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Ind. Eng. Chem. Res. 2009, 48, 1650–1659
Degradation of Chemical Warfare Agents by Reactive Polymers† Lev Bromberg,‡ Heidi Schreuder-Gibson,§ William R. Creasy,| David J. McGarvey,⊥ Roderick A. Fry,| and T. Alan Hatton*,‡ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, U.S. Army Natick Soldier Research, DeVelopment & Engineering Center, Macromolecular Science Team, Natick, Massachusetts 01760-5020, Science Applications International Corporation, P.O. Box 68, Edgewood Chemical Biological Center, Aberdeen ProVing Ground, Maryland 21010, and U.S. Army Edgewood Chemical and Biological Center, Research and Technology Directorate, Aberdeen ProVing Ground-Edgewood Area, Maryland 21010
Nucleophilic hydrolysis of chemical warfare agents (CWA), S-2-(diisopropylamino)ethyl O-ethyl methylphosphonothioate (VX), O-pinacolyl methylphosphonofluoridate (soman, or GD), and isopropyl methylphosphonofluoridate (sarin, or GB) by polyacrylamidoxime (PANOx) and poly(N-hydroxyacrylamide) (PHA) has been demonstrated. The reactive PANOx and PHA were obtained by one-step oximation of polyacrylonitrile and polyacrylamide, respectively. The polymers were converted to their respective oximate salts at pH values greater than the pKa of oximate or amidoximate groups of 7.5 and 10.8, respectively. Although the PANOx and PHA exhibited spontaneous hydrolysis at ambient temperature and humidity, the conversion of the hydroxamate into the unreactive carboxylic groups was insignificant even at prolonged storage, so that the polymers maintained reactivity at ambient conditions. When exposed to ambient air or 100% humidity, the polymers imbibed up to 65 wt % water, which dramatically enhanced the polymer reactivity toward the CWA under study. The half-lives of VX in heterogeneous hydrolysis, which appeared to be pseudo-firstorder in the polymer dispersions, were measured to be from 0.093 to 4.3 and 7.7 h in the presence of PANOx and PHA, respectively. The rates of hydrolytic activity of PANOx for VX exhibited a strong dependency on the degree of conversion of the amidoxime to amidoximate groups. The half-life of GB was less than 3 min. Only a minor presence of the toxic VX degradation product, S-[2-(diisopropylamino)ethyl]methylphonothioate (EA-2192), was detected in the course of degradation by the reactive polymers. The efficiency, ease of synthesis, and nontoxic nature of the PANOx and PHA polymers make them attractive materials in decontamination and as components of reactive barriers. Introduction Decommissioning nerve agent stockpiles, counteracting nerve agent attacks, and remediation of organophosphate (OP) spills all require development of reactive yet cost-efficient materials for the preparation of decontamination wipes, air filters, column packing, protective wear, and self-decontaminating paints and coatings.1 We focused on nonenzymatic, chemical methods of OP degradation such as hydrolysis using polymeric amines and/ or nanoparticles functionalized by R-nucleophilic agents.2-4 Herein, we specifically concentrated on the performance of functional polymers as reactive barriers toward chemical warfare agents (CWA) such as S-2-(diisopropylamino)ethyl O-ethyl methylphosphonothioate (VX), O-pinacolyl methylphosphonofluoridate (GD, or soman), and isopropyl methylphosphonofluoridate (sarin, or GB) (Table 1). VX is the most persistent CWA, given its low volatility and Henry’s law constants (Table 1). Moreover, it is the most resistant CWA to water hydrolysis and hence to degradation in aqueous media.5 In water, VX undergoes a slow, but selective, hydrolysis to yield relatively nontoxic ethyl methylphophonate (EMPA).6 On the other hand, basic hydrolysis is nonselective, † Any opinions, findings, conclusions and recommendations expressed in this article are those of the authors and do not necessarily reflect the view of the U.S. Army Research Office. * To whom correspondence should be addressed. Tel.: 617-253-4588. Fax: 617-253-8723. E-mail:
[email protected]. ‡ Massachusetts Institute of Technology. § Natick Soldier Research. | Science Applications International Corporation. ⊥ U.S. Army Edgewood Chemical and Biological Center.
