STRAIN-INDUCED RING-OPENING POLYMERIZATION OF FERROCENYLORGANOCYCLOTRIPHOSPHAZENES - A NEW SYNTHETIC ROUTE TO POLY(ORGANOPHOSPHAZENES)

The strained transannular ferrocenylcyclotriphosphazenes N3P3(OCH2CF3)4(eta-C5H4)2Fe, N3P3R(OCH2CF3)3(eta-C5H4)2Fe [R = OPh, R = Me, R = Ph (R geminal to Cp), and R = Ph (nongeminal to Cp)], and N3P3R2(OCH2CF3)2(eta-C5H4)2Fe [R = Ph (geminal to Cp) and R = Ph (nongeminal to Cp)] undergo ring-opening polymerization when heated at 250-degrees-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(eta-C5H4)2Fe, N3P3Me(OCH2CF3)3(eta-C5H4)2Fe, and N3P3Ph2(OCH2CF3)2(eta-C5H4)2Fe (Ph groups nongeminal to Cp) also polymerize at 250-degrees-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(eta-C5H4)2Fe undergoes ring expansion to the corresponding cyclic hexamer when heated at 250-degrees-C in the presence or absence of (NPCl2)3, but it does not polymerize. When heated in the absence of (NPCl2)3, N3P3(OCH2CF3)4(eta-C5H4)2Fe, N3P3(OPh)(OCH2CF3)3(eta-C5H4)2Fe, and N3P3Ph(OCH2CF3)3(eta-C5H4)2Fe (Ph nongeminal to Cp) also undergo ring expansion to form the corresponding cyclic hexamers. The Lewis acid BCl3 initiates the ring-opening polymerization of N3P3(OCH2CF3)4(eta-C5H4)2Fe and catalyzes the ring expansion of N3P3(OPh)4(eta-C5H4)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.