Designing and Refining Ni(II)diimine Catalysts Toward the Controlled Synthesis of Electron-Deficient Conjugated Polymers

Intramolecular Palladium Catalyst Transfer on Benzoheterodiazoles as Acceptor Monomers and Discovery of Catalyst Transfer Inhibitors

Intramolecular catalyst transfer on benzoheterodiazoles was investigated in Suzuki-Miyaura coupling reactions and polymerization reactions with t Bu3 PPd precatalyst. In the coupling reactions of dibromobenzotriazole, dibromobenzoxazole, and dibromobenzothiadiazole with pinacol phenylboronate, the product ratios of monosubstituted product to disubstituted product were 0/100, 27/73, and 89/11, respectively, indicating that the Pd catalyst undergoes intramolecular catalyst transfer on dibromobenzotriazole, whereas intermolecular transfer occurs in part in the case of dibromobenzoxazole and is predominant for dibromobenzothiadiazole. The polycondensation of 1.3 equivalents of dibromobenzotriazole with 1.0 equivalent of para- and meta-phenylenediboronates afforded high-molecular-weight polymer and cyclic polymer, respectively. In the case of dibromobenzoxazole, however, para- and meta-phenylenediboronates afforded moderate-molecular-weight polymer with bromine at both ends and cyclic polymer, respectively. In the case of dibromobenzothiadiazole, they afforded low-molecular-weight polymers with bromine at both ends. Addition of benzothiadiazole derivatives interfered with catalyst transfer in the coupling reactions.

Side-Chain Engineering in Hydrophilic n-Type π-Conjugated Polymers for Enhanced Reactivity

Minor changes to side chains in conjugated polymers (CPs) can have pronounced effects on polymer properties by altering backbone planarity, solubility, and interaction with ions. Here, we report the photocontrolled synthesis of hydrophilic CPs from Grignard monomers and find that switching from alkyl to oligo(ethylene glycol) (OEG) side chains changes their photoreactivity. Specifically, installing hydrophilic side chains on the same monomer core yields higher molecular weight polymers and allows polymerization to proceed with lower-energy red light. Additionally, we discover a side chain decomposition pathway for N-OEG monomers, which are prevalent in CP research. Decomposition can be overcome by adding an extra methylene unit in the side chains without compromising polymer molecular weight or hydrophilicity. Importantly, this polymerization does not require transition metal catalysts and is a promising approach to the preparation of n-type conjugated block copolymers.

Recent advances in n-type and ambipolar organic semiconductors and their multi-functional applications

Organic semiconductors have received broad attention and research interest due to their unique integration of semiconducting properties with structural tunability, intrinsic flexibiltiy and low cost. In order to meet the requirements of organic electronic devices and their integrated circuits, p-type, n-type and ambipolar organic semiconductors are all necessary. However, due to the limitation in both material synthesis and device fabrication, the development of n-type and ambipolar materials is quite behind that of p-type materials. Recent development in synthetic methods of organic semiconductors greatly enriches the range of n-type and ambipolar materials. Moreover, the newly developed materials with multiple functions also put forward multi-functional device applications, including some emerging research areas. In this review, we give a timely summary on these impressive advances in n-type and ambipolar organic semiconductors with a special focus on their synthesis methods and advanced materials with enhanced properties of charge carrier mobility, integration of high mobility and strong emission and thermoelectric properties. Finally, multi-functional device applications are further demonstrated as an example of these developed n-type and ambipolar materials.

Rethinking Catalyst Trapping in Ni-Catalyzed Thieno[3,2-b]thiophene Polymerization

Catalyst-transfer polymerization (CTP) is a chain-growth method used to synthesize conjugated polymers. Although CTP works well for most donor-type monomers, the polymerization stalls with thieno[3,2-b]thiophene when using Ni catalysts. Previous reports have rationalized this result by suggesting that the catalyst is trapped in a Ni0 π-complex with the highly electron-rich arene. In this study, evidence is provided that the catalyst trap is more likely a NiII complex that arises from oxidative insertion of Ni0 into the C-S bonds of thieno[3,2-b]thiophene. This result is consistent with the known reactivity of Ni0 complexes toward S-heteroarenes and is supported herein by 31P nuclear magnetic resonance spectra acquired in situ, as well as data collected from small-molecule model reactions and density-functional theory simulations of the polymerization. We propose that this C-S insertion pathway and related off-cycle reactions may be relevant to understanding or enabling the CTP of other monomers with fused thiophenes.

