State-Dependent Photochemical and Photophysical Behavior of Dithiolate Ester and Trithiocarbonate Reversible Addition-Fragmentation Chain Transfer Polymerization Agents

The rise in popularity of photochemically initiated reversible addition-fragmentation chain transfer (RAFT) polymerization (photoRAFT) along with the broad spectrum of proposed, and possible, initiation mechanisms result in the need for careful characterization of the photophysical properties of some common RAFT agents. Direct irradiation of the RAFT agent as a means to generate radicals, also known as the photoiniferter mechanism, is one commonly proposed mechanism. The current study shows that dithioesters and trithiocarbonates have the lowest singlet and triplet excited-state energy levels that are close to, or lower than, the C-S bond dissociation energies. Excitation of these agents into their S-1 band results in negligible radical production, while excitation into the S-2 band or higher results in the decomposition of dithioesters and trithiocarbonates, resulting in radical formation but with low quantum yields. Likewise, there is significant literature precedence for an electron transfer initiation mechanism, PET-RAFT. It is shown that the dithioesters and trithiocarbonates all show peak reduction potentials at ca. -1.0 V (vs SCE). However, transient absorption spectroscopy studies of the electron transfer from a mediator show that these reactions occur rapidly only when the mediator potential is more negative than -1.2 V (vs SCE).