Highly Efficient Approach to High-Molecular Weight Polyhydroxyurethanes

Conventional methods for synthesizing thermoplastic polyurethanes (TPUs) typically rely on isocyanates and tin-based catalysts, both of which pose significant environmental and safety risks. To mitigate these issues, alternative routes for synthesizing nonisocyanate polyurethanes (NIPUs) have been explored since the 1980s. However, most reported approaches yield polymers with molecular weights below 35 kg/mol and only a few exceptions reaching 50-70 kg/mol, often requiring oligomeric monomers or postmodification. This limitation has been a major barrier to the broader application and production of NIPUs. In this study, we demonstrate a direct polyaddition strategy for synthesizing high-molecular-weight poly-(hydroxyurethane)-s (PHUs, a subclass of NIPUs) from low-mass difunctional monomers without relying on macromonomers, oligomers, catalysts, or postcondensation/modification steps. The key to this achievement was the rational design of the monomers. The reactivity of cyclic carbonates was enhanced by incorporating aromatic rings into the structure of 7,7,7',7'-tetramethyl-6,6',7,7'-tetrahydro-5,5'-spirobi-[indeno-[5,6-d]-[1,3]-dioxole]-2,2'-dione. Model reactions of this bis-(cyclic carbonate) with secondary amines revealed that steric and electronic effects are decisive with alicyclic secondary amines markedly outperforming their linear analogues. Further pairing of activated aromatic bis-(cyclic carbonate) with alicyclic secondary diamines enabled nearly quantitative monomer conversion under mild, catalyst-free conditions (at 50 °C), yielding linear PHUs with a record-high degree of polymerization (DP_n) of up to 220 and a number-average molecular weight (M _n) of 105 kg/mol. This strategy was further extended to another aromatic cyclic carbonate, 5,5'-(9H-fluorene-9,9-diyl)-bis-(benzo-[d]-[1,3]-dioxol-2-one), producing PHUs with M _n values of up to 100 kg/mol.