Rapidly Reprocessable Cross-Linked Polyhydroxyurethanes Based on Disulfide Exchange

Abstract

Polymer networks that are cross-linked by dynamic covalent bonds often sacrifice the robust mechanical properties of traditional thermosets in exchange for rapid and efficient reprocessability. Polyurethanes are attractive materials for reprocessable cross-linked polymers because of their excellent mechanical properties, widespread use, and ease of synthesis, but their syntheses typically rely on harmful isocyanate precursors. Polyhydroxyurethanes (PHUs), derived from amines and cyclic carbonates, are promising alternatives to traditional polyurethanes. PHU networks are reprocessable via transcarbamoylation reactions even in the absence of external catalysts, but this process occurs over hours at temperatures above 150 °C. We have dramatically shortened the reprocessing times of PHU networks by incorporating dynamic disulfide bonds. Using cystamine as a comonomer gives materials with similar thermal stability and mechanical properties to other rigid cross-linked PHUs. Despite their excellent mechanical properties, these materials show rapid stress relaxation and have characteristic relaxation times as low as 30 s at 150 °C. This property enables reprocessing with quantitative recovery of cross-link density as measured by DMTA after only 30 min of elevated-temperature compression molding. Disulfide incorporation is a promising approach to obtain reprocessable, cross-linked PHU resins that are not derived from isocyanates.