Molecular Modeling of the Polymerization Behavior of Poly(vinylimidazole) in an Ionic Liquid Solution

Imidazole-based polymers, such as poly(vinylimidazole), may provide useful properties for membrane-based CO₂ capture applications, so it is becoming increasingly important to understand the formation mechanisms and underlying molecular structure of these materials.  One of the challenges in the photopolymerization of 1-vinylimidazole (VIm) are slow rates and incomplete monomer conversion.  We recently discovered that polymerization rate and overall conversion could be dramatically improved when VIm is in the presence of a bulky salt, lithium bis(trifluoromethylsulfonyl)imide [Li⁺][Tf₂N^-].  In this work, molecular dynamics simulations are used to model the intermolecular conformational behavior and thermophysical properties of the VIm-[Li⁺][Tf₂N^-] complexes.  Molecular conformations of different ratios of [VIm] to salt were characterized in order to identify the structural ordering induced by [Li⁺][Tf2N^-] on the polymerization of [VIm].  The same structural analyses were also performed on systems containing poly(vinylimidazole) [PVIm] in order to explore the conformations and thermophysical properties at different stages of the polymerization reaction.  The calculated properties from the molecular dynamics simulations are compared against experimentally-derived structural features and found to be in agreement. Radial distribution functions indicate that [Li⁺] chelates with the “pyridine-like” nitrogen of the imidazole ring in both [VIm] and [PVIm], while the fluorine atoms of [Tf₂N^-] have strong interactions with the vinyl groups.  Also, as the relative concentration of the salt increases, the probability of vinyl groups aggregating in favorable conformations to polymerization tends to increase (at all temperatures studied). Furthermore, electronic structure calculations have also been performed to identify electrostatic interactions in the systems, such as partial charge assignments and potential dipole interactions with [VIm] and [PVIm].