Di Stefano, Giuseppina (2025) Alternative synthetic strategies for the production of water based isocyanates-free polyurethanes that are sustainable for the environment and in terms of industrial and economic scalability. [Tesi di dottorato]

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Abstract

The development of non-isocyanate polyurethanes (NIPUs) has gained significant attention over the past decade as a promising route toward more sustainable and environmentally friendly polymeric materials. Unlike conventional polyurethanes synthesized from toxic isocyanate monomers, NIPUs are mainly produced through the aminolysis reaction between cyclic carbonates and amines. However, one of the main limitations of NIPUs lies in their relatively low molecular weight, which is difficult to control due to the stepwise nature of the aminolysis process. This constraint limits the attainment of good mechanical properties, thereby restricting the range of potential applications. This thesis investigates the synthesis of novel classes of waterborne NIPUs and their structural, thermal, mechanical, and elastic behavior, providing insights into the structure–property relationships that govern their performance. The research work elucidates strategies to overcome the main challenges in NIPU synthesis, offering viable and sustainable alternatives to conventional isocyanate chemistry while maintaining desirable thermal and mechanical characteristics. The first part of the research explores both catalyst-free and organocatalyzed routes aimed at elucidating the kinetics and mechanism of the aminolysis reaction between bio-based diglycerol dicarbonate and aliphatic or polyether diamines of varying chain lengths. The catalyst-free route led to the formation of waterborne hybrid epoxy–NIPUs, which exhibited soft and highly flexible behavior with excellent elastic recovery, particularly for systems containing long aliphatic diamines. Kinetic investigations revealed the key role of organocatalysts in modulating the reaction rate, demonstrating that heterogeneous basic catalysts provide an effective balance between catalytic activity, processability, and sustainability, particularly when using polyether diamines of high molecular weight. This optimized approach enabled the preparation of stable aqueous dispersions and transparent hybrid NIPU films. The incorporation of an epoxy-based chain extender markedly influenced crystallinity and mechanical performance, leading to tunable stiffness and elasticity. The second part extends the synthetic approach toward high molecular weight carbonate-functionalized polymers as reactive precursors for water-dispersible NIPUs. Dicarboxyl-terminated precursors were first synthesized and subsequently functionalized with glycerol carbonate to introduce terminal dicyclic carbonate groups. This strategy enabled the preparation of NIPUs with distinct ionic characters, allowing a systematic comparison of how molecular architecture affects dispersion stability, thermal transitions, and mechanical response. In particular, cationic NIPUs were obtained by incorporating tertiary amine groups as internal dispersing sites, followed by neutralization with acetic acid and dispersion in water, producing stable colloidal systems and flexible films. This system exhibited a semi-crystalline behavior and good tensile performance, with notable tensile strength and elongation at break in combination with elastic properties. On the contrary, anionic NIPUs, showed a significant advantage in processability, as the presence of anionic groups enabled direct dispersion in water without requiring a separate neutralization step. These amorphous samples exhibited typical elastomeric behavior, characterized by lower strain at break and pronounced strain hardening, particularly at reduced carbonate-to-amine ratios. Finally, in preliminary studies examined alternative strategies to extend the chemical versatility of NIPUs were examined. A first approach involved the synthesis of PEG-modified polyesters to obtain neutral water-dispersible systems without ionic stabilizers; the second investigated transcarbamoylation reactions of di-tert-butyl dicarbamates (Boc-dicarbamates) as innovative, isocyanate-free routes to urethane linkages. Preliminary results validated the feasibility of both methods and their potential for developing new classes of functional NIPUs. Overall, this work establishes a comprehensive and sustainable framework for the synthesis of waterborne NIPUs, demonstrating that their thermal and mechanical properties can be precisely tuned by tailoring the molecular architecture of the cyclic carbonate precursor and selecting the appropriate synthetic route. The findings advance the understanding of green polyurethane chemistry and open new perspectives for the development of eco-friendly coatings, adhesives, and flexible polymeric materials.

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