Molecular insights into the effect of small molecule impurities in the ring-opening polymerization of [PCl2N]3 using quantum mechanical analysis

Abstract

This is the first theoretical investigation that systematically analyzes the interactions between the hexachlorophosphazene, [PCl₂N]₃, and small molecule impurities H₂O and HCl in a 1 : 1 stoichiometric ratio. Utilizing both ab initio methods and seven DFT functionals in conjunction with the triple-ζ basis set, the pivotal structures in proposed reaction mechanisms are fully characterized and the energy change for each step was determined at the CCSD(T)/aTZ‖MP2/aTZ level of theory. Our QM calculations show that [PCl₂N]₃ can be hydrolyzed via a single-step mechanism with an activation energy of ca. 180 kJ mol⁻¹, or be ring-opened by HCl through a two-step mechanism, in which the rate-determining step has an activation energy of ca. 120 kJ mol⁻¹. Because the activation energy of these two reactions is notably lower than that of the ring-opening polymerization and the ring–ring expansion equilibrium (which requires ca. 240 kJ mol⁻¹ of energy determined at a comparable DFT level of theory), our study indicates that even trace amount of H₂O and HCl can significantly interfere with the polymerization process. Beyond revealing new mechanistic details, our calculations also indicate that all selected functionals can provide reasonable electronic structures to describe the reaction progress. On the other hand, while each of the functionals investigated here excels in closely matching the CCSD(T)/aTZ‖MP2/aTZ energy barriers for certain steps in the reaction, the B3LYP functional is capable of providing the most consistent results. This establishes that the B3LYP functional can be suitable for investigating phosphazene reactions as a computationally efficient and robust quantum mechanical approach while maintaining near–ab initio accuracy.