Inorganic Polymers

Abstract

The thermal ROP route requires the synthesis and careful purification of the cyclic trimer [Cl 2 PN] 3 and the use of elevated temperatures where control of molecular weight is very difficult and cross-linking can take place at high conversion, which can limit the yield. INORGANIC POLYMERSVol. 3to phosphazene polymers, which may well facilitate more rapid and extensive commercialization.Monomer Synthesis. Both the ROP and condensation routes to poly(dichlorophosphazene) require high purity monomers. For the traditional ROP process, ultrahigh purity [NPCl 2 ] 3 is required. The most common method for its preparation is through the reaction of PCl 5 with NH 4 Cl in a high boiling halogenated solvent, such as chlorobenzene or tetrachloroethane, at 150 • C. The yields can approach 70-80% under specialized conditions, but 50% is more typical (25). The phosphoranimine most commonly used in the aforementioned condensation route to poly(dichlorophosphazene) is Cl 3 P NSi(CH 3 ) 3 . The synthesis of this compound has been described (21) and is based on a modification of an earlier procedure (26) involving the reaction of PCl 5 with LiN(Si(CH 3 ) 3 ) 2 at −78 • C in hexane. However, the purification required by this method is a challenge and leads to an overall yield of <40%.It has also been reported that the reaction of N(Si(CH 3 ) 3 ) 3 with PCl 5 can be manipulated to maximize yields of either the cyclic trimer (76%) or the pure phosphoranimine monomer (40%) through variations in reaction conditions (25). These reactions use milder conditions than previously reported methods, with higher yields. In addition, the major side product, (CH 3 ) 3 SiCl, can be recycled to form one of the starting materials, N(Si(CH 3 ) 3 ) 3 .N-Silylphosphoranimines with alkyl and aryl substituents can be synthesized by reaction of commercial starting materials such as PCl 3 , ((CH 3 ) 3 Si) 2 NH, and Grignard reagents by way of a three-step process, with typical yields of 60-70%. These precursors are also accessible via a "one-pot" synthesis, which requires only isolation of the final product with yields of about 75% (3).Uses. Some of the most useful polyphosphazene derivatives are fluoroalkoxy derivatives, and amorphous copolymers are very useful as flame-retardant, hydrocarbon-solvent, and oil-resistant elastomers and have found aerospace and automotive applications. Polymers such as the amorphous comb polymer poly[bis(methoxyethoxyethoxy)phosphazene] (8) are of considerable interest as components of polymeric electrolytes in battery technology (1). Vol. 3 INORGANIC POLYMERS 35Polyphosphazenes are also of interest as biomedical materials and bioinert, bioactive, membrane-forming, and bioerodable materials (1).