The construction of a stable physical–chemical multi-crosslinking structure through a simplified FROMP strategy synergistically enhances the flame retardancy and mechanical properties of PDCPD

The enhancement of the flame retardancy of polydicyclopentadiene (PDCPD) while maintaining its excellent mechanical properties has long been an important and critical technical challenge for many years. In this contribution, we designed and synthesized a new flame-retardant monomer containing double norbornene groups (NB-PDP) which can undergo the frontal ring-opening metathesis polymerization (FROMP) with dicyclopentadiene (DCPD) and 5-dicyclopentadiene-2-carboxylic acid (NB-COOH). Through the formulation optimization, the flame retardancy and mechanical properties of the copolymers could be easily regulated. To investigate the influencing mechanism of the NB-PDP and NB-COOH on the properties of the copolymers, the thermodynamics and kinetics of the FROMP, as well as the micro-structures, mechanical properties, and flame-retardant performance of the PDCPD/NB-PDP/NB-COOH copolymers were systematically studied. The findings suggest that the integration of NB-PDP and NB-COOH resulted in a diverse array of physical and chemical cross-linking networks within the system. Consequently, the tensile strength of the copolymers reached a maximum of 63.1 MPa and the elongation at break achieved up to 28.5%, representing the increases of 43.0% and 154.0% compared to that of PDCPD without any modification, respectively. It is worth mentioning that except the flame-retardant NB-PDP, NB-COOH could also serve as the carbon source to enhance the char formation and further improve the flame-retardant properties, such as the limiting oxygen index (LOI), peak heat release rate (PHRR), total heat release (THR), and total smoke production (TSP). These phenomena indicate that the material exhibits excellent mechanical properties and conspicuous flame retardancy. This work provided an efficient method for the preparation of the intrinsically flame-retardant PDCPD materials and a new strategy for the constructing of the thermosetting materials with excellent comprehensive performance.