Synthesis of High-Tg Nonisocyanate Polyurethanes via Reactive Extrusion and Their Batch Foaming

Polyurethanes (PUs) are common commodity plastics that cover a wide range of applications. However, PUs are based on isocyanates, the toxic compounds that pose respiratory and dermal hazards. As a more sustainable alternative, polyhydroxyurethanes (PHUs) have gained attention as PU substitutes. Nevertheless, their utility is often limited by low molecular weights and inferior properties. To overcome these shortcomings, the present study focuses on the application of reactive extrusion (REx) to the synthesis of thermoplastic PHUs, allowing rapid production without compromising molecular weight. First, a series of cyclic carbonate monomers were synthesized; in particular, a BPA-based bis(cyclic carbonate) was prepared from the commercial bis(epoxide), while two additional bis(cyclic carbonates) were prepared from the relevant bisphenols via sequential epoxidation and carbonation with CO2. Spectroscopic analyses confirm the successful synthesis of high-purity monomers. Second, PHUs were prepared via REx of the synthesized bis(cyclic carbonates) with various diamines following optimization of the process using a model system with the aim of maximizing the molecular weight and yield. The resulting series of linear PHUs possessed molecular weights of up to 21800 g/mol (as assessed via GPC), high strengths (up to 60 MPa as determined by quasi-static tensile testing), and attractive thermal properties (Tg up to 110 °C via DSC; Tonset > 195 °C via TGA). The viscoelastic and rheological behavior of selected PHUs is also reported. Finally, PHUs with comparable properties to traditional PUs were foamed, and the resulting foams were shown to possess densities and compressive and thermal properties comparable to conventional rigid PU foams.