Continuous Copolymerization of Vinylidene Fluoride with Hexafluoropropylene in Supercritical Carbon Dioxide: Low Hexafluoropropylene Content Semicrystalline Copolymers

The copolymerization of vinylidene fluoride with hexafluoropropylene (HFP) was carried out in supercritical carbon dioxide by precipitation polymerization using a continuous stirred tank reactor. Copolymers with ca. 10 mol % HFP were synthesized at 40 °C and pressures in the range of 207−400 bar using perfluorobutyryl peroxide as the free radical initiator. The effects of feed monomer concentration and reaction pressure were both explored at otherwise constant conditions. The rate of polymerization (Rp) and the number-average molecular weight (Mn) increased linearly with the total monomer concentration up to about 6 M, the highest concentration investigated. The Rp and the Mn were strongly influenced by the reaction pressure. An 80% increase in both Rp and Mn was observed when the reaction pressure rose from 207 to 400 bar. The molecular weight distributions of the synthesized copolymer showed a long tail that increased to become a broad shoulder with increasing total monomer concentration. This tail increased with increasing reaction pressure. The data suggest that the carbon dioxide-rich fluid phase is the primary locus of polymerization.