Development of an Arylthiobismuthine Cocatalyst in Organobismuthine-Mediated Living Radical Polymerization: Applications for Synthesis of Ultrahigh Molecular Weight Polystyrenes and Polyacrylates

Bismuth in Radical Chemistry and Catalysis

Whereas indications of radical reactivity in bismuth compounds can be traced back to the 19th century, the preparation and characterization of both transient and persistent bismuth-radical species has only been established in recent decades. These advancements led to the emergence of the field of bismuth radical chemistry, mirroring the progress seen for other main-group elements. The seminal and fundamental studies in this area have ultimately paved the way for the development of catalytic methodologies involving bismuth-radical intermediates, a promising approach that remains largely untapped in the broad landscape of synthetic organic chemistry. In this review, we delve into the milestones that eventually led to the present state-of-the-art in the field of radical bismuth chemistry. Our focus aims at outlining the intrinsic discoveries in fundamental inorganic/organometallic chemistry and contextualizing their practical applications in organic synthesis and catalysis.

Organocatalyzed Photo-Controlled Synthesis of Ultrahigh-Molecular-Weight Fluorinated Alternating Copolymers

Ultrahigh-molecular-weight (UHMW) polymers with tailored structures are highly desirable for the outstanding properties. In this work, we developed a novel photoorganocatalyzed controlled radical alternating copolymerizations of fluoroalkyl maleimide and diverse vinyl comonomers, enabling efficient preparation of fluorinated copolymers of predetermined UHMWs and well-defined structures at high conversions. Versatility of this method was demonstrated by expanding to controlled terpolymerization, which allows facial access toward fluorinated terpolymers of UHMWs and functional pendants. The obtained copolymers exhibited attractive physical properties and furnished thermoplastic, anticorrosive and (super)hydrophobic attributes as coatings on different substrates. Molecular simulations provided insights into the coating morphology, which unveiled a fluorous protective layer on the top surface with polar groups attached to the bottom substrate, resulting in good adhesion and hydrophobicity, simultaneously. This synthetic method and customized copolymers shed light on the design of high-performance coatings by macromolecular engineering.

Synthesis, Isolation, and Characterization of Two Cationic Organobismuth(II) Pincer Complexes Relevant in Radical Redox Chemistry

Herein, we report the synthesis, isolation, and characterization of two cationic organobismuth(II) compounds bearing N,C,N pincer frameworks, which model crucial intermediates in bismuth radical processes. X-ray crystallography uncovered a monomeric Bi(II) structure, while SQUID magnetometry in combination with NMR and EPR spectroscopy provides evidence for a paramagnetic S = 1/2 state. High-resolution multifrequency EPR at the X-, Q-, and W-band enable the precise assignment of the full g- and ²⁰⁹Bi A-tensors. Experimental data and DFT calculations reveal both complexes are metal-centered radicals with little delocalization onto the ligands.

Controlled Synthesis of High-Molecular-Weight Polystyrene and Its Block Copolymers by Emulsion Organotellurium-Mediated Radical Polymerization

Structurally controlled high-molecular-weight (HMW) polystyrenes (PSts) and block copolymers consisting of HMW PSt segments were successfully synthesized by emulsion organotellurium-mediated radical polymerization (TERP). The hydrophilicity of the organotellurium group of TERP chain transfer agents (CTAs) was important for success, and CTAs 1b and 1c with di- and tetraethylene glycol units were suitable. By using 1b and 1c and using hexadecyltrimethylammonium bromide (CTAB) as the surfactant, PSts with MWs over 1 million and with low dispersity (Đ < 1.6) were synthesized with >96% monomer conversion. Because of the high monomer conversion, high end-group fidelity, and rapid monomer diffusion to polymer particles, HMW block copolymers with low dispersity were successfully synthesized by adding a second monomer after converting the first monomer without isolating the macroinitiators. Despite recent developments in reversible-deactivation radical polymerization (RDRP), the synthesis of HMW polymers, particularly PSts and block copolymers, has been a formidable challenge. This method provides a valuable route for fabricating polymer materials based on HMW PSts.

Organopnictogen(III) bis(arylthiolates) containing NCN-aryl pincer ligands: from synthesis and characterization to reactivity

