Photoinduced Ion-Pair Inner-Sphere Electron Transfer-Reversible Addition-Fragmentation Chain Transfer Polymerization

Photoredox-mediated reversible deactivation radical polymerization (RDRP) is a promising method of precise synthesis of polymers with diverse structures and properties. However, its mechanism mainly based on the outer-sphere electron transfer (OSET) leads to stringent requirements for an efficient photocatalyst. In this paper, the zwitterionic organoboranes [L₂B]⁺X^- are prepared and applied in reversible addition-fragmentation chain transfer (RAFT) polymerization with the photoinduced ion-pair inner-sphere electron transfer (IP-ISET) mechanism. The ion-pair electron transfer mechanism and the formation of the radical [L₂B]^• are supported by electron paramagnetic resonance (EPR) radical capture experiments, ¹H/¹¹B NMR spectroscopy, spectroelectrochemical spectroscopy, transient absorption spectroscopy, theoretical calculation, and photoluminescence quenching experiments. Photoluminescence quenching experiments show that when [CTA]/[[L₂B]⁺] ≥ 0.6, it is static quenching because of the in situ formation of [L₂B]⁺[ZCS₂]^-, the real catalytic species. [L₂B]⁺[C₃H₇SCS₂]^- is synthesized, and its photoluminescence lifetime is the same as the lifetime in the static quenching experiment, indicating the formation of [L₂B]⁺[ZCS₂]^- in polymerization and the IP-ISET mechanism. The matrix-assisted laser desorption ionization time-of-flight mass (MALDI-TOF MS) spectra show that the structure of [C₃H₇SCS₂] was incorporated into the polymer, indicating that ion-pair electron transfer occurs in catalytic species. The polymerization shows high catalytic activity at ppb catalyst loading, a wide range of monomers, excellent tolerance in the presence of 5 mol % phenolic inhibitors, and the synthesis of ultrahigh-molecular-weight polymers. This protocol with the IP-ISET mechanism exhibits a value in the development of new organic transformations and polymerization methods.