85744, United States § International Business Machines Corporation, Austin, Texas 78758, United States ‡

S Supporting Information *

ABSTRACT: The extent to which oxidative degradation of macromolecules can be delayed is generally limited by the low solubility of antioxidants in most polymers. This can be surmounted by synthesizing macromolecules with covalently attached antioxidant functionalities, but these are frequently expensive. Here, we demonstrate a simple click modification of polybutadienes (PDB) with 3,6-dichloro-1,2,4,5-tetrazine (DCT) that, in addition to modifying and stiffening the polymer chains, releases nitrogen gas to foam the solidifying polymers and generates dihydropyridazine groups that transform them into macromolecular antioxidants. Tetrazines react by a cycloaddition/cycloreversion reaction (Carboni−Lindsey reaction) with the CC bonds to install 1,4-dihydropyridazine groups that increase the mass and rigidity of the butadiene macromolecules. The 1,4-dihydropyridazine group is an effective antioxidant that donates two hydrogen atoms per ring to combine with radicals and forms an aromatic pyridazine ring whose white fluorescence under UV permits visual monitoring of oxidation. Foams made by reacting liquid hydroxyl-terminated polybutadienes with DCT stabilize with thermoset formation through substitution reactions between the hydroxyl and dichlorodihydropyridazine groups. ethynyl,13 allyl,14 or dicyclopentadiene groups15 and with CC bonds in carbon nanotubes16 and graphene,17 the Carboni−Lindsey cycloaddition has not been used to modify the more mundane CC bonds of common macromolecules such as polybutadiene or styrene−butadiene rubber. In addition, aromatization of dihydropyridazines to pyridazines provides two hydrogen atom equivalents for terminating radical chain reactions, just as with 1,4-cyclohexadiene18 or 1,4dihydropyridine19 antioxidants (Figure 1b). Here, we describe the first application of tetrazine click chemistry to chemically modify polydienes with antioxidant dihydropyridazine groups while foaming the solidifying polymers with nitrogen from the Carboni−Lindsey reaction.15

1. INTRODUCTION Antioxidants protect polymers against oxidative damage to the extent that their eventual depletion dictates the lifetime of the polymer.1,2 The amount of antioxidants (hindered phenols, aryl amines, or phosphites) used is limited by their poor solubility in polymers and their attenuation of the polymers’ mechanical properties through plasticization.3 One powerful method for circumventing both of these limitations is to synthesize macromolecular antioxidants.4,5 This also eliminates the potential environmental or health problems of blooming or leaching of antioxidants2 but increases the cost of the polymers with synthesis of specialized monomers. An alternative approach is simple chemical modification of existing commodity polymers to install antioxidant groups in numbers sufficient to substantially increase the lifetime of the polymer.6−8 This goal is now achievable through the click reaction of 1,2,4,5-tetrazines with polymers such as polybutadiene (PBD) bearing reactive CC bonds. Tetrazine-alkene click chemistry is based on the Carboni−Lindsey reaction,9 an inverse electron demand [4π + 2π] cycloaddition of an electron-rich alkene with a 1,2,4,5-tetrazine as an electron-poor diene, followed by a [4π + 2π]-cycloreversion, affording a dihydropyridazine and a molecule of nitrogen (Figure 1a). Generally applied to click or bio-orthoganol modification of macromolecules with norbornenyl,10,11 trans-cyclooctenyl,12 © 2017 American Chemical Society

2. RESULTS AND DISCUSSION 2.1. Click Modification of Polybutadienes. 3,6-Dichloro1,2,4,5-tetrazine (DCT) reacts as soon as it is dissolved in polybutadiene, giving rise to near orange solutions bubbling with nitrogen from the second step of the Carboni−Lindsey reaction (Figure 2). The product of the reaction of DCT with poly(1,4-butadiene) can be described as a copolymer of butadiene and 4,5-dimethylene-3,6-dichloro-1,4Received: July 15, 2017 Revised: August 30, 2017 Published: August 30, 2017 7953

DOI: 10.1021/acs.chemmater.7b02973 Chem. Mater. 2017, 29, 7953−7960

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Chemistry of Materials

Figure 1. Carboni−Lindsey reaction (a) of polybutadiene with tetrazines as reactive modifiers and chemical blowing agents for the production of poly(dihydropyridazine) foams, shown completely converted. The dihydropyridazine acts as a two H atom donor antioxidant (b) oxidizing to the polypyridazine, poly(1,4-BD-co-MCP).

Figure 2. Carboni−Lindsey reaction of poly-1,2-BD (a), poly-1,4-BD (b), and hydroxy-terminated poly(1,4-BD−OH (c) with DCT. Cross-linking between dichloropyridazine groups was observed over time in poly(1,4-BD-co-MCHP) and during the reaction with the hydroxyl terminated poly(1,4-BD) to afford thermosets.