Salt elimination reactions between organopnictogen(III) dichlorides, RPnCl₂ [R¹ = 2,6-(Me₂NCH₂)₂C₆H₃, Pn = Sb (1), Bi (2); R² = 2,6-{MeN(CH₂CH₂)₂NCH₂}₂C₆H₃, Pn = Sb (3), Bi (4); R³ = 2,6-{O(CH₂CH₂)₂NCH₂}₂C₆H₃, Pn = Sb (5), Bi (6)] and 2 equivalents of KSC₆H₃Me₂-2,6 afforded the isolation of a series of new NCN-chelated monoorganopnictogen(III) bis(arylthiolates), RPn(SC₆H₃Me₂-2,6)₂ [R¹, Pn = Sb (7), Bi (8); R², Pn = Sb (9), Bi (10); R³, Pn = Sb (11), Bi (12)]. Compounds 7 and 8 are unstable upon exposure to a dry O₂ atmosphere and their aerobic decomposition yields the monoorganopnictogen(III) oxides, cyclo-[2,6-(Me₂NCH₂)₂C₆H₃Pn(μ-O)]₂ [Pn = Sb (13), Bi (14)] with concomitant formation of the corresponding disulfide, ArS-SAr (Ar = C₆H₃Me₂-2,6). The oxidative addition of elemental sulfur or selenium to 7 undergoes a similar reaction path and gives stable heterocyclic species cyclo-[2,6-(Me₂NCH₂)₂C₆H₃Sb(μ-E)]₂ [E = S (15), Se (16)]. The reaction of 12 with I₂ (1 : 1 molar ratio) gives the diiodide [2,6-{O(CH₂CH₂)₂NCH₂}₂C₆H₃]BiI₂ (17), along with the S-S oxidative coupling by-product, ArS-SAr. The use of an excess of iodine affords the crystallization of a 2 : 1 iodine adduct of 17 (17·0.5I₂), built through halogen bonding. All new compounds were characterized by multinuclear NMR spectroscopy and ESI-MS as well as single crystal X-ray diffraction (except compounds 9 and 10).

Bismuth Redox Catalysis: An Emerging Main-Group Platform for Organic Synthesis

Bismuth has recently been shown to be able to maneuver between different oxidation states, enabling access to unique redox cycles that can be harnessed in the context of organic synthesis. Indeed, various catalytic Bi redox platforms have been discovered and revealed emerging opportunities in the field of main group redox catalysis. The goal of this perspective is to provide an overview of the synthetic methodologies that have been developed to date, which capitalize on the Bi redox cycling. Recent catalytic methods via low-valent Bi(II)/Bi(III), Bi(I)/Bi(III), and high-valent Bi(III)/Bi(V) redox couples are covered as well as their underlying mechanisms and key intermediates. In addition, we illustrate different design strategies stabilizing low-valent and high-valent bismuth species, and highlight the characteristic reactivity of bismuth complexes, compared to the lighter p-block and d-block elements. Although it is not redox catalysis in nature, we also discuss a recent example of non-Lewis acid, redox-neutral Bi(III) catalysis proceeding through catalytic organometallic steps. We close by discussing opportunities and future directions in this emerging field of catalysis. We hope that this Perspective will provide synthetic chemists with guiding principles for the future development of catalytic transformations employing bismuth.

Two Faces of the Bi−O Bond: Photochemically and Thermally Induced Dehydrocoupling for Si−O Bond Formation

The diorgano(bismuth)alcoholate [Bi((C₆H₄CH₂)₂S)OPh] (1‐OPh) has been synthesized and fully characterized. Stoichiometric reactions, UV/Vis spectroscopy, and (TD‐)DFT calculations suggest its susceptibility to homolytic and heterolytic Bi−O bond cleavage under given reaction conditions. Using the dehydrocoupling of silanes with either TEMPO or phenol as model reactions, the catalytic competency of 1‐OPh has been investigated (TEMPO=(tetramethyl‐piperidin‐1‐yl)‐oxyl). Different reaction pathways can deliberately be addressed by applying photochemical or thermal reaction conditions and by choosing radical or closed‐shell substrates (TEMPO vs. phenol). Applied analytical techniques include NMR, UV/Vis, and EPR spectroscopy, mass spectrometry, single‐crystal X‐ray diffraction analysis, and (TD)‐DFT calculations.

Molecular bismuth(iii) monocations: structure, bonding, reactivity, and catalysis

Structurally defined, molecular bismuth(iii) cations show remarkable properties in coordination chemistry, Lewis acidity, and redox chemistry, allowing for unique applications in synthetic chemistry.

Main-Group Metal Complexes in Selective Bond Formations Through Radical Pathways

Recent years have witnessed remarkable advances in radical reactions involving main-group metal complexes. This includes the isolation and detailed characterization of main-group metal radical compounds, but also the generation of highly reactive persistent or transient radical species. A rich arsenal of methods has been established that allows control over and exploitation of their unusual reactivity patterns. Thus, main-group metal compounds have entered the field of selective bond formations in controlled radical reactions. Transformations that used to be the domain of late transition-metal compounds have been realized, and unusual selectivities, high activities, as well as remarkable functional-group tolerances have been reported. Recent findings demonstrate the potential of main-group metal compounds to become standard tools of synthetic chemistry, catalysis, and materials science, when operating through radical pathways.

