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Polyphosphazene
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<h2 id="synthesis">Synthesis</h2> <p>The method of <a href="/facts/Chemical_Synthesis/3KXsVVwC">synthesis</a> depends on the type of polyphosphazene. The most widely used method for linear polymers is based on a two-step process.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a><a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a><a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> In the first step, <a href="/facts/Hexachlorocyclotriphosphazene/NA3lsYxu">hexachlorocyclotriphosphazene</a>(NPCl2)3 is heated in a sealed system at 250 °C to convert it to a long chain linear polymer with typically 15,000 or more <a href="/facts/Repeating_unit/WIxA68X4">repeating units</a>. In the second step the <a href="/facts/Chlorine/y27UwYBM">chlorine</a> atoms linked to phosphorus in the polymer are replaced by organic groups through reactions with <a href="/facts/Alkoxide/RqG0rm1v">alkoxides</a>, <a href="/facts/Phenols/xs1CtNTa">aryloxides</a>, <a href="/facts/Amines/SN7W44fv">amines</a> or <a href="/facts/Organometallic/bhqFz2P0">organometallic</a> reagents. Because many different <a href="/facts/Reagent/c3GLInCu">reagents</a> can participate in this <a href="/facts/Macromolecule/039ULSLg">macromolecular</a> <a href="/facts/Substitution_reaction/zpqsbWAo">substitution reaction</a>, and because two or more reagents may be used, a large number of different polymers can be produced.. Variations to this process are possible using <a href="/facts/Poly(dichlorophosphazene)/2P8PyPLN">poly(dichlorophosphazene)</a> made by <a href="/facts/Condensation_reaction/wMG0F6La">condensation reactions</a>.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>Another synthetic process uses Cl3PNSiMe3 as a precursor:<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p> n Cl3PNSiMe3 --> [Cl2PN]n + ClSiMe3 <p>Because the process is a <a href="/facts/Living_cationic_polymerization/h5EEYVaz">living cationic polymerization</a>, block copolymers or comb, star, or dendritic architectures are possible.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a><a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> Other synthetic methods include the condensation reactions of organic-substituted phosphoranimines.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a><a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a><a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a><a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> </p><p>Cyclomatrix type polymers made by linking small molecule phosphazene rings together employ difunctional organic reagents to replace the chlorine atoms in (NPCl2)3, or the introduction of <a href="/facts/Allyl/CDxPwyvF">allyl</a> or <a href="/facts/Vinyl_group/SyXSgVnw">vinyl</a> <a href="/facts/Substituent/6y9IE6G0">substituents</a>, which are then <a href="/facts/Polymerization/1hmBz7Ur">polymerized</a> by <a href="/facts/Radical_polymerization/CbG5UqAd">free-radical</a> methods.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> Such polymers may be useful as coatings or <a href="/facts/Thermosetting_polymer/EagnsydT">thermosetting</a> <a href="/facts/Synthetic_resin/v9gh4HDh">resins</a>, often prized for their thermal stability. </p> <h2 id="properties-and-uses">Properties and uses</h2> <p>The linear high polymers have the <a href="/facts/Molecular_geometry/7USG5zDn">geometry</a> shown in the picture. More than 700 different <a href="/facts/Macromolecule/039ULSLg">macromolecules</a> that correspond to e group]]s or combinations of different side groups. In these polymers the properties are defined by the high flexibility of the <a href="/facts/Backbone_chain/9Q3so597">backbone</a>. Other potentially attractive properties include radiation resistance, high <a href="/facts/Refractive_index/yjkzvgeE">refractive index</a>, <a href="/facts/Ultraviolet/kncBUqn5">ultraviolet</a> and <a