Thiol-ene reactions are versatile organic reactions between a thiol (-SH group) and an ‘ene’ (an electron rich vinyl group, C=C) that proceed via an alternating step growth and chain transfer mechanism. Additionally, the photoinitiated reaction is observed to be rapid, selective, and high yielding in the fabrication of polymeric materials.1 Covalent binding of low melting point salts, ionic liquids (ILs), directly onto polymers classifies them as polymeric ionic liquids (PILs) and imparts these materials with electrolytic properties such as ion conductivity.2 The kinetics of the direct reaction between thiols and allylic ILs have not been studied in detail and would provide a framework for the selection of monomers for PILs with readily tailored properties. Our studies show that the use of heterocyclic amine-based ILs, such as imidazolium and pyridinium, and thiols, such as mercaptopropionates, yield the fastest thiol-allylic IL rates for formation of ideal ionic thioether products as observed by NMR and FTIR spectroscopy. Furthermore, all of the thiol-allylic IL reactions studied are chain transfer limited by nature. Model spectroscopic studies show that the use of high polarity or chlorinated aprotic solvents promotes photoinitiated disulfide formation which competes with the thiol-ene reaction and shifts the overall reaction kinetics from first order to second order overall. These model studies demonstrate the scope and limitations of the thiol-allylic IL reaction and can be directly applied to the synthesis of both linear and crosslinked PILs with readily tuned physicochemical properties as will be showcased in the design of thiol-ene PIL networks as membranes for alkaline fuel cells3 or separations.

Taking charge of thiol-allylic ionic liquid photopolymerizations

253rd National Meeting (2017)

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