Kinetics of the Homopolymerization of Vinylidene Fluoride and Its Copolymerization with Hexafluoropropylene in Supercritical Carbon Dioxide: The Locus of Polymerization

In previous studies, the continuous polymerization of vinylidene fluoride (VF2) and mixtures of VF2 with hexafluoropropylene (HFP) was carried out in supercritical carbon dioxide (ScCO2) using a continuous stirred-tank reactor (CSTR). Most of the polymerizations were heterogeneous; i.e., polymer particles precipitated during the reaction. However, some were homogeneous, especially at higher HFP concentrations. In this study. the data from the earlier experiments have been tested against three kinetic models to determine the primary locus of the heterogeneous polymerizations. The first model, the "solution polymerization" model, is based on the assumption that all of the polymerization reactions take place in the continuous, CO-rich phase, with no reaction in the polymer phase. In the second model, the "surface polymerization" model, chain initiation occurs in the continuous phase, while chain propagation and termination occur in a thin zone oil the surface of the polymer particles. The third model, the "interior polymerization" model, is similar to the "surface polymerization" model, except that propagation and termination take place uniformly throughout the polymer particles. For all polymer compositions. the Solution polymerization model is able to describe the experimental data for both the rate of polymerization and the number-average molecular weight quite welt. On the other hand, both the surface and the interior polymerization models consistently fail to fit the experimental results. This analysis Suggests that the CO2-rich continuous phase is the main locus of polymerization in the precipitation polymerization of VF2 and VF2/HFP mixtures in scCO(2) over the Current range of the experimental data.