Synthesis Of Conjugated Polymers Research Articles

Room Temperature Synthesis of a Well-Defined Conjugated Polymer Using Direct Arylation Polymerization (DArP).

While a major improvement to the sustainability of conjugated polymer synthesis, traditional direct arylation polymerization (DArP) still requires high temperatures (typically >100 °C), necessitating a significant energy input requirement. Performing DArP at reduced or ambient temperatures would represent an improvement to the sustainability of the reaction. Here we describe the first report of a well-defined conjugated polymer synthesized by DArP at room temperature. Previous efforts toward room temperature DArP relied on the use of a near-stoichiometric silver reagent, an expensive coinage metal, which makes the reaction less cost-effective and sustainable. Here, room temperature polymerizations of 3,4-ethylenedioxythiophene (EDOT) and 9,9-dioctyl-2,7-diiodofluorene were optimized and provided molar mass (Mn) up to 11 kg/mol PEDOTF, and performing the reaction at the standard ambient temperature of 25 °C provided Mn up to 15 kg/mol. Model studies using other C-H monomers of varying electron density copolymerized with 9,9-dioctyl-2,7-diiodofluorene provided insight into the scope of the room temperature polymerization, suggesting that performing room temperature DArP is highly dependent on the electron richness of the C-H monomer.

Indophenine Reaction as a Method for Synthesizing Conjugated Polymers with Sub-1 eV Optical Bandgaps

Traditionally, conjugated polymer syntheses have relied heavily on metal-catalyzed cross-coupling reactions. These cross-couplings require appropriately functionalized monomers, increasing the length and complexity of the overall synthesis; in addition, some of the commonly used reagents (e.g., trialkyltin chlorides) are highly toxic. Even after a polymer has been successfully prepared, residual metal impurities can also compromise its optoelectronic properties and performance in devices. As such, there is a demand for metal-free approaches to conjugated polymer synthesis. In this work, the indophenine reaction was used to polymerize thiophene and bifunctional isatin comonomers, with only sulfuric acid used as the catalyst. The structural influence of the comonomers was evaluated using different thiophenes and bifunctional isatins. The resulting polymers have very wide absorption bands that span the UV/vis/NIR regions, with optical bandgaps as low as 0.76 eV. The polymers show electrochromic behavior, as determined through spectroelectrochemical measurements.

Precise control of conjugated polymer synthesis from step-growth polymerization to iterative synthesis

Conjugated polymers with excellent optoelectronic properties, easy processability, and flexible physical properties are versatile in various applications. In recent years, the precise synthesis of conjugated polymers has attracted much attention due to the close relationship between structure and property. From the perspective of the polymerization mechanism, the synthetic methods of conjugated polymers can be classified into step-growth polymerization, chain-growth polymerization, and iterative synthesis. From step-growth polymerization to iterative synthesis, polymer synthesis becomes more controllable. In this review, we introduce these three synthetic approaches to prepare conjugated polymers with controllable structures based on transition metal-catalyzed cross-coupling reactions and further reveal the control over chemical structures, including length, regioregularity, sequence, and topology. The precise control over the synthesis of conjugated polymer is of great significance for the synthesis of functional polymer with a specific structure, which paves a way for fabricating high-performance devices.

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.

A Chain‐Growth Mechanism for Conjugated Polymer Synthesis Facilitated by Dinuclear Complexes with Redox‐Active Ligands

Conjugated polymers are widely used in energy conversion and sensor applications, but their synthesis relies on imprecise step-growth or narrow-scope chain-growth methods, typically based on transition metal (TM)-catalyzed cross-coupling. Here we report that a dinickel complex with a redox-active naphthyridine diimine ligand accesses new chain-growth mechanistic manifolds for both donor and acceptor conjugated polymers, represented by poly(3-hexylthiophene), poly(2,3-bis(2-ethylhexyl)thienopyrazine), and poly(2-(2-octyldodecyl)benzotriazole). For the latter, our method is particularly effective: we achieve high degrees of polymerization (DP) (>100) with moderate dispersities (Đ) of ≈1.4. Mechanistic analysis supports a radical/radical anion chain-growth mechanism with organometallic intermediates instead of TM-catalyzed cross-couplings. Hence, our work develops new mechanisms for conjugated polymer synthesis and furnishes insights about the elementary reactivity of dinuclear complexes.

