Trichloroethoxy-Substituted Polyphosphazenes: Synthesis

Nov 5, 2012 - For example, high performance elastomers,(13-18) dental materials,(19-21) and cardiovascular applications(22, 23) have been reported. He...
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Trichloroethoxy-Substituted Polyphosphazenes: Synthesis, Characterization, and Properties Chen Chen, Xiao Liu, Zhicheng Tian, and Harry R. Allcock* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: High polymeric organophosphazenes have been synthesized with trichloroethoxy side groups either as the sole substituents or as cosubstituents with trifluoroethoxy, phenoxy, or methoxyethoxyethoxy groups. Initially, small-molecule model compound studies were carried out between sodium trichloroethoxide and hexachlorocyclotriphosphazene at ambient temperature to yield the fully substituted product hexakis(trichloroethoxy)cyclotriphosphazene as a prelude to the synthesis of high polymeric trichloroethoxy homo- and cosubstituted phosphazenes. The cyclic trimeric and polymeric species were characterized by 1H and 31P NMR, GPC, and DSC techniques. Physical property comparisons were made with the longestablished elastomers that contain both trifluoroethoxy and longer chain fluoroalkoxy side groups by the use of DSC, TGA, and limited oxygen index tests. The introduction of trichloroethoxy side groups further improves the resistance of these polymers to combustion.

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INTRODUCTION Polyphosphazenes are hybrid inorganic−organic polymers with a backbone of alternating phosphorus and nitrogen atoms and with two organic, organometallic, or inorganic side groups linked to each skeletal phosphorus atom.1−3 Most poly(organophosphazenes) are synthesized by chlorine replacement reactions carried out on high molecular weight poly(dichlorophosphazene), (NPCl2)n, an approach that permits the properties of a polymer to be fine-tuned by the introduction of different substituents or different ratios of two or more cosubstituents. Several hundred different polyphosphazenes have been synthesized by this approach. As a result of their structural diversity, applications such as elastomers, biodegradable or biostable medical materials, fire retardants, solid polymer electrolytes, membranes, and optical or electro-optical materials have been developed.1,4−6 The incorporation of fluorine-containing organic side groups into polyphosphazenes has long been of great interest. In earlier reports, we and others described a series of 2,2,2trifluoroethoxy homo- and cosubstituted polyphosphazenes.7−12 They have been studied for diverse applications. For example, high performance elastomers, 13−18 dental materials,19−21 and cardiovascular applications22,23 have been reported. Here, we describe the synthesis of a new group of polyphosphazenes with 2,2,2-trichloroethoxy side groups, which are analogues of the 2,2,2-trifluoroethoxy derivatives. The objective was to compare some of the most important properties of these polymers with those of their fluorinated counterparts. Fire resistance is an important attribute of these polymers. Halogenated organic small molecules (mainly chlorinated and brominated species) represent some of the most widely used fire retardants.24 Phosphorus and nitrogen are also common elements found in fire-resistant and fire-retardant materials,25 and their presence in the backbone of polyphosphazenes is a significant factor in the interest in these polymers.26 The © 2012 American Chemical Society

polymers described in this paper possess the synergistic influence of phosphorus, nitrogen, chlorine, and fluorine that could significantly improve their behavior for a wide range of fire-resistant applications under challenging conditions.

