Study on controlled free-radical polymerization in the presence of dithiobenzoic acid (DTBA)

Dithiocarboxylic Acids: An Old Theme Revisited and Augmented by New Preparative, Spectroscopic and Structural Facts

The unstable dithiocarboxylic acids dithioacetic acid, 2-methyl-dithiopropionic acid, 2,2-dimethyl-dithiopropionic acid and dithiobenzoic acid were synthesized and characterized by NMR spectroscopy and GC/MS. The stable dithiocarboxylic acids 2,4,6-trimethyl benzoic acid, 2,4,6-tri-iso-propylbenzoic acid and 2,6-dimesityl benzoic acid were synthesized, isolated and characterized by spectroscopic methods and in parts by mass spectrometry and X-ray crystallography. The new data were used to re-evaluate literature data on the synthesis, spectroscopy and structural data of dithiocarboxylic acids as a fundamental class of organic compounds in general.

On the Mechanism of Radical Polymerization of Methyl Methacrylate with Dithiobenzoic Acid as Mediator

Dithiobenzoic acid (DTBA) induces controlled polymerization behaviour in methyl methacrylate polymerization at 60°C, accompanied by a pronounced induction period of several hours. DTBA is partially transformed during this induction period into a dithioester with a tertiary ester group moiety, which constitutes an efficient reversible addition–fragmentation chain transfer (RAFT) agent. The transformation reaction is proposed to proceed via a hydrogen abstraction from DTBA by radicals and subsequent termination of the formed phenylcarbonothioylsulfanyl radical with propagating radicals. The proposed reaction scheme was implemented into a computer model, by which the rate coefficient of the hydrogen abstraction from DTBA and of the reinitiation of the intermediate phenylcarbonothioylsulfanyl radical was estimated. The model is in agreement with all of the species observable by electrospray ionization mass spectrometry, with the extent of the experimental induction periods, and with the absolute concentrations of dithioesters that act as efficient RAFT agents during the polymerization. A protocol that uses a cocktail of initiators is introduced, by which the induction period in DTBA-mediated polymerization is effectively eliminated.