Chain-End Functionality: The Key Factor toward Fluoropolymer Thermal Stability

The aqueous radical homopolymerization of vinylidene fluoride (VDF), as well as its copolymerizations with hexafluoropropylene (HFP) or perfluoromethyl vinyl ether (PMVE), and its terpolymerization with HFP and PMVE, initiated by trifluoromethyl radicals (•CF3), generated from the ammonium persulfate (APS) and potassium trifluoromethyl sulfinate (CF3SO2K) redox system are presented. The optimization of the experimental conditions in terms of initial reactant molar ratios, temperature, and reaction time was achieved. The best results were obtained at 60 °C, in water, without any surfactant, using 0.9 equiv of APS as the oxidant with respect to CF3SO2K. PVDF, poly(VDF-co-HFP), poly(VDF-co-PMVE), or poly(VDF-ter-PMVE-ter-HFP) copolymers were obtained in high yields (varying between 89 and 100%) and with molar masses up to 113,000 g/mol. The 1H and 19F NMR spectroscopies revealed that, under optimized conditions, the formed chain ends were exclusively CF3 or CF2H. The influence of the nature of the chain end on the thermal stability showed that PVDFs terminated by CF3 were more stable than their analogues bearing sulfate end groups by 120 °C. In addition, the prepared CF3-PVDFs had 53% of crystallinity (compared to 47% for PVDF initiated from APS alone), which offers excellent resistance to conventional organic solvents including dimethyl sulfoxide, acetone, and dimethyl formamide.