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EXPERIMENTAL SECTION

Reagents and Equipment. All the synthesis reactions were carried out under a dry argon atmosphere using standard Schlenk line techniques. Tetrahydrofuran (THF) (EMD) and diethyl ether were dried using a solvent purification system in which the solvents pass through columns of molecular sieves under a dry argon atmosphere.27 2,2,2-Trichloroethanol (Aldrich), 2,2,2-trifluoroethanol (Aldrich), and di(ethylene glycol)methyl ether (Aldrich) were distilled over calcium hydride. Phenol was purified by sublimation under vacuum. Methanol (EMD), dichloromethane (DCM) (EMD), hexanes (EMD), anhydrous sodium sulfate (EMD), and sodium hydride (60% dispersion in mineral oil, Aldrich) were used as received. Poly(dichlorophosphazene) 5 was prepared via the thermal ring-opening polymerization of recrystallized and sublimed hexachlorocyclotriphosphazene (Fushimi Pharmaceutical Co., Japan, or Ningbo Chemical Co., China) in evacuated Pyrex tubes at 250 °C.7 31 P and 1H NMR spectra were obtained with use of a Bruker 360 WM instrument operated at 145 and 360 MHz, respectively. 31P NMR spectra were proton decoupled. 1H shifts are reported in ppm relative to tetramethylsilane at 0 ppm, and 31P shifts are reported in ppm relative to 85% H3PO4 at 0 ppm. Glass transition temperatures were measured with a TA Instruments Q10 differential scanning calorimetry apparatus at a heating rate of 10 °C/min, a scan range from −100 to 300 °C, and a sample size of ca. 10 mg. Gel permeation chromatograms were obtained using a Hewlett-Packard HP 1100 gel permeation chromatograph equipped with two Phenomenex Phenogel linear 10 columns and a Hewlett-Packard 1047A refractive index detector. The samples were eluted at 1.0 mL/min with a 10 mM solution of tetra-n-butylammonium nitrate in THF, and the elution Received: August 30, 2012 Revised: October 16, 2012 Published: November 5, 2012 9085

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mineral oil) in THF. The resultant clear solution of sodium phenoxide was added dropwise to a solution of polymer 5 in THF (300 mL), and the mixture was stirred for 24 h at room temperature. A solution of sodium 2,2,2-trichloroethoxide in THF, prepared by dropwise addition of 2,2,2-trichloroethanol to a suspension of sodium hydride (60% dispersion in mineral oil) in THF, was added to the reaction mixture mentioned above. After stirring for 48 h at room temperature, the reaction mixture was concentrated under reduced pressure and precipitated from THF into water five times and from THF into hexanes twice. The polymers were dried under high vacuum to yield products 10−12. The amounts of polymer 5 (4.64 g, 40.0 mmol) used for the synthesis of polymers 10−12 were kept constant. For polymer 10, phenol (5.65 g, 60.0 mmol), sodium hydride (60% dispersion in mineral oil, 2.40 g, 60.0 mmol), and THF (60 mL) were used to prepare sodium phenoxide. 2,2,2-Trichloroethanol (3.86 mL, 40.0 mmol), sodium hydride (60% dispersion in mineral oil, 1.60 g, 40.0 mmol), and THF (40 mL) were used to prepare sodium 2,2,2trichloroethoxide. The product was isolated as a gray elastomer, which was soluble in THF and chloroform. Yield: 8.10 g (78%). For polymer 11, phenol (3.76 g, 40.0 mmol), sodium hydride (60% dispersion in mineral oil, 1.60 g, 40.0 mmol), and THF (30 mL) were used to prepare sodium phenoxide. 2,2,2-Trichloroethanol (5.78 mL, 60.0 mmol), sodium hydride (60% dispersion in mineral oil, 2.40 g, 60.0 mmol), and THF (60 mL) were used to prepare sodium 2,2,2trichloroethoxide. The product was isolated as a white leathery solid, which was soluble in THF and slightly soluble in chloroform. Yield: 8.59 g (74%). For polymer 12, phenol (1.88 g, 20.0 mmol), sodium hydride (60% dispersion in mineral oil, 800 mg, 10.0 mmol), and THF (20 mL) were used to prepare sodium phenoxide as the first substituent. 2,2,2Trichloroethanol (7.71 mL, 80.3 mmol), sodium hydride (60% dispersion in mineral oil, 3.20 g, 80.0 mmol), and THF (60 mL) were used to prepare sodium 2,2,2-trichloroethoxide as the second substituent. The product was isolated as a white leathery solid, which was soluble in THF and slightly soluble in chloroform. Yield: 8.80 g (71%). Synthesis of Poly[(2,2,2-trichloroethoxy)1.77(phenoxy)0.23phosphazene] (13). Phenol (1.13 g, 12.0 mmol) was added portion-wise to a suspension of sodium hydride (60% dispersion in mineral oil, 480 mg, 12.0 mmol) in THF (40 mL). After the formation of a clear solution of sodium phenoxide, this solution was added dropwise to a solution of polymer 5 (6.95 g, 60.0 mmol) in THF (400 mL), and the mixture was stirred for 24 h at room temperature. In a separate flask, 2,2,2-trichloroethanol (13.9 mL, 144 mmol) was added dropwise to a suspension of sodium hydride (60% dispersion in mineral oil, 5.76 g, 144 mmol) in THF (100 mL). After the formation of a clear solution of sodium 2,2,2-trichloroethoxide, this solution was added dropwise to the reaction mixture described above, and the mixture was stirred for 48 h at room temperature. The reaction mixture was then concentrated under reduced pressure, precipitated from THF into water five times and from THF into hexanes twice, and then dried under high vacuum to yield a yellow leathery solid, which was soluble in THF and chloroform. Yield: 15.1 g (77%). Synthesis of Poly[(2,2,2-trichloroethoxy)x(2-(2methoxyethoxy)ethoxy)yphosphazenes] (14−16). Polymers 14−16 were synthesized by following a similar procedure as described for the synthesis of polymers 10−12. The amounts of polymer 5 (1.16 g, 10.0 mmol) used for the synthesis of polymers 14−16 were kept constant. The polymers 14−16 were purified by dialysis (MWCO 12 000−14 000) against water, methanol, and methanol/dichloromethane (1/4) and dried under high vacuum. For polymer 14, 2-(2-methoxyethoxy)ethanol (1.72 mL, 15.1 mmol) and sodium hydride (60% dispersion in mineral oil, 600 mg, 15.0 mmol) were used to prepare sodium 2-(2-methoxyethoxy)ethoxide as the first substituent. 2,2,2-Trichloroethanol (0.961 mL, 10.0 mmol) and sodium hydride (60% dispersion in mineral oil, 400 mg, 10.0 mmol) were used to prepare sodium 2,2,2-trichloroethoxide

