Bis-urea gelators

Fig. 4.9 a Polymerizable bis-urea gelator G22 capable of producing highly porous dried gel b TEM micrographs of butyl acetate gel of G22 before polymerization (5 mg/mL, bar 1,000 nm) c SEM of a benzene gel of G22 after irradiation (5 mg/mL, bar 400 nm). Reprinted with permission from Ref. [59]. Copyright 1997 Ameriean Chemieal Society... 

A novel approach to polymerized gel systems has been followed by our group. Based on the design principles described in the previous section, we successfully prepared a new polymerizable bis-urea gelator 35, which is capable of gelating a many different organic solvents [68]. On the other hand, the bis-amide derivative does not gel... 

Scheme 5.5 Bis (urea) gelators used to form gels for crystal growth of pharmaceuticals.

Estroff. L.A. Hamilton. A.D. Efleetive gelation of water using a series of bis-urea dieaiboxylie aeids. Angew. Chem., Int. Ed. 2000, 39. 3447. 

Figure 11 Mono- and linear bis-urea compounds tested for gelation.

Only in the case of 24 in hexadecane a gel was obtained. In all other solvents precipitation occurred after cooling of the solutions to room temperature. On the other hand the bis-urea compounds 25 and 26 are potent gelators for a wide range of solvents... 

TABLE 1 Gelating capability of linear bis-urea compounds [a]... 

The bis-urea compounds presented have many properties in common with other gelators. They are, however, very easy to synthesize, and many structural variations are possible without losing the gelating ability. For these reasons, the bis-urea compounds are not only excellent model compounds to study gelation phenomena in more detail, but also are excellent building blocks for the development of functional gels. 

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. 

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