dihydropyridazine:poly(1,4-butadiene-co-4,5-dimethylene-3,6dichloro-1,4-dihydropyridazine) or poly(1,4-BD-co-MCHP) (Figure 2b). The product from the poly(1,2-butadiene) is poly(1,2-butadiene-co-4-vinyl-3,6-dichloro-1,4-dihydropyridazine) or poly(1,2-BD-co-VCHP) (Figure 2a). After oxidation of the dihydropyridazine rings, poly(1,3-butadiene-co-4,5-dimethylene-3,6-dichloropyridazine) or poly(BD-co-MCP) (Figure 2d) and poly(1,2-butadiene-co-4-vinyl-3,6-dichloropyridazine) or poly(BD-co-VCP) were obtained. Because polybutadienes are liquids above 0 °C, direct reaction with DCT could be conducted without solvent. 3,6Diphenyltetrazine, dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate (DMDT), and 3,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1,2,4,5tetrazine all were found to be relatively insoluble in the liquid polybutadienes, but DCT dissolved at concentrations up to 25 mol % (41 wt %). The deep orange, viscous solutions immediately bubbled with nitrogen (Figure 2a) from the

exothermic reactions, became increasingly viscous, and eventually solidified. Polybutadienes, click modified with 25 mol % DCT solidified as opaque, orange foams in about 0.5−1 h (Figure 2b). Tetrazine-alkene click chemistry generates about 26 cm3 N2 from 0.175 g of DCT used to modify 0.268 g of polybutadiene at 25 mol % conversion. In practice, the foam expands from less than 0.5 cm3 to just over 5 cm3. With curing overnight, the color of the polymeric foams fades to tan at room temperature with the consumption of the DCT or to dark brown when cured at 60 °C (Figure 2e) or when treated with ammonia due to nucleophilic attack on the dihydropyridazine rings. Foams made from poly-1,2-polybutadiene with molecular weights over 3 kg/mol were thermoplastic and soluble in aromatic, chlorinated, and ethereal solvents. After weeks at room temperature, these thermoplastic materials gradually cross-linked and became increasingly insoluble. Foams made 7954

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Chemistry of Materials Table 1. Conversion of Polybutadienes with DCT to Foams or Polymers polymer poly(1,4/1,2-BD-coMCHP)-25 poly(1,2-BD-coVCHP)-25 poly(1,2-BD-co-VCP)100 poly(1,4-BD-coMCHP)-25 poly(1,4-BD-co-MCP)100 poly(1,4-BDOH-coMCHP)-12.5 poly(1,4-BDOH-coMCHP)-25 poly(1,4-BDOH-coMCHP)-45

conc DCT mol %

polybutadiene type, MN, MW, Tg

25

80% 1,4-, 20% 1,2-, MN 12294, MW12899, Tg −100 °C

25

1,2-, MN 3042, MW 4414, Tg −20 °C

100

1,2-, MN 3042, MW 4414, Tg −20 °C

25

1,4-cis/trans, MN 6494, MW 22209, Tg −100 °C

100

1,4-cis/trans MN 6494, MW 22209, Tg −100 °C

solvent-free or solution:product solvent free: foam/ collapsed solvent free: foam/ brittle solution: white ppt.

MN, MW, (g/mol)

Tg (°C) −30

23

15234, 15805 4200, 5984

51

4519, 6551

85

25

8184, 36238

95

48

9975, 17432

53

insol.

insol.

extent conver. % 24

5

12.5

hydroxyl terminated, PDI, Tg −100 °C

solvent free: foam/ collapsed solution: light brown ppt solvent free: foam

25

hydroxyl terminated PDI, Tg −100 °C

solvent free: foam

insol

insol.

45

45

hydroxyl terminated PDI, Tg −100 °C

solvent free: foam

insol.

insol.

100

Figure 3. Solution 1H NMR of poly(1,2-BD-co-VCHP)-25 (a), poly(1,2-BD-co-VCP)-100 (c), and poly(1,4-BD-co-MCHP)-25 (e) and solid state 13 C NMR spectra of poly(1,2-BD-co-VCHP)-25 (b), poly(1,2-BD-co-VCP)-100 (d), and poly(1,4-BD-co-MCP)-100 (f).

butadiene) with excess DCT. The orange color of the foams came from residual DCT and was noticeably stronger with higher loadings. The residual tetrazine could be eliminated by extraction or heating in air, but the intractable materials could only be analyzed by elemental analyses and solid state techniques. 2.2. Structure of Click Modified Polymers. The extent of click modification (Table 1) was estimated using 1H NMR to compare new dihydropyridazine resonances to the residual butadiene alkene (cis: δ 5.51, trans: δ 5.64, vinyl: δ 4.94 and 5.40) and allylic (cis: δ 2.08, trans δ 2.06) or vinyl backbone (δ 1.18 and 2.11) peaks. In all 3 of the soluble polymers made with 25 mol % DCT, 23−25 mol % of the butadiene repeat units were converted into vinyldihydropyridazine groups in poly(1,2BD-co-VCHP) (Figure 3a) or dimethylenedihydropyridazine groups in poly(1,4-BD-co-MCHP) (Figure 3e). Where both 1,2- and 1,4-regiochemistries were present in poly(1,4/1,2-BD-

from solvent-free click modification of poly-1,4-butadiene collapsed in a few hours after forming to give transparent, dark brown films (Figure 2b). Similarly, foams prepared with 10 mol % or less DCT or from lower molecular weight poly-1,2butadiene also collapsed and were not investigated further. Click modification of hydroxyl-terminated polybutadienes with 12.5, 25, or 45 mol % DCT (Figure 2e) afforded insoluble, thermoset foams, poly(1,4-BDOH-co-MCHP), presumably cross-linking through nucleophilic attack of the hydroxyl groups on dichlorodihydropyridazine groups. This cross-linking chemistry is slow compared to the Carboni−Lindsey reaction and foaming of the polymers but is considerably faster than cross-linking observed with poly(1,2-BD-co-VCHP) or poly(1,4-BD-co-MCHP) without hydroxyl substituents. If the crosslinking reaction was faster than the cycloaddition chemistry, precipitates would form instead of foams. This is observed in the solution reaction of poly(1,2-butadiene) or poly(1,47955

