Limits to expanding the PN-F series of polyphosphazene elastomers

Mixed-substituent fluoroalkoxyphosphazene polymers bearing ∼15% 1H,1H,2H,2H-perfluorooctan-1-oxy or 1H,1H,2H,2H-perfluorodecan-1-oxy side groups together with trifluoroethoxy cosubstituent groups were synthesized. The low reactivity of the long-chain fluoroalkoxides and their limited solubility in organic solvents prevented higher levels of substitution. Moreover, the sodium alkoxides with two methylene residues adjacent to the oxygen proved to be unstable in solution due to elimination of NaF and precipitation of side products, and this limited the time available for chlorine replacement reactions. The resulting cosubstituent polymers were characterized by proton nuclear magnetic resonance (¹H-NMR), ³¹P-NMR, ¹⁹F-NMR, gel-permeation chromatography, and differential scanning calorimetry. Unlike homo- or mixed-substituent fluoroalkoxyphosphazene polymers, such as [NP(OCH₂CF₃)₂]_n (a microcrystalline thermoplastic, T_g ∼ -63C, T_m ∼ 242C) or [NP(OCH₂CF₃)(OCH₂(CF₂) _xCF₂H)]_n (PN-F, a rubbery elastomer, T _g ∼ -60C, but no detectable T_m), the new polymers are gums (T_g ∼ -50C, but no detectable T_m) with molecular weights in the 10⁵ g/mol rather than the 10⁶ g/mol range. POLYM. ENG. SCI., 54:1827-1832, 2014.