href="/facts/Visible_light/jeofpMn4">visible</a> transparency, and its <a href="/facts/Fireproofing/t6MKThD7">fire resistance</a>. The side groups exert an equal or even greater influence on the properties since they impart properties such as <a href="/facts/Hydrophobe/Ifya0CXQ">hydrophobicity</a>, <a href="/facts/Hydrophile/k8p95WYu">hydrophilicity</a>, <a href="/facts/Color/XDvW7ESP">color</a>, useful biological properties such as <a href="/facts/Biodegradation/d1UW4dfR">bioerodibility</a>, or <a href="/facts/Polyelectrolyte/3A3jLXkh">ion transport</a> properties to the polymers. Representative examples of these polymers are shown below. </p> <h3>Thermoplastics</h3> <p>The first stable <a href="/facts/Thermoplastic/5KJB8dwN">thermoplastic</a> poly(organophosphazenes), isolated in the mid 1960s by <a href="/facts/Harry_R._Allcock/G6vDbEGM">Allcock</a>, Kugel, and Valan, were macromolecules with trifluoroethoxy, <a href="/facts/Phenols/xs1CtNTa">phenoxy</a>, <a href="/facts/Methoxy/KbrAmwQc">methoxy</a>, <a href="/facts/Alkoxy_group/6b40PtAp">ethoxy</a>, or various amino side groups.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a><a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a><a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> Of these early species, poly[bis(trifluoroethoxyphosphazene], [NP(OCH2CF3)2]n, has proved to be the subject of intense research due to its <a href="/facts/Polymer/eF32E50K">crystallinity</a>, high hydrophobicity, biological compatibility, fire resistance, general radiation stability, and ease of fabrication into films, <a href="/facts/Microfiber/X2U3bow7">microfibers</a> and <a href="/facts/Nanofibers/TyFPW0jN">nanofibers</a>. It has also been a substrate for various <a href="/facts/Surface_science/WTENdutJ">surface reactions</a> to immobilize biological agents. The polymers with phenoxy or amino side groups have also been studied in detail. </p> <h3>Phosphazene elastomers</h3> <p>The first large-scale commercial uses for linear polyphosphazenes were in the field of high technology <a href="/facts/Elastomer/kCSD0udN">elastomers</a>, with a typical example containing a combination of trifluoroethoxy and longer chain fluoroalkoxy groups.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a><a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a><a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a><a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> The mixture of two different side groups eliminates the <a href="/facts/Polymer/eF32E50K">crystallinity</a> found in single-substituent polymers and allows the inherent flexibility and <a href="/facts/Elasticity_(physics)/wcJoIFg1">elasticity</a> to become manifest. <a href="/facts/Glass_transition/yVl3QzsT">Glass transition</a> temperatures as low as -60 °C are attainable, and properties such as oil-resistance and <a href="/facts/Hydrophobe/Ifya0CXQ">hydrophobicity</a> are responsible for their utility in land vehicles and <a href="/facts/Aerospace/T6kTLUyV">aerospace</a> components. They have also been used in biostable biomedical devices.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> </p><p>Other <a href="/facts/Pendant_group/5UBRDNDC">side groups</a>, such as non-fluorinated alkoxy or <a href="/facts/Oligomer/v7oDY8zu">oligo</a>-alkyl ether units, yield hydrophilic or hydrophobic elastomers with glass transitions over a broad range from -100 °C to 100 °C.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> Polymers with two different aryloxy side groups have also been developed as elastomers for fire-resistance as well as <a href="/facts/Thermal_insulation/j7tVT4uv">thermal</a> and <a href="/facts/Soundproofing/hKbTnCMq">sound insulation</a> applications. </p> <h3>Polymer electrolytes</h3> <p>Linear polyphosphazenes with <a href="/facts/Oligomer/v7oDY8zu">oligo</a>-<a href="/facts/Polyethylene_glycol/tC7xBFwr">ethyleneoxy</a> side chains are gums that are good solvents for salts such as lithium <a href="/facts/Trifluoromethanesulfonate/6sbvDSyt">triflate</a>. These solutions function as <a href="/facts/Electrolyte/4YE8bIVa">electrolytes</a> for lithium ion transport, and they were incorporated into fire-resistant <a href="/facts/Rechargeable_battery/A6zUkPkW">rechargeable</a> <a href="/facts/Lithium-ion_polymer_battery/0I2uusoW">lithium-ion polymer battery</a>.