(247g) Exploring of the Pre-Polymerization Coordination of 1-Vinylimidazole

Authors: 
Abedini, A., The University of Alabama
Hamilton, J. R., The University of Alabama
Turner, C. H., The University of Alabama
Whitley, J. W., University of Alabama
Bara, J. E., University of Alabama

In bulk photopolymerization, it can be difficult to maintain a high reaction rate and achieve complete conversion. To alleviate these challenges, several methods have been explored to improve the mass transfer of the monomers in the reaction mixtures. One of the most prevalent strategies is to increase the local monomer concentration in the mixture, both before polymerization and during the polymerization process. Since the thermochemical properties of the solution will change during polymerization, the availability of monomers to react with active sites, both before and during the reaction, will affect the overall reaction rate and conversion of the polymerization [1].

Poly(vinylimidazole) (poly-VIm) has a multiple range of applications, including: coating electrodes [2], adhesive development [3], fuel cell membranes [4], CO2[5][6] and metal ion adsorption [7], and biomedical application[8].  Also, it is one of the ionic liquid (IL) coordinated polymers that has received a great deal of attention in radical polymerization.  Previous experimental studies have indicated that cations (Zn2+, AL3+, Li+, and Mg2+) can increase both the reaction rate and the monomer conversion in radical polymerization, since the cations can form favorable complexes with the monomers [1]. While most of the past polymerization work has dealt with finding conventional solvents and salts to enhance polymerization [9], more recent research has considered ILs as promising additives for these reactions [10]. These recent experimental results indicate that polymerizable monomers coordinated with ILs, formed from weakly coordinating anions and cations, can enhance polymerization reaction kinetics [1].  Lithium bistriflimide (LiTf2N) is one of the ILs that has been investigated experimentally in the polymerization of VIm. This IL had a substantial effect on increasing reaction kinetics, since mass transfer is a major limitation in this photopolymerization reaction. The addition of LiTF2N leads to 100% monomer conversion in the 1VIm: 1Tf2N solution, as reported by Whitley et al. [1]. Different concentrations of LiTf2N with VIm show interesting behavior. Though the initial reaction rate of the 1:1 (VIm:LiTf2N) solution is lower than neat VIm polymerization, the final monomer conversion for the 1:1 solution is higher and reaches 100% conversion after about 30 min.  The initial reaction rate for the 1:1 molar concentration is lower in comparison to the other molar concentrations (2:1 and 3:1) [1].

This observed behavior (higher conversion and lower initiating reaction) needs to be fundamentally explored. Currently, there is no direct explanation for this phenomenon, and the molecular-level interactions in this type of system are unknown.  While Li+ has been experimentally found [1] to interact with 3 Tf2N- anions, we have conducted a thorough investigation of the Li+ cation interactions in the VIm:LiTf2N system, in comparison to another cation (Na+) and other anions (BF4- and PF6-).  In order to develop a broad understanding of the polymerization dynamics, polymer structure, and properties, the molecular-level pre-polymerization structure of this mixture needs to be clearly quantified. In this work, molecular dynamics simulations of different VIm:LiTf2N solutions show that Li+ locally enriches key reaction sites involved in polymerization.  This local enhancement of site-site interactions is suggested to play a vital role in the experimentally observed improvement in polymerization behavior. References:

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