resulting in up to 22% of the toxic S-[2-(diisopropylamino)ethyl]methylphonothioate (EA-2192).6,7 Hydrolytic action toward organophosphates by low-molecular weight oximates such as hydroxamic acids8-11 and amidoximes12-14 has been established. Primary amines and imines are also capable of degrading OP compounds via the general SN2 mechanism of base catalysis.15,16 Amidoximes act as strong nucleophiles in deacylation and dephosphorylation of esters, but their efficiency (especially in dephosphorylation) is enhanced dramatically by conversion into corresponding amidoximate ions14 (Scheme 1). Oximes typically dissociate in the pH range of 7-10,2,3,14 generating oximate ions, and thus it is feasible to apply them in aqueous solutions that are only slightly basic, such as oceans, lakes, and other natural reservoirs. It has been pointed out14 that since the amidoximes are much less acidic than the corresponding oximes, with pKa values generally in excess of 11-12, the practical value of the amidoximes in dephosphorylation (when carried out in aqueous milieu) would be insignificant, because they are deprotonated only in a strongly alkaline environment. However, we believe that polymeric amidoximes, in a form of cross-linked polyamidoximate salts, can function as effective dephosphorylation agents by generating amidoximate ions on the surface of the polymeric salt particle, without the excess of hydroxyl ions present. Such reactive polymers (if cost-effective and nontoxic) would be promising materials for modification of surfaces such as in textiles, coatings, and sorbents. Reports on chemical hydrolysis of CWA and analogues mediated by polymers have been limited to polymeric amine-copper(II) complexes17 and alkaline hydroly-
10.1021/ie801150y CCC: $40.75 2009 American Chemical Society Published on Web 12/23/2008
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1651 Table 1. Structure and Properties of CWA Used in This Study
CWA molecular weight boiling point, °C vapor pressure at 25 °C, mm Hg water solubility, g/L Henry’s law constant, atm × m3/mol
GB
GD
140.1 182.2 158 198 2.10 (20 °C) 0.40 miscible 1.52 × 10-7
1
VX 267.4 298, decomposes 0.0007
21 (20 °C) 30 4.6 × 10-6 3.5 × 10-9
Scheme 1. Dissociation of Polyacrylamidoxime into Polyacrylamidoximate
sis in latex dispersions, wherein the substrate (organophosphonate) concentrations in the latex particle are higher than those in the bulk phase because of the preferential partition, thus accelerating the rate of the hydrolysis within the polymer phase.18 Catalytic properties of polymers bearing oxime functionality toward ester hydrolysis in water have been noted,19 but tested with esters such as p-nitrophenyl acetate, degradation of which is a poor predictor of the activity toward CWA. In this work, we studied the processes of CWA degradation by polymeric N-hydroxylated amidine, polyacrylamidoxime (PANOx), as well as poly(N-hydroxyacrylamide) (polyhydroxamic acid, or PHA), polymers capable of generating nucleophilic oximate groups in each of their respective units by dissociation of the amidoxime or hydroxamic groups, respectively. Both PANOx and PHA are eco-friendly, cost-effective, nontoxic polymers that are the products of essentially one-step oximation of ubiquitous polyacrylonitrile and polyacrylamide. The ability of PANOx and PHA, predominantly in fibrous form or as resins, to strongly bind uranyl (UO22+), Cu2+, Hg2+, Pb2+, Pd2+, and some other heavy metal cations, useful in purification of water resources and remediation of contaminated groundwater has been emphasized, and biocompatibility and biological activity of PHA have been described.20-37 Nevertheless, the potential utilization of PANOx and PHA in degradation of CWA and capture of the products of such decomposition have not been established. Herein, we present a study of the hydrolysis of GB, GD, and VX by PANOx and PHA in the presence of water. The issue of the effect of the amidoxime-to-amidoximate group conversion in the PANOx under study was addressed. It appeared that PANOx and PHA hydrolyzed the CWA avidly, as described below. Experimental Section Materials. Polyacrylonitrile (PAN) (Mw ) 150 kDa) was purchased from Polysciences, Inc. and used as received. Polyacrylamide (PAAm, 50 wt % aqueous solution, average Mw ) 10 kDa), hydroxylamine hydrochloride (98%), and anhydrous
Scheme 2. Conversion of PAN to PANOx by Reaction of PAN with Hydroxylamine