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Chemistry of Materials co-MCHP), DCT showed no selectivity. In poly(1,2-BD-coVCHP), the backbone methylene (1.5 ppm) and methine resonances (1.8 ppm) are partially obscured by the 1,2butadiene backbone methylene and methine. The dihydropyridazine ring methylene (2.75 ppm) is just downfield from the 1,2-butadiene methine. The backbone methylenes in poly(1,4BD-co-MCHP) samples (Figure 3e) were also found at 1.4 and 2 ppm, while the ring methine peak was near 2.5 ppm. Tautomerization of 4,5-dihydropyridazines to the 1,4-dihydropyridazines, well-known from reaction of small molecule alkenes with tetrazines, was confirmed by NH resonance between 8 and 10 ppm in all of the click modified polymers. The molecular weight of the modified polybutadienes increased proportionally to what would be expected for the amount of DCT used (Table 1). To achieve higher conversions of the carbon−carbon double bonds, modification of polybutadienes with DCT in tetrahydrofuran or methylene chloride was used, followed by oxidation of the dihydropyridazine groups with dimethyldioxirane to the more stable and less reactive pyridazine groups. Dimethyldioxirane was chosen to reduce side products because its byproduct is acetone. Epoxidation of residual alkenes appears to be slow relative to aromatization of the dihydropyridazine. Aromatization to the pyridazines prevented cross-linking by reaction of the iminyl chloride and chloroenamine groups in the 1,4-dihydropyridazines and allowed for solution NMR characterization. With solvent free modification of polybutadienes, no more than 25% DCT could be dissolved in the polybutadiene. Attempts to click modify all of the CC bonds in the polybutadiene were performed in tetrahydrofuran or methylene chloride solution with 100 mol % DCT, but the conversions maxed out at 50%. Further attempts to push conversions to higher levels with more DCT or longer reaction times led to insoluble precipitates. Isolating the polymers at 50% conversion resulted in faster cross-linking than observed in the 25 mol % modified polymers from the solvent-free click modifications. Cross-linking is likely due to nucleophilic substitution of the chloride groups of iminyl groups in dihydropyridazine rings or through nucleophilic substitution (SNAr) of the chloride groups on DCT or chlorotetrazine groups. When polybutadiene does not have hydroxyl groups, the nucleophiles could be enamine or imine groups in dihydropyridazine rings attacking iminyl chloride groups in dichlorodihydropyridazine rings on other macromolecules (Figure 2c). When the polybutadienes have hydroxyl substituents, cross-linking may also occur through SNAr on DCT as there are between two and three hydroxyl groups per polymer chain. Oxidizing the dihydropyridazine groups before isolating the polymer converts the reactive dihydropyridazine groups to more stable, aromatic dichloropyridazine groups. Adding dimethyldioxirane (DMD) to the polymer solution, afforded poly(1,4-BD-co-MCP) and poly(1,2BD-co-VCP) as dramatically less reactive and water-stable polymers (Figure 2d). In the spectrum of poly(1,2-BD-coVCP), the aliphatic region of the spectrum (1−3.5 ppm) is one continuous complex absorption arising from overlapping peaks and line broadening (Figure 3c). The NH peak from the dihydropyridazine precursor was replaced by a new singlet from the pyridazine ring’s sole hydrogen. In poly(1,4-BD-co-MCP) samples, this peak is not expected or observed. Solid state 13C NMR was required for the insoluble thermosets, and its excellent signal-to-noise was helpful in providing spectral details of the soluble polymers prepared by solution polymerizations. In the solid state NMR spectra

(Figures 3b, d, and f), the peaks from the unreacted butadiene repeat units are present and labeled with asterisks. In the materials with the dihydropyridazine groups intact (Figure 3b), the spectrum, resonances for the backbone methylenes and methines from both butadiene residues, and dihydropyridazine groups appear between 20 and 45 ppm. The carbons in the dihydropyridazine rings lie downfield around the butadiene CC resonances. In samples exposed to water during workup, an additional peak near 170 ppm from a dihydropyridazinone amide carbonyl is observed (Figures 3b and S13) and corroborated by low chlorine measurements in elemental analyses. In the spectrum for poly(1,2-BD-co-VCP) (Figure 3d), the two carbons bearing chloride groups are at 158 and 148 ppm, while the aromatic CH is at 130 ppm with the quaternary carbon where the ring is attached to the polymer backbone is at 140 ppm. The minor peak at 100 ppm may indicate that a small amount of epoxidation of the alkene bonds did occur. In the spectrum for poly(1,4-BD-co-MCP)-100, there are only the two expected pyridazine carbons at 158 and 140 ppm and another small peak at 105 ppm that may be due to a small amount of epoxide. 2.3. Properties and Microstructure. DCT click conversion of the flexible and disordered polybutadienes into more rigid and massive poly(dichlorodihydropyridazines) and poly(dichloropyridazines) changed the liquid precursors into solid elastomers, thermoplastics, and thermosets. The nature of the product was dependent mostly on how much of the DCT was used to modify, what the glass transition temperature was for the starting polymer modified, and the availability of nucleophilic hydroxyls or nitrogens. Cis-poly(1,4-butadiene) has a Tg of −100 °C,20 while its DCT-modified product has a Tg of 53 °C. The predominantly vinyl poly(1,2-butadiene) has a Tg of −20 °C21 and its DCT derivative after oxidation has a Tg of 85 °C. We were unable to measure the glass transition temperature for the thermoset derived from the hydroxyterminated polybutadiene which is a mixture of cis and trans 1,4-butadiene repeat units. Conversion to the linear, oxidized poly(1,4-BD-co-MCP) or the poly(1,4-BD-co-VCP) afforded thermoplastics. Thermogravimetric analysis of the polymers with dihydropyridazine groups revealed decomposition starting at 100 °C lower than polybutadiene in nearly every case save PBDOH-co-MCHP made from hydroxyl-terminated poly-BD (Figures S38−S41). The pore sizes for these foams were polydisperse and as large as millimeters in diameter, while SEM imaging indicated the presence of an open cell network with smaller pores in the range of 10s to 100s of micrometers (Figure 4). The nonuniformity of cell size and shape is likely attributed to the absence of a stabilizing surfactant in their formation. The PBDOH-co-MCHP foams were tested and found to be insoluble in methanol, hexanes, toluene, chloroform, and THF. Swelling was observed in toluene, chloroform, and THF, with the largest observed expansion occurring in THF. Thermoset foams could be formed in molds and cubic samples of 10 mm3 were prepared for density measurements and compression testing. Samples of poly(BDOH-co-MCHP)-25 with 25 mol % DCT loading were found to have a density of 0.22 g/cm3, and compression testing provided a compressive strength of 40.67 ± 1.81 kPa at 25% deformation and Young’s modulus of 0.436 ± 0.013 MPa. 2.4. Dihydropyridazines as Free Radical Inhibitors. Oxidation of nonfluorescent 1,4-dihydropyridazines in the click modified polymers removes two hydrogens and affords 7956

