Lexan, synthesis

Draw a stepwise mechanism for the following reaction, which is used to prepare bisphenol A, a monomer used in Lexan synthesis. 

Much the most important polycarbonate in commercial terms is made from 2,2-di(4-hydroxyphenyl)propane, commonly known as bisphenol A. This polymer was discovered and developed by Farbenfabriken Bayer [92], The synthesis and properties of this and many other polycarbonates were described by Schnell in 1956 [93], The polymer became available in Germany in 1959, and was given the trade name Makrolon by Bayer (in the USA, Merlon from Mobay). General Electric (GE) independently developed a melt polymerisation route based on transesterification of a bisphenol with DPC [94], Their product, Lexan, entered the US market in 1960. The solution polymerisation route using phosgene has since been displaced by an interfacial polymerisation. 

Polycarbonates and Polyurethanes The chemistry of carbonic acid derivatives is particularly important because two large classes of polymers are bonded by linkages containing these functional groups the polycarbonates and the polyurethanes. Polycarbonates are polymers bonded by the carbonate ester linkage, and polyurethanes are polymers bonded by the carbamate ester linkage. Lexan polycarbonate is a strong, clear polymer used in bulletproof windows and crash helmets. The diol used to make Lexan is a phenol called bisphenol A, a common intermediate in polyester and polyurethane synthesis. 

The synthesis of step-growth polymers—polytimides such as nylon and Kevlar, polyesters such as Dacron, polyurethanes such as spandex, and polycarbonates such as Lexan (Section 30.6)... 

Two methods are in common use for the synthesis of Lexan transesterification and a phosgene-based method. Polycarbonates produced via... 

In i960 the commercialization of LEXAN polycarbonate was well underway and our Chemical Development Operation then began looking for the next candidate for commercial development. They settled on PPO which by this time was fairly well characterized and had an attractive property profile. Around this time also a fresh Ph.D. chemical engineer from the University of Illinois) Jack Welch, was hired and soon became a key player in the development. A miniplant was soon in operation. A major hurdle to commercialization of PPO was the unavailability of the monomer, therefore an effort was mounted to find an attractive synthesis. Dr. Stephen Hamilton made the key discovery of a "hot tube" reaction which produced the monomer in high yield by simply passing phenol. rd methanol over a magnesium oxide catalyst at 400 deg. C. With a monomer process in hand a separate business... 

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