Among the available organic compounds, thioketones exhibit unique properties and can undergo various photochemical reactions under visible light. In particular, the recent discovery that thioketones can function as efficient spin traps for carbon radicals has significantly increased research interest in this area. Several controlled/living radical polymerization (CLRP) for the synthesis of polymer with predictable properties such as molecular weight have been developed. Nevertheless, despite the efforts, the scope of the monomers and thioketones structures used in mediating radical polymerization has not been fully explored in literature. Therefore, the development of simple and efficient controlled/living radical polymerization methods for the synthesis of polymers is highly desirable.
In recent research paper in the Polymer Chemistry journal, researchers at the Beijing University of Chemical Technology: Haoyu Yu (Ph.D. Student), Jianwei Shao (Ph.D. Student), Dong Chen (Ph.D. Student), Professor Li Wang, and Professor Wantai Yang investigated the polymerization behavior of methyl methacrylate mediated by four different aromatic thioketones with substituents exhibiting different electronic effects: 2-trifluoromethyl-9H-xanthene-9-thione/TfXT, 9H-xanthene-9-thione, 10-phenylacridine-9(10H)-thione and 2,7-bis(dimethylamino)-9H-xanthene-9-thione. Specifically, the effects of the four aromatic thioketones on the radical polymerization of methyl methacrylate were evaluated and compared.
Prior to this work, the authors had previously developed a simple and feasible cycloketyl radical-mediated polymerization method to synthesize polymers. It utilizes cycloketyl xanthone (CX) radicals originating from the thermal- or photo-induced decomposition of 9,9′-bixanthene-9,9′-diol (BIXANDL) for initiating radical polymerization and reversibly deactivating propagating radicals. Inspired by that method, the authors anticipated that xanthene-9-thione with substituents of different electronic effects could generate reactivity-adjustable radicals, and could significantly influence the polymerization behavior by modifying the stability and susceptibility of thioketones towards radical attacks. Based on this approach, the main objective of the current study was to expound on the previous findings. In their current approach, they conducted an electrospray ionization mass spectroscopy to study in detail the underlying polymerization mechanism of methyl methacrylate mediated thioketones under different conditions. Finally, the feasibility of this approach was validated by using it to synthesize quasi-block copolymer of methyl methacrylate and tert-butyl methacrylate.
Results showed that all the polymerizations exhibited pseudo-first-order and decelerated kinetics. However, the deceleration effect was significant when the substituents changed from a strong electron-donating group to a strong electron-withdrawing group. Interestingly, TfXT exhibited the best performance among the four thioketones. It perfectly controlled the polymerization, especially when the molar ratio of TfXT to the initiator was greater than 2. Moreover, the ESI-Q-TOF-MS analysis revealed in detail the mechanism involving the rapid capturing of propagating radicals and the subsequent cross-termination with other propagating radicals. Furthermore, the presented method was successfully applied in the synthesis of an amphiphilic quasi-block copolymer of methyl methacrylate and methacrylic acid which was the hydrolysate of the above-mentioned copolymer, thus validating its feasibility. It was worth noting that the produced amphiphilic quasi-block copolymer could easily self-assemble into spherical particles in either methanol or water with relatively good stability.
In summary, the authors successfully synthesized four aromatic thioketones, with substituents exhibiting different electronic effects. These thioketones were compared in mediating the radical polymerization of methyl methacrylate in which TfXT exhibited the best performance compared to the others. The approach was simple, feasible, and could be successfully used to produce quasi block copolymers. In a statement to Advances in Engineering, Professor Wantai Yang pointed out that the presented method would highly contribute to the design of controlled/living radical polymerization methods with enhanced control features for possible industrial applications.
Yu, H., Shao, J., Chen, D., Wang, L., & Yang, W. (2020). Aromatic thioketone-mediated radical polymerization of methacrylates and the preparation of amphiphilic quasi-block copolymers. Polymer Chemistry, 11(18), 3251-3259.