Mechanistic Insights into Oxygen Tolerance of Graphitic Carbon Nitride-Mediated Heterogeneous Photoinduced Electron Transfer-Reversible Addition Fragmentation Chain Transfer Polymerization

Here, heterogeneous photoinduced electron transfer-reversible addition fragmentation chain transfer (PET-RAFT polymerization of methyl methacrylate (MMA) was carried out under 405 nm irradiation without prior deoxygenation of the reaction medium using urea-based graphitic nitride (U-g-C3N4) as a photocatalyst. The impact of each constituent, including U-gC(3)N(4), a chain transfer agent (CTA), and a tertiary amine (TA), on the control of the polymerization and its tolerance toward oxygen was discussed. We found that the U-g-C3N4/CTA photocatalytic system is efficient to mediate PET-RAFT polymerization under a low quantity of oxygen. Adding a moderate quantity of the TA to the reaction medium was suitable to enhance its oxygen tolerance. However, an excess of the TA compared to the CTA caused the loss of the polymerization control since the TA can generate an excess of radicals that dominate the CTA-derived chains leading to the formation of irreversibly terminated chains. Detailed mechanisms according to our results and the literature were proposed. Excellent end-group fidelity was demonstrated by performing successive chain extensions of the resulting PMMA to yield pentablock PMMAs. Sustainability of the U-g-C3N4-mediated PET-RAFT polymerization was demonstrated both by performing polymerization under sunlight and by reusing the photocatalyst several times without degrading its photocatalytic efficiency.

Automated summary

The study successfully utilized U-g-C3N4 as a photocatalyst for PET-RAFT polymerization, investigating the effects of different components on polymerization control and oxygen tolerance, and proposing detailed mechanisms. It was found that adding a moderate quantity of tertiary amine could enhance oxygen tolerance, but excess would lead to loss of polymerization control.