Polycarbonates from CO

Making monomeric and polymeric organic carbonates by using CO as one of the raw materials is important for two reasons. Aromatic polycarbonates are well established as engineering plastics with an annual worldwide production of about 2 million tons. Aliphatic polycarbonates are under vigorous developmental efforts as they may have similar potential applications. 

Second, as shown by reaction 4.7.4.1, some of the established processes for making organic carbonates involve the use of phosgene, a highly toxic and hazardous reagent. Development of an alternative process that avoids the use of phosgene and utilizes CO would therefore be very welcome. 

The reaction of an epoxide such as propylene oxide with CO can lead to a mixture of cyclic and polymeric carbonates. This is shown by reaction 4.7.4.2. 

The polymerization mechanism consists of alternate insertions of COj and epoxide into the metal oxygen bond. Insertions of two 

In both 4.63 and 4.64, the polymer chain is hnked to the metal through an oxygen atom, but in 4.63 it bonds to a carbonate and in 4.64 to an alkoxo group. Polymer chain terminations in both the cases are assisted by A, the Lewis acid, which can accept the electron pair on the oxygen atom. 

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But not only palladium(O) complexes can activate CO or O2, also palla-dium(II) complexes have been reported to be active in the presence of carbon monoxide or dioxygen as it was shown in the direct synthesis of polycarbonate from CO and phenol or bisphenol A [79,80]. The authors could confirm the positive influence of the NHC ligand comparing the activity and reactivity of the palladium-carbene complex with the corresponding PdBr2 catalyst. The molecular weights and yields of the polycarbonates improved with increasing steric hindrance of the substituents in the l,T-position of the car-bene complex. 

Figure 4.14 General mechanism for polycarbonates from CO and epoxides. X is a nucleophile such as halide.

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