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Figure 4. Cross-section (a), microscopic close-up of poly(1,4-BD-coMCHP) foam cellular structure (b), foam surface fluorescing white (c), and nonfluorescent core of foam surrounded by fluorescent outer layer (d).

aromatic pyridazines that fluoresce under ultraviolet light. Because 3,6-dichloropyridazine fluoresces in dilute solution near 390 nm,22 the white fluorescence (λmax = 506 nm) observed on the foams outer layer and observed in solid samples of 3,6-dichloropyridazine (Figures 4 and S2) may be due to emissions from exciplexes formed at the high chromophore concentrations. It is established that air will oxidize dihydropyridazine groups.23 Surface oxidation of the poly(1,4-BD-co-MCHP) thermoset foams is clearly visible in Figure 4c. When the foam was cut in half (Figure 4d), it was possible to distinguish between the fluorescent white, oxidized layer surrounding the nonfluorescent, unoxidized center of the foam. In samples where residual DCT was present, the center of the foam fluoresced orange. Confirmation of the antioxidant properties of dihydropyridazines was obtained by observing their influence at 1 mol % on the polymerization and gelation of 1,4-divinylbenzene and styrene with 1 mol % AIBN at 70 °C under nitrogen (Figure 5). The dihydropyridazines were generated in situ by adding 1 mol % DCT or DMDT to the styrene/divinylbenzene solutions, which changed from red to yellow as the cycloadditions progressed. Without antioxidant, the monomers polymerized to form gels in 11 min (Figure 5a). With 1 mol % BHT, gelation was delayed until 13 min (Figure 5b). With 1 mol % DCT added to the styrene to form the dihydropyridazine then mixed with divinylbenzene and polymerized, gels formed after 20 min (Figure 5c). With 1 mol % DMDT, gelation was delayed 120 min (Figure 5d). The click modified polybutadienes have far greater quantities of dihydropyridazine groups than in these gelation studies that should provide unusually high levels of antioxidant protection.

Figure 5. Evaluation of antioxidant properties of BHT, dichlorodihydropyridazine, and dimethyl dihydropyridazine dicarboxylate by inhibition of free radical polymerization and gelation of styrene/ divinylbenzene. Aldrich. Hydrazine hydrate (100%, 64% hydrazine) and trichloroisocyanuric acid (99%) were from Acros Organics. Methylene chloride (certified ACS stabilized) and 1,4 dioxane (certified ACS) were from Fischer Chemical. Acetonitrile (HPLC grade) and methanol (GR ACS) were from EMD Millipore. Hexanes (ACS grade) were from Macron, and chloroform-d (CDCl3, 0.05% v/v TMS) and dimethyl sulfoxide-d6 (D, 99.9%) were from Cambridge Isotope Laboratories. 3,6-Dichloro-1,2,4,5-tetrazine was prepared according to literature procedures.23−26 DMDO was synthesized immediately prior to use following the procedure of Mikula et al.27 3.2. Instrumentation. 1H and 13C nuclear magnetic resonance (NMR) spectra were obtained using either a Bruker AVIII 400 MHz or a Bruker DRX 500 MHz spectrometer with chemical shifts referenced to TMS (δ 0.00) ppm for 1H and CDCl3 (δ 77.0 ppm) for 13 C. CP-MAS was obtained using a Varian VNMRS 400 MHz NMR spectrometer operating at 100.5246 MHz for 13C (9.6 T static magnetic field). The NMR probe was a 1.6 mm triple resonance T3 high-speed MAS probe (Varian, Palo Alto, CA). All 13C spectra were collected at 20 kHz MAS using 2 ms contact time with rampedamplitude CP (1H power of 86 kHz matching conditions) with 131 kHz TPPM 1H high power decoupling. Adamantane was used as an external chemical shift reference with the 38.6 ppm for 13C spectrometer. Size exclusion chromatography (SEC) was performed in a tetrahydrofuran (THF) mobile phase on either an Acquity APC system (Waters) using three 4.6 mm 150 mm Acquity APC XT columns (450 Å, 2.5 μm; 125 Å, 2.5 μm; and 45 Å, 1.7 μm) connected in series and a refractive index detector calibrated with polystyrene standards or a Waters 1515 isocratic pump running three 5-μm PLgel columns (Polymer Laboratories, pore size 104, 103, and 102 Å) at a flow rate of 1 mL/min with a Waters 2414 differential refractometer and a Waters 2487 dual-wavelength UV−vis spectrometer calibrated with polystyrene standards. Thermogravimetric analysis (TGA) was performed with a Q50 TGA (TA Instruments) from 40 to 800 °C at a rate of 20 °C/min under a nitrogen atmosphere. Differential scanning calorimetry (DSC) was conducted with a Q100 DSC (TA Instruments) with heat cool heat cycles at a heating rate of 20 °C/min and a