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a><a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a><a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> The same polymers are also of interest as the electrolyte in <a href="/facts/Dye-sensitized_solar_cell/yJsvA64H">dye-sensitized solar cells</a>.<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> Other polyphosphazenes with <a href="/facts/Sulfonate/ApbgQMXB">sulfonated</a> aryloxy side groups are proton conductors of interest for use in the membranes of <a href="/facts/Proton_exchange_membrane_fuel_cell/FH4ktLw7">proton exchange membrane fuel cells</a>.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> </p> <h3>Hydrogels</h3> <p>Water-soluble poly(organophosphazenes) with oligo-ethyleneoxy side chains can be <a href="/facts/Cross-link/zeQ9hEAA">cross-linked</a> by <a href="/facts/Gamma_ray/5Drf913J">gamma-radiation</a>. The cross-linked polymers absorb water to form <a href="/facts/Gel/gBFE0s9G">hydrogels</a>, which are responsive to temperature changes, expanding to a limit defined by the cross-link density below a <a href="/facts/Lower_critical_solution_temperature/DJQmzsIq">critical solution temperature</a>, but contracting above that temperature. This is the basis of controlled permeability membranes. Other polymers with both oligo-ethyleneoxy and carboxyphenoxy side groups expand in the presence of <a href="/facts/Valence_(chemistry)/ck7cHByj">monovalent</a> <a href="/facts/Cation/wrbwhF3z">cations</a> but contract in the presence of di- or tri-valent cations, which form ionic cross-links.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a><a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a><a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a><a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a><a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a> Phosphazene hydrogels have been utilized for controlled drug release and other medical applications.<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a> </p> <h3>Bioerodible polyphosphazenes</h3> <p>The ease with which properties can be controlled and fine-tuned by the linkage of different side groups to polyphosphazene chains has prompted major efforts to address <a href="/facts/Medical_research/lSVBFxDJ">biomedical</a> materials challenges using these polymers.<a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a> Different polymers have been studied as <a href="/facts/Macromolecule/039ULSLg">macromolecular</a> <a href="/facts/Drug_carrier/Cf9nhPLT">drug carriers</a>, as membranes for the <a href="/facts/Drug_delivery/vDfboXeT">controlled delivery of drugs</a>, as biostable <a href="/facts/Elastomer/kCSD0udN">elastomers</a>, and especially as tailored <a href="/facts/Biodegradation/d1UW4dfR">bioerodible</a> materials for the regeneration of living <a href="/facts/Bone/kwJaR6IQ">bone</a>.<a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a><a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a><a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a><a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a> An advantage for this last application is that poly(dichlorophosphazene) reacts with <a href="/facts/Amino_acid/WDxYwBny">amino acid</a> ethyl <a href="/facts/Esters/FoUAIIw4">esters</a> (such as ethyl <a href="/facts/Glycine/3nXI1hqW">glycinate</a> or the corresponding ethyl esters of numerous other amino acids) through the <a href="/facts/Amines/SN7W44fv">amino</a> terminus to form polyphosphazenes with amino acid ester side groups. These polymers <a href="/facts/Hydrolysis/JTvwIHPs">hydrolyze</a> slowly to a near-neutral, pH-<a href="/facts/Buffer_solution/zrLz9eaa">buffered solution</a> of