Comprehensive Mechanistic Analysis of the Ring-Opening Polymerization of [PCl2N]3 Using Quantum Mechanical Calculations

The most popular method to synthesize polychlorophosphazenes, the parent of a prominent class of inorganic polymers, is the ring-opening polymerization (ROP) of [PCl2N]3. In contrast to the accepted (SN1-initiated) ROP mechanism that begins with heterolytic P-Cl bond cleavage in [PCl2N]3, our quantum mechanical (QM) calculations suggest that the ROP can proceed through a SN2-like route in which one [PCl2N]3 can be attacked by a neighboring [PCl2N]3 and hence transform through a four-center transition state (4C PNPCl TS), yielding a cyclic chlorophosphazene with a linear tail, termed a “tadpole”. Meanwhile, two [PCl2N]3 molecules can morph into [PCl2N]6 (RR expansion) through a different four-center transition state (4C PNPN TS) without the assistance of a bridging chlorine. As the activation energy of these processes follows the trend tadpole backbite < chain branching < ROP initiation ≤ RR initiation = RR expansion < chain propagation (all within 241.2 ± 16 kJ/mol), the ROP and RR mechanisms compete toward product formation. Not only does our pioneering QM calculations unveil the pivotal role of the bridging chlorine in the SN2 mechanism, it also explains its effect on reactivity of [PCl2N]3 species, underscoring the significance of halogen substituents in modulating polymerization. By comprehensively examining the ROP, RR, linear propagation, and ring closure processes, we attempt to resolve long-standing queries in chlorophosphazene research, elucidating the wide variability in reaction pot products and the necessity of halogen substituents in specific processes. Thus, this work characterizes a variety of four-center transition states for the first time and introduces a novel mechanistic process for polymerization. Finally, our work provides an explanation of the existence of the chlorinated tadpole.