2025 AIChE Annual Meeting

Modeling Crosslinked Thiol-Yne Polymerizations

A growing interest in holographic polymers requires researchers to gain greater control over the polymerization process in an effort to generate improved holographic properties (increase refractive index and decrease haze). As such, a strong focus has been aimed towards improving models for holographic polymerizations due to their ability to quickly and efficiently explore possible reactions and predict favorable conditions. One promising approach uses thiol-yne step growth photopolymerization (also known as thiol-yne “click” chemistry) to create holographic polymers. Thiol-yne is remarkable due to the high theoretical crosslink density, which is twice that of analogous thiol-ene systems. While the mechanism of thiol-yne is well established, the reaction kinetics of thiol-yne polymerizations are less understood, and the impact of traditional control mechanisms; temperature, concentration, stoichiometry, and light intensity are not known. Additionally, viscosity increases beyond the gel point, in combination with radical termination kinetics, prevent the polymerization from reaching high conversion (greater than 80%), even at high temperatures. Here, previous experimental results and the reaction mechanism are used to develop novel models for the thiol-yne photo-polymerization in an effort to predict the conditions necessary to achieve higher conversion and superior control over local conversion for improved image clarity. Previous models have investigated chemical reactivity of individual reaction steps, but fail to account for the variables involved in bulk polymerizations, such as concentration, stoichiometry, and diffusion limitations due to high crosslink density and viscosity. This study incorporates reaction kinetics, radical population balances, and diffusion constraints via free volume theory to create the first comprehensive model for thiol-yne polymerizations. Simulating the polymerization provides a deeper understanding of reaction kinetics while qualitatively relating reaction parameters to final material properties such as crosslink density. Additionally, novel acceleratory behavior, termed intermediate accelerated kinetics, is predicted for thiol-yne systems. This is defined as a temporary increase in thiol consumption rate as the reaction progresses. Further investigation into “click” chemistry will focus on improving models that expand our understanding of the fundamental processes of reaction and diffusion in highly crosslinked polymerizations. This has applications to other polymerizations (e.g. polyester and polyurethane) and will allow for greater control of local conversion; which is important in the continued development of polymer printing processes (e.g. StereoLithogrAphy) and holographic polymers for use in augmented reality, security, and information storage.