We report a tough, semicrystalline, ternary thiol ene polymer system containing linear dithiols, cross-linking trithiols, and spiroacetal alkene units in the main chain backbone that is synthesized by "click" ultraviolet photopolymerization in a one-step, solvent-free process. We varied the cross-link density to tune crystallinity and microstructure; in turn, thermomechanical properties such as yield strength, glass transition temperature, failure strain, and stress-strain behavior could be modified and controlled. Thiol-enes containing 7.5 and 10 thiol mol % cross linker resulted in networks that balanced crystallinity, elasticity, and cross-linking to maximize toughness. These materials demonstrate how the presence of spiro units throughout a polymer's backbone creates semicrystalline networks of substantial toughness from traditionally weak chemistries such as thiol enes. This system can be synthesized in a neat, one-step photopolymerization process; as such, it illustrates the power of spirochemistry in designing photopolymers with tunable; robust thermomechanical properties.