Reports: AC3
46908-AC3 New Catalysts for Phosphazene Ring-Opening Polymerization
Following the success of using trialkylsilyl carboranes, e.g. R3Si(CHB11H5Br6), as room temperature catalysts for ring-opening polymerization (ROP) of cyclo-hexachlorotriphosphazene, cyclo-N3(PCl2)3, other group 14 elements have been investigated as alternative catalysts. Thus, trialkylgermanium and trialkyltin carboranes have been synthesized and characterized. X-ray crystal structures have been obtained. Group 14 periodic trends are evident in these structural data and a manuscript is in preparation. Surprisingly, trialklytin carboranes are inactive as ROP catalysts and the corresponding trialkylgermanium carboranes are barely active. This has been traced computationally to element-halogen bond strengths. Thus, trialkylsilyl carboranes remain the only good catalysts for phosphazene ROP and they have been subjected to detailed mechanistic investigation. Under conditions where all species remain soluble in o-dichlorobenzene, 31P NMR has been used to monitor intermediates and follow the progress of the ROP reaction. As a complement, mass spectrometric methods have been developed to identify and quantify all the components and intermediates of the reaction. This work is yet to be completed and published but the following conclusions can be drawn: (a) cyclic oligomeric phosphazenes (NPCl2)n up to large n are observed at quite early stages of the polymerization reaction, (b) their abundances are initially under kinetic control but they are eventually subject to thermodynamic control, catalyzed by the silyl carborane, (c) endgroups have been observed on linear polymer chains suggesting a living polymer mechanism. This work is being continued under a grant from NSF.