methanol (99+%) were obtained from Sigma-Aldrich Chemical Co. PAAm solution was frozen at -80 °C and lyophilized to dryness before use. CWA isopropyl methylphosphonofluoridate (sarin, or GB), pinacolyl methylphosphonofluoridate (soman, or GD), and S,2-diisopropylaminoethyl methylphosphonothioate (VX) were obtained from the Chemical Agent Standard Analytical Reference Material program at the Edgewood Chemical and Biological Center. The agent purities were determined by liquid NMR and confirmed by gas chromatography with thermal conductivity detection. Purities of GD, GB, and VX were 97, 97, and 95 wt %, respectively. All handling of the CWA was conducted in a chemical surety laboratory certified for supertoxic compounds. Caution. Median lethal inhalation concentration (LCt50) to 50% of exposed population for GB, GD, and VX is 100, 50, and 10 mg × min × m-3, respectively. Ct refers to the concentration of the vapor or aerosol in the air multiplied by the time the individual is exposed. These compounds are extremely hazardous and should only be handled by trained personnel following appropriate safety precautions. PANOx Synthesis. Polyacrylonitrile was modified to obtain PANOx via reaction of PAN with excess hydroxylamine hydrochloride in methanol. The reaction (Scheme 2) was carried out at 70-80 °C in sealed, glass VWR media bottles. In a typical synthesis, PAN 10.6 g (200 mmol) was dispersed in a bottle containing anhydrous methanol (200 mL) and hydroxylamine hydrochloride (17.2 g, 250 mmol) was added in a finely powdered form. The mixture was stirred gently at room temperature, then sodium hydroxide (10 g, 250 mmol) was added under stirring, and the bottle was sealed and kept at 75 °C for 24 h. No boiling was observed. The bottle was allowed to equilibrate at ambient temperature and opened, and solid contents were filtered off for further use. The resulting polymer was washed repeatedly at room temperature with methanol. Samples were withdrawn from the main fraction, washed with deionized water, snap-frozen, dried under 10-5 Torr vacuum, and subjected to elemental analysis. Calcd, based on 100% conversion of PAN into PANOx: C, 43.50; H, 7.44; N, 31.22; O, 17.83. Found: C, 43.05; H, 7.73; N, 31.11; O, 18.12. Yield of the oximated product was 97 mol % relative to the initial PAN. 1H (DMSO-d6): 8.38 (broad m, oxime protons), 7.12 (m, NH2), 2.12 (m, CH2 in the main chain), 1.73 ppm (broad m, CH in the main chain); 13C(DMSO-d6): 158 ppm (m, >CdN- carbon); FTIR (KBr): 2945 (stretching, CH2), 1645 (stretching, CdN), 1450 (scissors vibration, CH2), 1394 (scissors vibration, CH), 937 cm-1 (N-O stretch). Two techniques of PANOx post-treatment were implemented, as follows, resulting in two polymer batches (designated A and B) with different degrees of conversion of amidoxime groups to amidoximate. PANOx-A. This polymer fraction was equilibrated with deionized water under gentle shaking overnight (pH adjusted to 7.4), then the polymer was filtered off using filter paper (pore diameter ≈ 10 µm), snap-frozen, and dried under vacuum. PANOx-B. Polymer fraction was dialyzed against excess deionized water (molecular weight cutoff 12-14 kDa, external
1652 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 Scheme 3. Conversion of PAAm to PHA by Oximation of PAAm with Hydroxylamine
water pH adjusted to 12 by 0.3 M NaOH) followed by lyophilization until constant weight. The polymer fractions were stored at -20 °C and ground to submillimeter-sized particles using mortar and pestle before use. In a separate series of tests, the polymer was stored, in a finely ground form, in ventilated vials at room temperature and relative humidity of 30% for extended periods of time. A small fraction of the PANOx was deuterated by dialysis against excess D2O (membrane Mw cutoff, 12-14 kDa) and lyophilized. Poly(N-hydroxyacrylamide) Synthesis (Scheme 3). A 50 wt % aqueous solution of polyacrylamide (300 mL, 2.11 mol) was mixed with a freshly prepared solution of 150 g (2.16 mol) of hydroxylamine hydrochloride in 100 mL of deionized water. The resulting solution was stirred overnight at room temperature, and a solution of sodium hydroxide (85 g in 100 mL) was added followed by stirring for 3 days, again at room temperature. Release of ammonia by bubbling was observed. The resulting solution was frozen at -80 °C and lyophilized. The resultant PHA was dissolved in deionized water at 50 wt %, and the solution was dialyzed against excess deionized water (molecular weight cutoff, 6-8 kDa, pH adjusted to 7.5) and lyophilized. The yield of PHA was in excess of 90 mol % relative to the initial amount of PAAm (Scheme 3), with the losses occurring during the dialysis procedure. Dry PHA was stored at -20 °C. 1 H (D2O): 1.91 (1H, CH in the main chain), 1.39, 2H (CH2 in the main chain). FTIR (KBr): 3600 (broad, -NHOH), 3430 (broad, OH stretching), 3210 (amide NH stretching), 1670 (CdO, stretching), 1630 (CO-NH bending), 1450 (scissoring, CH2), 1425 (C-N stretch). Methods. Potentiometric titration was performed at 25 ( 1 °C using a 736 GP Titrino potentiometric titration system (Metrohm Ltd.). The results of the titration were expressed through the equivalence point recognition criterion (ERC) and shown in arbitrary units. The solution of PHA was brought to an initial pH of 11.76 and then titrated with 0.1 M HCl using an automatic titrator. The equivalence point was expressed in arbitrary units as a function of pH, and the data were processed by the built-in instrument software.38 The ζ-potential of a PANOx particle dispersion in 10 mM KCl was measured using a ZetaPALS system (Brookhaven Instruments Corporation). A 0.1-0.2 wt % dispersion was sonicated briefly, and the pH was adjusted by addition of minute quantities of 0.1 M aqueous NaOH or HCl solutions. Following a 2-h equilibration, the final pH was recorded and ζ-potential was measured (n ) 5). Liquid NMR measurements were performed at 25 ( 0.5 °C using a Bruker DRX 401 spectrometer. 1H and 13C resonance frequencies were 400.13 and 100.61 MHz, respectively. Proton
decoupling was applied throughout, and at least 15 000-20 000 scans were collected in the 13C NMR measurements. Polymer concentrations in D2O in 1H and 13C NMR measurements were set at 10 and 15 wt %, respectively, and the pD was adjusted to ∼10.0 by addition of minute quantities of NaOD. FTIR spectra were measured in KBr using a Nicolet 8700 spectrometer (Thermo Scientific Corp.) in the absorbance mode by accumulation of 256 scans with a resolution of 4 cm-1. The FTIR spectrometer was equipped with a Nicolet FTIR/TGA interface and a transfer line to monitor interferograms of the vapors evolving during the thermogravimetric analysis (TGA) heating scan with the sequence interval corresponding to the duration of the TGA ramp. Raman spectra were measured using a Kaiser Hololab 5000R Raman spectrometer (Kaiser Optical Systems, Inc.) with an excitation wavelength of 785 nm. TGA was conducted using a Q5000IR thermogravimetric analyzer (TA Instruments, Inc.). Samples were subjected to heating scans (20 °C/min) in a temperature ramp mode. Water content in hydrated samples was measured in triplicate. Kinetics of CWA Degradation. Kinetics of the CWA degradation were measured on PANOx and PHA samples hydrated by the following three different methods: 1. A weighed sample was placed in a sealed vial with a wet filter paper at room temperature. The water vapor in the vial reached 100% relative humidity, and the polymer absorbed water until it equilibrated. Water content in a fraction of each sample was measured by TGA as described above. 2. A weighed sample of polymer was added to a measured volume of water and allowed to absorb the liquid before the addition of CWA. 3. Polymer was equilibrated with ambient room air. Samples of the polymers were packed into 5-mm o.d. zirconium high-resolution magic-angle spinning (HRMAS) rotors and weighed. The weights of the polymer samples ranged between 35 and 85 mg, affording a large molar excess of the oximate groups compared to that of the CWA. The kinetic measurement commenced with the addition of 1 µL (∼1 mg) of a CWA by a pipet onto the polymer sample. The agents appeared to rapidly soak into the polymer samples without beading up. Rotors were capped and inserted into the instrument and spun up to the MAS spinning rate. Between the NMR experiments, the samples were removed and stored at room temperature. All experiments with CWA were performed using a Bruker Avance 500 spectrometer, operating at 202.46 MHz (31P). The HRMAS spectra were recorded with a 5-mm Bruker 31P gradient probe at ambient temperature (23 °C) using a 1-D proton decoupled decay experiment with the magic-angle spinning rate of 5000 Hz. The following parameters were applied: center frequency of the acquisition (O1P), 20 ppm (4049 Hz); sweep width, 200.8 ppm; number of scans, 128; excite pulse length (pw90), 8 µs; recycle delay, 3 s; total experiment run time, 7.5 min. Small relative distortions of the relative peak areas, due to the short recycle delay, were about the same as or less than the spectral signal-to-noise ratios. For the kinetic determinations, it was necessary to trade off precision with acquisition speed to obtain fast data acquisition. The presence, if any, of the VX decomposition byproduct, EA-2192, was tested as follows. The VX (1 µL) was deposited onto 34.6 mg of PANOx-B, and the hydrolysis was allowed to proceed for 1.5 h. Neither unreacted VX nor EA-2192 was detected in the sample by the HRMAS 31P using a long acquisition time. Then the polymer was removed from the HRMAS rotor, 1 mL of 2-propanol was added, and the
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1653 Scheme 4. Formation of Glutaroimide-Dioxime Cross-Links in PANOx by Loss of Ammonia
Figure 1. FTIR (KBr) (a) and Raman (b) spectra of as-prepared PANOx-A and parent PAN.
