Synthesis of Polyhydroxy [n]-Polyurethanes Derived from a Carbohydrate Precursor

Polyhydroxy [n]-polyurethanes with chains formed by sugar-derived units have been synthesized. The enantiomerically pure monomeric precursor 1-amino-1-deoxy-2,3:4,5-di-O-isopropylidene-d-galactitol (4) was prepared from d-galactono-1,4-lactone (1) by a three-step route. Acetonation of 1 with 2,2-dimethoxypropane gave methyl 2,3:4,5-di-O-isopropyldene-d-galactonate (2). The ester group of 2 was converted into the amide and reduced with LiAlH4 to afford 4. This amino alditol was treated with di-tert-butyltricarbonate to produce the intermediate isocyanate derivative as activated monomer for the polymerization. The reaction conditions were adjusted to prevent the formation of urea linkages during the polycondensation. Thus, polymerization of the isocyanate monomer in THF and in the presence of Zr(acac)4 as catalyst afforded the linear [n]-polyurethane 6, which upon purification was practically free of urea linkages. Removal of the acetal protecting groups of 6 with 10:1 TFA−water, under conditions that did not produce hydrolysis of the polymer chain, led to the polyhydroxy [n]-polyurethane 8, having all the hydroxyl groups free. Polyurethanes 6 and 8 are examples of polyhydroxy [n]-polyurethanes entirely obtained from a sugar precursor. They were isolated as crystalline materials that exhibited high melting temperatures and thermal stability up to 230 °C. Accordingly, the SEM analysis of 6 and 8 revealed surfaces with morphologies characteristic of crystalline polymers.