the following structure formula: ##STR4## wherein said monomer and said prepolymer are thermoplastic and wherein R in said monomer is selected from phenylene radicals; --SAS-- radicals wherein S is a sulfur atom and A is an aromatic or aromatic aliphatic hydrocarbon group; bisphenol radicals containing no or at least one halogen substituent; diethoxy containing intermediate imide and tetravalent a