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558 Chemical Reviews, 1999, Vol. 99, No. 2 Kendall et al. Scheme 9. Synthesis of Polycarbonate from Bisphenol A and Diphenyl Carbonate in the Presence of Supercritical CO2 Polycarbonate synthesis from bisphenol A and diphenyl carbonate catalyzed by tetraphenylphos- phonium tetraphenyl borate was performed in the presence of supercritical CO2 by DeSimone.157,158 The authors noted that although diphenyl carbonate is soluble in CO2, only a slight excess (1.005 equiv) was required for the reaction because, once a degree of polymerization of two is achieved, removal of diphenyl carbonate is not significant. The system was heated to 150 °C to melt the reactants and then heated at 160 °C under a slow flow of argon, followed by pressurization with CO2 and heating to 270 °C for 1 h. High molecular weight polymer (up to 1.3 × 104 g/mol) was achieved at 270 °C and 296 bar CO2. The high solubility of phenol in CO2 allows for its efficient removal, and its recovery was used to monitor the reaction progress. Beckman showed that exposure of thin films of bisphenol A polycarbonate to CO2 at 50 °C to 87 °C and up to 600 bar for 12 h, resulted in crystalline polymer.155 The crystallinity was observed as an endotherm at about 210-230 °C in the heating cycle of the differential scanning calorimetry (DSC) of the sample. Usually the crystallization of polycarbonate is induced by organic solvents. DeSimone obtained similar CO2-induced crystallization results on amor- phous polycarbonate chips157 and demonstrated that the CO2-crystallized chips could be chain extended to high molecular weight using solid-state polymer- ization methods. The use of CO2 could potentially allow the molecular weight of polycarbonate to be increased by solid-state polymerization in super- critical CO2. Melt-phase polycondensation reactions are com- monly used to prepare poly(ethylene terephthalate) (PET). PET is an important plastic that sees wide- spread use in fiber, film, and food packaging applica- tions for materials. Bis(hydroxyethyl) terephthalate (BHET) was converted to poly(ethylene terephtha- late) (PET) using an Sb2O3 catalyst and temperatures of 250-280 °C under a variable flow (2-10 mL/min) of CO2 at 207 bar.157,159 Molecular weights produced under these conditions varied from 3 × 103 to 6.3 × 103 g/mol. Polymer molecular weight, determined from intrinsic viscosity measurements, increased significantly with flow rate and/or reaction time. However, the molecular weights reported here are less than those normally produced by vacuum melt- phase polymerization (about 2 × 104 g/mol).160 Be- cause the ethylene glycol condensate is soluble in CO2 up to 2-3 wt %, it is expected that the condensate could be effectively removed from the swollen poly- mer product to result in a higher molecular weight. However, the CO2 solubility of ethylene glycol is lower than that of phenol (vide supra) and may partially account for the lower effectiveness of su- percritical CO2 in PET synthesis compared to poly- carbonate synthesis. Polyamides have also been synthesized in the melt phase in the presence of supercritical CO2.157,158 Because primary amines react with CO2 to form carbamates, the nylon salt route was used (see Scheme 10). A 1:1 salt of hexamethylenediamine and adipic acid was heated at 220 °C for 2 h and then at This strategy has been employed in the synthesis of polycarbonates, polyesters, and polyamides. There are two industrially important routes to polycarbonates: interfacial reactions in methylene chloride using phosgene, and melt transesterification of bisphenol and diphenyl carbonate.151 The latter avoids the use of phosgene and methylene chloride,152 but the high viscosity of the melt limits the molecular weight attained.151 In fact, the chain stiffness, which adds to the commercial value of polycarbonates, causes the high viscosity.153 The utility of supercriti- cal CO2 in producing high molecular weight polycar- bonates by melt polymerization is 2-fold: CO2 solu- bilizes the phenol (about 12 wt % at 272 bar and 100 °C)154 to extract the byproduct, driving the reaction to higher conversion, and plasticizes the polycarbon- ate155 to lower its viscosity, facilitating the process- ing.151 Odell studied the melt polymerization of bisphenols (such as bisphenol A, bisphenol P, bisphenol AF, and bisphenol Z) with diphenyl carbonate in CO2 (see Scheme 9).151,156 The reactor was first heated to 70 °C to melt the reactants. The system was then filled with CO2 and heated to the target temperature. Temperatures between 180 and 250 °C and pressures of 207-241 bar were used to obtain number average molecular weights ranging from 2.2 × 103 to 1.1 × 104 g/mol (Mw ) 4.5 × 103 to 2.7 × 104 g/mol). While molecular weight increased with increasing temper- ature, the temperature and pressure parameters were selected to provide sufficient extraction of phenol condensate while minimizing removal of diphenyl carbonate starting material. The plasticiza- tion of the polymer by CO2 allows for much easier stirring versus the vacuum system. Like in the vacuum system, the rate did not depend on the choice of bisphenol used. In the vacuum system, tempera- ture is used to lower melt viscosity and drive the reaction, but in the CO2 case the high temperature is used to increase solubility of byproducts without extracting the reactants and drive the reaction. In addition, to improve polymer molecular weight, a dispersant was employed.151 (Polycarbonate A)-b- poly(dimethylsiloxane) was used to produce a micro- cellular foam in the synthesis of polycarbonate A in CO2. The authors speculated that a dispersed poly- mer may allow for more effective removal of phenol because of higher polymer surface area.PDF Image | Polymerizations in Supercritical Carbon Dioxide
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