Polycarbonate transesterification production process

It is not surprising, given the concurrent research and commercialization efforts that were focused on bringing BPA-based polycarbonates to the market in the late 1950s, that a patent interference arose. Bayer was eventually awarded the first U.S. patents on BPA-based polycarbonates [35] and for the interfacial method of production. GE was issued a patent covering the melt transesterification production process [36]. 

Polycarbonates from DPC and bisphenol A (Scheme 6.12) are produced commercially.21 The technology starts with formation of EC from C02 and ethylene oxide. EC is then transformed to DMC in a transesterification reaction. DPC is manufactured by the reaction of DMC with phenol in the presence of [Pb(OPh)2] as catalyst. The final products are the polycarbonate and the ethylene glycol coproduct of EC transesterification. The process does not use phosgene or other halogenated raw materials or solvents. 

Transesterification of diphenyl carbonate and bisphenol A. The final step in the nonphosgenation process for polycarbonates is the reaction of bisphenol A (BPA) and the carbonate ester, diphenyl carbonate (DPC). Research has focused on the transesterification melt process because it has the advantage over the conventional interfacial process of allowing the reaction of the diphenyl carbonate and bisphenol A to take place completely in the liquid phase. The disadvantage of this approach is that elevated temperatures are needed to ensure that unreacted DPC and BPA are completely volatilized from the product. Only a lower molecular weight (30,000-50,000) polymer can be made in this way. Typical molecular weights for polycarbonate produced by phosgenation in the interfacial pro-... 

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. 

The most practical nonphosgene process for manufacturing polycarbonates is the transesterification of diphenylcarbonate (DPC) with bisphenol-A. A nonphosgene process for the melt polymerization production of aromatic polycarbonates [reaction (14)], making use of EniChem technologies for the production of DMC and DPC as intermediates, has been commercially established and will account in a short time for about 300,000 ton/yr polycarbonates. 

Polycarbonates of numerous bisphenols have been extensively studied. However, most commercial polycarbonates are derived from bisphenol A. At first, both direct-reaction and melt-transesterification processes were employed (Figure 4). In direct-reaction processes, phosgene reacts directly with bisphenol A to produce a polymer in a solution. In transesterification, phosgene is first reacted with phenol to produce diphenyl carbonate, which in turn reacts with bisphenol A to regenerate phenol for recycle and molten, solvent-free polymer. Transesterification is reported to be the least expensive route. It was phased out, however, because of its unsuitability to produce a wide range of products. 

The transesterfication process, shown in Eq. (11), is no longer practiced industrially because of the difficulty in producing a wide variety of polycarbonate resins with this process. As a result, the Schotten-Baumann synthesis currently dominates commercial production [5]. However, the transesterification process may experience a resurgence if nonphosgene routes to polycarbonates are commercialized because some of the nonphosgene chemistry under development takes advantage of the transesterification route. 

Although there is an extensive volume of work on the polymerization of polycarbonate and polycarbonate copolymers, today there are only two basic commercial condensation processes used to produce the majority of commercial product interfacial and melt transesterification. For a more in-depth review of these two processes as well as others that are not commercially significant, the following resource is recommended [49]. 

The polymer is exposed to an extensive heat history in the melt process. Early work on transesterification technology was troubled by thermal-oxidative reactions of the polymer, especially in the presence of basic catalysts (8-11). Early polycarbonates prepared by Fox and others via the melt process had noticeable brown colors. More recent work on catalyst systems, more reactive carbonates, and modified processes have improved the process to the point where formation of color and product decomposition can be effectively suppressed. Polymers with color at least as good as interfacially prepared materials can now be prepared commercially. One of the key requirements for the transesterification process is the use of clean starting materials. Methods for the purification of both BPA and diphenyl carbonate have been developed and patented. Activated carbonates that form high molecular weight polycarbonate at equilibrium in solution at or below room temperature have also been reported, although they are chiefly only of academic interest (66,67). 

Other polymerization techniques. An analogue of the transesterification process has also been demonstrated, nsing the diacetate of BPA (72). Transesterification of the diacetate with dimethyl carbonate using a titanate catalyst produces polycarbonate with methyl acetate as the hy-product. Removal of the methyl acetate from the equilibrium drives the reaction to completion. Because of the problem of recycling methyl acetate hack to BPA-diacetate, the process has not been commercialized. 

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