Tunable and Photomodifiable Nonisocyanate Polyurethanes from Lignin-Based Cyclic Carbonates Bearing α,β-Unsaturated Ketone.

Lignin-based nonisocyanate polyurethanes (NIPUs) with tunable properties and photomodifiable capability were prepared through polyaddition of diamines and lignin-based bis-cyclic carbonates bearing α,β-unsaturated ketone. These bis-cyclic carbonates were synthesized from lignin-based phenols via Aldol condensation, O-glycidylation and cycloaddition of CO2. Model reaction of vanillin-based cyclic carbonate (BVKCC) and n-hexylamine confirmed efficient aminolysis of cyclic carbonate and partial aza-Michael addition of α,β-unsaturated ketones at ambient temperature. Significantly, this aza-Michael adduct reverted quantitatively to the original α,β-unsaturated ketone upon heating to 100 °C. Aminolysis kinetics of BVKCC and n-hexylamine was investigated by 1H NMR and DFT calculation. TGA, DSC and tensile tests revealed that these NIPUs exhibited excellent thermal and mechanical properties with Td5%, Tg and tensile strength reaching up to 318 °C, 60.2 °C and 44.5 MPa, respectively. Crucially, the retained α,β-unsaturated ketone enables facile postpolymerization modification via UV-triggered [2 + 2] cycloaddition, leading to a significant enhancement of thermal properties.