Precision Synthesis of Conjugated Polymers Using the Kumada Methodology

Since the discovery of conductive poly(acetylene), the study of conjugated polymers has remained an active and interdisciplinary frontier between polymer chemistry, polymer physics, computation, and device engineering. One of the ultimate goals of polymer science is to reliably synthesize structures, similar to small molecule synthesis. Kumada catalyst-transfer polymerization (KCTP) is a powerful tool for synthesizing conjugated polymers with predictable molecular weights, narrow dispersities, specific end groups, and complex backbone architectures. However, expanding the monomer scope beyond the well-studied 3-alkylthiophenes to include electron-deficient and complex heterocycles has been difficult. Revisiting the successful applications of KCTP can help us gain new insight into the CTP mechanisms and thus inspire breakthroughs in the controlled polymerization of challenging π-conjugated monomers.In this Account, we highlight our efforts over the past decade to achieve controlled synthesis of homopolymers (p-type and n-type), copolymers (diblock and statistical), and monodisperse high oligomers. We first give a brief introduction of the mechanism and state-of-the-art of KCTP. Since the extent of polymerization control is determined by steric and electronic effects of both the catalyst and monomer, the polymerization can be optimized by modifying monomer and catalyst structures, as well as finding a well-matched monomer-catalyst system. We discuss the effects of side-chain steric hindrance and halogens in the context of heavy atom substituted monomers. By moving the side-chain branch point one carbon atom away from the heterocycle to alleviate steric crowding and stabilize the catalyst resting state, we were able to successfully control the polymerization of new tellurophene monomers. Inspired by innocent role of the sterically encumbered 2-transmetalated 3-alkylthiophene monomer, we introduce the treatment of hygroscopic monomers with a bulky Grignard compound as a water-scavenger for the improved synthesis of water-soluble conjugated polymers. For challenging electron-deficient monomers, we discuss the design of new Ni(II)diimine catalysts with electron-donating character which enhance the stability of the association complex between the catalyst and the growing polymer chain, resulting in the quasi-living synthesis of n-type polymers. Beyond n-type homopolymers, the Ni(II)diimine catalysts are also capable of producing electron-rich and electron-deficient diblock and statistical copolymers. We discuss how density functional theory (DFT) calculations elucidate the role of catalyst steric and electronic effects in controlling the synthesis of π-conjugated polymers. Moreover, we demonstrate the synthesis of monodisperse high oligomers by temperature cycling, which takes full advantage of the unique character of KCTP in that it proceeds through distinct intermediates that are not reactive. The insight we gained thus far leads to the first example of isolated living conjugated polymer chains prepared by a standard KCTP procedure, with general applicability to different monomers and catalytic systems. In summarizing a decade of innovation in KCTP, we hope this Account will inspire future development in the field to overcome key challenges including the controlled synthesis of electron-deficient heterocycles, complex and high-performance systems, and degradable and recyclable materials as well as cutting-edge catalyst design.

Redox‐active BIAN‐based Diimine Ligands in Metal‐Catalyzed Small Molecule Syntheses**

α‐Diimine ligands have significantly shaped the coordination chemistry of most transition metal complexes. Among them, bis(imino)acenaphthene ligands (BIANs) have recently been matured to great versatility and applicability to catalytic reactions. Besides variations of the ligand periphery, the great versatility of BIAN ligands resides within their ability to undergo facile electronic manipulations. This review highlights key aspects of BIAN ligands in metal complexes and summarizes recent contributions of metal‐BIAN catalysts to syntheses of small and functionalized organic molecules.

