Hexachlorocyclopendadiene Hexachlorocyclopendadiene [77-47-4]  is a reactive intermediate and is used to manufacture flame retardant for use in the wire and cable industry as well as the preparation of chlorendic anhydride, which is used as a reactive intermediate in the manufacture of flame retardant, unsaturated polyesters.

Tetrachlorphthalic anhydride   Tetrachlorphthalic anhydride  [117-08-8] (TCPA) is manufactured by the ferric chloride catalyzed chlorination of phthalic anhydride. The relatively low chlorine content and the lower flame retardant efficiency of the aromatic chlorides limit use to unsaturated polyester resin formulations that do not require a high degree of flame retardancy.

Chlorendic acid  Chlorendic acid [115-28-6] (HET Acid) and its anhydride [115-27-5] are widely used flame retardants. Chlorendic acid is synthesized by a Diels-Alder reaction of maleic anhydride and hexachlorocyclopentadiene   in toluene  followed by hydrolysis of the anhydride using aqueous base (60). The anhydride can be isolated directly from the reaction mixture or can be prepared in a very pure form by dehydration of the acid. The principal use of chlorendic anhydride and chlorendic acid has been in the manufacture of unsaturated polyester resins. Because the esterification rate of chlorendic anhydride is similar to that of phthalic anhydride , it can be used in place of phthalic anhydride in commercial polyester formulations. Besides having a degree of fire resistance, polyesters containing chlorendic anhydride often exhibit a significant degree of UV stability and corrosion resistance The double bond in chlorendic acid is not reactive as a cross-linking site; hence, reactive monomers such as maleic anhydride must be included in the polyester backbone to achieve cross-linking. Other applications are in alkyds, polyurethanes, and epoxy resins.