Probing the Mechanism of the PCl5−Initiated Living Cationic Polymerization of the Phosphoranimine Cl3PNSiMe3 using Model Compound Chemistry

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• 作者：Vivienne Blackstone^&#8224 ; ; Alan J. Lough^‡ ; ; Martin Murray^&#8224 ; ; Ian Manners^&#8224 ;
• 刊名：Journal of the American Chemical Society
• 出版年：2009
• 出版时间：March 18, 2009
• 年：2009
• 卷：131
• 期：10
• 页码：3658-3667
• 全文大小：267K
• 年卷期：v.131,no.10(March 18, 2009)
• ISSN：1520-5126

文摘

New insight into the mechanism of the ambient temperature PCl₅-initiated living cationic chain growth polycondensation of the N-silylphosphoranimine Cl₃PNSiMe₃ (1) to give poly(dichlorophosphazene), [NPCl₂]_n, has been provided by studies of model compound chemistry. Investigations of the reactivity of Cl⁻ salts of the proposed cationic intermediates [Cl₃PNPCl₃]⁺ ([2]⁺) and [Cl₃PN−PCl₂NPCl₃]⁺ ([6]⁺) toward Ph₃PNSiMe₃ (3a) provided evidence that under the usual polymerization conditions that involve a high monomer to initiator ratio, propagation occurs at both chain ends. However, analogous studies of near stoichiometric processes suggested that propagation is faster at one chain end, particularly when the chains are short. In addition, experiments involving [Ph₃PNPPh₃][PCl₆] ([9][PCl₆]) and the N-silylphosphoranimines R₃PNSiMe₃ 3a (R = Ph) and 3b (R = p-CF₃C₆H₄), showed that the [PCl₆]⁻ anion, which is formed in the early stages of the polymerization and has hitherto been assumed to be an innocent spectator counteranion, is actually reactive under the reaction conditions and can initiate oligomerization and polymerization. Finally, the absence of reactions between phosphoranimines 3b or 1 with the Cl⁻ salts of the cations [Ph₃PN−PCl₂NPPh₃]⁺ ([10a]⁺), [Ph₃PN−(PCl₂N)₂PPh₃]⁺ ([5]⁺), and [Ph₃PN−(PCl₂N)₃PPh₃]⁺ ([8]⁺) with P−Cl bonds located internally but not at the chain ends have shown that chain branching reactions are unlikely to be significant during the polymerization. These results identify key factors that complicate the living PCl₅-initiated chain growth polycondensation of 1 and potentially lead to a loss of control over molecular weight and broaden the molecular weight distributions, but also indicate that the polymer formed is essentially linear rather than branched.