dispersion was vortexed for 30 min. The extract was diluted 10-fold by 2-propanol and subjected to an LC/MS study as described previously.39 LC/MS measurements were accomplished using an Agilent Series 1100 LC/MSD spectrometer with electrospray ionization and equipped with quaternary pump, autosampler, degasser, and variable wavelength detector. In liquid chromatography, a Phenomenex Aqua C18 (150 mm × 4.6 mm, 5 µm) column with a mobile phase mixture consisting of 90% aqueous formate buffer and 10% acetonitrile was utilized. A calibration curve was generated using standards of EA-2192. Extraction efficiency of the EA-2192 from the PANOx was not established.40 Results and Discussion Polymer Characteristics. The composition of PANOx and PHA, in terms of the oxime functionality, depends on the conditions of the conversion of the parent polymers into the corresponding oximate species as well as on the extent of the subsequent polymer hydrolysis. Our synthetic procedures were optimized with respect to the extent of conversion of the chosen precursor polymers, PAN and PAAm, into PANOx and PHA, respectively. FTIR and Raman spectra of as-prepared PANOx samples and their parent PAN are shown in Figure 1. As is seen, the characteristic stretching vibration band of the nitrile side group, observed in both FTIR and Raman spectra at 2240-2245 cm-1, disappeared as a result of the nitrile group consumption in the oximation, indicating a conversion close to 100%. Instead, strong signals appeared at 1645-1654 cm-1 corresponding to the stretching vibration of the CdN group of the amidoxime.41 The IR signal at around 1646 cm-1 was rather broad and could be deconvolved into a very strong band at 1646 cm-1, indicative of the oximate ion (>CdNO-) stretch, and a weaker band at 1600 cm-1 corresponding to the NH2 scissors mode. The latter disappeared in a deuterated PANOx sample (data not shown). The Raman signals corresponding to the oximate ion stretch and amino group scissors vibration were similarly seen at 1654 and 1600 cm-1, respectively. These observations showed that the PANOx sample contained ionized
oximate groups. No discernible differences were observed by vibrational spectroscopy between PANOx-A and PANOx-B fractions. Elemental analysis of the PANOx-A fraction (found: C, 42.05; H, 6.22; N, 28.13; Na, 4.48; O, 19.12) corresponded to approximately 71 mol % of amidoxime, 14 mol % of sodium amidoximate, and 7 mol % each of sodium hydroxamate and glutaroimide-dioxime groups (calcd: C, 41.15; H, 6.58; N, 28.59; Na, 5.03; O, 18.66). In contrast, elemental analysis of the PANOx-B sample resulting from the oximation followed by dialysis against water with pH 12 (Experimental Section) corresponded to approximately 86 mol % sodium amidoximate and 7 mol % each of sodium hydroxamate and glutaroimide-dioxime groups (Found: C, 35.31; H. 5.45; N, 24.23; Na, 18.16; O, 16.85. Calcd: C, 35.47; H, 5.03; N, 24.64; Na, 18.78; O, 16.08). Therefore, it appeared that the PANOx samples prepared via different post-treatment procedures possessed significantly different degrees of conversion of amidoxime to sodium amidoximate groups. That is, in PANOx-A only 16% of available amidoxime groups were converted, whereas in the PANOx-B sample 100% of the available amidoxime groups were converted to amidoximate. The presence of the glutaroimide-dioxime links42 (Scheme 4) can explain the observed insolubility of the PANOx particles in water and most organic solvents, except for dimethylsulfoxide. Liquid NMR characterization of the PANOx was thus carried out in DMSO-d6, where 13C NMR signals at 120-121 ppm corresponding to the nitrile carbon (-CtN) in PAN43 disappeared and signals around 148 ppm corresponding to the imine carbon (>CdN) group of PANOx appeared. These results were consistent with the full conversion of the nitrile groups of PAN. The PHA samples resulting from our synthesis were readily soluble in water. 13C NMR spectra of the PHA measured in D2O (Figure 2) featured signals at 38.7 and 172.3 ppm that could be assigned to the methyne carbons in the backbone next to the hydroxamic side chains and to the carbonyl groups of the Nhydroxyacrylamide unit, respectively. These signals were absent in the spectrum of the parent polyacrylamide. The characteristic signal of the >CdO group in the PAAm spectrum around 179.4 ppm was slightly shifted upfield, and a new signal in the PHA spectrum observed at 182.5 ppm corresponded to the carbonyl of the carboxylate group.44,45 These observations indicate some degree of the PHA hydrolysis leading to the acrylic acid side chain formation. The content of carboxylic groups and overall composition of PANOx and PHA were further quantified using pH-dependent ζ-potential measurements and potentiometric titration, respectively (Figure 3), along with elemental analysis. The PHA titration revealed two distinct pKa values at 4.8 and 7.6, corresponding to the carboxylic and hydroxamic acid groups, respectively. Analogously, the points of inflection in the
1654 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 Scheme 5. Hydrolysis of the Polymer’s Amidoxime Group into Hydroxamic and Acrylic Acid Groups30-32 and Corresponding pKa Values Obtained in the Present Work
Figure 2. 100.61 MHz 13C NMR spectra of PHA and parent PAAm in D2O. Mw of polymers ) 10 kDa; solution concentration, 12 wt %, 22 000 scans, proton decoupling applied throughout.
Figure 3. Typical pH dependencies of ζ-potential of 0.1 wt % PANOx-B particle suspension (open points) and potentiometric titration of 0.2 wt % solution of PHA (solid line) obtained in degassed 10 mM KCl solutions. The results of the potentiometric titration were expressed through the ERC and shown in arbitrary units.
