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

Abstract

This study focuses on the synthesizing of liquidterminated carboxyl fluoropolymers (LTCFs) from poly-(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) through microwave (MW)-assisted oxidative degradation. An indepth 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 19 F nuclear magnetic resonance ( 19 F-NMR). The analysis reveals that the dehydrofluorination reaction primarily occurs on the HFP-VDF-HFP sequence, resulting in two sequence structures: C�C Zaitsev and C�C Hofmann . 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 (K HF reaction) and conversion (K C−C reaction) are in competition. When H 2 O 2 is enough, the degradation of the P(VDF-HFP) copolymer predominantly follows the K C−C reaction and partially the K HF reaction. The opposite is true for low H 2 O 2 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 K HF reactions, creating conjugated polyene sequences of certain lengths. Meanwhile, the rapid degradation of H 2 O 2 expedites C�C oxidative breaking, producing the COOH functional groups.