3.0. EXPERIMENTAL SECTION 3.1. Materials. All chemicals were purchased from the chemical suppliers and used without further purification. Polybutadiene (containing 20% 1,2 addition, 80% cis- and trans- 1,4, listed avg Mn ∼ 5000), polybutadiene (listed avg Mn ∼ 3000), polybutadiene (predominantly 1,2-addition), polybutadiene (hydroxyl functionalized, listed avg Mn ∼ 1200), and guanidine hydrochloride were from 7957

DOI: 10.1021/acs.chemmater.7b02973 Chem. Mater. 2017, 29, 7953−7960

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Chemistry of Materials cooling rate of 5 °C/min. Data from the second heating cycle are reported. Elemental analyses for C, H, N, and Cl were acquired from ALS Environmental services and those for C, H, and N from Numega Resonance Laboratories. Infrared (IR) spectra were obtained with a Thermo/Nicolet Avatar 360 FT-IR using a Voltex, Inc. HeNe Laser source using either a Harrick MVP-Pro Single Reflection ATR Microsampler, NaCL plates, or in KBr pellet form. Compression testing was performed using an Instron 5542. 3.3. Click Modifications. 3.32.1. Poly(1,4-butadiene-co-(4-vinyl (6%))-4,5-dimethylene (18%)-3,6-dichloro-1,4-dihydro-pyridazine) Collapsed Foam: PBD-co-VMCHP-25. To a 20 mL glass scintillation vial was added poly-1,2-1,4-butadiene (0.270 g, 4.996 mmol) and 3,6dichloro-1,2,4,5-tetrazine (0.175 g, 1.161 mmol). The mixture was stirred at room temperature until the tetrazine was completely dissolved and gas generation could be observed in the bright orange foaming liquid. The foam continued to expand for an additional 30 min. After 24 h, the color of the foam had changed to a light peachyellow color with a small amount of what appeared to be unreacted orange dichlorotetrazine specks contained within it, and the foam was observed to fluoresce yellow under UV light. Mass of product after 24 h (0.401 g). After 1 week, the foam had collapsed into an orangebrown intractable resin. 1H NMR (499 MHz, Chloroform-d, δ): 9.13 (s, 0.3H), 5.39 (d, J = 21.7 Hz, 2H), 5.12 (s, 0.1H), 4.97 (s, 0.3H), 2.85 (s, 0.2H), 2.62 (s, 0.3H), 2.45 (s, 0.2H), 2.30 (d, J = 29.8 Hz, 0.2H), 2.06 (d, J = 22.8 Hz, 4H), 1.67 (d, J = 41.8 Hz, 1H), 1.42 (s, 0.2H), 1.28 (s, 0.4H); 13C NMR (126 MHz, CDCl3, δ): 142.75, 140.70, 129.72, 114.58, 46.42, 43.82, 38.28, 34.23, 32.81, 32.63, 29.53, 29.15, 27.52, 27.32, 24.40, 23.95; IR (ATR, SiO2): ν = 3199 (br, ν s (NH)), 3121 (br, ν s (NH)), 3007, 2919, 2851, 1681 (s, νs(dihydropyridazine CC)), 1448, 1311, 1175, 1076, 969, 914, 742, 666, 626, 575, 553 cm−1; GPC (PS stds, THF): Mn, Mw, PDI = (peak 1) 25361, 27089 Da, 1.07 (peak 2) 11826, 12001 Da, 1.02; Anal. Calcd for C1070H1434Cl114N114: C 64.47, H 7.25, Cl 20.27, N 8.01; found: C 65.47, H 7.69, Cl 12.21, N 9.62. 3.3.2. Poly(1,4-butadiene-co-4,5-dimethylene-3,6-dichloro-1,4-dihydro-pyridazine)-25 Collapsed Foam: PBD-co-MCHP-25. To a glass scintillation vial (20 mL volume) was added poly-1,4-butadiene (0.270 g, 5.00 mmol) and 3,6-dichloro-1,2,4,5-tetrazine (0.188 g, 1.25 mmol). The mixture was blended with a glass stir rod until a bright orange foaming paste was generated. The reaction was left to proceed for 24 h under ambient conditions. The orange foam ceased growing after 3 h and became a light tan color after 24 h (0.432 g). After 72 h, the foam had collapsed into a brown intractable resin. 1H NMR (499 MHz, Chloroform-d, δ) 9.01 (s, 0.1H), 6.74 (s, 0.3H), 5.92−4.82 (m, 2.2H), 3.57 (s, 0.1H), 3.02−2.70 (m, 0.6H), 2.65 (s, 0.3H), 2.55−2.25 (m, 0.7H), 2.11 (q, J = 3.4, 2.8 Hz, 4H), 1.90−0.85 (m, 1.4H); 13C NMR (126 MHz, CDCl3, δ): 140.64, 129.71, 27.53, 27.39, 27.35, 27.32, 27.22, 26.25; IR (ATR, SiO2): ν = 3208 (br, νs(NH)), 3110 (br, νs(NH)), 3007, 2924, 2857, 2148, 1680 (s, νs(dihydropyridazine C C)), 1530, 1452, 1401, 1310, 1220, 1173., 1076, 971, 912, 888, 829, 7423, 674, 624, 589, 567, 552 cm−1; GPC: (PS stds, THF): Mn, Mw, PDI = 8184, 36238 Da, 4.43. Anal. Calcd: C2064H2046Cl420N420: C 64.06, H 6.57, Cl 21.05, N 8.32; found: 64.43, H 8.10, Cl 16.92 from difference, N 10.55. 3.3.3. Poly(1,2-butadiene-co-4-vinyl-3,6-dichloro-1,4-dihydropyridazine-25 Thermoplastic Foam: PBD-co-VCHP-25. To a glass scintillation vial (20 mL volume) was added poly-1,2-butadiene (0.270 g, 5.00 mmol) and 3,6-dichloro-1,2,4,5-tetrazine (0.188 g, 1.25 mmol). The mixture was blended with a glass stir rod until a bright orange foaming paste was generated, and the reaction was left to proceed for 24 h under ambient conditions. The orange foam ceased growing after 2 h and retained its shape after formation (0.416 g) 1H NMR (400 MHz, Chloroform-d, δ) 8.58 (s, 0.1H), 5.45 (d, J = 173.7 Hz, 1.2H), 4.95 (d, J = 10.3 Hz, 2.0H), 3.23−2.28 (m, 0.5H), 2.06 (d, J = 62.2 Hz, 1.5H), 1.68−0.84 (m, 2.7H); 13C NMR (20 kHz, CPMAS): δ 170.18, 156.41, 143.39, 129.94, 115.96, 39.64; IR (ATR, SiO2): ν = 3241(br, νs(NH)), 3077, 2973, 2919, 2846, 1892, 1834, 1682 (s, νs(dihydropyridazine CC)), 1636, 1560, 1452, 1418, 1377, 1312, 1237, 1137, 1077, 996, 910, 830, 756, 668, 627, 611, 571, 547, 488, 474 cm−1; GPC: (PS stds, THF): Mn, Mw, PDI = 4200, 5984 Da,

