Aromatic urethanes

Aromatic urethane Poly-1,4-butane diol (spandex)... 

The reaction of nitrobenzenes with an aliphatic, araliphatic, or cycloaliphatic alcohol and carbon monoxide in the presence of a DBU-containing catalyst mixture yielded aromatic urethanes 43 (77GEP2614101 80GEP2808980 80GEP2808990 80GEP2838754). 

Aromatic urethanes and dialkyl carbonates were simultaneously prepared by the reaction of aromatic nitro compounds with alcohols and carbon monoxide over a catalyst mixture containing DBU (82JAP(K)32251). 

TABLE 6. Process for the synthesis of aromatic urethanes (from ref. 34). 

In addition to modulation of the general reactivity by choice of substituents with different electron demand, Umemoto and coworkers described a method of increasing the selectivity of the fluorination, particularly for phenols and aromatic urethanes in the ortho-position [183b, 190], In the reaction of N-fluoropyridinium-2-sulfonate the electrophilic fluorine is supposedly directed by a combination of Jt n donor-acceptor interaction and electrostatic (hydrogen) bonding to a specific position (Scheme 2.85). 

The aromatic urethane hexaacrylates making up the remainder of the formulation behave in a nearly analogous fashion as shown in Figure 7. 

Figure 7. Aromatic urethane hexaacrylatephotocures at 0°C (dark gray), 30°C (light grey),...

Figure 8. Aromatic urethane hexaacrylate cure exotherm from 5 second flash excitation at 50° C.

Although the polyurethanes form useful adhesive bonds, particularly between metals and elastomers, their use in the aerospace industry for bonding purposes is limited. Because polyurethanes tend to depolymerize above 120 °C and are subject to hydrolysis, and because aromatic urethanes autoxidize when exposed to thermal or UV light (13). epoxies are the preferred bonding material. Recently they were studied as launch seals for both land and sea missile launch tubes and were found to be superior to seals based on neoprene rubber (14). The chemical reaction for this application is proposed to be that between isocyanates and amines (Reaction 3). 

Both MDI and TDI, being aromatic diisocyanates, yield urethane polymers that tend to yellow on prolonged exposure to sunlight, presumably due to oxidation of some terminal aromatic amine (derived from these isocyanates). MDI also possesses a methylene group that is susceptible to oxidation via a proton abstraction mechanism involving autoxidation of the aromatic urethane groups to a quinoneimide structure as proposed by Schollenberger et al. (24. 25). 

Aromatic urethanes have been stabilized by complexation of peroxides in the polymer and the effect of prior y-irradiation has been investigated, on polyurethanes.The ageing of polyurethane coatings has been investigated, and light-stable integral skin foams have been developed for polyurethanes. Creep behaviour of polyurethanes on ultraviolet exposure has also been investigated. ... 

Aromatic urethane model compounds (Urethanes I and II) were studied in the crystalline form to identify the characteristic Amide bands in the interface region when freed of any interactions with urea or polyether segments. Aliphatic urethane model compounds (Urethanes III and IV) were studied to identify the influence of aromatic rings on particular bands. The 1800-1200 cm-1 regions of aromatic Urethane I and aliphatic Urethane III are shown in Figure 3. 

Homogeneous 1 2 and 1 4 molar mixtures of aliphatic Urethanes III or IV with polypropylene glycol (2000 MW) are similar to the aromatic Urethane I or II mixtures in the Amide I and II regions. However, the Amide III band absorbs at 1245 cm-1, with a weaker 1260-cm 1 shoulder. The 1210-cm 1 shoulder due to vibrations of the aromatic ring is missing. Also, the 1320-cm 1 shoulder present in the aromatic mixtures is absent in the aliphatic mixtures. 

Selection of the prepolymer is guided by the desired film-forming properties. For example, acrylated epoxy resins typically provide hard, solvent-resistant films with fast cure at relatively low cost. Acrylated urethanes may be chosen for their excellent film flexibility and toughness. Aliphatic urethanes are used in preference to aromatic urethanes for stringent exterior weatherability requirements. The acrylated acrylics offer excellent nonyellowing properties when a soft film can be used. 

Chem. Descrip. Aromatic urethane acrylate Uses Urethane-acrylic in radiation-curable coatings, lithographic inks, overprint varnishes... 

Chem. Descrip. Aromatic urethane acrylate resin/SR 306 (tripropylene glycol diacrylate) blend, < 400 ppm MEHQ inhibitor Uses Urethane-aciylic produdng hard, fast curing, solv. resist. UV/EB cured prods., e.g., coatings (metal, paper, wood) and inks (flexo and gravure)... 

Chem. Descrip. Aromatic urethane acrylate with SR-351 Uses Urethane-aciylic for inks (screen, offset, lithographic), electronics, and wood, metal, and plastic coatings Features Hydrophobic good pigment wetting Properties APHA 100 color vise. 1200 cps (60 C)... 

Chem. Descrip. Aromatic urethane aciylate oligomer Uses Urethane-acrylic tor UV-curable metal coatings, plastic coatings, inks... 

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