times were calibrated with polystyrene standards. The X-ray diffraction data for cyclic trimeric compound 4 and polymer 6 were obtained with use of a Bruker AXS APEX X-ray diffractometer and a Rigaku DmaxRapid microdiffractometer. Thermal decomposition traces were obtained with use of a PerkinElmer TGA 7 thermogravimetric analyzer. Heating occurred at a rate of 20 °C/min from 50 to 800 °C with the air as purge gas at a flow rate of 50 mL/min. Limiting oxygen indices (LOIs) were determined through the use of a device constructed in accordance with ASTM-D-2863-00. Test specimens were prepared by dissolving polymers in THF and casting the solutions onto flat BYTAC substrates. The thickness of the films was 0.5 ± 0.1 mm. The films were shaped into “Type VI” specimens in accordance with ASTM-D-2863-00. Poly[bis(2,2,2-trifluoroethoxy)phosphazene], poly[bis(phenoxy)phosphazene], and poly[bis(2-(2methoxyethoxy)ethoxy)phosphazene] were synthesized according to reported procedures.7,8 Synthesis of Hexakis(2,2,2-trichloroethoxy)cyclotriphosphazene (4). 2,2,2-Trichloroethanol (1.54 mL, 16.0 mmol) was added dropwise to a suspension of sodium hydride (60% dispersion in mineral oil, 640 mg, 16.0 mmol) in THF (30 mL). After formation of the sodium salt, hexachlorocyclotriphosphazene (695 mg, 2.00 mmol) was added to the solution, and the mixture was stirred for 14 h at room temperature. The reaction mixture was concentrated under reduced pressure. The oily residue was dissolved in dichloromethane, washed with water, dried over anhydrous sodium sulfate, concentrated under vacuum, and recrystallized from hexanes/ethyl acetate to yield a colorless crystalline solid. Yield: 1.76 g, 1.72 mmol (86%). 1H NMR (360 MHz, CDCl3) δ: 4.60 (m, 2 H). 31P NMR (145 MHz, CDCl3) δ: 15.46 (s, 100%). Synthesis of Poly[bis(2,2,2-trichloroethoxy)phosphazene] (6). 2,2,2-Trichloroethanol (2.41 mL, 25.0 mmol) was added dropwise to a suspension of sodium hydride (60% dispersion in mineral oil, 1.00 g, 25.0 mmol) in THF (40 mL). After the formation of a clear solution of sodium 2,2,2-trichloroethoxide, the solution was added dropwise to a solution of polymer 5 (1.16 g, 10.0 mmol) in THF (80 mL), and the mixture was stirred for 20 h at room temperature. The reaction mixture was concentrated under reduced pressure, precipitated from hot THF into water 5 times and from hot THF into hexanes twice, and dried under high vacuum to yield a white solid, which is soluble in hot THF. Yield: 1.74 g, 5.09 mmol (51%). Synthesis of Poly[(2,2,2-trichloroethoxy)x(2,2,2-trifluoroethoxy)yphosphazenes] (7−9). Sodium hydride (60% dispersion in mineral oil) was washed with anhydrous ether twice before being suspended in THF (50 mL). Varying ratios of 2,2,2-trichloroethanol and 2,2,2-trifluoroethanol were added sequentially and dropwise to a suspension of sodium hydride in THF (40 mL). After the formation of a clear solution of the sodium salts, this solution was added dropwise to a solution of polymer 5 in THF (200 mL), and the mixture was stirred for 48 h at room temperature. The reaction mixture was concentrated under reduced pressure, precipitated from THF into water five times, and dried under high vacuum to yield the solid polymers 7−9. The amounts of polymer 5 (2.32 g, 20.0 mmol) and NaH (60% dispersion in mineral oil, 1.92 g, 47.9 mmol) used for the synthesis of polymers 7−9 were kept constant. For polymer 7, 2,2,2-trichloroethanol (1.16 mL, 12.0 mmol) and 2,2,2-trifluoroethanol (2.59 mL, 36.0 mmol) were used for the synthesis. The product was isolated as a yellowish, waxy elastomer, which was soluble in THF and acetone. Yield: 5.20 g (97%). For polymer 8, 2,2,2-trichloroethanol (2.31 mL, 24.0 mmol) and 2,2,2-trifluoroethanol (1.73 mL, 24.0 mmol) were used for the synthesis. The product was isolated as a yellowish, waxy elastomer, which was soluble in THF and acetone. Yield: 5.25 g (90%). For polymer 9, 2,2,2-trichloroethanol (3.47 mL, 36.0 mmol) and 2,2,2-trifluoroethanol (0.864 mL, 12.0 mmol) were used for the synthesis. The product was isolated as a white fibrous solid, which was soluble in THF and acetone. Yield: 5.79 g (92%). Synthesis of Poly[(2,2,2-trichloroethoxy)x(phenoxy)yphosphazenes] (10−12). Phenol was added portion-wise to a suspension of sodium hydride (60% dispersion in 9086

