Effect of Kinetically-Distinct Crosslinking on Temporal Mechanical Property Development in Photopolymerized Networks

The degree of crosslinking in polymer networks dictates key mechanical properties including stiffness, toughness, and self-assembly. Crosslinking density has been patterned to form multi-materials with local variations in mechanical properties. This patterning can be achieved through a material system where crosslinking density can be varied with light dose. Thiol-ene click chemistry occurs under light-induced radical polymerization and has the potential for tunability. In addition to being consistent, high yield, and stereoselective, prior work has shown thiol-ene systems are controllable with judicious choice of thiol and ene monomers. In this work, we investigate the kinetics of various ternary thiol-ene systems as a function of light exposure. Using ¹H-NMR spectrometry and Fourier Transfer Infrared (FTIR) spectroscopy, we measure functional group conversion at various illumination times, probing the kinetics of thiol addition to both internal and terminal ene groups. In our investigation, we elucidate how steric hindrance, isomerization, and monothiol structure act in the radical addition. We find that thiols will preferentially add to terminal ene groups, and trans-oriented internal acrylate groups homopolymerize on a similar time scale. We correlate these kinetics to the crosslinking density of polymer networks formed using dithiols in this ternary system and the emergent mechanical properties. Ultimately, we present a methodology to directly correlate mechanistic studies of polymerization to bulk physical properties, which is anticipated to have applications in additive manufacturing and designer polymer networks.