Linear polyurethanes derived from alditols and diisocyanates

A set of linear [m,n]‐type polyurethanes was synthesized by polycondensation in solution from hexamethylene diisocyanate and 4,4′‐methylene‐bis(phenyl isocyanate) with alditols. Threitol, arabinitol, and xylitol bearing the secondary hydroxy groups blocked as methyl ethers were used. Either regioregular or nonregioregular polymers (depending on the configuration of the alditol) were obtained in high yields and with number‐average molecular weights within the 20,000–30,000 range. All these polyurethanes were amorphous with Tg being highly dependent on the aliphatic or aromatic nature of the diisocyanate used, but scarcely depending on the chemical structure of the alditol moiety. They were found to be stable up to near 300 °C, decomposing at higher temperatures through a complex three‐stage mechanism. Polyurethanes obtained from threitol did not show significant enhancement of hydrolytic degradability as compared with polyurethanes obtained from 1,4‐butanediol. Conversely, polyurethane prepared from xylitol and hexamethylendiisocyanate was found to be almost fully hydrolyzed in 1 month when incubated in water either at 80 °C and pH 7.4 or at 37 °C and pH 10. It was concluded that the alditol size seems to be of prime importance in determining the hydrodegradability of these sugar containing polyurethanes.