Aromatic diisocyanates

Other Preparative Reactions. Polyamidation has been an active area of research for many years, and numerous methods have been developed for polyamide formation. The synthesis of polyamides has been extensively reviewed (54). In addition, many of the methods used to prepare simple amides are appHcable to polyamides (55,56). Polyamides of aromatic diamines and aUphatic diacids can also be made by the reaction of the corresponding aromatic diisocyanate and diacids (57). 

In certain niche applications, aliphatic isocyanates, such as isophorone diisocyanate (IPDI), hexamethylene diisoeyanate (HDI), methylene 4,4 -biscyclo-hexylisocyanate (H12MDI), and polymeric versions of these diisocyanates, are used, e.g., in instances where light stability or reduced reactivity is needed. These isocyanates usually cost more than the aromatic diisocyanates. Thus, they are used in adhesive areas that can Justify the higher costs. 

Aromatic diisocyanates such as toluene 2,4-diisocy-anate (TDI) and methylene di-p-phenylene isocyanate (MDl) are usually used. Aliphatic diisocyanate such as hexanediisocyanate (HDI), although it has the advantage that the TPU synthesized from it is softer and not prone to turning yellow, is seldom used due to its high cost. 

Figure 19.3 is an aromatic diisocyanate. The observed successive losses of 28 and 56 Daltons are similar to the losses found with quinone or anthraquinone. 

Poly(boronic carbamatejs were prepared by alkoxyboration polymerization of diisocyanates with mesityldimethoxyborane (scheme 33).59 The polymers obtained have boronic carbamate functions in their repeating units and can be expected to be novel reactive polymers. First, alkoxyboration polymerization between mesityldimethoxyborane and 1,6-hexamethylene diisocyanate was examined, and the optimized reaction conditions were bulk reactions at 140°C. Both aliphatic and aromatic diisocyanates gave the corresponding polymers. When aromatic diisocyanates were employed, the... 

A number of studies have dealt with the photo-induced discoloration and degradation of polyurethanes based on aromatic diisocyanates such as toluene diisocyanate (TDI - represents a mixture of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate isomers) and methylene 4,4-diphenyl diisocyanate (MDI) Cl, 2, 3,... 

Polymer Matrix Effects. In order to approximate the environment experienced by the arylcarbamate moieties in coatings based on aromatic diisocyanates, we chose to study the photochemistry of alkyl N-arylcarbamates in polymethacrylate (PMMA) and polypropyl-methacrylate (PPMA) films. First, however, 2a and 3a were irradiated in ethyl propionate (a model solvent for PMMA and PPMA) to determine the effect of the solvent polarity (dielectric) on the photolysis of the carbamates. Upon excitation at 280 nm, where the solvent absorbance was negligible, is 0.006 for 2a and 0j) is 0.005 for 3a. These values are significantly smaller... 

Up to this point we have discussed only carbamates la-4a with a single carbamate group on the phenyl ring as model systems for aromatic polyurethane photodecomposition. In polyurethane coatings based on the aromatic diisocyanate TDI two carbamate groups are attached to the phenyl ring. Furthermore commercially available TDI is actually a mixture of 2,4-toluene diisocyanate (2,4-TDI) and 2,6-toluene diisocyanate (2,6-TDI) which when formulated give 2,4- and 2,6-biscarbamates. Model systems for these species would then be biscarbamates of 2,4-TDI and 2,6-TDI (as shown below) and not carbamates such as la-4a. 

In Figure 6, the larger values of A b for systems 4 and 5 compared with the other systems illustrate the increased opportunities for intramolecular reaction in tetrafunctional compared with trifunctional systems. Further, the smaller values of b for system 5 compared with those for system 1, with the same value of v, probably indicated that equation(3) relatively undercounts the opportunities for intramolecular reaction for tetrafunctional as compared with trifunctional reactants, so that smaller values of b are required in compensation. System 3, based on aromatic diisocyanate, gives the largest values of b, characteristic of its stiffer chain structure. 

It has been shown recently that the selective reductive homo-coupling polymerization of aromatic diisocyanates via one electron transfer promoted by samarium iodide in the presence of hexamethylphosphoramide [PO(NMe2)3] (HMPA) can produce poly(oxamide)s in nearly quantitative yield (Scheme 9). 

Elastomers resulting from a series of organic reactions, e.g., a polyglycol ester of adipic acid reacted with an aromatic diisocyanate and cured with water, glycol or further reaction with an aromatic diisocyanate. These elastomers have high tensile strength, very high abrasion resistance, and lower hysteresis than natural rubber. A major use is in the manufacture of foamed polymers. 

Aromatic copolyesters, 13 372 Aromatic diacyl peroxides, 14 283 Aromatic diisocyanates, urethanes obtained from, 25 462—463 Aromatic epoxies, 10 347-348 Aromatic ethers, 10 574 Aromatic fluorocarbon production, global, 11 871-872... 

Flash Photolysis of Aromatic Diisocyanate-Based Polyurethanes... 

The laser flash photolysis of aromatic diisocyanate based polyurethanes in solution provides evidence for a dual mechanism for photodegradation. One of the processes, an N-C bond cleavage, is common to both TDI (toluene diisocyanate) and MDI (methylene 4,4 -diphenyldiisocyanate) based polyurethanes. The second process, exclusive to MDI based polyurethanes, involves formation of a substituted diphenylmethyl radical. The diphenylmethyl radical, which readily reacts with oxygen, is generated either by direct excitation (248 nm) or indirectly by reaction with a tert-butoxy radical produced upon excitation of tert-butyl peroxide at 351 nm. 

Future work will be concerned with quantitative measurement of hydrogen abstraction rates of labile hydrogens in the carbamate moieties of several aromatic diisocyanate based polyurethanes. It is expected that experimental conditions will alter significantly the hydrogen abstraction rate. Emphasis will also be placed on measurement of transient intermediates in polyurethane films. Finally, extensive laser flash photolysis experiments will be conducted on polyurethanes based on both 2,A-toluenediisocyanate and 2,6-toluenediisocyanate. Preliminary data suggest that the placement of the methyl substituent can alter the nature of the transient intermediates formed. 

The isosorbide polyurethanes based on the aromatic diisocyanates P(I-TDI) and P(I-MDI), possess more rigid structures with both polymers forming brittle films and brittle compression moldings. Their glass transition temneratures are above their decomposition temperature of 260°C. The thermostability of isosorbide polyurethanes correspond to that of conventional polyurethanes with similar structure based on 1,4-cyclohexanedimethanol for which degradation temperature of 260°C has been determined. 

PUs are also widely used as coating materials sold as finished polymers, two-part systems, and prepolymer systems. Water-based PU systems are now available allowing easy home use. Aromatic diisocyanate-derived coatings generally offer poor external light stability while aliphatic-derived systems offer good light stability. 

A key factor in the preparation of polyurethanes is the reactivity of the isocyanates. Aromatic diisocyanates are more reactive than aliphatic diisocyanates, and primary isocyanates react faster than secondary or tertiary isocyanates. The most important and commercially most readily accessible diisocyanates are aliphatic and colorless hexamethylene-1,6-diisocyanate (HDI), isophorone diisocyanate (IPDI),and aromatic, brownish colored diphenylmethane-4,4 -diiso-cyanate (MDI), 1,5-naphthalenediisocyanate, and a 4 1 mixture of 2,4- and 2,6-toluenediisocyanates (TDI). 

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