Polycarbonates phosgenation process

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

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

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. 

Fukuoka, S. Kawamura, M. Komiya, K. Tojo, M. Hachiya, H. Hasegawa, K. Aminaka, M. Okamoto, H. Fukawa, L Konno, S. A novel non-phosgene polycarbonate production process using by-product CO2 as starting material. Green Chem. 2003, 5 (5), 497-507. 

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] ... 

This process also avoids the use of methylene chloride as a solvent and the co-production of NaCl salt. Another well-established application of DMC in the field of polycarbonates relates to the production of poly[diethyleneglycol bis(allylcarbo-nate)], a thermosetting resin used in the production of optical glasses and lenses. The non-phosgene process involves the intermediate formation of diallyl carbonate from DMC - whereas the traditional process was based on the use of diethyleneglycol bis(chloroformate) that in turn was obtained from phosgene - and allows high flexibility in terms of customer-tailored products. 

The commercial polycarbonate polymerization processes can be categorized according to whether phosgene (COCI2) or its derivative is utilized as a raw material. Thus, the processes are distinguished as employing either phosgenation or transesteriflcation. 

Asahi Chemical Industry Co., Ltd. has succeeded in developing alternative and innovative non-phosgene processes for producing isocyanates and polycaihonates in the pilot scales which are commercially viable. In the production of isocyanates, processes for both aromatic isocyanates, such as methylene diphenyl diisocyanate (MDI), and aliphatic isocyanates, such as hexamethylene diisocyanate (HDI) or isophoione diisocyanate (IPDI), have already been developed successfully. A part of those processes has already been reported (i), and the others will be reported in the near future. In this paper, the new iimovative process for producing polycarbonates is reported. 

Polycarbonates are well known to be typical amorphous polymers and to have excellent properties such as heat resistance, impact resistance, transparency, and dimensional stability (2-5). Polycarbonates, therefore, have been widely employed in various applications from nursing bottles to precision instruments (CDs, cameras, etc.), or in structural materials (for electrical applications, electronics, automobiles, construction applications, etc.). The global demand of polycarbonates has been growing more than 10% per year. The production capacity of polycarbonate world wide is about 1 million tons per year, and the boom in polycarbonate plant construction continues. Almost all of the polycarbonates, however, have been produced by the Phosgene Process . 

Asalti s polycarbonates are colorless with good transparency other excellent features include, for example, heat stability and leworkability, in contrast to polycarbonates produced by the phosgene process. Asahi s polycarbonates are hi er quality because they are from impurities such as chlorinated compounds, which are difficult to remove from polycarbonates obtained from the phosgene process and which have a negative effect on polymer properties. 

The Production Costs of Asahi s Polycarbonates. Asahi s new process is able to produce polycarbonate of higher quality than that of the conventional phosgene process and, more importantly, the production cost of the new process has been estimated to be completely competitive with that of the phosgene process. 

The transesterification of dimethyl carbonate with phenol was recognised early as an appropriate reaction step to replace the direct phosgene process for making chloride-free polycarbonates for optical applications (Scheme 21.11). 

Fukuoka, S. Fukawa, I. Tqjo, M. Oonishi, K. Hachiya, R Aminaka, M. Hasegawa, K. Komiya, K.A Novel Non-Phosgene Process for Polycarbonate Production from C02 Green and Sustainable Chemistry in Practice. Catal. Surv. Asia. 2010, 14,146-163. 

Polycarbonates are prepared commercially by two processes Schotten-Baumaim reaction of phosgene (qv) and an aromatic diol in an amine-cataly2ed interfacial condensation reaction or via base-cataly2ed transesterification of a bisphenol with a monomeric carbonate. Important products are also based on polycarbonate in blends with other materials, copolymers, branched resins, flame-retardant compositions, foams (qv), and other materials (see Flame retardants). Polycarbonate is produced globally by several companies. Total manufacture is over 1 million tons aimuaHy. Polycarbonate is also the object of academic research studies, owing to its widespread utiUty and unusual properties. Interest in polycarbonates has steadily increased since 1984. Over 4500 pubflcations and over 9000 patents have appeared on polycarbonate. Japan has issued 5654 polycarbonate patents since 1984 Europe, 1348 United States, 777 Germany, 623 France, 30 and other countries, 231. 

Phosgene addition is continued until all the phenoHc groups are converted to carbonate functionahties. Some hydrolysis of phosgene to sodium carbonate occurs incidentally. When the reaction is complete, the methylene chloride solution of polymer is washed first with acid to remove residual base and amine, then with water. To complete the process, the aqueous sodium chloride stream can be reclaimed in a chlor-alkah plant, ultimately regenerating phosgene. Many variations of this polycarbonate process have been patented, including use of many different types of catalysts, continuous or semicontinuous processes, methods which rely on formation of bischloroformate oligomers followed by polycondensation, etc. 

An analogue of the transesterification process has also been demonstrated, in which the diacetate of BPA is transesterified with dimethyl carbonate, producing polycarbonate and methyl acetate (33). Removal of the methyl acetate from the equihbrium drives the reaction to completion. Methanol carbonylation, transesterification using phenol to diphenyl carbonate, and polymerization using BPA is commercially viable. The GE plant is the first to produce polycarbonate via a solventiess and phosgene-free process. 

Taking the manufacture of polycarbonates as an example, two manufacturing routes are shown in Scheme 3.1. In the first route phosgene is reacted with bisphenol A in dichloromethane. The main environmental concern with this process is the large-scale use of phosgene. In the second... 

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