Polymerization of vinyl fluoride in ionic liquid and ionic solutions

- Vinyl fluoride (VF) is highly soluble in ionic liquids with fluorinated anions, and in aqueous LiTf2N salt solutions.
- VF polymerization in aqueous LiTf2N solutions produces high molecular weight polymer at low VF pressures (<1000 psi).
- VF solubility and monomer consumption rate increase with LiTf2N salt concentration in aqueous solution.
- The morphology of polyvinyl fluoride (PVF) synthesized in aqueous LiTf2N solutions consists of fractal-like aggregates.
- PVF polymer/aqueous LiTf2N mixtures form viscous pastes. Removal of salt from polymer via washing with water is difficult.

Poly(vinyl fluoride) films are extensively used as protective, easy-clean materials in the photovoltaic and aerospace industries. However, polymerization of vinyl fluoride (VF) monomer typically requires high pressures (>5000 psi) due to the low solubility of VF monomer in the aqueous polymerization medium. VF was found to exhibit high solubilities in fluorinated ionic liquids and concentrated aqueous ionic salt solutions at low pressures (<1000 psi). Polymerization of vinyl fluoride in the presence of ionic liquids or ionic salt solutions was pursued as a means of reducing the pressure needed to achieve high monomer conversion and molecular weight. The use of traditional ionic liquids possessing alkylimidazolium cations paired with fluorinated anions as solvents for VF polymerization resulted in poor conversion of vinyl fluoride, likely due to hydrogen atom abstraction from the alkyl substituents on the imidazolium cations. Polymerization of VF in the presence of lithium salts of the ionic liquid anions (trifluoromethylsulfonyl imide [Tf2N], triflate, and trifluoroacetate), on the other hand, resulted in high VF conversion and production of high molecular weight polymer at low VF pressures. The morphology of the resultant polymer consisted of fractal-like aggregates, and mixtures of the polymer with the aqueous ionic salt solutions formed viscous pastes.

TEM image of poly(vinyl fluoride) made in 80 wt.% LiTf2N + H2O with fractal morphology.269