Conversion of Hydroxamic Acids to Polyurethane
Conversion of diesters to dicarbamates via formation of the hydroxamic acid is a relatively seldom traveled route to polyurethanes. In part, this is surely because the synthesis is somewhat laborious with relatively modest yield. In part, it may also be because the route to urethane via hydroxamic acid is actually through an isocyanate intermediate. The reaction begins with displacement of the methyl ester with the hydroxamic acid by reaction with hydroxylamine hydrochloride (Fig. 12.14). The hydroxamic acid is converted to isocyanate by Lossen rearrange-
FIGURE 12.14 Reaction of a dimethyl ester with hydroxylamine hydrochloride to form the dihydroxamic acid, which through subsequent reactions forms a dicarbamate. Reaction of the dicarbamate with a polyol can form high-molecular-weight polyurethane.
FIGURE 12.15 Reaction of the polyhydroxamic acid with dimethyl carbonate resulting in a latent polyisocyanate through the Lossen rearrangement.
merit (Fig. 12.15). Rapid reaction with methanol leads to the biscarbamate [33, 34]. Subsequent reaction with a polyol results in the polyurethane polymer. High-functionality polyols and diesters of varying molecular weights can result in block copolymer formation.
FIGURE 12.16 Illustrative reaction of hydroxylamine and diphenyl carbonate forming polyurethane with phenol by-product. Removal of the phenol is required for attaining high molecular weight.
FIGURE 12.17 Direct reaction of ethanolamine and carbon dioxide to form oxazohdinone and subsequent reaction with ethanolamine to form hydroxyethyl lmidazohdinone. This side reaction can be avoided by using higher-molecular-weight alcoholamines or having branched structures that inhibit intramolecular cyclization.