Influence of Different Organic Side Groups on the Thermal Behavior of Polyphosphazenes: Random Chain Cleavage, Depolymerization, and Pyrolytic Cross-Linking

The thermal behavior of several polyphosphazenes was examined. The polymers were studied by thermogravimetric analysis between 50 and 1000 °C, by bulk pyrolysis in a tube furnace over the same temperature range, and by thermolysis in a closed system. The volatile products were analyzed by a combination of ³¹P NMR spectroscopy, vapor-phase chromatography, and mass spectrometry. Three distinct processes were identified: (1) random chain cleavage of the phosphazene backbone, (2) depolymerization to form small molecule cyclic phosphazenes, and (3) cross-linking reactions to form a network structure. During pyrolysis [NP(OCH₂CF₃)2]_n underwent random chain cleavage and depolymerization followed by volatilization of the small-molecule cyclic products. Species [NP(CH₃)₂]_n and [NP(CH₃)(C₆H₅)]_n depolymerized to cyclic oligomers, which then volatilized. The aryloxy-substituted phosphazenes [NP(OC₆H₅)₂]_n and [NP(OC₆H₄CH₃-p)₂]_n underwent random chain cleavage, depolymerization to cyclic oligomers, and cross-linking of these cyclic species. High-temperature pyrolysis of [NP(NHC₃H₇-n)₂]_n and other poly-(aminophosphazenes) and the transannular metallocene polymers [NP(OCH₂CF₃)₄(η-C5H₄)₂Fe]_n and [NP(OCH₂CF₃)₄(η-C₅H₄)₂Ru]_n resulted exclusively in cross-linking reactions. Possible reaction mechanisms are considered.