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as the second substituent. The product was isolated as a yellow gum, which was soluble in THF and chloroform. Yield: 1.91 g (49%). For polymer 15, 2-(2-methoxyethoxy)ethanol (1.14 mL, 9.99 mmol) and sodium hydride (60% dispersion in mineral oil, 400 mg, 10.0 mmol) were used to prepare sodium 2-(2-methoxyethoxy)ethoxide as the first substituent. 2,2,2-Trichloroethanol (1.93 mL, 20.0 mmol) and sodium hydride (60% dispersion in mineral oil, 800 mg, 20.0 mmol) were used to prepare sodium 2,2,2-trichloroethoxide as the second substituent. The product was isolated as a brownish gum, which was soluble in THF and chloroform. Yield: 2.10 g (57%). For polymer 16, 2-(2-methoxyethoxy)ethanol (0.590 mg, 5.00 mmol) and sodium hydride (60% dispersion in mineral oil, 200 mg, 5.00 mmol) were used to prepare sodium 2-(2-methoxyethoxy)ethoxide as the first substituent. 2,2,2-Trichloroethanol (1.93 mL, 20.0 mmol) and sodium hydride (60% dispersion in mineral oil, 800 mg, 20.0 mmol) were used to prepare sodium 2,2,2-trichloroethoxide as the second substituent. The product was isolated as a colorless gum, which was soluble in THF and chloroform. Yield: 2.40 g (68%).

