Phosgenation process

For this reaction to proceed it is obviously necessary to remove the hydrochloric acid formed, preferably by means of hydrohalide acceptor. 

The attractive possibility of dissolving the bis-phenol A in caustic soda solution and bubbling phosgene into it is not practical since the polymer is insoluble in the caustic soda and precipitates out at a low and variable molecular weight. 

Greater success has been achieved with organic solvents which are also hydrohalide acceptors, pyridine being a specific example. 

Typically in such a process the bis-phenol A is dissolved in about ten times its weight of pyridine and vigorously stirred at 25-35°C. Phosgene is then bubbled into the solution and in a few minutes the pyridine hydrochloride starts to precipitate. As polymer is formed the viscosity of the solution increases and eventually becomes too great for stirring. The polymer is then recovered by the addition of a solvent such as methyl alcohol which dissolves the pyridine hydrochloride but precipitates the polymer. 

A variation of this process involves the formation of a preformed pyridine-phosgene complex. Polymerisation will then be effected by adding a solution of bis-phenol A. 

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For methylene diphenyl diisocyanate (MDI), the initial reaction involves the condensation of aniline [62-53-3] (21) with formaldehyde [50-00-0] to yield a mixture of oligomeric amines (22, where n = 1, 2, 3...). For toluene diisocyanate, amine monomers are prepared by the nitration (qv) of toluene [108-88-3] and subsequent hydrogenation (see Amines byreduction). These materials are converted to the isocyanate, in the majority of the commercial aromatic isocyanate phosgenation processes, using a two-step approach. 

Fig. 3. Schematic of toluene diamine phosgenation process A, cold phosgenator B, hot phosgenator C, wash column D, solvent distillation E, preflasher F, evaporator G, TDI distillation H, phosgene removal I, HCl absorber and K, phosgene decomposition.

Fig. 6. Schematic of hexamethylene diamine phosgenation process A, HMD A tanks B, phosgene solution tanks C, phosgenation reactor D, secondary...

High molecular weight polycarbonates may be produced without undue difficulty by the phosgenation process. The basic reaction is as shown in Figure 20.5. 

Because of the cost of pyridine the phosgenation process may be carried out with a mixture of pyridine and a non-hydrohalide-accepting solvent for the polymer and the growing complexes. Suitable solvents include methylene dichloride, tetrachlorethane and chloroform. Although unsubstituted aromatic hydrocarbons may dissolve the solvent they are not effective solvents for the acid chloride-pyridine complexes. 

In a phosgene processing factory, phosgene concentrations were measured over an 8-mo period with a device capable of detecting phosgene at 0-0.5 ppm (Henschler 1971). Positive values, of relatively short duration, were recorded on only 32 of 240 d 22 d, 0.05 ppm 6 d, between 0.06 and 0.1 ppm 3 d, between 0.1 and 0.5 ppm 1 d, 0.5 ppm (of short duration). For longer time... 

A number of methods for the manufacture of polycarbonates are available but the melt process and the phosgenation process are the most important. 

The demonstrated utility of these high-reactive isocyanates prompted us to search for a phosgenation process more economical than the oxalyl chloride route. Our attempts succeeded in the development at a laboratory scale of a method based on the reaction of phosgene with N,N-disilyl amides as depicted in scheme 161 (Ref. 215). 

However, the phosgenation process has much more value in less simple products. We previously have treated in section 3-2-2-1 the reaction of excess phosgene with glycerol which affords a monochloroformate containing a five membered cyclic carbonate function. 

Summarizing, from the investigations in this field it can be concluded that from an industrial viewpoint the direct carbonylation of nitroaromatics to isocyanates represents no economically feasible alternative, for the conventional phosgenation process, for the following reasons ... 

The commercialization of phosgene processes based on hydrogen chloride, rather than dichlorine, would permit, for example, isocyanate plants to be sited at locations not dependant on chloralkali-producing facilities or large chlorine-consuming (e.g. vinyl chloride) plant. However, it is not known for certain whether any of these processes are currently commercially operated. 

The main disadvantages of this phosgene process are 1) The high toxicity and corrosiveness of phosgene 2) The use of copious amounts of methylene chloride solvent (10 times the weight of the product) This solvent is water soluble, so it contaminates the wash water and 3) The complex cleanup to remove ionic materials. 

Haba, O. Ueda, M. Kuze, S. Synthesis of polycarbonate from dimethyl carbonate and bisphenol-A through a non-phosgene process. J. Polym. Sci. A, Polym. Chem. 1999,37,2087-2093. 

DMC has been proven to perform advantageously as a substitute for phosgene in several reactions. A non-phosgene process for the melt polymerization production of aromatic polycarbonates has been established commercially [69, 72] ... 

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