The polymerization of vinylidene fluoride in supercritical carbon dioxide was studied in a continuous stirred tank reactor using diethylperoxydicarbonate as the free radical initiator. Experiments were carried out to investigate the effect of inlet monomer concentration, temperature, average residence time, and agitation on the polymerization rate, the average molecular weights, and the molecular weight distribution of the poly(vinylidene fluoride). A homogeneous kinetic model that includes inhibition due to chain transfer to monomer predicted the polymerization rates reasonably well. However, imperfect mixing, rather than a chemical effect, may have caused the apparent inhibition observed at high monomer concentrations. At inlet monomer concentrations greater than about 1.5 M, broad and bimodal molecular weight distributions were observed. An extended homogeneous kinetic model that includes chain transfer to polymer predicted the polydispersities reasonably well. This model also predicted a region of inoperability that matched the experimental results. However, the extended homogeneous model could not account for the bimodal distributions.

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.

The kinetics of the surfactant-free precipitation polymerization of vinylidene fluoride (VF2) in supercritical carbon dioxide have been studied in a continuous stirred autoclave. Diethyl peroxydicarbonate was used as the free-radical initiator. The stirring rate and agitator design had no effect on the rate of polymerization (R p ) or on the weight-average molecular weight (M w ) of the formed poly(vinylidene fluoride) (PVDF). The fractional conversion of VF2 ranged from 7 to 26%, and R p was as high as 27 × 10 -5 mol/L‚s at 75°C and at a VF2 feed concentration of 2.5 mol/L. The PVDF polymer was collected as a dry, "free-flowing" powder and had M w 's up to 150 kg/mol and melt flow indices as low as 3.0 at 230°C. Homogeneous, free-radical kinetics provided a reasonable basis for describing the rate of polymerization, despite the heterogeneous nature of the system. The order of the reaction with respect to the monomer was found to be 1.0, and the order with respect to the initiator was 0.5. The experimental data suggest that an inhibitor is present in the monomer or that the monomer itself acts as an inhibitor.

No abstract