Strain-induced ring-opening polymerization of ferrocenylorganocyclotriphosphazenes: A new synthetic route to poly(organophosphazenes)

Article abstract of DOI:10.1021/ja00025a026

The strained transannular ferroxenylcyclotriphosphazenes N₃P₃(OCH₂ CF₃)₄(η-C₅H₄)₂Fe, N₃P₃R(OCH₂CF₃) ₃(η-C₅H₄)₂Fe [R = OPh, R = Me, R = Ph (R geminal to Cp), and R = Ph (nongeminal to Cp)], and N₃P₃R₂(OCH₂CF₃) ₂(η-C₅H₄)₂Fe [R = Ph (geminal to Cp) and R = Ph (nongeminal to Cp)] undergo ring-opening polymerization when heated at 250°C in the presence of a small amount (1%) of (NPCl₂)₃, which functions as a polymerization initiator. The! cyclic trimers N₃P₃(OPh)(OCH₂CF₃) ₃(η-C₅H₄)₂Fe, N₃P₃Me(OCH₂CF₃) ₃(η-C₅H₄)₂Fe, and N₃P₃Ph₂(OCH₂CF₃) ₂(η-C₅H₄)₂Fe (Ph groups nongeminal to Cp) also polymerize at 250°C but in the absence of (NPCl₂)₃. These transformations are the first examples of uncatalyzed ring opening polymerization of cyclic phosphazenes that lack phosphorus-halogen bonds. By contrast, the sterically crowded cyclotriphosphazene N₃P₃(OPh)₄(η-C₅H ₄)₂Fe undergoes ring expansion to the corresponding cyclic hexamer when heated at 250°C in the presence or absence of (NPCl₂)₃, but it does not polymenze. When heated in the absence of (NPCl₂)₃, N₃P₃(OCH₂CF₃) ₄(η-C₅H₄)₂Fe, N₃P₃(OPh)(OCH₂CF₃) ₃(η-C₅H₄)₂Fe, and N₃P₃Ph(OCH₂CF₃) ₃(η-C₅H₄)₂Fe (Ph nongeminal to Cp) also undergo ring expansion to form the corresponding cyclic hexamers. The Lewis acid BCl₃ initiate the ring-opening polymerization of N₃P₃(OCH₂CF₃) ₄(η-C₅H₄)₂Fe and catalyzes the ring expansion of N₃P₃(OPh)₄(η-C₅H ₄)₂Fe. Possible explanations for the differences in thermal behavior are given. The implications of these results for the mechanisms of phosphazene ring-opening polymerization and ring-ring equilibration are also discussed.