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Abstract: As the largest class of inorganic macromolecules, polyphosphazenes have unique backbones that contain alternating nitrogen and phosphorus atoms. Starting from the chlorinated parent polymer, polyphosphazenes have tunable properties and hence are widely applied in different fields. However, in contrast to the intense functionalization and application studies, few works have reported the polymerization mechanism to explain the intricacies of the reaction process. Despite the advancements in sophisticated analytical chemistry techniques and cutting-edge technologies, seizing and analyzing the high-energy reaction intermediates remains a formidable challenge although numerous attempts have been made. As a result, the lack of understanding of the reaction mechanism has significantly plagued the refinements and developments of polymer synthesis and hence hindered the bulk use of novel materials. This presentation will give a quick walk through of the recent efforts in exploration of the monomer formation, ring-opening (ROP) and ring-ring expansion (RR) of [PCl₂N]₃ using quantum mechanical calculations. These findings can provide invaluable guidance for further experimental mechanistic studies through instrumental analysis, and also shed light on some mysteries that have remained elusive in over 200 years of chlorophosphazene research. Specifically, what are potential "tadpole" compounds, what can trigger crosslinking reactions and why there is a coexistence of linear and cyclic products in polymer synthesis.