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The incorporation of flame retardants (FRs) is often applied to reduce the fire risk of flexible polyurethane foam (FPUF), however, the addition of FRs usually brings mechanical loss, and the emigration of FRs during service life results in a decrease in flame retardancy. In this work, a hyperbranched FR (HPNP) containing nitrogen and phosphorous was synthesized via transesterification between dimethyl methyl phosphonate (DMMP) and triethanolamine. The distribution of HNPN in FPUF was simulated by the Materials Studio software. The flammability was characterized by the open flame test, horizontal combustion test, limiting oxygen index (LOI) and cone calorimeter test. The FPUF sample containing only 4 wt% HPNP self-extinguished immediately after the removal of the ignitor. For the FPUF containing 8 wt% HPNP sample, the LOI was increased to 26.3% from 17.9%. The total heat release and total smoke production were reduced by 65% and 75%, respectively compared to that of the control FPUF sample. More importantly, the radical trapping action in the gas phase and the charring promotion in the condensed phase were simulated and verified based on the experimental data. Furthermore, the interfacial energy calculation and molecular dynamics simulation suggested that the improved compatibility of FPUF/HPNP resulted in the superior durability of HPNP in FPUF. In addition, the FPUF samples containing HPNP also showed enhanced mechanical properties. It was expected the hyperbranched HPNP would be an ideal additive to produce flame retardant FPUF with excellent comprehensive performance. [Display omitted] • A hyperbranched flame retardant (HPNP) was synthesized by transesterification. • FPUF containing 8 wt%HPNP showed self-extinguish effect with an LOI of 26.3%. • Total smoke release of FPUF was greatly reduced by the presence of 8 wt% HPNP. • The addition of HPNP also enhanced the mechanical properties of FPUF. • The improved compatibility of HPNP and FPUF suppressed the emigration of HPNP. [ABSTRACT FROM AUTHOR]