compounds which are bis - are prepared by a new process in which a halogen - containing precursor is heated in the presence of a nickel compound , a reducing agent , for example , metallic zinc , and an organic solvent . the resulting compound is useful in a process for the preparation of a copolymer , in which the compound is reacted with alkali to form a tetraacid , the acid is dehydrated to form a dianhydride , and the dianhydride is reacted with a second dianhydride and a diamine under copolymer - forming conditions .