Computer-aided design of a destabilized RAFT adduct radical: Toward improved RAFT agents for styrene-block-vinyl acetate copolymers

High-level ab initio molecular orbital calculations indicate that a fluorine Z substituent significantly destabilizes the RAFT adduct radical, R'SO·(Z)SR, relative to known Z substituents. This destabilization of the RAFT adduct radical lowers the fragmentation enthalpy relative to normal dithioesters, but without stabilizing the C=S bond of the product RAFT agent, as in xanthate- or dithiocarbamate-mediated polymerization. On the basis of these calculations, it is predicted that, provided appropriate R groups are chosen, RAFT agents containing fluorine Z substituents (i.e., S=C(F)SR, fluorodithioformates, or "F-RAFT" agents) should provide a basis for improved control of monomers with reactive propagating radicals (such as vinyl acetate) and should have the advantage that their C=S bonds remain reactive enough for control of monomers with more stable propagating radicals (such as styrene) and hence the production of styrene-vinyl acetate copolymers.