Complex Effects of the Sweep Fluid on Solid-State Polymerization: Poly(bisphenol A carbonate) in Supercritical Carbon Dioxide

The effects of the sweep fluid on solid-state polymerization (SSP) of poly(bisphenol A carbonate) (BPA-PC) were investigated. Prepolymers with two different number-average molecular weights, PCP6C ( M n = 3800 g/mol) and PCP9C ( M n = 2400 g/mol), were synthesized using melt transesterification. Solid-state polymerization of these prepolymers was carried out at temperatures in the range of 150−190 °C with supercritical carbon dioxide (scCO 2 ) and N 2 as the sweep fluids. It was found that scCO 2 at 207 bar could either increase or decrease the rate of SSP relative to the rate in atmospheric N 2, depending on the prepolymer molecular weight. At 190 °C, the molecular weights of the polymers synthesized from the higher-molecular-weight prepolymer (PCP6C) were higher with scCO 2 as the sweep fluid compared to those of the polymers synthesized with N 2 . In contrast, at the same temperature of 190 °C, the molecular weights of the polymers synthesized from the lower-molecular-weight prepolymer (PCP9C) were lower with scCO 2 compared to those of the polymers synthesized with N 2 . This apparently contradictory effect can be understood in terms of competition between rate-increasing effects of scCO 2, such as greater chain mobility and a higher byproduct diffusion coefficient caused by plasticizing of the polymer by scCO 2, and rate-diminishing effects associated with scCO 2, such as lamellar thickening, higher crystallinity, and perhaps increased occlusion of end groups in crystalline regions.