New Insights into Poly(Dichlorophosphazene) Synthesis via Me ₃ Si─N═PCl ₃ Polymerization: An Experimental and Theoretical Study

ABSTRACT

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) ([NPCl ₂ ] _n ), is notoriously delicate and poorly understood, limiting broader adoption. Herein, we reinvestigate the controlled polymerization of Me ₃ Si─N═PCl ₃ 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 Ph ₃ PCl ₂ , yielding [NPCl ₂ ] _n chains of varying lengths. Experiments and DFT calculations indicate that initiation proceeds via three key steps: (i) dissociation of Ph ₃ PCl ₂ into [Ph ₃ PCl] ⁺ and Cl ⁻ ions, dependent on the reaction medium; (ii) nucleophilic attack of Cl ⁻ on Me ₃ Si─N═PCl ₃ , producing [N═PCl ₃ ] ⁻ and eliminating Me ₃ SiCl; (iii) reaction of [N═PCl ₃ ] ⁻ with [Ph ₃ PCl] ⁺ to form Ph ₃ P─NPCl ₄ , which dissociates to generate additional reactive species, propagating chain growth. The elimination of Me ₃ SiCl, concurrent with [N═PCl ₃ ] ⁻ 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.