A novel click-chemistry approach to flame retardant polyurethanes

The low thermo-oxidative properties of PU foams somewhat limits their practical application, particularly as heat sensitive materials. The introduction of a covalently linked flame retardant organophosphonate ester into the PU foam was achieved using the CuAAC 'click' reaction of an alkyne-polyol and azidoalkylmonophosphonate. These functionalised materials were prepared in four steps: first, a number of azidoalkyl monophosphonate compounds were formed via nucleophilic substitution of bromoalkylphosphonates with NaN3; next, polyols bearing terminal alkyne groups were prepared by anionic ring opening copolymerization between propylene oxide and glycidyl propargyl ether; followed by 'clicking' the azidoalkylphosphonate to the polyol and finally, synthesis of the PU foam with 2.4 wt% of “click-polyol”. The functionalised PU foam demonstrated a well-formed polyhedral cell structure and an increase in the fire resistance, according to the SEM and thermogravimetric analysis, respectively. Even after thermal treatment at 400 °C, the new PU foam material displayed enhanced flame resistant properties by forming a char layer on the surface of the polymer, whilst maintaining its polyhedral structure.