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RESULTS AND DISCUSSION Background. The two polyphosphazenes containing fluoroalkoxy side groups shown as 1 and 2 have been studied in considerable detail.7−9 Polymer 1 is a microcrystalline, hydrophobic thermoplastic film- and fiber-former, while 2 is a high performance elastomer. Both are resistant to combustion. In this work we have explored the possibility that the fluoroalkoxy side chains in 1 be replaced by chloroalkoxy units, in single-substituent species such as 1, and the synthesis of copolymers containing chloroalkoxy side groups with trifluoroethoxy, phenoxy, or methoxyethoxyethoxy side groups.

Figure 1. Crystal structure of compound 4. H atoms have been omitted. For the sake of clarity the figure does not show the disorder in chlorine atom positions. Selected bond lengths in Å: N1−P1 1.566(4), N1−P2 1.573(4), N2−P2 1.574(4), N2−P3 1.572(4), N3− P3 1.573(4), N3−P1 1.577(4), O1−P1 1.564(3), O2−P1 1.571(3), O3−P2 1.563(3), O4−P2 1.592(3), O5−P3 1.575(3), O6−P3 1.564(3).

with complete replacement of the chlorine atoms as shown in Scheme 2. These results indicate that sodium 2,2,2-trichloroethoxide possesses a high reactivity in chlorine replacement reactions and in this respect is comparable to sodium 2,2,2trifluoroethoxide. This reflects both the high solubility of their sodium salts in THF and the relatively small steric hindrance. Most of the syntheses of other organophosphazene polymers require elevated temperatures or prolonged reaction times to compete the reactions, but the higher reactivity of the halogenated nucleophiles is a clear advantage. Following the facile isolation of polymer 6, a series of related polymers were synthesized in which 2,2,2-trichloroethoxy groups were cosubstituted together with 2,2,2-trifluoroethoxy (TFEO), phenoxy (PhO), or 2-(2-methoxyethoxy)ethoxy (MEEO) side groups in various ratios. These three cosubstituents were selected because they cover a range from unhindered hydrophobic side groups (2,2,2-trifluoroethoxy), through hydrophobic hindered units (phenoxy), to relatively unhindered hydrophilic cosubstituents [2-(2-methoxyethoxy)ethoxy], and also because these are three of the most extensively studied side groups in polyphosphazene chemistry in terms of both fundamental research and applications.29 Polymers 7−9 were synthesized by the simultaneous addition of the solutions of sodium 2,2,2-trifluoroethoxide and sodium 2,2,2-trichloroethoxide to polymer 5. Simultaneous addition of the two nucleophiles was used because of the similar reactivity of the sodium salts. For the synthesis of polymers 10−16, the sodium salts of the nonchlorinated substituents, which have relatively lower reactivity, were introduced first, and an excess of sodium 2,2,2-trichloroethoxide was then added to polymer 5 to complete the substitution (Scheme 2). All the chlorine atoms of polymer 5 readily underwent replacement by both the relatively hindered or unhindered side groups. Because of the mild reaction condition employed, no significant side group exchange effects were detected.10,30−32 Characterization of High Polymers. The ratios of side groups of the cosubstituted polymers were determined by integrations of 1H NMR spectra, as shown in Table 1. In most

To this end we have followed a protocol with a proved utility. New reactions were first examined using small molecule cyclic phosphazenes, which are easier to purify and characterize than their high polymeric analogues. Following optimization of these reaction conditions, the same or similar conditions are then transferred to the high polymeric level. Cyclic Trimeric Phosphazene Model Studies. The synthesis of homosubstituted trimeric cyclic phosphazene 4 was carried out by treatment of hexachlorocyclotriphosphazene (3) with an excess amount of the sodium 2,2,2-trichloroethoxide to produce the fully substituted crystalline hexakis(2,2,2trichloroethoxy)cyclotriphosphazene (4), as shown in Scheme 1.28 The structure of 4 was confirmed by 1H NMR and 31P Scheme 1. Synthesis of 2,2,2-Trichloroethoxy Homosubstituted Trimeric Cyclic Phosphazene 4

