Linear Polyphosphazenes

Hexachlorocyclotriphosphazene (cycHc trimer) is a respiratory irritant. Nausea has also been noted on exposure (10). Intravenous and intraperitoneal toxicity measurements were made on mice. The highest nonlethal dose (LDq) was measured as 20 mg/kg (11). Linear chloropolymer is also beUeved to be toxic (10). Upon organic substitution, the high molecular weight linear polymers have been shown to be inert. Rat implants of eight different polyphosphazene homopolymers indicated low levels of tissue toxicity (12). EZ has been found to be reasonably compatible with blood (13), and has lower hpid absorption than fiuorosihcone. 

Cyclophosphazenes are a fascinating group of inorganic heterocyclic compounds whose chemistry is multi-faceted, well developed and reasonably well understood. They are closely related to the linear poly-phosphazenes this relationship is unlike any other existing between ring-polymer systems. Although cyclic siloxanes and polysiloxanes have a close interrelationship, the number and types of cyclophospha-zene derivatives that are known, together with their exact counterparts in polyphosphazenes, underscore the utility of cyclophosphazenes as models for the more complex polyphosphazenes. The literature on cyclophosphazenes has appeared earlier in the form of books (1,2), chapters of books (3-5), authoritative compilations of data (6,7), and several reviews (8-21). The current literature on this subject is reported annually in the Specialist Periodic Reports published by the Royal Society of chemistry (22). This review deals mostly with chlorocyclo-... 

This chapter covers the literatures of phosph(v)azenes. The general pattern of development in this area is similar to that observed in previous yearly reviews with additional interest being shown in polyphosphazenes and in a variety of molecular orbital calculations of both linear and cyclic phosphazenes. 

An important developing area that lies in the region between polymer chemistry, ceramic science, and metals, involves the search for new electrically-conducting solids. Linear polymers may conduct electricity by electronic or ionic mechanisms. As will be discussed, polyphosphazenes have been synthesized that, depending on the side group structure, conduct by either of these two processes. 

Polyphosphazenes are the most important and the most thermally characterized of the phosphorus-containing inorganic polymers. Linear, cycloli-near, and cross-linked cyclomatic polymers based on phosphazene structures have been produced. The repeating units of some polyphosphazenes are as follows ... 

In addition to linear polyphosphazenes with one type of side group, as shown in 3.1, other molecular architectures have also been assembled. These include polyphosphazenes in which two or more different side groups, R1 and R2, are arrayed along the chain in random, regular, or block distributions (3.2-3.4). Other species exist with short phosphazene branches linked to phosphorus atoms in the main chain (3.5,3.6). Also available are macromolecules in which carbon or sulfur replace some of the phosphorus atoms in... 

Polymers that consist of metal atoms joined together in a linear array are quite rare. This is because such systems prefer to collapse to three-dimensional clusters—literally very small chunks of metal stabilized on the surface by ligands. However, many of the intriguing electrical properties proposed for linear metallo polymers might be realized if a non-metallo polymer, such as a polyphosphazene, were used as a template and scaffold to stabilize a string of metal atoms and prevent cluster formation (structures 3.67 and 3.68). This has been accomplished only in a very preliminary and tentative way,... 

The molecular architecture of a polyphosphazene has a profound influence on properties. For example, linear and tri-star trifluoroethoxy-substituted polymers with the same molecular weight (1.2 x 104 or higher) have strikingly different properties.138 The linear polymers are white, fibrous materials that readily form films and fibers, whereas the tri-arm star polymers are viscous gums. One is crystalline and the other is amorphous. Cyclolinear polymers are usually soluble and flexible. Cyclomatrix polymers are insoluble and rigid. Linear polymers can be crystalline, but graft or comb polymers are usually amorphous. 

The principal polyphosphazenes that have been used in hydrogels are those with linear or branched ethyleneoxy side chains, aryloxy groups with carboxylic acid substituents, or mixed-substituent polymers that bear hydrophilic methylamino side groups plus a hydrophobic cosubstituent such as phenoxy or trifluoroethoxy. Cross-linking is usually accomplished by gamma-ray irradiation or, in the case of the carboxylic acid functional species, by treatment with a di- or tri-valent cation. Here, we will consider another example based on MEEP (3.79), a polymer that is well suited to the clean method of radiation cross-linking. 

A related synthetic strategy by Allcock and Welker65 is illustrated in reaction (19). Here, borazine side groups are linked to a polyphosphazene chain. This reduces the volatility of the borazine and allows facile fabrication of the linear high polymer. Subsequent pyrolysis at temperatures up to 1,000 °C results in breakdown of the phos-phazene carrier backbone (possibly via the formation of volatile phosphorus nitride) and formation of boron nitride. 

Multisubsituted linear polyphosphazene polymers, (II), having high ion conductivity at ambient temperature were prepared by Allcock [3] and used as gel polymer electrolytes. 

Figure 1 Different types of polyphosphazenes including linear classical macromolecules (1), star structures (2), block copolymers (3), cyclolinear polymers (4), and comb or graft copolymers (5) and (6). Not shown are cyclomatrix materials in which cyclic trim eric rings are connected in three dimensions ...

The Backbone. The linear inorganic backbone imparts an unusual combination of properties. First, perhaps unexpectedly in view of the unsaturated structure, the skeletal bonds have a low barrier to torsion (perhaps as low as 0.1-0.5KcaF repeating unit), which becomes translated into one of the most flexible backbones known throughout polymer chemistry. This means that some polyphosphazenes have glass-transition temperatures (Tg) as low as -100 °C. It also means that, in the absence of microcrystallinity, numerous polymers of this type are rubbery elastomers. This is a key property for... 

Cyclophosphazenes (46) nndergo both thermal ringopening polymerization with formation of linear polymers (4 7) see Polyphosphazenes) and ring-ring eqnilibria (resnlting in ring expansion). 

This review covers phosphazene literature over the period June 2001 to December 2002 Chemical Abstracts Vols. 135, 136 and 137), and discusses linear phosphazenes including compounds derived thereof (Section 2), cyclophos-phazenes (Section 3) and polyphosphazenes (Section 4). Structural data have been summarized in Section 5. References have been given in Section 6. 

Species that combine the properties of organosilicon compounds and phos-phazenes are prepared by the linkage of organosilicon side groups to a small molecule cyclic or linear high polymeric phosphazene skeleton. This is particularly important for high polymeric derivatives in which hybrid properties typical of polysiloxanes (silicones) and polyphosphazenes - may be obtained. 

Amino, alkoxy, and aryloxy polyphosphazenes are typically prepared by nucleophilic displacement reactions of poly(dihalophosphazenes). Analogous reactions with organometallic reagents, however, result in chain degradation and cross linking rather than in linear, alkyl, or aryl substituted poly(phosphazenes). The thermolysis of appropriate silicon-nitrogen-phosphorus compounds can be used to prepare fully P—C bonded poly(organophosphazenes). The synthesis of two of these materials and their Si—N—P precursors is described here. 

New methods have been put forward for the preparation of the basic polyphosphazene (NPCl2)n. A one-pot reaction of PCI5 and (NH4)2S04 (molar ratio about 4.5 1) delivers linear (NPCb) without significant cross-linking. 

Many papers have appeared on the chemistry of linear phosphazenes, varying from electron-rich ligands in various systems to starting materials in the preparation of organo-substituted polyphosphazenes. 

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