Selective Photoactivation: From a Single Unit Monomer Insertion Reaction to Controlled Polymer Architectures

Abstract

Here, we exploit the selectivity of photoactivation of thiocarbonylthio compounds to implement two distinct organic and polymer synthetic methodologies: (1) a single unit monomer insertion (SUMI) reaction and (2) selective, controlled radical polymerization via a visible-light-mediated photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) process. In the first method, precise single unit monomer insertion into a dithiobenzoate with a high reaction yield (>97%) is reported using an organic photoredox catalyst, pheophorbide a (PheoA), under red light irradiation (λmax = 635 nm, 0.4 mW/cm(2)). The exceptional selectivity of PheoA toward dithiobenzoate was utilized in combination with another catalyst, zinc tetraphenylporphine (ZnTPP), for the preparation of a complex macromolecular architecture. PheoA was first employed to selectively activate a dithiobenzoate, 4-cyanopentanoic acid dithiobenzoate, for the polymerization of a methacrylate backbone under red light irradiation. Subsequently, metalloporphyrin ZnTPP was utilized to selectively activate pendant trithiocarbonate moieties for the polymerization of acrylates under green light (λmax = 530 nm, 0.6 mW/cm(2)) to yield well-defined graft co-polymers.

Unraveling Photocatalytic Mechanism and Selectivity in PET‐RAFT Polymerization

“…There are three distinct energy transfer mechanisms that could potentially mediate the RAFT agent activation for subsequent polymerization: i) Förster resonant energy transfer from photocatalyst to RAFT agent. [4] The initial conditions are set up so as to mimic the highly vibrationally excited photocatalyst (i.e., after transition from the excited electronic state to the ground state) in an ambient-temperature bath consisting of one adjacent RAFT agent and several solvent molecules. ii) Dexter electron exchange between photocatalyst and RAFT agent.…”

“…This is usually done through complex photoredox and energy transfer processes and has offered a profound challenge to researchers [1][2][3] who tried to model this natural phenomenon. [4][5][6][7][8][9][10][11] As the name suggests, these photoredox catalysts harnesses the energy of visible light to accelerate a chemical reaction through electron transfer processes. Over the subsequent century, interest has grown in finding new systems that are capable of absorbing light and mediating chemical reactions for the production of fine chemicals and advanced materials.…”

“…[4][5][6][7][8][9][10][11] As the name suggests, these photoredox catalysts harnesses the energy of visible light to accelerate a chemical reaction through electron transfer processes. [5,[14][15][16][17][18][19][20][21][22][23] Inspired by this, Boyer and co-workers have recently utilized photoredox catalysts, [4,8,17,18] such as pheophorbide a (PheoA) and zinc tetraphenylporphine (ZnTPP), to mediate photoinduced electron/energy transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization (Scheme 1). [12] The property of compatibility has led to some recent developments in the field of visible-light photocatalysis, where different catalysts are utilized to perform complex organic reactions in a single pot.…”

“…benzodithioate: CPD, cumyl dithiobenzoate: CDB), xanthate (methyl 2-[(ethoxycarbonothioyl)sulfanyl] propanoate: Xanthate), and trithiocarbonate (3-benzylsulfanyl-thiocarbonylthiosulfanyl propionic acid: BSTP, 2-(n-butyltrithiocarbonate)-propionic acid: BTPA) moieties, the details of which are given specifically in Figure 1c-h, PheoA displays exceptional catalytic selectivity toward CPADB compared to either xanthates or trithiocarbonates; [4] whereas ZnTPP selectively activates the trithiocarbonates for the polymerization of methacrylates, styrenes, etc. [17,29,30] Two important mechanistic questions are thrown up by these developments: i) Ought the "ET" in PET-RAFT to stand for electron transfer (Scheme 1A) or energy transfer (Scheme 1B)?…”

Organocatalyzed Photocontrolled Radical Polymerization of Semifluorinated (Meth)acrylates Driven by Visible Light

“…Furthermore,c ontrol of the polymerization of semifluorinated monomers with an external stimulus, [8] such as visible light, has rarely been possible. [5] Recently,p hotoredox catalysis has found widespread application in ATRP, [9] RAFT/iniferter polymerization, [10] ring-opening metathesis polymerization, [11] and cationic polymerization, [12] thus providing arange of photo-CRP methods with excellent spatiotemporal control over chain growth. [13] Forthese processes,(meth)acrylates,acrylamides,and styrene derivatives are the most frequently investigated monomers.…”