Radical addition to thioketones: Computer-aided design of spin traps for controlling free-radical polymerization

An extensive study has been undertaken of the radical affinity of a number of thioketones (S=C(X)(Y)) with the aim of selecting combinations of X and Y that render the substrate suitable for the mediation of free radical polymerizations. Using high level ab initio molecular orbital calculations, enthalpies at 0 K were determined for the reactions R^. + S= C(X)(Y) → R-S-C^.(X)(Y) for R^. = CH₃, CH ₂OH, CH₂CN, and benzyl, in reactions with a variety of thioketones, including various combinations of X and Y taken from H, CH ₃, Ph, CN, OCH₃, C(CH₃)₃ and para-CN-Ph as well as several compounds in which the X and Y are bonded, namely xanthene-9-thione, fluorine-9-thione, and cyclopenta[def]phenanthrene-4-thione. The radical affinities of the various thioketones has been discussed in terms of the radical stabilization energies (RSEs) of the adduct radicals and stabilities of the S=C bonds. From these studies, the two thioketones S=C(CN)(Ph) and fluorene-9-thione were selected as being potentially suitable candidates for use in controlling free radical polymerizations due to their high radical affinities. However, based on transition state theory calculations of the rate coefficients for homo/ copolymerization of S=C(CN)(Ph) with itself and styrene at 333.15 K, this substrate was deemed to be unsuitable, as it was likely to undergo side reactions. Instead, the more-hindered fluorine-9-thione was identified as the ideal thioketone, and the equilibrium constants at 333.15 K for the reactions of the styryl and vinyl acetate dimer radicals with fluorine-9-thione were made. These two reactions, at 333.15 K, displayed equilibrium constants in the vicinity of 10¹⁴ L mol^-1 and 10¹⁶ L mol^-1, respectively, indicating that there is significant scope within the thioketone class of compounds to mediate free radical polymerizations via radical stability alone.