Low-Temperature Solid-State Polycondensation Synthesis of Non-Isocyanate Polyurethanes: A Pathway to High Molar Mass, Performance, and Reprocessability

Conventional melt polycondensation methods, while principally suited for producing non-isocyanate polyurethanes (NIPUs), are limited by transurethanization kinetics and rely on high-temperature protocols that ultimately compromise the performance of NIPUs. To boost their competitiveness as sustainable alternatives to isocyanate-based polyurethanes, we addressed this barrier by implementing a promising yet previously unexplored route - solid-state polycondensation (SSP) - for synthesizing semi-crystalline and CO2-rich non-isocyanate poly(carbonate-urethane)s (NIPCUs). The SSP was carried out on solid particles obtained through a brief melt prepolymerization, followed by crystallization and polymerization in the solid state, where volatile condensation co-products were efficiently removed by a flow of inert gas. By varying particle size and adapting the process conditions to the thermal properties of the prepolymer, it was possible to effectively promote the transurethanization reaction at temperatures as low as 80-130 °C. Characterization of the SSP products revealed a substantially higher molar mass and toughness, as well as superior aesthetic quality compared to melt-polymerized NIPCUs, suffering from degradation that counteracted effective chain extension. Moreover, the SSP method suppressed side reactions and thermal degradation while forming a highly ordered crystalline phase. This imparted mechanical integrity, allowing the NIPCUs to maintain high performance over multiple reprocessing cycles and demonstrate excellent recyclability potential.