The most popular method to synthesize polychlorophosphazenes, the parent of a prominent class of inorganic polymers, is the ring-opening polymerization (ROP) of [PCl 2 N] 3 . In contrast to the accepted (S N 1-initiated) ROP mechanism that begins with heterolytic P−Cl bond cleavage in [PCl 2 N] 3 , our quantum mechanical (QM) calculations suggest that the ROP can proceed through a S N 2-like route in which one [PCl 2 N] 3 can be attacked by a neighboring [PCl 2 N] 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 [PCl 2 N] 3 molecules can morph into [PCl 2 N] 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 S N 2 mechanism, it also explains its effect on reactivity of [PCl 2 N] 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.