Photobase-catalyzed thiol–ene click chemistry for light-based additive manufacturing†

Abstract

Photo-mediated additive manufacturing from liquid resins (vat photopolymerization) is a rapidly growing field that will enable a new generation of electronic devices, sensors, and soft robotics. Radical-based polymerization remains the standard for photo-curing resins during the printing process due to its fast polymerization kinetics and the range of available photoinitiators. Comparatively, there are fewer examples of non-radical chemical reactions for vat photopolymerization, despite the potential for expanding the range of functional materials and devices. Herein, we demonstrate ionic liquid resins for vat photopolymerization that utilize photo-base generators (PBGs) to catalyze thiol-Michael additions as the network forming reaction. The ionic liquid increased the rate of curing, while also introducing ionic conductivity to the printed structures. Among the PBGs explored, 2-(2-nitrophenyl)-propyloxycarbonyl tetramethylguanidine (NPPOC-TMG) was the most effective for the vat photopolymerization process wherein 250 μm features were successfully printed. Lastly, we compared the mechanical properties of the PBG catalyzed thiol-Michael network versus the radical polymerized network. Interestingly, the thiol-Michael network had an overall improvement in ductility compared to the radical initiated resin, since step-growth methodologies afford more defined networks than chain growth. These ionic liquid resins for thiol-Michael additions expand the chemistries available for vat photopolymerization and present opportunities for fabricating devices such as sensors.