ζ-potential of the PANOx dispersion vs pH curve (Figure 3) indicated the charging behavior of this polymer related to various ionizable groups such as amino, carboxylic, hydroxamic, and amidoxime. Positive charges on PANOx particles below pH 5.5 were related to the protonated amino groups of the amidoxime, while ionization of the hydroxamic and oxime groups could be identified with pKa around 7.5 and 10.8, respectively. The observed pKa of the amidoxime groups in PANOx corresponded well with the pKa values reported for the low molecular weight PANOx analogues. For instance, pKa of the amidoxime group was reported to be 10.7-10.8 for succinediamidoxime.46 Hydrolytic conversion of the amidoxime into hydroxamic acid followed by the hydrolysis of the latter into the acrylic acid is depicted in Scheme 5. Elemental analysis of the PHA resulting from our synthetic procedure and dialyzed against water with pH 7.5 (Experimental Section) (found, C, 37.54; H, 4.75; N, 13.48; Na, 10.66; O, 33.57) afforded the following monomer unit composition: sodium hydroxamate, 40 mol %; hydroxamic acid, 28 mol %; acrylamide, 20%, and sodium acrylate, 12 mol % (calcd, C, 37.82; H, 4.44; N, 13.53; Na, 10.62; O, 33.59). Hence, the yield of the oximation was approximately 80%. The extent of the hydroxamate group hydrolysis was not significant, however, as our synthesis was conducted at room temperature.
Elevated temperature would result in extensive hydrolysis of the formed PHA. As explained above, oximation of the PANOx was close to 100% conversion, but at elevated temperature in the absence of water some loss of ammonia occurred resulting in the cross-links (Scheme 4). Heating PANOx in the presence of water would convert it into PHA as a first step (Scheme 5). Given the “transient” nature of the PANOx and PHA polymers that are prone to the hydrolytic reactions, we evaluated the effects of the polymer storage at ambient temperature and humidity conditions. Accelerated degradation at elevated temperatures was studied using thermogravimetric analysis. Figure 4 depicts TGA of as-received PAN and as-synthesized PHA and PANOx polymers, as well as TGA of the PHA and PANOx samples equilibrated at 100% humidity. As is seen, the water content in the hydrophobic PAN was below 1% and the polymer decomposed sharply in the 309-344 °C range, losing more than 35% of its weight. In contrast, PANOx, which initially contained about 1.4 wt % of the hydration water, decomposed via a threestep thermo-oxidative degradation at temperatures above 160, 217, and 305 °C, with the weight loss of 8.7, 25.9, and 48.4%, respectively. These observations, indicating lower thermal stability of the PANOx than its parent PAN, corresponded well with previously published data.22 Analogously, as-prepared PHA showed several-step degradation with onsets at 123, 181, and 325 °C with weight losses of 7.7, 13.7, and 36.5%, respectively. Samples of both PANOx and PHA were very hygroscopic, exhibiting contents of water as much as 63 ( 8 and 53 ( 9 wt %, respectively, after equilibration in a saturated water vapor atmosphere at 100% humidity at room temperature. FTIR spectra of the vapors evolving from the hydrated PANOX and PHA exhibited clearly identifiable ammonia bands at 3334, 964, and 930 cm-1, starting at 80-99 °C (Figure 5). Ammonia became the most prominent vapor evolving in all PANOx and PHA samples at 150-180 °C, indicating vigorous hydrolytic processes at these temperatures, with the conversion of PANOx to PHA and further into poly(acrylic acid) before the polymer decomposition. These processes, accelerated by the elevated temperature in the TGA experiments, also occurred at ambient temperature and humidity upon storage of the as-prepared
Figure 4. Thermogravimetric analysis of representative samples of PAN, PHA, and PANOx-B polymers. PAN, PHA, and PANOx-B designate asreceived or as-prepared samples. PHA hydrated and PANOx-B hydrated designate as-prepared samples equilibrated at 100% humidity and room temperature for two weeks. Equilibration was carried out in desiccators in atmosphere saturated by vapors of deionized, distilled water.
