High performance polydicyclopentadiene engineering materials with intrinsic flame retardancy: flexible copolymerization of flame-retardant elements and multiple interactions within aggregated structures

Improving the flame-retardant performance of polydicyclopentadiene (PDCPD) while maintaining mechanical properties has remained a technical challenge. In this study, a novel synchronous enhancement strategy for intrinsic flame retardancy and reinforcement based on frontal ring-opening metathesis polymerization (FROMP) was proposed, in which the newly synthesized phosphorus-containing norbornene flame retardant (NB-DPPC) and 5-norbornene-2-carboxylic acid (NB-COOH) were embedded into the cross-linked networks of PDCPD. The effects of NB-COOH and NB-DPPC on the thermodynamics and kinetics of the polymerization, as well as the crosslinked structure, mechanical properties, and flame retardancy of the copolymers, were systematically investigated. The results showed that by introducing NB-COOH into the PDCPD system improved the mechanical properties and flame resistance of the material. Specifically, the tensile strength and elongation at break of the specific copolymer reached 71.81 MPa and 123.62%, which were 50.07% and about 12 times higher than those of PDCPD, respectively. Meanwhile, the maximum limiting oxygen index (LOI) of the material reached 30.2%, resulting in an increase of 51.76%. According to the carbon layer structure and combustion gas composition, the flame retardant mechanism was further investigated. This work established a efficient method for the preparation of intrinsic flame retardant PDCPD and provided a strategy for constructing PDCPD materials with excellent comprehensive performance.