Direct Radical Copolymerization of Vinylidene Fluoride with Vinylphosphonic Acid and Characterization of the Resulting poly(VDF-<i>co</i>-VPA) Copolymers

Poly(VDF-co-VPA) copolymers were prepared via the free radical copolymerization of vinylidene fluoride (VDF) and vinylphosphonic acid (VPA). Solution polymerization of VDF with VPA in organic solvent was carried out in different reaction media and feed ratios to study the factors influencing such a copolymerization. Hydrophobic and hydrophilic fractions of the resulting copolymers were separated and purified from the total product mixture through filtration and dialysis, each of both revealing VDF-rich and VPA-rich copolymers, respectively. The presence of both comonomer units, functional groups, and molar mass of the resulting copolymers was characterized by 1H, 19F, and 31P NMR and Fourier-transform infrared (FT-IR) spectroscopy, gel permeation chromatography (GPC) after methylation, as well as by matrix-assisted laser desorption/ionization (MALDI). Diffusion-ordered spectroscopy (DOSY) and 2D NMR characterization results further proved the formation of the random copolymers. The modified thermal behavior of the products monitored by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) corroborated VPA insertion (releasing much char at high temperature) into VDF units. The gradient copolymers not observed in the radical copolymerization of VDF with vinyl dimethylphosphonate may originate from the reactivity difference of VPA against VDF and its unique anhydride formation mechanism in the polymerization.