A Chain-Growth Mechanism for Conjugated Polymer Synthesis Facilitated by Dinuclear Complexes with Redox-Active Ligands.

Conjugated polymers are widely used in energy conversion and sensor applications, but their synthesis relies on imprecise step-growth or narrow-scope chain-growth methods, typically based on transition metal (TM)-catalyzed cross-coupling. Here we report that a dinickel complex with a redox-active naphthyridine diimine ligand accesses new chain-growth mechanistic manifolds for both donor and acceptor conjugated polymers, represented by poly(3-hexylthiophene), poly(2,3-bis(2-ethylhexyl)thienopyrazine), and poly(2-(2-octyldodecyl)benzotriazole). For the latter, our method is particularly effective: we achieve high degrees of polymerization (DP) (>100) with moderate dispersities (Đ) of ≈1.4. Mechanistic analysis supports a radical/radical anion chain-growth mechanism with organometallic intermediates instead of TM-catalyzed cross-couplings. Hence, our work develops new mechanisms for conjugated polymer synthesis and furnishes insights about the elementary reactivity of dinuclear complexes.

Circular Discovery in Small Molecule and Conjugated Polymer Synthetic Methodology.

Simple and efficient methods are a key consideration for small molecule and polymer syntheses. Direct arylation polymerization (DArP) is of increasing interest for preparing conjugated polymers as an effective approach compared to conventional cross-coupling polymerizations. As DArP sees broader utilization, advancements are needed to access materials with improved properties and different monomer structures and to improve the scalability of conjugated polymer synthesis. Presented herein are considerations for developing new methods of conjugated polymer synthesis from small molecule transformations, exploring how DArP has successfully used this approach, and presenting how emerging polymerization methodologies are developing similarly. While it is common to adapt small molecule methods to polymerizations, we demonstrate the ways in which information gained from studying polymerizations can inform and inspire greater advancements in small molecule transformations. This circular approach to organic synthetic method development underlines the value of collaboration between small molecule and polymer-based synthetic research groups.

The Reactivity of Ni Complexes in Conjugated Polymer Synthesis

The Reactivity of Ni Complexes in Conjugated Polymer Synthesis

Recycling Heterogenous Catalysts for Multi-Batch Conjugated Polymer Synthesis via Direct Arylation Polymerization.

Despite the inherent sustainability direct arylation polymerization (DArP) offers through a C-H activation pathway, the use of expensive homogeneous Pd catalysts remains problematic for large-scale conjugated polymer (CP) synthesis. Herein, the first report on the recycling of heterogeneous catalysts for CP synthesis using DArP is presented. We found SiliaCat Pd-DPP to be a highly efficient and recyclable catalyst for multi-batch CP synthesis providing CPs with molecular weights (Mn) up to 82 kg/mol even after being recycled three times. Batch-to-batch variations were further optimized to afford up to five batches of polymers with a Mn of 25 ± 2.5 kg/mol without structural disparity. Significantly, this work discloses among the most sustainable CP synthesis protocols to date and presents the critical concept of catalyst-recycling to the important field of organic semiconducting polymers, which potentially enables access to truly low-cost flow chemistry for industrial-scale CP synthesis.

Aldol Polymerization to Construct Half-Fused Semiconducting Polymers

Conventional conjugated polymer synthesis involves costly transition-metal catalysts and even toxic reagents, causing serious environmental challenges around the world. The comonomer linkages of semiconducting polymers are mostly single bonds; charge delocalization and transport have been hindered due to rotation around these single bonds, leading to energetic and conformational disorders. Here, we have developed a novel programmable half-fused polymer library by introducing aldehyde chemistry to promote a very simple and cheap Lewis acid-mediated aldol condensation that affords conjugated polymers with continuous double bonds and single bonds as the linkages between the polymer repeat units, maintaining a coplanar conjugated backbone to eliminate rotation owing to the intramolecular noncovalent interactions. In addition, this innovative concept not only shows an excellent substrate generality but also affords unprecedented polymers with unique properties, allowing us to change the variable conjugated length and side-chain engineering of the backbone to modulate the optoelectronic properties and tune the delocalization of frontier molecular orbitals. These polymers also exhibit very high number-averaged molecular weights (Mn) while showing good solution processability, which makes them applicable as high-performance thin-film transistor materials with reduced conformational disorder. Herein, our results demonstrate that the half-fused design strategy can effectively extend the scope of semiconducting polymers. The low synthetic complexity can be utilized to design organic electronic materials for large-scale production and commercialization.

