Preparation of novel high T-g (220-280 degrees C) macrocyclic oligomers in high yield by the reaction of 1,2-dihydro-4-(4-hydroxyphenyl)(2H)phthalazin-1-one with activated difluoro-monomers is described. The reaction, conducted under pseudo-high dilution conditions, produces cyclic oligomers in 90-97% isolated yield. Detailed structural characterization of these novel oligomers by the combination of NMR, MALDI-TOF-MS, GPC, and reverse-phase HPLC confirm the cyclic nature and reveal the composition of these cyclic oligomers. MALDI-TOF-MS which enables the detection of oligomers with mass up to 6000 Da, is shown to be a very powerful tool for determination of and the proof of the cyclic nature of the cyclic oligomers. The MALDI results provide answers to the possible combinations of monomer units in the cyclic oligomeric components for random co-cyclic oligomers. Rheological measurement of cyclic oligomers 3c shows that the cyclic oligomers are thermally stable in the melt and the molten cyclic oligomers essentially behave like Newtonian fluids. At 340 degrees C and 100 s(-1) the steady-state shear viscosity of the molten cyclic oligomers 3c is only about 14 poise. Ring-opening polymerization of the co-cyclic oligomers 4 to a high molecular weight polymer with M(w) = 87,000 is achieved by heating at 340 degrees C for 45 min in the presence of a nucleophilic initiator.