Polyphosphazenes bonding

The physical properties of polyphosphazene depend on the nature and the number of substitutes. However, the flexibility of the P-N backbone is the property in common. Because of the weakness of the rotation energy around the N-P bond (3.38 and 21.8 kJ/mol, respectively for... 

In fact, fluorinated polyphosphazenes are usually considered to be extremely stable towards chemical agents and aggressives due to the presence of C-F bonds in the side phosphorus substituents. PTFEP, for instance, appears to be completely insensitive to several, most common, solvents (aliphatic and aromatic hydrocarbons, alcohols and water), to acids (e.g. acetic acid), and to bases (e.g. pyridine and concentrated NaOH solutions), although some decomposition could be observed in triethylamine and in concentrated H2SO4 [41]. Phos-phazene fluoroelastomers, moreover, are known to be completely insoluble in aromatic solvents [533] and petroleum-resistant materials [502-506,552]. 

These discoveries generated a lot of effort over the successive 25 years in the preparation of especially designed drug delivery systems for the controlled release of radioactive progesterone [654], colchicine [656], naproxen [657,673, 674], mitomycin C [675-677], inulin [678], trimethoprin [657], succinylsul-fathiazole [657], ethacrynic acid [653], and steroids [633], regardless of whether these drugs are physically trapped in polyphosphazene matrices, or chemically bonded to the polymer skeleton. 

The question of electronic conductivity in the polyphosphazenes inevitably raises questions regarding the electronic structure of the phosphazene linkage.7-12 This matter has been the subject of controversy in the literature, but experimentally the situation is now well known.4,13 In spite of the fact that the phosphazene backbone is fully conjugated, bond equalized and possesses bond lengths which are indicative of partial double bond character, the evidence suggests that these are localized systems. 

N-Silylphosphinimines. Until recently, no general method has been available which allows complete incorporation of the desired substituents before polymerization. The synthesis of polyphosphazenes with direct phosphorus-carbon bonds has been possible only in a few cases (25,60). A new method which holds promise involves the synthesis of suitably constructed N-silylphosphinimines which upon heating, eliminate substituted silanes to give polyphosphazenes (61). This procedure was... 

The parent polymer by itself is not a useful material owing to the extreme hydrolytic sensitivity of the P-Cl bond. However, this feature has been turned around and used as an advantage. Nucleophilic substitution of the chlorines in the polymer results in substituted polyphosphazenes which are hydrolytically stable. Also, using this method the polymer architecture and properties are readily fine-tuned by a subtle variation of the substituent. Over three hundred types of polyphosphazenes have been synthesised by this method. Assembly of organic polymers containing cyclo-phosphazenes as pendant groups is another approach that is gaining importance [6]. 

The observed low Tg s of most polyphosphazenes are consistent with the low barrier to internal rotation predicted for them and indicate the potential these polymers have for elastomeric applications, Theoretical calculations, based on rotational isomeric models assuming localized it bonding, predict the lowest ( 100 cal per mol of repeating unit) known polymer barrier to rotation for the skeletal bonds of polydifluorophosphazene,... 

A theoretical analysis of the conformational energies of PDCP is presented. The results indicate that the bond pair P—N—P possesses a considerable conformational freedom, whereas the bond pair N-P-N is relatively rigid. This difference explains the low glass transition temperatures and large end-to-end distances measured for polyphosphazenes. All the calculated magnitudes are extremely sensitive to the energy Eler) that controls the statistical weight of the conformations tg, to, tg , gt, ct, and g t, relative to tt for the bond pair P-N-P. A qualitative explanation for this sensitivity is discussed. 

The spacer units in 3.60 are assembled from polyphosphazenes that bear p-bromophc-noxy side groups via a lithiation reaction, and treatment with a diorganochlorophosphine to give 3.62. The chemistry is summarized in reaction sequence (45).107 Polymer 3.62 coordinates to a variety of metallo species,108 including osmium cluster compounds and cobalt carbonyl hydroformylation catalysts. When used as a polymeric hydroformylation catalyst, this latter species proved how stable the polyphosphazene backbone is under the drastic conditions often needed for these types of reactions. The weakest bonds in the molecule proved to be those between the phosphine phosphorus atoms and the aromatic spacer groups. 

In the following sections, four different aspects of polyphosphazene structure will be reviewed briefly side-group disposition chain conformation skeletal bonding and skeletal flexibility. 

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