Improving the efficiency and sustainability of catalysts for direct arylation polymerization (DArP)

AbstractIn the past decade, direct arylation polymerization (DArP) has rapidly developed as a sustainable synthetic protocol for cost‐effective, atom‐economical preparation of conjugated polymers. By circumventing monomer functionalization with toxic transmetallating reagents such as organostannane and organoboron required for Stille‐Migita and Suzuki‐Miyaura polymerization methods, DArP proceeds through a metal‐catalyzed CH activation pathway for the preparation of high‐performance conjugated polymer materials. This review evaluates the development of several classes of efficient catalysts/catalytic systems from small‐molecule studies to polymerizations, including the mechanisms involved in these transformations and how they inspire catalyst and monomer design for defect‐free conjugated polymer synthesis. Recent advances in developing more sustainable first‐row transition metal catalysts for DArP are also highlighted, and the fundamental understanding of these efficient and sustainable catalysts should motivate the pursuit for the next generation of catalytic design to enable more effective and environmentally friendly conjugated polymer synthesis.

P-Cymene: A Sustainable Solvent that is Highly Compatible with Direct Arylation Polymerization (DArP).

For over a decade, Direct Arylation Polymerization (DArP) has been demonstrated to be an eco-friendly, facile, and low-cost alternative to conventional methodologies such as Stille polymerization for conjugated polymer synthesis. By accessing through a C-H activation pathway, DArP offers a reduction of synthetic steps while eliminating the generation of stoichiometric, highly toxic organotin byproducts. However, as the major component in these reactions, the solvents most prevalently employed for DArP are hazardous and produced from unsustainable sources, such as dimethylacetamide (DMA), tetrahydrofuran (THF), and toluene. Although the use of sustainable alternative solvents such as 2-MeTHF and cyclopentyl methyl ether (CPME) has recently emerged, drawbacks of ethereal solvents include the need for a pressurized reaction setup as well as potential peroxide formation. While aromatic solvents are superior in solubilizing conjugated polymers, very little has been done in searching for more sustainable, benign alternatives for this class of solvent. Herein, we report the application of a sustainable, naturally sourced, high-boiling aromatic solvent, p-cymene, to DArP for the first time. p-Cymene was found to display excellent solubilizing ability in the synthesis of a broad scope of alternating copolymers with Mn up to 51.3 kg/mol and yields up to 96.2%, outperforming those prepared using CPME and toluene. Structural analysis revealed the exclusion of defects in these polymers prepared using p-cymene as the solvent which, in the case of a 2,2'-bithiophene monomer, is challenging to access through the use of conventional solvents for DArP, such as DMA and toluene.

Improving the Kumada Catalyst Transfer Polymerization with Water-Scavenging Grignard Reagents.

Conjugated polymers have received widespread interest as optoelectronic materials. Recently, these macromolecules have been adopted for biologically relevant applications, such as sensors, imaging agents, and drug delivery vectors. A major limitation of the chemistry used to prepare these classes of compounds is that the resultant polymers themselves are not tolerant to water or are not inherently water-soluble. For example, the most controlled method of conjugated polymer synthesis, the Kumada catalyst transfer polymerization (KCTP), requires stringent drying of monomers, catalysts, and other reagents. Here, we describe an approach to use a water-scavenging Grignard reagent to alleviate many of the shortcomings that currently hinder the synthesis of water-soluble conjugated polymers. This method shows improved polymerization performance in both traditional conjugated polymer synthesis as well as more challenging syntheses of polar hygroscopic polymers that are of interest for biological applications.

