Crystallization and solid-state polymerization of poly(bisphenol A carbonate) facilitated by supercritical CO2

Poly(bisphenol A carbonate) was synthesized by solid-state polymerization (SSP) using supercritical CO2 to induce crystallinity in low molecular weight polycarbonate beads. The CO2-induced crystallization was studied as a function of time, temperature, molecular weight, and pressure. There was an optimum temperature for crystallization which depended on the molecular weight of the polymer. The molecular weight and percent crystallinity of the polymer produced by SSP were determined as a function of time and radial position in the bead. The molecular weight and percent crystallinity were strong functions of the particle radius, probably because of the slow diffusion of phenol out of the polymer particles. Nitrogen and supercritical CO2 were used as sweep fluids for the SSP process. The polymerization rate was always higher in supercritical CO2 at otherwise comparable conditions. We hypothesize that supercritical CO2 plasticizes the amorphous regions of the polymer, thereby increasing chain mobility and the rate of phenol diffusion out of the polymer. This permits the reaction temperature to be reduced, thereby suppressing side reactions that lead to color body formation. These advantages result in higher molecular weight product with good optical clarity when supercritical CO2 is the sweep fluid.