The 3D printing of polyesters is most commonly performed using polylactide filaments for material extrusion (formerly known as fused-deposition modelling) or by introducing methacrylate end-groups to polyesters, which can be polymerized by a radical mechanism. However, cyclic esters can also be polymerized via cationic ring-opening photopolymerization, which opens up the possibility of using high-resolution stereolithography as the method of choice to form polyester networks in situ during the printing process. Hereby, we present the first approach to 3D print polyesters starting directly from ε-caprolactone via stereolithography at elevated temperatures, by introducing low amounts of crosslinks into the material to provide stability to the structure but maintain its degradable character and especially semicrystallinity. Photorheology tests showed that high temperatures and the introduction of crosslinkers led to increased reactivity and fast gelation. By performing tensile tests, dynamic mechanical thermal analysis and simultaneous thermal analysis the material properties were evaluated depending on the amount and type of crosslinker. It was shown that decreasing the amount of crosslinker significantly increased the crystallinity and influenced the mechanical properties as well as shape memory properties. Finally, successful 3D printing of a polyester was performed using 10 mol% of a crosslinker with oxetane moieties and 90 mol% ε-caprolactone at 120 °C.

The polymerization of cyclic esters to form polyesters is a thoroughly investigated topic and can be performed via anionic, cationic or metal‐mediated techniques. While there have been attempts to apply light‐induced on‐demand initiation techniques, in most investigations photobase generators are used. In this work the focus lies on photoacid generators, which – contrary to many photobase generators – are easily stored, thermally stable, reactive, less sensitive towards water and oxygen inhibition and commercially available in various forms. However, in most cases the low reactivity of the monomers requires long polymerization times. Therefore, this study provides a basic study of the temperature‐dependent cationic photopolymerization of cyclic esters and shows that polyesters can be formed in less than 5 min of irradiation at elevated temperatures and without solvents, leading to highly crystalline products. In addition, the temperature‐dependent crystallinity, molecular weights and dispersities of the polymers are investigated and presented. © 2021 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.