Reactions of Polycarbamates
Carbamates can be prepared by reaction of alcohols with other carbamates or by reactions of alcohols with urea . The simplest carbamate is methyl carbamate and is prepared by reaction of methanol with urea (Fig. 12.11). The carbamate can be subsequently reacted with a single aldehyde to produce a hemiaminal or with two
FIGURE 12.11 Reaction of urea and methanol to form methyl carbamate.
FIGURE 12.12 Reaction of methyl carbamate and formaldehyde to make the hemiaminal and subsequently the aminal. Polyurethane can be formed when a polycarbamate and a polyaldehyde are reacted together. Boxes indicate the urethane groups.
FIGURE 12.13 Reaction of a polyol with methyl carbamate or urea to produce a multifunction polycarbamate that can form a cross-linked polyurethane when reacted with a polyaldehyde.
aldehydes to produce an aminal (Fig. 12.12). As noted in an early patent, the initial reaction with methanol can be equally performed on a polyol to produce a polycarbamate (Fig. 12.13), which can be subsequently reacted with a polyaldehyde to create a three-dimensional polyurethane cross-linking structure . The catalyzed reaction can proceed at room temperature that makes the reaction particularly suitable as a potential replacement for some isocyanate-derived products. While formation of the aminal is possible in controlled small-molecule model studies, it is less common for addition polymerization making cross-linked polyurethane where hemiaminal formation predominates.
The range of potential polycarbamate building blocks via aminal chemistry is limited only by imagination since the number of possible polyol reagents is immense and the urea feedstock is relatively cost-effective. On the other hand, the number of polyaldehydes is relative small and therefore potentially expensive. Furthermore, aldehyde propensity to oxidize in the presence of oxygen is a potential complication and must be accounted for in reactant stoichiometries in a similar manner that MDI dimerization must be accounted for in preparation of MDI-based foams.