BiTEMPS methacrylate dynamic covalent cross-linker providing rapid reprocessability and extrudability of covalent adaptable networks: high-yield synthesis with strong selectivity for disulfide linkages

Abstract

The dialkylamino disulfide-based dynamic cross-linker bis(2,2,6,6-tetramethyl-4-piperidyl methacrylate) disulfide, also known as BiTEMPS methacrylate (BTMA), has been of recent interest in the preparation of various reprocessable cross-linked polymers, otherwise known as covalent adaptable networks (CANs), by free-radical polymerization or free-radical reactive processing. Here, we revised the synthesis of BTMA to produce “BTMA-S2”, i.e., BTMA with a significantly higher yield, higher purity, and less color, with ≥95% disulfide linkages compared to “BTMA-S_n”, i.e., a previous version of BTMA that was synthesized with a relatively low yield, lower purity, and more color with a mixture of oligosulfide linkages. We used a low level (5 mol%) of this BTMA-S2 to synthesize CANs with n-hexyl methacrylate (BTMA-S2-HMA CANs). The BTMA-S2-HMA CANs recover their original cross-link densities after reprocessing. Additionally, compared to the BTMA-S_n-HMA CANs, the BTMA-S2-HMA CANs exhibit much faster stress relaxation at elevated temperatures, which manifests in rapid reprocessability. Specifically, BTMA-S2-HMA CANs made with 5 mol% BTMA-S2 can be reprocessed by compression molding at 130 °C for 5 min with full recovery of cross-link density. This is a factor of six faster reprocessing than analogous BTMA-S_n-CANs made with BTMA-S_n. Additionally, we demonstrate facile melt extrusion at 180 °C of the BTMA-S2-HMA CAN material with full recovery of cross-link density after extrusion.