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1655
Figure 5. FTIR spectra of vapors evolving in TGA runs of hydrated PANOx-B at 99 and 179 °C and aqueous 30% ammonia at 49 °C. Characteristic ammonia bands were observed at 3334, 964, and 930 cm-1 corresponding to the N-H stretching modes, symmetric bending, and rotational bands.47
samples. Thus, a PANOx fraction kept at ambient temperature and humidity for six months, snap-frozen by liquid nitrogen and lyophilized (found: C, 36.38; H, 5.08; N, 19.91; Na, 17.35; O, 21.28), contained approximately 56 mol % sodium amidoximate, 30 mol % sodium hydroxamate, and 7 mol % each of acrylic acid and glutaroximide groups (calcd: C, 36.12; H, 4.78; N, 20.24; Na, 17.62; O, 21.23), with either a ca. 5% loss of nitrogen or gain of oxygen, respectively, compared to the initial sample. Importantly, however, these changes, albeit significant, would not lead to any marked deterioration of the ability of the “aged” PANOx to generate nucleophilic oximate groups, because of the apparent lack of significant conversion of the hydroxamic to the acrylic acid groups. Similarly, a PHA sample kept at ambient temperature and humidity for six months, snapfrozen by liquid nitrogen and lyophilized (found: C, 38.20; H, 4.05; N, 10.02; Na, 10.26; O, 37.47), gave the following estimated monomer unit composition: sodium hydroxamate, 40 mol %; hydroxamic acid, 28 mol %; acrylic acid, 20 mol%; and sodium acrylate, 12 mol % (calcd: C, 38.03; H, 3.99; N, 9.98; Na, 10.01; O, 37.99). A loss of approximately 3.5% nitrogen and a 4% increase in oxygen content can be attributed to the conversion of residual acrylamide groups in the asprepared PHA to acrylic acid. The apparent absence of any degradation of hydroxamic groups under these conditions is a favorable indication that this polymer retains its reactivity on aging. Kinetics and Mechanisms of GD and VX Degradation. Sufficient stability of the PANOx and PHA polymers under ambient conditions and the presence of reactive amidoxime and/ or hydroxamic functionalities called for further testing of the polymer performance in degrading CWA. Several representative 31 P HRMAS NMR spectra shown in Figure 6 illustrate the kinetics of soman (GD) degradation. GD was added to PANOx powder equilibrated at 100% relative humidity and spectra measured versus time. The four resonance peaks in the 26-32 ppm range are from GD, corresponding to the doublet (31P split by F, JP-F ) 1040 Hz) for each of the two diastereoisomers,48 or a doublet of doublets.48,49 The relative intensity of the two GD doublets decreased over time, while the single signal belonging to pinacolyl methylphosphonic acid (PMPA)48,50 centered at 25.2 ppm increased. Signals of other products of GD degradation that appeared at 34.3 and 33.8 ppm were tentatively assigned to the reaction of GD with the polymer oxime group,50 although positive identification could not be made because of the transient nature of these products, since they continued to react and the signals decreased in intensity. The products were bound to the polymer sample, and therefore they could not be extracted for confirmation by
Figure 6. Representative 202.64 MHz 31P HRMAS NMR spectra of soman (GD) deposited onto hydrated PANOx-A as a function of time. Total PANOx-A weight, 73 mg; water content, 54 ( 9 wt %; initial GD amount, 1 µL. PMPA stands for pinacolyl methylphosphonic acid.
Scheme 6. Degradation of GB and GD by PANOx or PHA in the Presence of Water
other techniques. The chemical shifts of these products were comparable to results reported for reactions of potassium 2,3butanedione monooximate with GD, which produced 31P resonances at 38.6 and 38.2 ppm, with a doublet corresponding to the diastereomers.50 These signals are also close to the ones that were previously assigned to pinacolyl methylphosphonic acid peroxide at 39.1 and 38.2 ppm.51 Analogous hydrolytic reactions were observed with hydrated PHA samples. Overall, the 31P spectra were consistent with the hydrolysis of the GD via nucleophilic attack of the polymeric oximate ion (either amidoximate or hydroxamate) on the P-F bond (Scheme 6). The final product (PMPA) may be bound into the polymer matrix, but the narrow NMR signals indicate that at least some of the products are mobile on the surface. Analogously, the 31P HRMAS NMR spectra of VX deposited onto hydrated PANOx (Figure 7) or PHA samples indicated facile degradation of VX (signal at 61 ppm) via formation of ethyl
Figure 7. Representative 202.64 MHz 31P HRMAS NMR spectra of VX deposited onto hydrated PANOx-A as a function of time. Total PANOx weight, 88 mg; water content, 54 ( 9 wt %; initial VX amount, 1 µL. EMPA stands for ethyl methylphosphonic acid.