“In-water” direct arylation polymerization (DArP) under aerobic emulsion conditions

To address the issue of generating large amounts of organic waste from conjugated polymer synthesis, the first direct arylation polymerization (DArP) protocol under emulsion conditions is disclosed with a 10-fold reduction of organic solvent utilized.

Air‐tolerant poly(3‐hexylthiophene) synthesis via catalyst‐transfer polymerization

AbstractThe discovery of catalyst‐transfer polymerization and its further developments have led to unprecedented control over the length and sequence of conjugated polymers. However, the methods themselves are technically challenging to perform due to the air‐ and moisture‐sensitivities of the monomers and catalysts. Herein, we report a catalyst‐transfer polymerization method that affords poly(3‐hexylthiophene) in high yields without using an inert atmosphere. The synthesis capitalizes on a rapid Negishi cross‐coupling using a moisture‐tolerant organozinc monomer mediated by an air‐stable Pd precatalyst. This simple method should make conjugated polymer synthesis more accessible to a broader range of researchers and may be generalizable to other monomer scaffolds.

Approaches for improving the sustainability of conjugated polymer synthesis using direct arylation polymerization (DArP)

Emerging strategies to enhance the sustainability of Direct Arylation Polymerization (DArP) are discussed, illustrating the great potential of this method.

A critical review of reactive vapor deposition for conjugated polymer synthesis

A critical review of reactive vapor deposition as a tool for conjugated polymer synthesis.

CH Activation as a Shortcut to Conjugated Polymer Synthesis.

Direct (hetero)arylation polymerization exploits the palladium-catalyzed activation of aromatic CH bonds for the atom-economical synthesis of conjugated polymers for a wide range of applications. This account outlines how direct arylation methodologies overcome many of the limitations of contemporary polymerization techniques at both the research and production scale, and explains how monomer design and reaction conditions must be tailored to ensure high polymer molecular weight, yield, and structural integrity. Current research aims to improve further this reaction's profile as a sustainable methodology while at the same time making it competitive with the Migita-Stille and Miyaura-Suzuki polymerizations both in scope of accessible structures and synthetic efficiency. This feature article charts the recent developments and future directions of CH activation research as it moves toward becoming at once an industrially feasible, environmentally friendly, and synthetically powerful polymerization technique.

Interface Engineering Strategies for Fabricating Nanocrystal-Based Organic–Inorganic Nanocomposites

Hybrid organic–inorganic nanocomposites have attracted considerable attention because they have the advantages of both conjugated polymers (CPs) and nanocrystals (NCs). Recent developments in the interfacial engineering of CP–NC organic–inorganic nanocomposites enabled the formation of an intimate contact between NCs and CPs, facilitating electronic interactions between these two constituents. To design CP–NC nanocomposites, several approaches have been introduced, including ligand refluxing, direct grafting methods, direct growth of NCs in proximity to CPs, and template-guided strategies. In this review, the general reactions of ligand exchange processes, purification methods, and characterization techniques have been briefly introduced. This is followed by a highlight of recent advances in the synthesis of hybrid CP–NC nanocomposites and newly developed inorganic surface treatments, as well as their applications. An outlook for future directions in this area is also presented.

Elucidating the Role of Diphosphine Ligand in Nickel-Mediated Suzuki–Miyaura Polycondensation

Synthesizing conjugated polymers using controlled polymerization techniques offers tremendous flexibility in the preparation of well-defined semiconducting materials. Suzuki–Miyaura coupling has emerged as a powerful method for these polymerizations since it can be conducted under mild conditions, a range of boron reagents can be employed, and it is exceptionally functional group tolerant. We have analyzed the Ni(dppp)Cl2 mediated Suzuki–Miyaura chain-growth polycondensation of 3-hexylthiophene and discovered that a portion of the precatalyst hydrolyzes at the outset of the reaction. Surprisingly, this loss of catalyst is critical to achieving a controlled polymerization as it releases free ligand into the reaction mixture, suppressing chain transfer. The identity of the ligand is also critical, as PPh3 and dppe do not produce the same behavior. Overall, the study offers an improved understanding of conjugated polymer synthesis when using weaker nucleophiles such as boronate esters or boronic acids in Suzuki–Miyaura cross-coupling.