Entropy-Driven Ring-Opening Polymerisation represents an attractive mechanism to produce high-performance polymeric materials as it can be performed using neat, low-viscosity precursors and without the production of by-products or release of volatiles. Macrocyclic oligomers (MCOs) of polyether ketone ketone (PEKK) were synthesised and investigated as an in situ method of forming this high-performance thermoplastic. Cyclic oligomers were successfully synthesised by pseudo-high dilution methods, and the reaction conditions were optimised through careful addition of starting materials and carbonate base selection. These novel compounds were characterised, X-ray crystal structures were obtained, and the synthesis method was extended from the homopolymers to MCOs with the structural isomers predominantly used in industry. PEKK formed from MCOs were characterised by DSC, TGA and GPC and found to have similar glass transitions and molecular weight averages to those of a commercial PEKK polymer.

Understanding how molecular weight distribution (MWD) influences the mechanical and fracture behaviour of additively manufactured PA12 is essential for establishing reliable process–structure–property relationships. An approach to minimize anisotropy has been to specifically target the feedstock materials with the goal of enhancing interlayer adhesion. In this study, polyamide 12 (PA12) grades of MWD were investigated to determine the effect of MWD on the interlayer bond formation and anisotropy of the printed samples. Three different grades of PA12 were printed into tensile coupons and double cantilever beam (DCB) specimens and the mechanical properties were assessed to study interlayer bond strength using standard fracture toughness technique. A significant improvement in mechanical properties and fracture toughness were observed with higher print temperatures attributed to strong interlayer adhesion owing to longer interlayer bonding times (verified through the Lumped Capacity model and thermocouple data). Fracture toughness assessment also demonstrated that amongst samples printed at the same temperature, PA12 with a lower average MWD provided the highest fracture toughness (G _1C ) values, attributed to the faster diffusion of short polymer chains across the interlayer interface. Tailoring the MWD of feedstock polymers utilized in 3D printing as a strategy to optimize performance and lower anisotropy was demonstrated to be a feasible pathway. These findings were utilized by developing a PA12 blend of broad MWD which provided the highest overall mechanical properties and G _1c , demonstrating the usefulness of incorporating a high proportion of fast diffusing short polymer chains in the development of polymeric feedstock materials. Additionally, for large scale 3D printing applications, where maintaining high chamber temperatures is impractical, these results suggest that optimizing MWD can be an alternative strategy to improve interlayer adhesion and mechanical properties without relying solely on elevated print temperatures.