NMR spectra and by a single-crystal X-ray diffraction study, as shown in Figure 1. We attribute the conformation of the side groups in each molecule to crystal packing forces rather than intramolecular interactions. Synthesis of High Polymers. In keeping with the behavior of the small-molecule model system, polymer 5 reacted with an excess of sodium 2,2,2-trichloroethoxide to yield polymer 6 9087

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Scheme 2. Synthesis of 2,2,2-Trichloroethoxy Homo- and Cosubstituted High Polymers

Thermal Properties and Combustion Resistance. Thermogravimetric analysis (TGA) is commonly employed to estimate thermal decomposition behavior. It can also serve as an indirect indicator of polymer flammability when air is used as the purge gas. From the results shown in Table 2, the onset temperature of decomposition for poly[bis(2,2,2trifluoroethoxy)phosphazene] was 239 °C, and the char yield was less than 2% when the temperature was further increased to 500 °C. This probably reflects the facile thermal depolymerization of this polymer into cyclic oligomers.33 By contrast, the 2,2,2-trichloroethoxy homosubstituent polymer 7 showed no weight loss until 322 °C and had higher char yields at 500 and 800 °C. Similarly, the incorporation of 2,2,2-trichloroethoxy groups in polymers 8−10 also resulted in improved thermal stability and higher char yields at elevated temperatures. Compared to phenoxy and 2-(2-methoxyethoxy)ethoxy homosubstituted polymers, the mixed-substituent polymers 10−13 and 14−16 still possessed similar thermal stabilities and comparably high char yields. This suggests that the incorporation of 2,2,2-trichloroethoxy units as cosubstituents has no negative effect on thermal stability or formation of residues. The limited oxygen index (LOI) value refers to the minimum concentration of oxygen that will just support the combustion of a material in a flowing mixture of oxygen and nitrogen. This is a generally accepted indicator of polymer flammability that is widely used in many applications. Table 2 shows the LOI values of selected polymers. Polymer 6 was too brittle to be fabricated into self-standing specimens that meet the requirement of ASTM-D-2863-00. Polymers 14−16 and poly[bis(2-(2methoxyethoxy)ethoxy)phosphazene] are amorphous gums. It has been demonstrated that poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] is a fire-resistant electrolyte for rechargeable lithium battery systems.34−36 2-(2-Methoxyethoxy)ethoxy substituents serve as ionically conductive side groups, and the presence of the 2,2,2-trichloroethoxy unit may further improve

cases, the actual ratios of side groups are close to the aniticipated values based on the stoichiometries of the nucleophiles. These results further indicate that the reactivity of sodium 2,2,2-trichloroethoxide in chlorine replacement reactions is high enough to compete with sodium 2,2,2trifluoroethoxide present in the synthesis of polymers 7−9 or to complete the replacement of P−Cl bonds in the presence of bulky substituents for the synthesis of polymers 10−13. As shown in Table 1, the molecular weights and GPCderived broad polydispersities of the polymers are typical of macromolecules derived from the form of polymer 5 synthesized via the thermal ring-opening polymerization of hexachlorocyclotriphosphazene. Polymer 6 has a significantly higher glass transition temperature (Tg) (12.4 °C) than the value for poly[bis(2,2,2-trifluoroethoxy)phosphazene] (−66 °C).7,8 Bulkier 2,2,2-trichloroethoxy side groups raise the Tg by restricting the torsional motions of the polymer chain through steric interactions. The strong optical birefringence, observed in the X-ray diffraction pattern, as well as the Tm of polymer 6, indicated a high level of crystallinity for polymer 6. In the mixed-substituted polymers, the introduction of 2,2,2trichloroethoxy side groups raises the glass transition temperatures to an extent that depends on the loading and structures of the other side groups. The presence of two different side groups lowers the symmetry and retards microcrystallization. These effects are reflected in the decreased Tm and improved solubility of the mixed-substituted polymers. Homosubstituted polymer 6 is a white, brittle, film-forming material, which dissolves in hot THF but is only slightly soluble in cold THF. Incorporation of as little as 13% of the nonchlorinated cosubstituents in polymer 13 dramatically improves the solubility and processability of the polymers. Mixed-substituted polymers 7−16 dissolve readily in cold THF, chloroform, or acetone. Polymers 7−13 possess various degree of elasticity and also have good film-forming abilities. Polymers 14−16 are amorphous gums which presumably can be cross-linked to generate elastomers. 9088

