9. Conclusions

Monosaccharide-based polyurethanes (PU) have been usually prepared by step-growth polymerization from diisocyanates and sugar-based diols but, in the last years, and due to the toxicity of stannous catalysts and common aromatic diisocyanates, other eco-friendly methods have been developed and many works have been reported on the synthesis of isocyanate-free PU, also called non-isocyanate PU (NIPU). Although most syntheses of high-molecular-weight linear polyurethanes involve monosaccharide derivatives having the hydroxyl groups appropriately blocked, several PU with enhanced hydrophilicity and hydrolytic degradability have been synthesized using sugar-derived monomers with free hydroxyl groups or by deprotection of PU prepared from O-protected sugar-based monomers.

In order to enhance degradability, a series of homo- and copolyurethanes containing the hydrophilic monomer [triethylene glycol (TEG)] was synthesized. The introduction of disulfide linkage into the polymer backbone of novel reduction-sensitive biodegradable sugar-based polyurethanes has also been reported. Thus, a faster glutathione-mediated degradation method was accomplished under milder degradation conditions.

The chemical modification of PU is important in order to search further applications. Click chemistry (CC) is the most widely used approach to achieve these chemical transformations; for instance, the thiol-ene/yne coupling reaction and the alkyne-azide cycloaddition reaction. In this sense, di- and tri-O-allyl- and tri-O-propargyl-L-arabinitol derivatives have been used as versatile materials for the preparation of tailor-made polyurethanes with varied degrees of functionalization.

We can conclude that it is viable to prepare monosaccharide-based polyurethanes analogous to the more qualified conventional polymers, but having a larger functionality and an enhanced hydrophilicity and degradability. However, in most cases, the high costs that represent the preparation of the monomers restrict the application of these polymers to the biomedical and other specialized fields.
