Preparation and characterization of bio-based non-isocyanate polyurethanes (NIPUs) using terpenes and CO₂

Abstract

The growing demand for sustainable alternatives to toxic phosgene-based polyurethanes (PUs) derived from fossil fuel resources has driven the synthesis of non-isocyanate polyurethanes (NIPUs). Therefore, the present research explores the synthesis of such systems through the incorporation of the citrus-derived terpene limonene. Particularly, this work explores a novel synthesis route based on the controlled partial carbonation of limonene diepoxide (LDO), rather than complete carbonation, enabling the remaining epoxide groups to be used for the subsequent polyol functionalization. Through this approach, the aim is to tailor the morphological and thermal properties of limonene-derived NIPUs by incorporating polyols with varying chemical structures and molecular weights.

A potassium iodide (KI)-impregnated heterogeneous catalyst was prepared and characterized for use in the supercritical carbonation of LDO, enabling potential catalyst recovery. Partial carbonation (~50%) was optimized at varying catalyst loadings (20-60 wt%) to identify the optimal balance between catalytic and material efficiency. The remaining epoxide groups retained after partial carbonation were utilized for functionalization with five commercial polyols varying in chemical structure and molecular weight. The final NIPUs were synthesized with the vegetable oil-based diamine Priamine 1075, while polymer development and polyaddition optimization were monitored by Fourier transform infrared spectroscopy (FTIR). Lastly, the morphological and thermal properties of the synthesized NIPUs were compared to evaluate the influence of polyol functionalization on their material properties.

Partial carbonation was successfully achieved (47.67 ± 0.71%), while preserving the remaining epoxide groups required for the functionalization stage. The functionalization of the carbonated product was evaluated using FTIR spectroscopy, where the key observation was the decrease in the epoxide related bands in the 700-900 cm⁻¹ region, indicating the opening of the remaining epoxide groups with the polyols. Gel Permeation Chromatography (GPC) revealed shifts to lower retention times and increased peak molecular weight (Mp) values of the functionalized products, suggesting changes in the molecular characteristics after functionalization. Furthermore, urethane bond formation was confirmed by the development of the characteristic carbonyl absorption band at approximately 1740 cm⁻¹, indicating the successful synthesis of limonene-derived NIPUs. The NIPUs exhibited clear differences in morphological and thermal behavior. Polyethylene glycol (PEG)-based NIPUs were characterized by crystalline domain organization and self-supporting networks, while polypropylene glycol (PPG)-based NIPUs were mainly amorphous and softer in appearance. Differential Scanning Calorimetry (DSC) revealed melting/crystallization transitions for PEG-based NIPUs, with these thermal characteristics becoming more pronounced with increasing polyol molecular weight. Additionally, glass transition temperatures (Tg) were observed for both PEG- and PPG-based NIPUs. The bio-based alternative was characterized by pronounced structural heterogeneity while maintaining thermal behavior comparable to the PEG- and PPG-functionalized NIPUs. Additionally, all synthesized NIPUs remained thermally stable up to around 200 °C, followed by a multistep degradation process.

The results confirm that tailored NIPUs can be successfully obtained through controlled partial carbonation of LDO followed by polyol functionalization. Furthermore, the chemical structure and molecular weight of the introduced polyols were shown to largely influence the morphology and thermal behavior of the synthesized materials, demonstrating that the developed route represents a promising strategy to produce sustainable and bio-based PU alternatives with controlled property tailoring.