1656 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 Scheme 7. Degradation Pathways of VX in the Presence of Alkali7,52,56,57 versus Those in the Presence of Wet PANOx and PHA
methylphosphonic acid (bound or free EMPA, signal at 26 ppm) by the cleavage of the electrophilic P-S bond (Scheme 7). The chemical shift corresponding to VX on PANOx (61 ppm) was calibrated externally relative to 85% H3PO4. The chemical shift of the VX signal in liquids is solvent-dependent, varying from 61.7 ppm in water to 55.4 ppm in tert-butyl alcohol and to 57 ppm in dueterated chloroform.52 In solids, the chemical shift was reported to be as low as 47 ppm on activated charcoal,53 although a range of 50-53 ppm is more typical for an aprotic, inorganic solid, and 60-63 ppm has been observed for protonated VX.53-55 On concrete, the chemical shift of VX is about 63 ppm.7,54 The chemical shift for VX observed in the present work is consistent with protonation or solvation of VX by water. The NMR bands did not exhibit any spinning sidebands indicative of a chemical shift anisotropy expected for phosphonates that are not motionally averaged. It must be noted that if the binding is strong enough to completely restrict the motion of the 31P, the corresponding signal may become so broad that it would be “lost” in the baseline, and thus we cannot rule out the possibility that some strong binding between the products of the VX hydrolysis and the polymer occurred. Interestingly, in that regard the reaction of GD with PHA demonstrated some unique features. After reaction with PHA, the signals of the GD products were not observable. In this case, the reactant CWA signal decreased as the reaction progressed, but the corresponding product peaks did not appear, consistent with the notion that the majority of the reaction product was strongly bound to the polymer, making it too immobile to be detected by the HRMAS NMR method. It may be possible to use cross-polarization magic-angle spinning (CP-MAS) to observe these products, but that was beyond the scope of the present study. The 31P signal of the sarin (GB) reaction product, isopropyl methylphosphonic acid (IMPA), with PANOx-B was detected at 26.5 ppm, but the reaction was so rapid that it was not possible
to monitor the progress of the GB hydrolysis. The reaction occurred rapidly even when the PANOx-B polymer was hydrated at ambient relative humidity, with no liquid water added. This implies that the reaction half-life was 0.97). As discussed above, in the presence of PHA, the signals of the GD products were not clearly discernible, and thus in this particular case the time-dependent kinetic parameters were estimated from the decline in the absolute GD signal integration (∆ln(Ft)/∆t ≈ ∆Is/∆t). Previous unpublished results indicated that the HRMAS NMR signal is stable enough to allow this type of kinetic measurement to be accomplished with confidence. Reactions of polymer samples that had been dried were not attempted with the CWA, but the presence of water is important for the reaction to proceed to the acid products. Figure 8
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1657 Table 2. Observed Rate Constant (kobs) and Half-Life (t1/2) for the Polymer-Enhanced Hydrolysis of GD, VX, and GB systema
Figure 8. Kinetics of VX degradation at 20 °C on solid PANOx-A before and after addition of liquid water. As-prepared PANOx sample (initial weight, 43.7 mg) contained less than 2% water; at the time indicated 10 mg of deionized water was added to the polymer. The time-dependent relative degree of conversion (Ft) was calculated using signal integration in corresponding 31P spectra.
Figure 9. Representative kinetics of GD and VX degradation at 20 °C on hydrated PANOx-A, PANOx-B, and PHA samples. Initial weight of the hydrated PANOx-A sample was 88.4 mg in experiment with VX and 72.9 mg in experiment with GD; PHA sample weighed 37.5 mg. PANOX-B samples weighed 39.4 and 34.6 mg in experiments with GD and VX, respectively, and 15 µL of liquid water was added to each PANOx-B sample before the CWA addition. Numbers designate the following substrate/ polymer combinations: 1, VX/PHA; 2, VX/PANOx-A; 3, GD/PANOx-A; 4, GD/PANOx-B; 5, VX/PANOx-B. Water content in the depicted PANOx-A and PHA samples was 54-61 wt % and 54 ( 7 wt %, respectively.
demonstrates the effect of the presence of water on the kinetics of VX degradation on the dry PANOx-A sample. No appreciable VX hydrolysis occurred within 50 h in the presence of dry PANOx-A sample (water content < 2 wt %). With about 18% water in the sample, the hydrolysis started and appeared to be pseudo-first-order, as was evidenced from the linearity of the plot (R2 > 0.98). The slope of the linear fits in Figure 8 yielded the observed rate constant, kobs, of 1.4 × 10-5 s-1. This result underscores the importance of the presence of water as a medium for dissociation of the amidoxime and/or hydroxamic groups to generate corresponding nucleophilic anions for the CWA hydrolysis to occur. It should be noted that because of the hygroscopicity of the PANOx and PHA samples, their prolonged exposure to air at ambient conditions resulted in vapor absorption leading to 15-25 wt % water contents per sample, which was sufficient for these samples to hydrolyze the CWA. However, hydration of these polymers at 100% humidity yielded water contents at 45-65 wt % levels, which further accelerated the hydrolysis (Figure 9). Notably, in all experiments, the hydrolysis rates with PANOx-B samples fully converted to sodium amidoximate were significantly greater than with the PANOx-A sample, where less nucleophilic amidoxime groups dominated. The observed rates and half-lives of GD, GB, and VX are collected in Table 1. The polymer-induced degradation of VX
PANOx-A PANOx-B PHA PANOx-A PANOx-B PHA PANOx-B Cu(II) complex of poly(4-vinylpyridine quaternized with ethyl bromide and 4-chloromethyl-4′-methyl-2,2′bipyridine in aqueous buffer, pH 7.0 and 25 °C17
CWA GD GD GD VX VX VX GB GD
kobs (s-1) 1.9 3.6 4.7 4.5 2.1 2.5 >4 1.7
× × × × × × × ×
-4
10 10-4 10-6 10-5 10-3 10-5 10-3 10-4
t1/2 (h) 1.0 0.54 41 4.3 0.093 7.7