100th Anniversary of Macromolecular Science Viewpoint: Achieving Ultrahigh Molecular Weights with Reversible Deactivation Radical Polymerization

Synthetic strategies for achieving ultrahigh molecular weights via reversible deactivation radical polymerization are discussed from the mechanistic, kinetic, and experimental aspects, and their applications as high-performance materials are highlighted. Further development of this field requires continuous effort to improve livingness and polymerization efficiency under greener conditions.

Polymer grafting on graphene layers by controlled radical polymerization

In situ controlled radical polymerization (CRP) is considered as an important approach to graft polymer brushes with controlled grafting density, functionality, and thickness on graphene layers. Polymers are tethered with chain end or through its backbone to the surface or edge of graphene layers with two in situ polymerization methods of "grafting from" and "grafting through" and also a method based on coupling reactions known as "grafting to". The "grafting from" method relies on the propagation of polymer chains from the surface- or edge-attached initiators. The "grafting through" method is based on incorporation of double bond-modified graphene layers into polymer chains through the propagation reaction. The "grafting to" technique involves attachment of pre-fabricated polymer chains to the graphene substrate. Here, physical and chemical attachment approaches are also considered in polymer-modification of graphene layers. Combination of CRP mechanisms of reversible activation, degenerative (exchange) chain transfer, atom transfer, and reversible chain transfer with various kinds of grafting reactions makes it possible to selectively functionalize graphene layers. The main aim of this review is assessment of the recent advances in the field of preparation of polymer-grafted graphene substrates with well-defined polymers of controlled molecular weight, thickness, and polydispersity index. Study of the opportunities and challenges for the future works in controlling of grafting density, site-selectivity in grafting, and various topologies of the brushes with potential applications in stimuli-responsive surfaces, polymer composites, Pickering emulsions, coating technologies, and sensors is also considered.

Photoinduced controlled radical polymerization of methyl acrylate and vinyl acetate by xanthate

A block copolymer of PMA-b-PVAc was successfully synthesized using photo-induced RAFT polymerization with a xanthate.

Reversible chain transfer catalyzed polymerization (RTCP) in nitrogen-based solvents without additional catalysts

N,N-Dimethylformamide (DMF) andN-methyl-2-pyrrolidone (NMP) as typical nitrogen-based solvents were used as the catalyst for RTCP without additional catalyst, which could also be carried out in the presence of a limited amount of air.

Highly Controlled Organotellurium-Mediated Living Radical Polymerization (TERP) in Ionic Liquids (ILs). The New Role of ILs in Radical Reactions

Ionic liquids (ILs) served perfectly as solvents for the organotellurium-mediated living radical polymerization (TERP) of methyl methacrylate (MMA), methyl acrylate (MA), and styrene. The reaction rate of polymerizing MMA and MA was significantly increased as previously reported, and the controllability of the polydispersity index (PDI) was also improved by a great margin. The TERP of MMA can now give poly(methyl methacrylates) (PMMAs) with PDIs less than 1.1 and nearly full conversion in a half hour without the presence of (TeMe)₂. The kinetic study revealed that the improved control could be ascribed to a faster degenerative chain transfer (DT) reaction which plays a key role in the control of PDI for TERP. Besides the polar effect of ILs, the existence of Lewis acid-base interaction between ILs and the Te atom was proven by UV-vis spectroscopy and ¹²⁵Te NMR results. Such positive interaction lowered the activation energy of the DT process.

Synergistic Interaction Between ATRP and RAFT: Taking the Best of Each World

This review covers recent developments on the combination of atom transfer radical polymerization (ATRP) and reversible addition–fragmentation chain transfer (RAFT) polymerization to produce well controlled (co)polymers. This review discusses the relative reactivity of the R group in ATRP and RAFT, provides a comparison of dithiocarbamate (DC), trithiocarbonate (TTC), dithioester (DTE), and xanthate versus bromine or chlorine, and an optimization of catalyst/ligand selection. The level of control in iniferter polymerization with DC was greatly improved by the addition of a copper complex. New TTC inifers with bromopropionate and bromoisobutyrate groups have been prepared to conduct, concurrently or sequentially, ATRP from Br-end groups, ATRP from the TTC moiety, and RAFT polymerization from the TTC moiety, depending on the combination of monomer and catalyst employed in the reaction. The use of concurrent ATRP/RAFT (or copper-catalyzed RAFT polymerization or ATRP with dithioester leaving groups), resulted in improved control over the synthesis of homo- and block (co)polymers and allowed preparation of well-defined high-molecular-weight polymers exceeding 1 million. Block copolymers that could not be prepared previously have been synthesized by sequential ATRP and RAFT polymerization using a bromoxanthate inifer. A simple, versatile, and one-step method involving atom-transfer radical addition–fragmentation (ATRAF) for the preparation of various chain transfer agents (including DC, DTE, and xanthate) in high purity is discussed and a one-pot, two-step polymerization starting with a RAFT agent synthesized by ATRAF, followed by polymerization, is demonstrated.