Photocontrolled Radical Polymerization from Hydridic C–H Bonds

Abstract

Given the ubiquity of C-H bonds in biomolecules and polymer backbones, the development of a photocontrolled polymerization from a C-H bond would represent a powerful strategy for selective polymer conjugation precluding several synthetic steps to introduce complex functionality. We have developed a hydrogen-atom abstraction strategy that allows for a controlled polymerization from a C-H bond using a benzophenone photocatalyst, a trithiocarbonate-derived disulfide, and visible light. We perform the polymerization from a variety of ethers, alkanes, unactivated C-H bonds, and alcohols as well as showcase the applicability of the method to several monomer classes. Our method lends itself to photocontrol which has important implications for building advanced macromolecular architectures. Finally, we demonstrate that we can graft polymer chains controllably from poly(ethylene glycol) showcasing the potential application of this method for controlled grafting from C-H bonds of commodity polymers. Main Text: Reversible-deactivation radical polymerizations (RDRPs) represent one of the most versatile strategies for controlling macromolecular architecture. Numerous synthetic methods have been developed to obtain control over chain length and dispersity. Reversible addition-fragmentation chain transfer (RAFT) has emerged as a powerful tool for controlling radical polymerizations. 1-4 Additionally, photoinduced electron transfer (PET-RAFT) processes have been established as a means to exact temporal and spatial control, finding numerous applications in photopatterning for designing complex 3D architectures. 5-10 To employ these methods, however, a prefunctionalized initiator must be installed into a macromolecule, biomolecule, or small molecule for a controlled polymerization. Despite the ubiquity of C-H bonds, a method for controlling polymerization from a C-H bond has not been demonstrated. 11,12 The development of such a strategy could allow the direct formation of protein-polymer bioconjugates or drug-polymer conjugates without prefunctionalization. 13 Furthermore, complex macromolecular architectures could be controllably accessed via grafting from existing polymers in a single step with spatial and temporal control. 14

Degradable Vinyl Random Copolymers via Photocontrolled Radical Ring‐Opening Cascade Copolymerization**

“…We reasoned that performing the copolymerization at lower temperatures would provide a key opportunity to modulate the reactivity ratios of vinyl comonomers. Therefore, we turned our attention to light‐mediated polymerization techniques, as recent works have demonstrated that they are versatile tools to mediate controlled polymerization following radical, [34–42] cationic, [43–47] and metathesis pathways [48–50] at ambient temperature (Figure 1B) [51] . In particular, we envisioned that the photoinduced electron/energy transfer‐reversible addition/fragmentation chain transfer (PET‐RAFT) polymerization developed by Boyer and co‐workers [52–56] could be employed to mediate the radical ring‐opening cascade copolymerization (rROCCP) [57, 58] of the macrocyclic allylic sulfone with acrylates or acrylamides (Figure 1C).…”

Degradable Vinyl Random Copolymers via Photocontrolled Radical Ring‐Opening Cascade Copolymerization**

“…[6,33] We reasoned that performing the copolymerization at lower temperatures would provide a key opportunity to modulate the reactivity ratios of vinyl comonomers. Therefore, we turned our attention to light-mediated polymerization techniques, as recent works have demonstrated that they are versatile tools to mediate controlled polymerization following radical, [34][35][36][37][38][39][40][41][42] cationic, [43][44][45][46][47] and metathesis pathways [48][49][50] at ambient temperature (Figure 1B). [51] In particular, we envisioned that the photoinduced electron/energy transferreversible addition/fragmentation chain transfer (PET-RAFT) polymerization developed by Boyer and coworkers [52][53][54][55][56] could be employed to mediate the radical ringopening cascade copolymerization (rROCCP) [57,58] of the macrocyclic allylic sulfone with acrylates or acrylamides (Figure 1C).…”

In Situ Generated Crude Trithiocarbonate for Visible Light‐Mediated RAFT Polymerization of Acrylates**

“…Although radical substitution of shelf‐stable bis(thioacyl) disulfides and was used to synthesize RAFT agent under thermal conditions in situ , [6] they were sometimes confronted with a low efficiency of initiation. Recently, instead of thermal initiators, benzophenone derived photoinitiator (4‐methoxyphenyl) (4‐(trifluoromethyl)phenyl) methanone was selected to generate radicals through a hydrogen abstraction process [7] . The trithiocarbonate generated in situ was implemented in the polymerization of methyl acrylate, and narrowly distributed polymers with targeted molecular weights were obtained.…”

“…Recently, instead of thermal initiators, benzophenone derived photoinitiator (4-methoxyphenyl) (4-(trifluoromethyl)phenyl) methanone was selected to generate radicals through a hydrogen abstraction process. [7] The trithiocarbonate generated in situ was implemented in the polymerization of methyl acrylate, and narrowly distributed polymers with targeted molecular weights were obtained. However, it was worth noting that bis(thioacyl) disulfides were obtained by oxidation of the corresponding carbodithionate salts, and it is necessary to remove the residual of oxidants.…”