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1.54/0.46 (1.50/0.50)

0.49/1.51 (0.50/1.50) 0.99/1.01 (1.00/1.00) 1.49/0.51 (1.50/0.50) 0.49/1.51 (0.50/1.50) 1.06/0.94 (1.00/1.00) 1.45/0.55 (1.50/0.50) 1.74/0.26 (1.80/0.20) 0.50/1.50 (0.50/1.50) 1.09/0.91 (1.00/1.00)

TCEO/ROa

−11.53 (s, 100%) −10.0 to −7.2 (m, 35.0%), −6.33 (s, br, 65.0%) −10.2 to −8.9 (m, 19.6%), −8.9 to −7.5 (m, 44.0%), −7.5 to −6.0 (m, 36.4%) −10.8 to −9.2 (m, 49.5%), −9.2 to −7.8 (m, 39.5%), −7.8 to −6.6 (m, 11.0%) −20.0 to −17.2 (m, 35.8%), −17.2 to −14.0 (m, 64.2%) −19.0 to −16.0 (m, 10.1%), −14.49 (s, br, 72.7%), −13.5 to −11.0 (m, 17.2%) −16.0 to −13.8 (m, 48.2%), −13.8 (m, 51.8%) δ −15.0 to −12.9 (m, 22.9%), −11.64 (s, 77.1.0%) −14.0 to −11.8 (m, 2.8%), −11.0 to −9.0 (m, 47.2%), −8.0 to −6.0 (m, 49.9%) −13.0 to −11.0 (m, 26.4%), −10.3 to −8.2 (m, 58.0%), −8.0 to −5.5 (m, 15.6%) −13.5 to −11.0 (m, 60.5%), −9.8 to −8.0 (m, 39.5%)

P NMRb

31

0.82 0.50 1.07 0.55 0.42

4.00−4.52 (m, 2.2 H), 7.00 (s, br, 5.0 H) 4.20−4.68 (m, 5.9 H), 7.0−7.3 (m, br, 5.0 H) 4.28−4.82 (m, br, 13.5 H), 7.1−7.4 (m, br, 5 H) 3.35 (s, 3.0 H), 3.50 (s, br, 2.0 H), 3.57−3.71 (m, br, 4.0 H), 4.03−4.20 (m, br, 2.0 H), 4.51−4.62 (m, br, 0.66 H) 3.34 (s, 3.0 H), 3.50 (s, br, 2.0 H), 3.55−3.73 (m, br, 4.0 H), 4.03−4.25 (m, br, 2.0 H), 4.53−4.69 (m, br, 2.33 H)

0.54

3.42

0.95

3.98−4.40 (m, 0.69 H), 7.00 (s, br, 5.0 H)

3.35 (s, 0.89 H), 3.50 (s, br, 0.59 H), 3.55−3.75 (m, br, 1.14 H), 4.10−4.30 (m, br, 0.56 H), 4.50−4.72 (m, br, 2.0 H)

2.64

1.12

4.67 (s, br, 0.51 H), 4.71 (s, br, 1.49 H)

3.02

3.08

2.58

3.28

2.75

2.84

2.21

1.62

4.62 (s, br, 1.01 H), 4.79 (s, br, 0.99 H)

2.37 2.31

PDI

1.45 2.28

Mw (×106)

4.78 (s) 4.57 (s, br, 1.51 H), 4.71 (s, br, 0.49 H)

