Continuous precipitation polymerization of vinylidene fluoride in supercritical carbon dioxide: Modeling the rate of polymerization

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 x 10(-5) mol/L s at 75 degreesC 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 degreesC. 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.