A Stereoselective Route to R-(+)-Limonene-Based Non-isocyanate Poly(hydroxyurethanes)

Bio-based non-isocyanate poly(hydroxyurethanes) (NIPUs) have recently gained significant interest due to increasing awareness of the health and environmental impacts caused by petroleum-based polyurethanes (PUs) synthesized from toxic isocyanates. R-(+)-limonene is a renewable resource derived from the citrus fruit rind. This research aimed to develop a high-yield route to NIPUs starting from commercially available R-(+)-limonene. The proposed synthesis route consists of 3 consecutive steps: (1) Synthesis of trans-limonene bis-epoxide (trans-LBE) by stereoselective di-bromohydration of R-(+)-limonene, using n-bromosuccinimide (NBS) in a 1:2 (R-(+)-limonene: NBS) molar ratio. A 97% yield of trans-LBE was achieved at 60 °C. (2) The resulting trans-LBE was further used as a substrate for trans-limonene bis-cyclic carbonate (LBC) synthesis via CO₂ cycloaddition catalysed by commercially available inexpensive tetrabutylammonium chloride (TBAC) as a homogeneous catalyst. A 98% conversion of trans-LBE was achieved to form LBC at 120 °C, 40 bar C after 48 h. (3) The copolymerisation of LBC and alkyl diamines afforded NIPU with promising glass transition (T_g) and decomposition temperatures (T_d). The trans-isomer of LBC affords polymers of higher molecular mass and strength. The most heat-resistant polymers obtained had a T_g of 55 °C and a T_d of 312 °C. Variation of LBC to diamine molar ratio revealed that a 1:1 ratio gives polymers of greater chain length whilst a 1:0.5 ratio increases thermal resistance and stiffness of the resulting brittle NIPU. Polymer modification is possible due to the presence of hydroxyl side chains, making these NIPUs promising for applications such as coatings and sealants.