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Polyurethane elastomers were prepared with hydroxytelechelic polybutadiene (HTPB) as polyol, modified 4,4′‐diphenylmethane diisocyanate (modified MDI) as liquid polyisocyanate, and phosphonate diols as chain extenders and flame retardant compounds. These phosphonate diols were synthesized by radical thiol–ene addition of allyl or vinyl dialkyl phosphonate to 3‐mercapto‐1,2‐propanediol. For various percentages of phosphorus (0 to 3%, w/w), polyurethane elastomers remain stable up to 250 °C. The percentage of residual char at 600 °C increases with increasing phosphorus content. For the soft segments, no variation in the glass transition temperature (Tg) is observed as the percentage of P increases, whereas the Tg of hard segments increases. Above 0.5% phosphorus content, the limiting oxygen index (LOI) becomes higher than the percentage of oxygen in the air. © 2003 Society of Chemical Industry

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