the amino acid, ethanol, phosphate, and ammonium ion. The speed of hydrolysis depends on the amino acid ester, with <a href="/facts/Half-life/kGVH8BAB">half-lives</a> that vary from weeks to months depending on the structure of the amino acid ester. <a href="/facts/Nanofiber/TyFPW0jN">Nanofibers</a> and porous constructs of these polymers assist <a href="/facts/Osteoblast/N6mMDQvR">osteoblast</a> replication and accelerate the repair of bone in animal model studies. </p> <h2 id="commercial-aspects">Commercial aspects</h2> <p>No applications are commercialized for polyphosphazenes. The cyclic trimer <a href="/facts/Hexachlorophosphazene/NA3lsYxu">hexachlorophosphazene</a> ((NPCl2)3) is commercially available. It is the starting point for most commercial developments. High performance <a href="/facts/Elastomer/kCSD0udN">elastomers</a> known as PN-F or Eypel-F have been manufactured for seals, <a href="/facts/O-ring/wp4x9hKD">O-rings</a>, and dental devices. An aryloxy-substituted polymer has also been developed as a fire resistant expanded foam for <a href="/facts/Thermal_insulation/j7tVT4uv">thermal</a> and <a href="/facts/Soundproofing/hKbTnCMq">sound insulation</a>. The patent literature contains many references to cyclomatrix polymers derived from cyclic trimeric phosphazenes incorporated into cross-linked resins for fire resistant <a href="/facts/Printed_circuit_board/muSR2p43">circuit boards</a> and related applications. </p> <h2 id="further-information">Further information</h2> <p><a href="https://sites.psu.edu/allcock/">"H. R. Allcock Research Group"</a>. Retrieved 2020-08-22. </p> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Allcock, H. R., Kugel, R. L. (1965). "Synthesis of High Polymeric Alkoxy and Aryloxyphosphonitriles". Journal of the American Chemical Society. 87 (18): 4216–4217. doi:10.1021/ja01096a056.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Allcock, H. R., Kugel, R. L. (1965). "Synthesis of High Polymeric Alkoxy and Aryloxyphosphonitriles". Journal of the American Chemical Society. 87 (18): 4216–4217. doi:10.1021/ja01096a056.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Allcock, H. R., Kugel, R. L., Valan, K. J. (1966). "Phosphonitrilic Compounds. VI. High Molecular Weight Poly(alkoxy- and aryloxyphosphazenes)". Inorganic Chemistry. 5 (10): 1709–1715. doi:10.1021/ic50044a016.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Allcock, H. R., Kugel, R. L. (1966). "Phosphonitrilic Compounds. VII. High Molecular Weight Poly(diaminophosphazenes)". Inorganic Chemistry. 5 (10): 1716–1718. doi:10.1021/ic50044a017.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"Allcock Research Group Web Site". <a href="https://sites.psu.edu/allcock/" target="_blank">https://sites.psu.edu/allcock/</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Allcock, Harry R. (2003). Chemistry and Applications of Polyphosphazenes. Wiley-Interscience. <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Allcock, H. R., Kugel, R. L. (1965). "Synthesis of High Polymeric Alkoxy and Aryloxyphosphonitriles". Journal of the American Chemical Society. 87 (18): 4216–4217. doi:10.1021/ja01096a056.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Allcock, H. R., Kugel, R. L., Valan, K. J. (1966). "Phosphonitrilic Compounds. VI. High Molecular Weight Poly(alkoxy- and aryloxyphosphazenes)". Inorganic Chemistry. 5 (10): 1709–1715. doi:10.1021/ic50044a016.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Allcock, H. R., Kugel, R. L. (1966). "Phosphonitrilic Compounds. VII. High Molecular Weight Poly(diaminophosphazenes)". Inorganic Chemistry. 5 (10): 1716–1718. doi:10.1021/ic50044a017.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>"Allcock Research Group Web Site". <a href="https://sites.psu.edu/allcock/" target="_blank">https://sites.psu.edu/allcock/</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Gleria, M., De Jaeger, R. and Potin, P. (2004). Synthesis and Characterization of Poly(organophosphazenes). New York: Nova Science Publishers.