Universal Suzuki-Miyaura Catalyst-Transfer Polymerization for Precision Synthesis of Strong Donor/Acceptor-Based Conjugated Polymers and Their Sequence Engineering

Catalyst-transfer polymerization has revolutionized the field of polymer synthesis due to its living character, but for a given catalyst system, the polymer scope is rather narrow. Herein we report a highly efficient Suzuki-Miyaura catalyst-transfer polymerization (SCTP) that covers a wide range of monomers from electron-rich (donor, D) to electron-deficient (acceptor, A) (hetero)arenes by rationally designing boronate monomers and using commercially available Buchwald RuPhos and SPhos Pd G3 precatalysts. Initially, we optimized the controlled polymerization of 3,4-propylenedioxythiophene (ProDOT), benzotriazole (BTz), quinoxaline (QX), and 2,3-diphenylquinoxaline (QXPh) by introducing new boronates, such as 4,4,8,8-tetramethyl-1,3,6,2-dioxazaborocane and its N-benzylated derivative, to modulate the reactivity and stability of the monomers. As a result, PProDOT, PBTz, PQX, and PQXPh were prepared with controlled molecular weight and narrow dispersity (Đ < 1.29) in excellent yield (>85%). A detailed investigation of the polymer structures using ¹H NMR and MALDI-TOF spectrometry supported the chain-growth mechanism and the high initiation efficiency of the SCTP method. In addition, the use of RuPhos-Pd showing excellent catalyst-transfer ability on both D/A monomers led to unprecedented controlled D-A statistical copolymerization, thereby modulating the HOMO energy level (from -5.11 to -4.80 eV) and band gap energy (from 1.68 to 1.91 eV) of the resulting copolymers. Moreover, to demonstrate the living nature of SCTP, various combinations of D-A and A-A block copolymers (PBTz-b-PProDOT, PQX-b-PProDOT, and PQX-b-PBTz) were successfully prepared by the sequential addition method. Finally, simple but powerful one-shot D-A block copolymerization was achieved by maximizing the rate difference between a fast-propagating pinacol boronate donor and a slow-propagating acceptor to afford well-defined poly(3-hexylthiophene)-b-poly(benzotriazole).

Precision synthesis of a fluorene-thiophene alternating copolymer by means of the Suzuki–Miyaura catalyst-transfer condensation polymerization: the importance of the position of an alkyl substituent on thiophene of the biaryl monomer to suppress disproportionation

The Suzuki–Miyaura coupling polymerization of PinB-F8T(3)-Br was accompanied by disproportionation, whereas that of PinB-F8T(4)-Br proceeded in a chain-growth polymerization manner to afford a well-defined fluorene-thiophene alternating copolymer.

Pairing Suzuki–Miyaura cross-coupling and catalyst transfer polymerization

Borylation strategies to make AB Suzuki–Miyaura monomers for use in catalyst-transfer polymerization with nickel or palladium catalysts.

Chain-Growth Polymerization of Benzotriazole Using Suzuki-Miyaura Cross-Coupling and Dialkylbiarylphosphine Palladium Catalysts

Electron-deficient (n-type) conjugated materials are commonly prepared via step-growth methods with limited control over the molecular weight and molecular weight distribution of the resulting polymers. In this communication, we demonstrate that Pd-dialkylbiarylphosphine catalysts enable the chain-growth polymerization of benzo[1,2,3]triazole using Suzuki-Miyaura coupling with molecular weight control and modest molecular weight distributions (Đ ∼ 1.2-1.6). The importance of a free ligand in the reaction mixture during polymerization was established by analysis of polymer samples using GPC and MALDI-TOF mass spectrometry. A block copolymer with poly(3-hexylthiophene) was also synthesized by sequential monomer addition. The success of these commercially available catalysts for polymerization of benzotriazole highlights their potential for chain-growth reactions with other bicyclic arenes in the future.

