Microwave-Assisted Oxidative Degradation of Poly(vinylidene fluoride-co-hexafluoropropylene) Copolymers: Preparation, Characterization, and Reaction Mechanism

This study focuses on the synthesizing of liquid-terminated carboxyl fluoropolymers (LTCFs) from poly­(vinylidene fluoride-co-hexafluoropropylene) (P­(VDF-HFP)) through microwave (MW)-assisted oxidative degradation. An in-depth analysis was conducted on the preparation process, the duration of MW irradiation, and the resulting changes in LTCFs. Chemical titration and gel permeation chromatography (GPC) revealed that the MW-assisted oxidative degradation was achieved in 150–240 s, which is several orders of magnitude shorter than heat (CH) oxidative degradation. The modifications in the structure of CC and CO bonds within LTCFs were investigated over a time range of 0 to 900 s using Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible spectrophotometry (UV–vis), and 19F nuclear magnetic resonance (19F-NMR). The analysis reveals that the dehydrofluorination reaction primarily occurs on the HFP-VDF-HFP sequence, resulting in two sequence structures: CCZaitsev and CCHofmann. The dehydrofluorination of the P­(VDF-HFP) copolymer followed Zaitsev’s rule mainly and Hofmann’s rule slightly. Furthermore, it was observed that the processes of CC bond formation (KHF reaction) and conversion (KC–C reaction) are in competition. When H2O2 is enough, the degradation of the P­(VDF-HFP) copolymer predominantly follows the KC–C reaction and partially the KHF reaction. The opposite is true for low H2O2 concentrations. To conclude, this study suggests a mechanism for the synthesis of LTCFs using MW-assisted oxidative degradation of P­(VDF-HFP). Under MW, the C atom adjacent to a CC bond readily undergoes autocatalytic KHF reactions, creating conjugated polyene sequences of certain lengths. Meanwhile, the rapid degradation of H2O2 expedites CC oxidative breaking, producing the COOH functional groups.