Coordinative Ring-Opening Polymerization of Limonene Carbamate Toward Phosgene- and Isocyanate-Free Polyurethane

Abstract

This study presents a phosgene- and isocyanate-free route for the synthesis of polyurethanes using (R)-limonene as a bio-based starting material. The synthesis of the cyclic limonene-based carbamate monomer LU is achieved in high yields using dimethyl carbonate as a sustainable, less hazardous phosgene surrogate and is verified by NMR, SC-XRD, ESI-MS, GC-MS, and elemental analysis. Polymerizations were carried out by coordinative ring-opening polymerization. Sn(Oct)₂ showed the best catalytic performance, achieving up to 93% conversion with molecular weights up to 16.0 kg mol⁻¹ at a polydispersity of 1.5. Detailed mechanistic insights were obtained by kinetic studies, end group determination via ESI-MS and stoichiometric ninhydrin experiments, N–H methylation of LU, kinetic isotope experiments, and ¹¹⁹Sn NMR measurements. The resulting semi-crystalline polyurethane exhibits promising thermal properties, including a decomposition temperature of 252 °C and a glass transition temperature above 150 °C. Depolymerization in solution was achieved in high yield using the same catalyst. This work describes a coordinative ring-opening polymerization approach using a terpene-based carbamate as monomer, an important step toward bio-based polyurethanes.