New Insights into Poly(Dichlorophosphazene) Synthesis via Me₃Si─N═PCl₃ Polymerization: An Experimental and...

Polyphosphazenes are a unique class of polymers, notable for their versatile functionalization and distinctive properties arising from their alternating phosphorus-nitrogen backbone. However, the synthesis of their primary precursor, poly(dichlorophosphazene) ([NPCl₂]_n), is notoriously delicate and poorly understood, limiting broader adoption. Herein, we reinvestigate the controlled polymerization of Me₃Si─N═PCl₃ using quantum mechanical calculations to elucidate its underlying mechanism. Guided by these insights, we optimized the experimental conditions and achieved significantly higher molecular weights than previously reported for this polymerization route. Polymerization occurs at ambient temperature only upon addition of a cationic initiator, such as Ph₃PCl₂, yielding [NPCl₂]_n chains of varying lengths. Experiments and DFT calculations indicate that initiation proceeds via three key steps: (i) dissociation of Ph₃PCl₂ into [Ph₃PCl]⁺ and Cl^- ions, dependent on the reaction medium; (ii) nucleophilic attack of Cl^- on Me₃Si─N═PCl₃, producing [N═PCl₃]^- and eliminating Me₃SiCl; (iii) reaction of [N═PCl₃]^- with [Ph₃PCl]⁺ to form Ph₃P─NPCl₄, which dissociates to generate additional reactive species, propagating chain growth. The elimination of Me₃SiCl, concurrent with [N═PCl₃]^- formation, constitutes the rate-limiting step. Accordingly, this type of polymerization can be described as a chloride-catalyzed cationic process. These findings provide a detailed mechanistic understanding of poly(dichlorophosphazene) formation and offer a basis for optimizing reaction conditions to achieve controlled polymerization.