H NMRb

1

−24.7

−42.8

−65.5

8.8

−2.1 58.1

28.6

−8.7 −6.0

85.4

132.5 21.2

Tm (°C)

−15.3

−43.4

12.4 −49.4

Tg (°C)

Aniticipated ratio is shown inside parentheses. bDeuterated solvents for acquiring NMR spectra: THF-d8 for polymer 6; acetone-d6 for polymers 7−9; CDCl3 for polymers 10, 13−16; THF-d8/CDCl3 1/ 1 (v/v) for polymers 11 and 12.

a

16

15

14

13

12

11

10

9

8

6 7

polymer

Table 1. Analytical Data for Homo- and Cosubstituted Polymers

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Notes

Table 2. LOI Values and TGA Data of Homo- and Cosubstituted Polymers polymer 6 1 7 8 9 [NP(OPh)2]n 10 11 12 13 [NP(OMEE)2]n 14 15 16

LOI >59 >48 >64 >69 >32 >36 >51 >56 >64

The authors declare no competing financial interest.

Tonseta (°C)

Tmaxb (°C)

Yc500c (%)

Yc800d (%)

322 239 271 305 306 377 323 343 342 308 238 235 244 259

407 416 342 350 386 432 386 388 389 369 269 264 271 311

20.2 1.5 5.2 10.6 18.1 35.5 38.0 32.9 28.9 24.6 30.1 27.4 30.3 22.5

1.8 1.2 3.5 6.8 14.2 16.5 30.7 24.1 24.1 13.2 5.2 1.7 9.3 2.1

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ACKNOWLEDGMENTS The authors thank Dr. Nichole Wonderling and Dr. Hemant Yennawar for help with the X-ray crystallography experiments.

a

Tonset is the onset temperature (temperature at 5% weight loss). bTmax is the temperature at the maximum degradation rate. cYc500 was the char yield at 500 °C. dYc800 was the char yield at 800 °C.

the combustion resistance of electrolytes. This aspect will be studied in future work for polymers 14−16. Overall, the polyphosphazenes discussed here possess excellent resistance to combustion, with the highest LOI values (above 60%) being found for polymers 9 and 13. The combustion resistance of these polymers depends mainly on the ratio between 2,2,2-trichloroethoxy and the nonhalogenated cosubstituents. Incorporation of as little as 25% 2,2,2trichloroethoxy substituents significantly increases the fire resistance of the cosubstituted polyphosphazenes, with the exception of polymer 7. Melting at high temperatures is a particular problem for thermoplastic polymers. This not only increases the burning surface area, which increases the intensity of a fire, but also leads to rapid fire spreading. Polymers that contain 2,2,2-trichloroethoxy substituents show both antidripping and intumescent properties. Poly[bis(2,2,2trifluoroethoxy)phosphazene] has a serious melt dripping problem during LOI tests. Melt dripping is suppressed and char formation is initiated after the incorporation of 25% 2,2,2trichloroethoxy side group. The highest char yield was achieved with a 50% loading of 2,2,2-trichloroethoxy side group for polymer 8. Poly[bis(phenoxy)phosphazene] and polymers 10− 13 followed the same trend but with a higher char yield. Only in oxygen atmospheres above 50% was the char yield decreased, as the flame temperature became higher.

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CONCLUSIONS The incorporation of 2,2,2-trichloroethoxy side groups into polyphosphazenes brings about significant changes to the properties. These changes include increased glass transition temperature, control of elastomeric properties, resistance to liquid flow, and significant improvements in resistance to thermal decomposition and combustion via both flame suppression and intumescence. Future research will be focused on the behavior of these polymers in composite materials and block copolymers.

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(35) Morford, R. V.; Kellam, E. C.; Hofmann, M. A.; Baldwin, R.; Allcock, H. R. Solid State Ionics 2000, 133, 171−177. (36) Fei, S. T.; Allcock, H. R. J. Power Sources 2010, 195, 2082−2088.

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dx.doi.org/10.1021/ma301822m | Macromolecules 2012, 45, 9085−9091