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160 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 3.5. Condensation polymerizations 3.5.1. Polyester, polyamides, polycarbonates Condensation polymerization [151] occurs through the step-wise addition of difunctional monomers to each other, usually in a reaction that produces a small molecule byproduct (water or alcohol, for example). Polyesterification (reaction of diol with diester or diacid) and polyamidation (diamine with diacid or diester) are two classic examples of great industrial importance. Because of the nature of these poly- merizations, there are key differences with respect to chain polymerizations. Condensation polymeriza- tions are usually endothermic, and hence heat must be applied to achieve high rate of reaction. Unlike chain polymerization, molecular weight builds slowly in condensation reactions. Indeed, the statistics of condensation polymerization show that the extent of reaction of the active end groups must reach at least 95% to create polymer chains of reasonable length. Because each condensation (chain building) reaction is governed by equilibrium, removal of the small molecule byproduct is crucial in achieving high extent of reaction and hence high chain length. Continuous industrial condensation polymerization processes all exhibit the same general elements [123]. The two monomers are added to the system in the cor- rect proportions and then heated and pumped into a U-shaped tubular reactor with the appropriate catalyst. Steam (or alcohol) is flashed from the reactor at its exit, and the resulting oligomer is pumped to a ‘fin- ishing stage’. Here, vacuum or flowing N2 is applied to remove the small molecule, while slow mixing cre- ates surface area to enhance the reaction rate. Here the oligomers are transformed to polymers. Tempera- tures in the process must be high enough to melt the polymer and hence temperatures of 520–570 K are not uncommon. Given the nature of condensation polymerizations, CO2 has been applied as a diluent/plasticizer to enhance the removal of the small molecule, hence increasing molecular weight [178]. By dissolving in the polymer melt, CO2 should reduce the viscosity and increase the rate of removal of the condensation byproduct. Clearly, for the process to be most suc- cessful, the small molecule should partition preferen- tially to the CO2 phase. The green aspect of such a scheme is that use of CO2 could allow better removal of the condensation byproduct at lower temperature, saving energy. The best example of this use of CO2 is probably the work of Kiserow and DeSimone on the CO2 -enhanced solid-state polymerization of polycarbonate. In bisphenol A polycarbonate produc- tion, diphenyl carbonate is reacted with bisphenol A to produce the polymer plus phenol. Many end users of polycarbonate (as well as nylon 6.6) prac- tice ‘solid-state polymerization’, where the purchased polymer is charged to a vacuum oven to increase molecular weight through additional reaction and byproduct removal. DeSimone showed that CO2 could be employed to remove phenol from polycarbonate oligomers at temperatures well below the Tg of the polymer (420 K), raising molecular weight substan- tially [179]. Later work [180] by Shi et al. showed that limitations to the increase in molecular weight are due primarily to an imbalance in the concentration of the two types of endgroup on the polymer (hydroxyl and terminal carbonate)—this is a common problem in the solid state polymerization of condensation polymers. A general problem with using CO2 to enhance con- densation byproduct removal is the low solubility of some common byproducts in carbon dioxide. Water, the most common byproduct in polyamide generation, is poorly soluble in CO2. In the formation of polyethy- lene terephthalate (the highest volume polyester), the polymer is formed via the self-condensation of the adduct of 2 mol of ethylene glycol and dimethyl terephthalate (see Scheme 6); the byproduct is hence ethylene glycol, also poorly soluble in CO2. Indeed, the use of CO2 to plasticize polymer melts and re- move condensation byproducts is sound, sustainable processing, but this technique will only be truly effec- tive if the byproduct is designed to partition strongly to CO2. Scheme 6.PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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