The work described in this thesis is concerned with the synthesis, isolation and characterisation of macrocyclic oligomers ("cyclomers") found in semi-crystalline poly(aryl ether ketone)s. A wide range of cyclomers were obtained in good yield (up to 92%) under pseudo-high dilution by nucleophilic condensation and their cyclic nature was confirmed by a range of analytical techniques. Selected macrocycles were used to produce polymers by ring-opening polymerisation in the melt. It was also shown that during a typical high-temperature synthesis of a poly(aryl ether ketone). such cyclic oligomers are incorporated into the polymer chain. Polymers with a high macrocyclic content were synthesised by high temperature polycondensation at low monomer concentration. These materials were used to assess the influence of macrocycles on polymer viscosity in the melt.