Tuning the Polymer Shell of Rose Bengal-Anchored Latex Synthesized by Dispersion Polymerization: Photoreactivity, Recovery, and Use in Continuous-Flow Photooxygenation

Abstract The immobilization of organic photocatalysts onto solid materials favors their recovery by postseparation from the reaction medium and their photostability. The aim of this study is to design supported photocatalysts addressing these challenges while offering the opportunity to implement a more energy-efficient continuous-flow photochemical process in a green organic solvent. Submicron polymer colloids, synthesized as stable latexes by polymerization in dispersed media, are well-suited to meet these criteria. In this study, surfactant-free dispersion polymerization-induced self-assembly (PISA) in ethanol was used to produce spherical core–shell particles decorated with Rose Bengal (RB) photosensitizer in the outer shell in order to generate singlet oxygen (1O2) under visible-light irradiation. By exploring three different polymers as particle stabilizers, we proposed to investigate the effect of the RB microenvironment on key properties such as photoreactivity, photostability, and reversible sedimentation. For this purpose, Rose Bengal methacrylic monomer (EMARB) was statistically copolymerized with different methacrylic monomers to produce macromolecular chain-transfer agents (macroCTA) by reversible addition–fragmentation chain-transfer (RAFT) polymerization. MacroCTAs were chain-extended in ethanol by RAFT dispersion polymerization of benzyl methacrylate (PBzMA) to produce self-assembled block copolymer latexes of 60 to 240 nm size. Both series of latex particles, based on either poly(2-hydroxypropyl methacrylate) (PHPMA) or poly(oligo(ethylene glycol) methyl ether methacrylate (POEGMA), exhibited a high quantum yield of 1O2 production close to free RB and low photobleaching rate. In comparison, the latex stabilized by poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) exhibited inferior properties due to electron transfer to the tertiary amine, which competed with energy transfer. The series of POEGMA- and PHPMA-based photoactive polymer colloids were efficiently implemented in a LED-driven continuous-flow millimetric reactor, leading to high rate of photooxygenation of biobased α-terpinene (100% conversion in 5 min). Finally, we highlighted that the RB-anchored latexes of intermediate diameters (100–170 nm) proved to be efficient supports for photosensitizer recovery by sedimentation–redispersion, regardless of the nature of the stabilizing polymer shell.