Photocontrolled Synthesis of n-Type Conjugated Polymers

Current approaches to synthesize π-conjugated polymers (CPs) are dominated by thermally driven, transition-metal-mediated reactions. Herein we show that electron-deficient Grignard monomers readily polymerize under visible-light irradiation at room temperature in the absence of a catalyst. The product distribution can be tuned by the wavelength of irradiation based on the absorption of the polymer. Conversion studies are consistent with an uncontrolled chain-growth process; correspondingly, chain extension produces all-conjugated n-type block copolymers. Preliminary results demonstrate that the polymerization can be expanded to donor-acceptor alternating copolymers. We anticipate that this method can serve as a platform to access new architectures of n-type CPs without the need for transition-metal catalysis.

Recent developments in the synthesis of regioregular thiophene-based conjugated polymers for electronic and optoelectronic applications using nickel and palladium-based catalytic systems

Thiophene-based conjugated polymers are important conjugated polymers due to their exceptional optical and conductive properties, over the past few decades many researchers have designed novel strategies to reach more efficient materials for electronic applications.

Homogenous Synthesis of Monodisperse High Oligomers of 3-Hexylthiophene by Temperature Cycling

Whereas monodisperse polymers are ubiquitous in Nature, they remain elusive to synthetic chemists. Absolute control over polymer length and structure is essential to imparting chemical functionality, reproducible properties, and specific solid-state behavior. Precise polymer length has proven to be extremely difficult to control. The most successful examples are generally similar to solid-phase oligo nucleotide or peptide synthesis, wherein the polymer is built up one unit at a time with each sequential monomer addition requiring purification and deprotection (or other functional group activation) step. We have discovered a stepwise homogeneous catalyst-transfer polymerization to prepare monodisperse oligo(3-hexylthiophene) using temperature to limit additions to one unit per chain per cycle. This is the first reported example of a one-pot synthesis of monodisperse oligomers that requires no additional purification or intermediate steps. It is our hope that the strategy of temperature cycling to "freeze" intermediates will be generalizable to other living polymerization techniques, such as other catalyst-transfer polymerization systems, and those where a resting state involves an association between the catalyst and growing chain.

The role of halogens in the catalyst transfer polycondensation for π-conjugated polymers

Catalyst transfer polycondensation is the only method to prepare π-conjugated polymers in a chain-growth manner, yet several aspects that underlie this polymerization are not fully understood. Here, we investigate the nickel-catalyzed polymerization mechanisms of a series of thiophene monomers bearing different halogen functionalities (Cl, Br, I). We have discovered the significant role that halogens and magnesium salts play in this polymerization. More specifically, the catalyst resting state changes depending on the type of halogenated monomer. For chlorinated monomers a mixture of Ni(ii)-dithienyl and dissociated Ni(phosphine) complexes are the resting states, which results in uncontrolled polymerization. For brominated monomers, a Ni(ii)-dithienyl complex is the resting state, which leads to controlled polymerization. For iodinated monomers, a Ni(ii)-thienyl iodide complex is the resting state, and notable inhibition by magnesium salt by-products is observed. The catalyst resting state changes to a Ni(ii)-dithienyl complex when a turbo Grignard reagent (i-PrMgCl·LiCl) is used. These findings are used to guide the design of a new monomer, 2-bromo-3-(2-ethylhexyl)-5-iodotellurophene, which enables the first controlled polymerization of a tellurophene monomer containing a sterically encumbered 2-ethylhexyl side chain. These insights are crucial for deepening the mechanistic understanding of Kumada cross coupling reactions and the controlled synthesis of π-conjugated polymers.

Evaluation of an external initiating Ni(ii) diimine catalyst for electron-deficient π-conjugated polymers

We have prepared, isolated, and evaluated the first Ni(ii) diimine catalyst able to externally initiate the Kumada catalyst transfer polymerization of an electron-deficient π-conjugated monomer.

Catalyst-dependent intrinsic ring-walking behavior on π-face of conjugated polymers

The balance between the ring-walking process and the oxidative-addition state are key determinants of catalyst mobility in catalyst-transfer condensation polymerization.

