(260c) Radical-Mediated Ring Opening Photopolymerization for Semi-Crystalline Thermoplastic Additive Manufacturing

Polymer processing approaches that employ in situ polymerization, such as the fabrication of fiber-reinforced polymer composites, the photocure of dental restorative materials, and the application of spray foam insulation, predominantly utilize thermosetting resins which, upon polymerization, afford cross-linked polymer networks. Nevertheless, in situ thermoplastic polymerization has become well-accepted in several areas, including the anionic ring opening polymerization of caprolactam in thermoplastic resin transfer molding and the free radical polymerization of methyl methacrylate in bone cements. One area where in situ thermoplastic polymerization holds significant promise is in stereolithographic additive manufacturing (SLA, also known as vat photopolymerization), a common approach for the three-dimensional (3D) printing of polymers based on the patterned photopolymerization of liquid resins, owing to the particular set of physical properties offered by these uncross-linked materials.

Conventional approaches to SLA employ liquid resin formulations based on multifunctional (meth)acrylates and epoxides that afford cross-linked polymeric networks upon polymerization. Nevertheless, the utilization of resins that yield semi-crystalline thermoplastics provides facile access to physical and mechanical properties that are otherwise difficult to attain, such as high toughness and resistance to solvent swelling as well as additional post-fabrication processing options that can be used to extend the life cycle of 3D printed polymers. A major challenge associated with SLA employing semi-crystalline thermoplastic resins is poor inter-layer adhesion, resulting in mechanically-compromised printed parts. Here, we report the SLA-based additive manufacturing of semi-crystalline thermoplastics utilizing the radical-mediated ring-opening photopolymerization of cyclic allylic sulfides. Upon irradiation, resin formulations based on these cyclic monomers polymerize and subsequently crystallize rapidly, enabling fast print speeds. Propagation during the ring-opening polymerization proceeds via an addition-fragmentation reaction, conserving the allylic sulfide functional group which then provides a mechanism for inter-layer covalent connectivity by enabling further addition-fragmentation chain transfer between polymer chains in adjacent layers. Stereolithographic printing with cyclic allylic sulfide resin formulations thus yields semi-crystalline polymer parts that retain good mechanical properties compared with their bulk counterparts.