The title composition, isoPEK, was prepared from 4-fluoro-3’-hydroxybenzophenone in N-methylcaprolactam at 225 degrees C. Cyclomer formation interfered with polymerization at lower temperatures in alternative solvents and dominated in dilute solutions. The 50/50 to 70/30 copolymers of PEK and isoPEK were prepared using 4-fluoro-4’-hydroxybenzophenone. An analogous composition, poly(oxy-1,3-phenylenecarbonyl-1,4- phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene) or isoPEKEK, was prepared from 4,4’-difluorobenzophenone and 3,4’-dihydroxybenzophenone using similar conditions. Thermal crystallization of the copolymers was successful, but isoPEK and isoPEKEK were only capable of solvent-induced crystallization. The semicrystalline materials were studied by thermal analysis and X-ray diffraction. The copolymers melted at 264-307 degrees C and possessed random microstructures with isomorphic crystallization of the PEK and isoPEK repeat units. IsoPEK and isoPEKEK melted, respectively, at the low temperatures of 180 and 193 degrees C. IsoPEK crystallized with both Form 1 and Form 2 unit cells, while isoPEKEK and the copolymers crystallized with the Form 1 unit cell. IsoPEK was also prepared by ring-opening polymerization of its cyclomers in the melt using cesium fluoride as catalyst. The 3,4’ catenation of the isoPEK repeat unit and the related monomers was used to rationalize the unique chemistry and properties of these polymers.