A multidisciplinary approach to the use of pyridinyl dithioesters and their N-oxides as CTAs in the RAFT polymerization of styrene. Not the chronicle of a failure foretold

The efficiency of three isomeric pyridinyl dithioesters and their N-oxides as chain transfer agents in the RAFT (reversible addition fragmentation chain transfer) polymerization of styrene was tested. Ortho (dithiopicolinates), meta (ditionicotinates), and para (dithioisonicotinates) isomers controlled the polymerization of styrene although with some retardation with respect to dithiobenzoates. The retardation, which was even greater for the N-oxides, was attributed to excessive stabilization of the dormant radical species intermediate in the RAFT process by the heteroaromatic rings as inferred from the measured reduction potentials of the compounds. Styrene polymerization was actually blocked at very low conversion in the case of the dithioisonicotinate N-oxide, and on the basis of ESR (electron spin resonance) studies it is suggested that in this case the dormant radical may actually act as a scavenger of the propagating radical. Although the knowledge beforehand of the reduction potential of a given CTA (chain transfer agent), from which the stability of the dormant radical it would form during the RAFT process could be estimated, might in principle allow one to foretell its performance, such predictions must be considered with caution.