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Inorg. Chem. 1999, 38, 280-283
A New Route to the Phosphazene Polymerization Precursors, Cl3PdNSiMe3 and (NPCl2)3 Harry R. Allcock,* Chester A. Crane, Christopher T. Morrissey, and Michael A. Olshavsky Department of Chemistry, The Pennsylvania State University, 152 Davey Laboratory, University Park, Pennsylvania 16802 ReceiVed May 12, 1998
An improved synthesis of the phosphazene polymerization precursors, hexachlorocyclotriphosphazene, (NPCl2)3 (1), and Cl3PdNSiMe3 (2) is reported. The addition of PCl5 to N(SiMe3)3 in methylene chloride at 40 °C produced a mixture of phosphazenes which contained 76% of 1. However, the addition of N(SiMe3)3 to PCl5 in methylene chloride at 0 °C, followed by the addition of hexane, provided 2 in 40% yield. The mechanism of the reaction is discussed.
Scheme 1
Introduction Polyphosphazenes form one of the largest classes of inorganicorganic macromolecules.1 The ease with which these polymers are synthesized has allowed the preparation of several hundred different structures. The simplest route to stable polyphosphazenes is via the replacement of the chlorine atoms in poly(dichlorophosphazene) by a wide variety of organic or organometallic substituents. Different side groups generate different property combinations, hence the selection of appropriate side groups allows the polymer solution properties and materials characteristics to be tailored easily. Various polyphosphazenes are being developed as biomaterials, solid polymer electrolytes, fire-resistant polymers, and elastomeric materials.1-4 The traditional method for the synthesis of poly(dichlorophosphazene) has been via the molten phase ring-opening polymerization of hexachlorocyclotriphosphazene (1), at 210250 °C.1 The polymer formed by this process has a high molecular weight (∼1 × 106), with a broad molecular weight distribution. The most common method for the preparation of 1 has been through the reaction of PCl5 with NH4Cl in a highboiling halogenated solvent, such as chlorobenzene or tetrachloroethane, at 150 °C. The yields of 1 can approach 7080% under specialized conditions, but a yield of 50% is more typical.5-7 The need for ultrahigh purity 1 for the polymerization reaction has placed some limits on the widespread use of this process. An alternative, ambient-temperature synthesis of poly(dichlorophosphazene) has recently been accomplished.8 A cationic initiator, such as PCl5, is added to the phosphoranimine monomer Cl3PdNSiMe3 (2) to yield medium molecular weight (1) Mark, J. E.; Allcock, H. R.; West, R. Inorganic Polymers; Prentice Hall: Englewood Cliffs, NJ, 1992; 61. (2) Allcock, H. R. Science 1992, 255, 1106. (3) Allcock, H. R. In Inorganic and Organometallic Polymers II; WisianNeilson, P., Allcock, H. R., Wynne, K. J., Ed.; ACS Symposium Series 572. American Chemical Society: Washington, DC, 1994; p 208. (4) Allcock, H. R. In Biodegradable Polymers as Drug DeliVery Systems; Langer, R., Chasin, M., Eds.; Marcel Dekker: New York, 1990. (5) Allcock, H. R.; Stinnett, S. J.; Tedder, J. B.; Adams, R. A. U.S. Patent 4,656,017, 1987. (6) Tanino, H.; Okamoto, T.; Ueyama, S.; Fujikawa, K.; Tsumiyama, T. U.S. Patent 4,522,689, 1985. (7) Fekete, T. M.; Start, J. F. U.S. Patent 4,046,857, 1977. (8) Allcock, H. R.; Crane, C. A.; Morrissey, C. T.; Nelson, J. M.; Reeves, S. D.; Honeyman, C. H.; Manners, I. Macromolecules 1996, 29, 7740.
(105) polymers (NPCl2)n (Scheme 1). This “living” cationic polymerization yields polyphosphazenes with narrow molecular weight distributions. Moreover, the molecular weights can be controlled through changes in the monomer-to-initiator ratio. This polymerization has also provided access to well-defined block copolymers and star polymers, both of which were unobtainable before the introduction of this method.9,10 The new approach also allows the formation of phosphazene-organic block copolymers. The key requirement for the broad utilization of this process is an improved synthesis of Cl3PdNSiMe3. The method described in published work8 involves the reaction of PCl5 with LiN(SiMe3)2 at -78 °C in hexane. The phosphoranimine was first synthesized from PCl5 and LiN(SiMe3)2 by Niecke and Bitter with a yield of 20%.11 Honeyman, Lough, and Manners improved the yield to 60% by lowering the reaction temperature from +10 to -78 °C.12 As was mentioned in a previous publication,8 the synthesis of pure 2 by the literature method is a challenge. The synthesis routinely produces ClN(SiMe3)2 as a side product, which is not separable from Cl3PdNSiMe3 by distillation. This compound is a powerful inhibitor of the polymerization. The method developed for purification uses the reaction of ClN(SiMe3)2 with PPh3 to form Ph3PdNSiMe3 which can easily be separated from the desired monomer 2 by vacuum distillation. The pure, isolated yield of 2 via this process tends to be low (