1.42; Anal. Calcd for C364H492Cl36N36: C 65.76, H 7.46, Cl 19.20, N 7.58; found: C 66.66, H 7.97, Cl 17.17 by difference, N 8.20. 3.3.4. Poly(1,2-butadiene-co-4-vinyl-3,6-dichloropyridazine)-100 Linear Polymer: PBD-co-VCP-100. A 50 mL 14/20 1-neck roundbottom flask was charged with poly-1,2-butadiene (0.216 g, 4.00 mmol), 3,6-dihloro-1,2,4,5-tetrazine (0.604 g, 4.00 mmol), dichloromethane (20 mL), and a Teflon coated magnetic stir bar. The bright orange solution was degassed with argon for 30 min and then refluxed for 72 h during which time the solution developed a slight dark brown color but remained transparent. The reaction was cooled to room temperature (23 °C) and a brown residue was observed to have formed on the sidewalls of the flask. A solution of freshly prepared dimethyl dioxirane in acetone (4.0 mL) was added to the flask, and the reaction was stirred for 1 h. During this time the solution became more transparent. After 1 h, the reaction solution was concentrated under reduced pressure before precipitating into hexanes. The precipitate was collected and dried with vacuum filtration and then dried further by heating to 45 °C under high vac. to yield a white powder (0.448 g). 1 H NMR (400 MHz, DMSO d6, δ): 8.04 (s, 1H), 5.31 (s, 1H), 4.86 (s, 2H), 3.62−0.78 (m, 8H); 13C NMR (20 kHz, CPMAS, δ): 157.18, 142.15, 129.40, 116.90, 101.96, 39.74, 17.92; FT-IR (ATR, SiO2): ν = 3074, 2922, 2852, 1696, 1639, 1560, 1452, 1419, 1380, 1326, 1136, 1080, 1051, 998, 916, 857, 833, 797, 760, 721, 693, 626, 614, 592, 573, 546, 493, 471, 449, 439, 419 cm−1; GPC: (PS stds, THF): Mn, Mw, PDI = 4616, 5930 Da, 1.28; Anal. Calcd for 58% conversion C424H399Cl96N96: C, 49.72; H, 3.93; Cl, 33.23; N, 13.13; found: C 50.63, H 4.83, Cl 30.78 by difference, N 13.76. 3.3.5. Poly(1,4-butadiene-co-4,5-dimethylene-3,6-dichloropyridazine)-100 Linear Polymer: PBD-co-MCP-100. A 50 mL 14/20 1-neck round-bottom flask was charged with poly-1,4-butadiene (0.216 g, 4.00 mmol), DCT (0.604 g, 4.00 mmol), dichloromethane (20 mL), and a Teflon-coated magnetic stir bar. The bright orange solution was degassed with argon for 30 min and then refluxed for 72 h, during which time the solution became an opaque dark brown color. The reaction was then cooled to room temperature (23 °C), and a separate solution of freshly prepared dimethyl dioxirane in acetone (4.0 mL) was added to the flask and stirred for 1 h. During this time, the reaction solution became slightly more transparent. After 1 h, the solution was concentrated under dynamic vacuum, dissolved in THF (2 mL), and precipitated into methanol (20 mL). The precipitate was collected and dried with vacuum filtration and then dried further by heating to 45 °C under high vac. to yield a light brown solid (0.407 g). 1 H NMR (499 MHz, DMSO-d6, δ) 5.38 (s, 2H), 2.77 (s, 3H), 2.20 (s, 2H), 1.87 (s, 2H), 1.37 (s, 3H). 13C NMR (20 kHz, CPMAS, δ) 157.68, 141.75, 129.74, 68.27, 27.58, 17.41; IR (ATR, SiO2): ν = 3009, 2944, 2866, 1637, 1527, 1447, 1396, 1311, 1151, 1075 1019, 917, 797, 739, 585 cm−1; GPC: (PS stds, THF): Mn, Mw, PDI = 9975, 17432 Da, 1.75; Anal. Calcd for 51% conversion C2064H2046Cl420N420: C 52.01, H 4.32, Cl 31.30, N 12.37, found: C 52.50, H 5.30, Cl 29.1 by difference, N 13.10. 3.3.6. Hydroxyl-Functionalized Poly(1,4-butadiene-co-4,5-dimethylene-3,6-dichloropyridazine)-12.5 Thermoset Foam: PBDOHco-MCHP-12.5. To a 20 mL plastic weighing cup was added hydroxylfunctionalized poly-1,4-butadiene (0.272 g, 5.04 mmol) and 3,6dichloro-1,2,4,5-tetrazine (0.094 g, 0.62 mmol). The mixture was stirred until the tetrazine was completely dissolved and gas generation could be observed in the bright orange foaming paste. The orange foam continued to rise under an open air environment at 25 °C for 30 min before ceasing to grow any further. After 24 h, the color of the foam had changed to a light tan color, and the foam was observed to fluoresce yellow under UV light. Mass of product after 24 h (0.3457 g, 99.1%) IR (KBr): ν = 3240 (br, νs(NH)), 3074, 3005, 2920, 2848, 2151, 1679 (s, νs(dihydropyridazine CC)), 1511, 1444, 1380, 1345, 1308, 1251, 1215, 1177, 1077, 995, 967, 912, 830, 727, 667, 624, 489 cm−1; Anal. Calcd for C94H132Cl3N6O6: C 72.91, H 8.59, Cl 6.87, N 5.43, O 6.20; found: C 72.23, H 8.82, Cl 7.56, N 5.65, O 5.74 by difference. 3.3.7. Hydroxyl-Functionalized Poly(1,4-butadiene-co-4,5-dimethylene-3,6-dichloropyridazine)-25 Thermoset Foam: PBDOHco-MCHP-25. To a 20 mL plastic weighing cup was added hydroxyl7958