{{cite book}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Template:Cite_book" target="_blank">/wiki/Template:Cite_book</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Rothemund, Sandra; Teasdale, Ian (2016). "Preparation of polyphosphazenes: A tutorial review". Chemical Society Reviews. 45 (19): 5200–5215. doi:10.1039/C6CS00340K. PMC 5048340. PMID 27314867. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5048340" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5048340</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Honeyman, C. H., Manners, I., Morrissey, C. T., Allcock, H. R. (1995). "Ambient Temperature Synthesis of Poly(dichlorophosphazene) with Molecular Weight Control". Journal of the American Chemical Society. 117 (26): 7035–7036. doi:10.1021/ja00131a040.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Allcock, H. R., Crane, C. A., Morrissey, C. T., Nelson, J. M., Reeves, S. D., Honeyman, C. H., Manners, I. (1996). ""Living" Cationic Polymerization of Phosphoranimines as an Ambient Temperature Route to Polyphosphazenes with Controlled Molecular Weights". Macromolecules. 29 (24): 7740–7747. Bibcode:1996MaMol..29.7740A. doi:10.1021/ma960876j.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Wisian-Neilson, P.; Neilson, R. H. (1980). "Poly(dimethylphosphazene), (Me2PN)n". Journal of the American Chemical Society. 102 (8): 2848–2849. doi:10.1021/ja00528a060. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Neilson, R. H., Wisian Neilson, P. (1988). "Poly(alkyl/arylphosphazenes) and their precursors". Chemical Reviews. 88 (3): 541–562. doi:10.1021/cr00085a005.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>Montague, R. A., Matyjaszewski, K. (1990). "Synthesis of Poly[bis(trifluoroethoxy)phosphazene] under Mild Conditions using a Fluoride Initiator". Journal of the American Chemical Society. 112 (18): 6721–6723. doi:10.1021/ja00174a047.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>Matyjaszewski, K., Moore, M. M., White (1993). "Synthesis of Polyphosphazene Block Copolymers bearing Alkoxyethoxy and Trifluoroethoxy Groups". Macromolecules. 26 (25): 6741–6748. Bibcode:1993MaMol..26.6741M. doi:10.1021/ma00077a008.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>Allen, C. W., Shaw, J. C., Brown, D. E. (1988). "Copolymerization of ((alpha-methylethenyl)phenyl)pentafluorocyclotriphosphazenes with Styrene and Methyl Methacrylate". Macromolecules. 21 (9): 2653–2657. Bibcode:1988MaMol..21.2653A. doi:10.1021/ma00187a001.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> <li id="fn:20"><p>Allcock, H. R., Kugel, R. L., Valan, K. J. (1966). "Phosphonitrilic Compounds. VI. High Molecular Weight Poly(alkoxy- and aryloxyphosphazenes)". Inorganic Chemistry. 5 (10): 1709–1715. doi:10.1021/ic50044a016.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li> <li id="fn:21"><p>Allcock, H. R., Kugel, R. L. (1966). "Phosphonitrilic Compounds. VII. High Molecular Weight Poly(diaminophosphazenes)". Inorganic Chemistry. 5 (10): 1716–1718. doi:10.1021/ic50044a017.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li> <li id="fn:22"><p>"Allcock Research Group Web Site". <a href="https://sites.psu.edu/allcock/" target="_blank">https://sites.psu.edu/allcock/</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></p></li> <li id="fn:23"><p>Rose, S. H. (1968). "Synthesis of Phosphonitrilic Fluoroelastomers". Journal of Polymer Science Part B: Polymer Letters. 6 (12): 837–839. Bibcode:1968JPoSL...6..837R. doi:10.1002/pol.1968.110061203. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></p></li> <li id="fn:24"><p>Singler, R. E., Schneider, N. S., Hagnauer, G. L. (1975). "Polyphosphazenes: Synthesis—properties—Applications". Polymer Engineering and Science. 15 (5): 321–338. doi:10.1002/pen.760150502.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></p></li> <li id="fn:25"><p>US 4945139, Charles H. Kolich; W. Dirk Klobucar & Jeffrey T. 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