Modeling Degradation of Thioester Networks Controlled by Oligomer Structures and Thiol-Thioester Exchange

The oligomeric structure of thioester-based covalent adaptable networks (CANs) was used to tune the bulk degradation of thioester networks via a thiol-thioester exchange (TTE) reaction. A statistical kinetic model for degradation was developed that considered the exchange reaction rate, the number of thioester links within oligomers, and the dispersity of these oligomer lengths. Model predictions indicated that the number of thioester links within the oligomers impacted the degradation rate by as much as 10-fold, while narrowing the oligomer dispersity impacted the degradation rate by up to 2-fold. To evaluate model predictions with experimental degradation studies, thioester oligomers were synthesized, photocured into films, and degraded by submerging them in a solution of 1 M butyl-3-mercaptopropionate, 0.3 M triethylamine, and acetone. Model predictions matched experimental results, showing that increasing thioester links in oligomers from one to four decreased the time for complete mass loss from 25 to 4 h while using only a single fitting parameter, the reaction rate constant k, which ranged from 0.0024 to 0.0040 M^-1 min^-1. An alternate route to tuning degradation was established by mixing oligomers containing either one or four thioester links in various molar ratios, which created blended, disperse CANs that mimicked the degradation profiles of monodisperse networks. Lastly, mass release studies using the model dye Nile red confirmed that thioester oligomers in CANs enable quantifiable mass release. This research demonstrates that the multifunctional oligomer structure within a single type and structure of a CAN represents a viable way to control the degradation profiles and release behavior achieved in these materials.