DOI: 10.1021/acs.chemmater.7b02973 Chem. Mater. 2017, 29, 7953−7960

Article

Chemistry of Materials

in the gel and smoke filling the vial. After removal from the oil bath, the product was observed to be an opaque peach solid. 3.4.4. DMET Sample: Styr, DVB, DMET, and AIBN. A 20 mL glass scintillation vial was charged with styrene (4.166 g, 40.00 mmol), dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate (0.131 g, 0.66 mmol), and a magnetic stir bar. The tetrazine began reacting with styrene immediately upon addition, and vigorous nitrogen bubbling was observed in the solution. The vial was capped with a rubber septum and degassed with argon for 1.5 h while stirring, during which time the solution became a transparent bright yellow color. After 1.5 h, divinylbenzene (1.736 g, 13.33 mmol) and AIBN (0.108 g, 0.66 mmol) were added to the flask. The vial was recapped, and the solution was purged with argon with stirring for an additional 30 min. The vial was kept under positive pressure with argon, transferred to a 70 °C oil bath, and left undisturbed. The solution was observed to have gelled after 120 min, and after 170 min, Trommsdorff autoacceleration led to fissures forming in the gel and smoke filling the vial. After removal from the oil bath, the product was observed to be an opaque yellow solid.

functionalized poly-1,4-butadiene (0.271 g, 5.03 mmol) and 3,6dichloro-1,2,4,5-tetrazine (0.189 g, 1.25 m mol). The mixture was stirred until the tetrazine was completely dissolved and gas generation could be observed in the bright orange foaming paste. The orange foam continued to rise under an open air environment at 25 °C for an approximately 30 min before ceasing to grow any further. After 24 h, the color of the foam had changed to a light peach-yellow color, and the foam was observed to fluoresce yellow under UV light. Mass of product after 24 h (0.408 g, 96.1%) 13C NMR (20 kHz, CPMAS): δ170.82, 147.36, 142.67, 130.56, 116.01, 42.51, 33.16, 28.34; IR (KBr): 3232 (br, νs(NH)), 3107, 3077, 3004, 2919, 2850, 2150, 1678 (s, νs(dihydropyridazine CC)), 1511, 1445, 1383, 1310, 1251, 1219, 1178,1135, 1075, 995, 968, 728, 665, 622, 488 cm−1; Anal. Calcd for C99H131Cl6N11O6: C 66.66, H 7.40, Cl 11.92, N 8.64, O 5.38; found: C 65.57, H 7.95, Cl 11.66, N 8.73, O 6.09 by difference. 3.3.8. Hydroxyl-Functionalized Poly(1,4-butadiene-co-4,5-dimethylene-3,6-dichloropyridazine)-45 Thermoset Foam: PBDOHco-MCHP-45. To a 20 mL plastic weighing cup was added hydroxylfunctionalized poly-1,4-butadiene (0.271 g, 5.02 mmol) and 3,6dichloro-1,2,4,5-tetrazine (0.337 g, 2.23 mmol). The mixture was stirred until the tetrazine was completely dissolved and gas generation could be observed in the bright orange foaming paste. The orange foam continued to rise under an open air environment at 25 °C for an approximately 30 additional minutes before ceasing to grow any further. After 24 h, the color of the foam had changed to a light peachyellow color, and the foam was observed to fluoresce yellow under UV light. Mass of product after 24 h (0.520 g, 95.4%) IR (KBr): 3349 (br, ν s (NH)), 3070, 3009, 2965, 2922, 2852, 2145, 1682 (s, νs(dihydropyridazine CC)), 1613, 1510, 1447, 1433, 1381, 1339, 1299, 1239, 1218, 1178, 1082, 1012, 996, 969, 914, 829, 757, 722, 664, 629, 563, 474 cm−1; Anal. Calcd for C108H132Cl14N20O6: C 56.33, H 5.78, Cl 21.55, N 12.17, O 4.17; experimental before CHCl3 extraction: C 59.31, H 7.01, Cl 15.38; N 11.66; after CHCl3 extraction found: C 64.15, H 7.55, Cl 11.99, N 9.28. 3.4. Antioxidant Gelation Delay Study. 