Continuous Copolymerization of Vinylidene Fluoride with Hexafluoropropylene in Supercritical Carbon Dioxide: High-Hexafluoropropylene-Content Amorphous Copolymers

Abstract

Copolymerization of vinylidene fluoride (VF2) and hexafluoropropylene (HFP) was carried out in supercritical carbon dioxide using a continuous stirred tank reactor. Three different HFP/VF2 molar feed ratios were studied, 59:41, 66:34, and 73:27, giving rise to amorphous copolymers containing about 23, 26, and 30 mol % HFP, respectively. The experiments were carried out at 40 °C with pressures in the range of 207–400 bar using perfluorobutyryl peroxide as the free radical initiator. Depending on the copolymer composition, the molecular weight, and the reaction pressure, either a homogeneous (solution) or a heterogeneous (precipitation) polymerization was observed. The effects of feed monomer concentration and reaction pressure were explored at otherwise constant conditions. The rate of polymerization (Rp) and the number-average molecular weight (Mn) increased linearly with the total monomer concentration, independent of the mode of polymerization, i.e., homogeneous or heterogeneous. Both Rp and Mn increased by about 20–30% when the reaction pressure was increased from 207 to 400 bar. This increase could be accounted for by the effect of pressure on the reaction rate constants. The molecular weight distributions were perfectly unimodal except for the lowest HFP-content copolymers at the highest monomer concentrations. The data suggest that the carbon-dioxide-rich fluid phase is the main locus of polymerization, even when the polymer precipitates during the reaction.