Phosphazene conjugation

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. 

Alfred Schmidpeter took his PhD at Munich University in 1960 with Egon Wiberg. His research contributions since then have been in different fields of organophosphorus and organoarsenic chemistry, such as phosphazenes, aminophosphine tautomers, oligocyclic phosphoranes, azaphospholes, two-coordinate phosphorus and arsenic in conjugated cyclic and acyclic systems. He was Professor of Inorganic Chemistry at the University of Munich since 1975 and retired in 1997. 

Through steric hindrance and conjugative effects, these ionic phosphonium salts are very stable to hydrolysis. This, coupled with the lipophilic nature of the cation, results in a very soft, loosely bound ion pair, making materials of this type suitable for use as catalysts in anionic polymerization [8 - 13]. Phosphazene bases have been found to be suitable catalysts for the anionic polymerization of cyclic siloxanes, with very fast polymerization rates observed. In many cases, both thermodynamic and kinetic equilibrium can be achieved in minutes, several orders of magnitude faster than that seen with traditional catalysts used in cyclosiloxane polymerization. Exploiting catalysts of this type on an industrial scale for siloxane polymerization processes has been prevented because of the cost and availability of the pho hazene bases. This p r describes a facile route to materials of this type and their applicability to siloxane synthesis [14]. 

The synthesis of oligomeric peralkylated polyaminophosphazenes based upon the P3 and P4 templates is an elegant (but complicated), hazardous, and expensive multi-step process. To exploit catalysts of this type in silicone synthesis on an industrial scale would require a much simplified and lower-cost synthetic protocol. One of the simplest and lowest-cost routes to a conjugated -P=N- template that forms the framework for the synthesis of phosphazene base materials is via phosphonitrilic chloride oligomers (Fig. 3). These are well-known acidic catalysts used in the silicone industry for the condensation polymerization of silanol-terminated polydimethylsiloxanes. Catalysts of this type are most commonly prepared by the reaction of PCI5 with NH4CI or HMDZ,... 

Branching of phosphazene bases leads to higher basicity, as evident in the P3 series where branched base 108 is more basic than linear base 101. This increase in basicity of branched compared to hnear stmctures has been ascribed mainly to effect of stability differences of the free bases, rather than of the cations, since linear conjugation in neutral phosphazenes is more effective than cross conjugation. Similarly, we could observe that in the P5 series linear base 117 is weaker than the branched 110, but in this case differences in resonance of the cations are more significant. 

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