3.4.1. Control Sample: Styrene (Styr), Divinylbenzene (DVB), and AIBN. A 20 mL glass scintillation vial was charged with styrene (4.167 g, 40.01 mmol), divinylbenzene (1.728 g, 13.27 mmol), AIBN (0.108 g, 0.66 mmol), and a magnetic stir bar. The vial was capped with a rubber septum, and the solution was purged with argon while stirring for 30 min. The vial was kept under positive pressure with argon, transferred to a 70 °C oil bath, and left undisturbed. The solution was observed to have gelled after 11 min, and after 13 min, Trommsdorff autoacceleration28,29 led to fissures forming in the gel and smoke filling the vial. After removal from the oil bath, the product was observed to be an opaque white solid. 3.4.2. Control Sample: Styr, DVB, butylatedhydroxytoluene (BHT), and AIBN. A 20 mL glass scintillation vial was charged with styrene (4.165 g, 39.99 mmol), divinylbenzene (1.745 g, 13.40 mmol), BHT (0.145 g, 0.66 mmol), AIBN (0.108 g, 0.66 mmol), and a magnetic stir bar. The vial was capped with a rubber septum, and the solution was purged with argon while stirring for 30 min. The vial was then transferred to a 70 °C oil bath and left undisturbed. The solution was observed to have gelled after 13 min, and the vial was removed from the oil bath, yielding a transparent colorless gel. 3.4.3. DCT Sample: Styr, DVB, DCT, and AIBN. A 20 mL glass scintillation vial was charged with styrene (4.162 g, 39.96 mmol), 3,6dichloro-1,2,4,5-tetrazine (0.100 g, 0.66 mmol), and a magnetic stir bar. The tetrazine began reacting with styrene upon addition, and vigorous nitrogen bubbling was observed in the orange solution. The vial was capped with a rubber septum and degassed with argon for 45 min while stirring, during which time the solution became a transparent dull peach color. After 45 m, divinylbenzene (1.732 g, 13.30 mmol) and AIBN (0.108 g, 0.66 mmol) were added to the flask. The vial was recapped, and the solution was purged with argon with stirring for an additional 30 min. The vial was kept under positive pressure with argon, transferred to a 70 °C oil bath, and left undisturbed. The solution was observed to have gelled after 20 min, and after 23 min, Trommsdorff autoacceleration led to fissures forming

4.0. CONCLUSION In summary, we prepared a new class of polymers through tetrazine click chemistry with polybutadienes. This chemistry can be used to prepare polymeric foams using the nitrogen from the Carboni−Lindsey reaction or nonporous elastomers, thermoplastics, or thermoset polymers from solution-based click chemistry. The 1,4-dihydropyridazine groups in the polymers act as a built-in antioxidant reacting with oxygen in the atmosphere or radicals in reactions to afford the fully aromatic, fluorescent pyridazine ring. This chemistry shows potential for use in generating foamed materials using other liquid polymers that contain alkene substituents and for making inexpensive antioxidant polymers that are highly resistant to oxidation. We are also currently in the process of determining other possible tetrazine candidates that function as chemical blowing agents with polydienes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02973. Experimental details describing tetrazine syntheses, NMR and IR spectra, and thermogravimetric analyses (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Douglas A. Loy: 0000-0001-7635-9958 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Brian Cherry for providing the excellent solid state NMR spectra, Tech Launch Arizona and the ACS Petroleum Research Fund (Grant 50941-ND7) for funding, and the IBM Ph.D Fellowship program.

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REFERENCES

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DOI: 10.1021/acs.chemmater.7b02973 Chem. Mater. 2017, 29, 7953−7960

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