The structure of the chlorophosphazenes used as catalyst for the polycondensation of siloxanoles to various high molecular weight silicone polymers in the industrial scale has been determined as [PCl3=N(-PCl2=N)x-PCl3]+[PCl6]-. mainly with x=1 and x=2. For better un derstanding of the catalytic mechanism, the reaction behaviour of these ionic phosphazenes with water and siloxanes has been studied. Thereby three further types of phosphazenes have been synthesized: PCl3=N(-PCl2=N)x-PCl2=O,HO-PCl2=N(-PCl2=N)x-PCl2=O and siloxy derivatives of the latter ≡Si-O-PCl2=N(-PCl2=N)x-PCl2=O. Although several P-Cl bonds could be attacked by the nucleophiles, the reactions were found to be extremely selective and the products were obtained in very high yields. Finally relations between phosphazene structure and catalytic activity have been ascertained to find the optimal catalyst structure.
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