Theoretical Study on the Mechanism and Regioselectivity of the Macromolecular Substitution Reactions of [NPCl₂]_n with Bifunctional Nucleophiles by a Combination of Quantum Mechanical and Molecular Dynamics Calculations

Abstract

A combination of molecular dynamics simulations and quantum chemistry calculations affords a good understanding of the reaction between [NPCl2] n and the bifunctional nucleophile aminophenol HO−C6H4−NH2 in THF solution and in the presence of alkali carbonates. This reaction is regiospecific at room temperature favoring the attack by the NH2 group, and only at higher temperatures the attack by the OH is activated. It is also explained why the reaction of HO−C6H4−NH2 with {[NP(O2C12H8)]1−x [NPCl2] x } n and Cs2CO3 at reflux proceeds exclusively by the OH giving aryloxyphosphazenes with terminal NH2 substituents. Molecular dynamics simulations have been performed on model systems consisting on [NPCl2] n chains surrounded by aminophenol molecules or ions obtained by deprotonation of either the OH or NH2 groups. Quantum mechanics calculations consisted in density functional theory (DFT) computation of the reaction mechanism of aminophenol with the phosphoranimine (NH2)Cl2P(=NH) as a simple model for the NPCl2 phosphazene units, using B3LYP approximation with 6-31G(d). Our results provide evidence to support that the macromolecular substitution reactions of poly(dichlorophosphazene) with aryloxide ions are diffusion controlled and governed by the atomic charges, while the reaction with amino-arenes are directed by hydrogen interactions between the NH2 hydrogens and the phosphazene nitrogen atoms that initiate a lower activation energy stepwise mechanism competing with a concerted mechanism. The combined methodology employed in this work seems to be useful to address the regiochemistry of the macromolecular substitution on polyphosphazenes with heterobifunctional nucleophiles which is a central issue in the design of polymers with predetermined chemical composition and structure.

An efficient and stable star-shaped plasticizer for starch: cyclic phosphazene with hydrogen bonding aminoethoxy ethanol side chains

“…This may not become a surprise as also other studies show a regioselectivity in the macromolecular substitution of bifunctional nucleophiles, with preferred substitution via an amine rather than an alcoholic hydroxyl. [46][47][48][49] In fact, the above suggested statistical mixture of both reaction topographies may even be favorable to promote plasticization, as suppression of regularity is attractive for suppressed crystallization and efficient plasticization. Also, whether the aminoethoxy ethanol NH 2 -(CH 2 ) 2 -O-(CH 2 ) 2 -OH reacts onto the phosphazene ring from its amine or hydroxyl group, the opposite peripheral hydroxyl or amine end groups can still allow hydrogen bonding with starch.…”

A Combined Experimental and Molecular Simulation Study of Factors Influencing the Selection of Antioxidants in Butadiene Rubber

“…On the micro scale, quantum mechanics (QM) simulations based on density functional theory 14,15 have been successfully applied to reveal the underlying reaction mechanisms that are difficult to obtain in experiments and to design reliable synthetic methodologies for diverse materials. 16 After using the correct experiments to identify the rate-determining reaction in thermal-oxidative aging, QM simulations can accurately calculate the free energy of the reaction. With molecular simulations, researchers can easily clarify the solubility and mobility of an antioxidant and the permeation process for O 2 during thermal-oxidative aging.…”

“…MD simulations help us obtain detailed insights about microstructures and provide direct quantitative information about the solubility parameters, mean square displacement, and other structural features of amorphous materials. On the micro scale, quantum mechanics (QM) simulations based on density functional theory , have been successfully applied to reveal the underlying reaction mechanisms that are difficult to obtain in experiments and to design reliable synthetic methodologies for diverse materials . After using the correct experiments to identify the rate-determining reaction in thermal-oxidative aging, QM simulations can accurately calculate the free energy of the reaction.…”

Synthesis and biomimetic mineralization of l‐proline substituted polyphosphazenes as bulk and nanofiber

“…Most of the studies involving bifunctional molecules in substitution reactions with phosphazenes and polyphosphazenes involve diamines, amino‐alcohols and amino‐phenols that have one of their reactive sites protected, and deprotected after substitution. A theoretical study about the reactivity of p ‐aminophenol in polyphosphazene substitutions showed that lower energy is required to substitute the chlorine atoms through the nitrogen than through the phenolic oxygen, at room temperature and using THF as solvent . Therefore, this substitution would be expected to happen almost exclusively through nitrogen.…”