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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.

PTFE latexes with particles in the submicrometer size range were employed as seeds in the emulsifier-free styrene emulsion polymerization to obtain PTFE−polystyrene (PS) core−shell nanospheres. Stable latexes were generally obtained. Neither residual PTFE nor secondary nucleation was observed, thus leading to PTFE−PS core−shell latexes. By appropriately choosing the ratio between the styrene monomer and the PTFE seed in the reaction mixture, it is possible to obtain spheres with predetermined sizes and narrow size distribution. In all cases, a slightly hemispherical morphology was observed. Peculiar effects related to the high degree of segregation of PTFE cores and their small size, and possibly to the PTFE/PS interface, were observed.

Benzyl (diethoxyphosphoryl)dithioformate (1) has been used to control the AIBN initiated radical polymerization of styrene. In a comparative study with the more popular RAFT transfer agent benzyl dithiobenzoate (2), compound 1 led to higher molecular weight polymers, although with a slightly worse polydispersity. During the polymerization process mediated by 1 a few radicals have been intercepted and characterized by means of ESR spectroscopy. The nature and relative amount of the various radical species varied as the polymerization proceeded, matching expectations based on the generally accepted RAFT mechanism. The present ESR results induce a reassessment of the identity of a radical species observed in a previous preliminary study.