Theoretical study on the P–N bond dissociation enthalpy in phosphamide and phosphoramidate flame retardants

Abstract

The phosphamide and phosphoramidate are well known as new types of phosphorus-nitrogen synergistic flame retardant in epoxy resin composites, rigid polyurethane foam and cotton fabric. The P–N bond breakage of the phosphamide and phosphoramidate flame retardants is involved in the flame-retardant process, which is very important for the flame-retardant effect. By comparing the precision of different density functional theory (DFT) on P–N BDEs and it is found that M06-2X provides the most accurate result with a minimum root mean square error (RMSE) of 5.3 kJ/mol. Therefore, the M06-2X method was used to study different types of phosphamide and phosphoramidate flame retardants, including DOPO-based flame retardants, diphenyl-phosphamide, phosphoramidate flame retardants and spirocyclic-bisphosphamide, bisphosphoramidate flame retardants. Through the preliminary verification of the calculation of BDE and the data including indexes of limiting oxygen index (LOI), temperature at 5 wt% loss (T5%) and temperature at maximum mass loss rate (Tmax) in the experimental literature, it is found that P–N BDEs have a certain correlation with the flame-retardant effect. Finally, based on the calculated BDE results and the substituent effect, new flame retardants based on these three parent structures of flame retardants were designed and the flame-retardant effects were predicted. The results show that the parent structures such as diphenyl-phosphoramidate and spirocyclic-bisphosphamide were beneficial to the flame-retardant effect of the flame retardants, and the structural unit on N atom such as aniline and thiazole were more favorable to the flame retardancy of the flame retardants.