Process for preparing flame retardant bisimide product

This invention relates to a process for preparing a white flame retardant product which principally contains a bisimide. The process comprises: providing an essentially water-free solution containing tetrabromophthalic anhydride and a solvent in a reaction vessel; forming a reaction mass by adding a diamine or a diamine salt to the solution at an addition rate such that, at the addition temperature, water formed during the addition is removed from the reaction mass substantially as it is formed; terminating the addition of the diamine when the molar ratio of the tetrabromophthalic anhydride initially present in the solution to the diamine or diamine salt added is at least about 2.00:1; maintaining the reaction mass at a temperature of at least 125 DEG C for at least 0.5 hr after the addition; and recovering from the reaction mass the flame retardant product.