Impact of Preferential π-Binding in Catalyst-Transfer Polycondensation of Thiazole Derivatives

Polymerizing electron-deficient arenes in a controlled, chain-growth fashion remains a significant challenge despite a decade of research on catalyst-transfer polycondensation. The prevailing hypothesis is that the chain-growth mechanism stalls at a strongly associated metal-polymer π-complex, preventing catalyst turnover. To evaluate this hypothesis, we performed mechanistic studies using thiazole derivatives and identified approaches to improve their chain-growth polymerization. These studies revealed a surprisingly high barrier for chain-walking toward the reactive C-X bond. In addition, a competitive pathway involving chain-transfer to monomer was identified. This pathway is facilitated by ancillary ligand dissociation and N-coordination to the incoming monomer. We found that this chain-transfer pathway can be attenuated by using a rigid ancillary ligand, leading to an improved polymerization. Combined, these studies provide mechanistic insight into the challenges associated with electron-deficient monomers as well as ways to improve their living, chain-growth polymerization. Our mechanistic studies also revealed an unexpected radical anion-mediated oligomerization in the absence of catalyst, as well as a surprising oxidative addition into the thiazole C-S bond in a model system.

Matchmaking in Catalyst-Transfer Polycondensation: Optimizing Catalysts based on Mechanistic Insight

Catalyst-transfer polycondensation (CTP) has emerged as a useful living, chain-growth polymerization method for synthesizing conjugated (hetero)arene-based polymers with targetable molecular weights, narrow dispersities, and controllable copolymer sequences-all properties that significantly influence their performance in devices. Over the past decade, several phosphine- and carbene-ligated Ni- and Pd-based precatalysts have been shown to be effective in CTP. One current limitation is that these traditional CTP catalysts lead to nonliving, non-chain-growth behavior when complex monomer scaffolds are utilized. Because these monomers are often found in the highest-performing materials, there is a significant need to identify alternative CTP catalysts. Recent mechanistic insight into CTP has laid the foundation for designing new catalysts to expand the CTP monomer scope. Building off this insight, we have designed and implemented model systems to identify effective catalysts by understanding their underlying mechanistic behaviors and systematically modifying catalyst structures to improve their chain-growth behavior. In this Account, we describe how each catalyst parameter-the ancillary ligand(s), reactive ligand(s), and transition metal-influences CTP. As an example, ancillary ligands often dictate the turnover-limiting step of the catalytic cycle, and perhaps more importantly, they can be used to promote the formation of the key intermediate (a metal-arene associative complex) and its subsequent reactivity. The fidelity of this intermediate is central to the mechanism for the living, chain-growth polymerization. Reactive ligands, on the other hand, can be used to improve catalyst solubility and accelerate initiation. Additional advantages of the reactive ligand include providing access points for postpolymerization modification and synthesizing polymers directly off surfaces. While the most frequently used CTP catalysts contain nickel, palladium-based catalysts exhibit a higher functional group tolerance and broader substrate scope (e.g., monomers with boron, magnesium, tin, and gold transmetalating agents). Overall, we anticipate that applying the tools and lessons detailed in this Account to other monomers should facilitate a better "matchmaking" process that will lead to new catalyst-transfer polycondensations.

Nickel(ii) complexes chelated by 2,6-pyridinedicarboxamide: syntheses, characterization, and ethylene oligomerization

Ni(ii) complexes chelated by a neutral tridentate amide ligand are conveniently prepared and are highly active for ethylene oligomerization.

Transition-metal-free controlled polymerization of 2-polyfluorophenyl-5-trimethylsilylthiophenes: the substituent impact of fluorine

A transition-metal-free polymerization of a series of 2-polyfluorophenyl-5-trimethylsilylthiophenes promoted by fluoride anions is reported.

Bulky α-diimine palladium complexes: highly efficient for direct C–H bond arylation of heteroarenes under aerobic conditions

Through the strategy to enhance the bulkiness on both the backbone and the N-aryl moieties, we designed and synthesized a type of bulky α-diimine palladium complex (i.e., {[Ar–NC(R)–C(R)N–Ar]PdCl₂, (Ar = 2-benzhydryl-4,6-dimethylphenyl)}, C1, R = H; C2, R = An; C3, R = Ph).