Nickel-Catalyzed Direct Arylation Polymerization for the Synthesis of Thiophene-Based Cross-linked Polymers

An earth-abundant nickel(II) bipyridine catalyst, combined with lithium hexamethyldisilazide as base, demonstrates its wide applicability in the direct arylation polymerization of di- and tri-thiophene heteroaryls with poly(hetero)aryl halides. With a nickel catalyst loading of 2.5 mol%, a series of twenty highly cross-linked organic polymers is obtained in 34 to 99 % yields. Using mixed polytopic coupling partners allows obtaining alternating and optically active thiophene-based solids with intrinsic porosity.

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 M_n 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.

An Efficient Precatalyst Approach for the Synthesis of Thiazole-Containing Conjugated Polymers via Cu-Catalyzed Direct Arylation Polymerization (Cu-DArP)

Over the past decade, direct arylation polymerization (DArP) has emerged as a facile and sustainable methodology for the synthesis of conjugated polymers. Recently, we developed Cu-catalyzed DArP (Cu-DArP) as a low-cost, Pd-free synthetic pathway, which enables conjugated polymers to be synthesized with high molecular weights and minimization of defects. However, the lack of study on the use of Cu-precatalysts in small-molecule direct arylation poses significant limitations for Cu-DArP to potentially overtake conventional Pd-catalyzed methodology, such as the low solubility and stability of the previously employed CuI. Therefore, in this report, we decide to explore the utility of a well-defined, easy-to-prepare, highly soluble, and stable precatalyst, Cu(phen)(PPh₃)Br, as an alternative to the CuI, 1,10-phenanthroline catalytic system previously used for Cu-DArP. Herein, we report a drastic improvement of Cu-DArP methodology for the synthesis of 5,5'-bithiazole (5-BTz)-based conjugated polymers enabled by an efficient precatalyst approach, affording polymers with good M_n (up to 16.5 kDa) and excellent yields (up to 79%). ¹H NMR studies reveal the exclusion of homocoupling defects, which further verifies the excellent stability of Cu(phen)(PPh₃)Br compared to CuI. Furthermore, we were able to decrease the catalyst loading from 15 mol % to only 5 mol % (M_n of 11.8 kDa, 64% yield), which is unprecedented when aryl bromides are employed for Cu-DArP. Significantly, 5-BTz was shown to be inactive under various of Pd-DArP conditions, which demonstrates the high compatibility of Cu-DArP as the only pathway for the C-H activation of the 5-BTz unit and a clear case demonstrating an advantage of Cu-DArP relative to Pd-DArP.

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.

Significant Difference in Semiconducting Properties of Isomeric All-Acceptor Polymers Synthesized via Direct Arylation Polycondensation

The direct arylation polycondensation (DArP) appeared as an efficient method for producing semiconducting polymers but often requires acceptor monomers with orienting or activating groups for the reactive carbon-hydrogen (C-H) bonds, which limits the choice of acceptor units. In this study, we describe a DArP for producing high-molecular-weight all-acceptor polymers composed of the acceptor monomers without any orienting or activating groups via a modified method using Pd/Cu co-catalysts. We thus obtained two isomeric all-acceptor polymers, P1 and P2, which have the same backbone and side-chains but different positions of the nitrogen atoms in the thiazole units. This subtle change significantly influences their optoelectronic, molecular packing, and charge-transport properties. P2 with a greater backbone torsion has favorable edge-on orientations and a high electron mobility μ_e of 2.55 cm²  V^-1  s^-1 . Moreover, P2-based transistors show an excellent shelf-storage stability in air even after the storage for 1 month.

Influence of an ester directing-group on defect formation in the synthesis of conjugated polymers via direct arylation polymerization (DArP) using sustainable solvents

We report the application of green solvents in DArP and the structure-dependent β-defect formation due to an ester directing group.

Synthesis of conjugated polymers using aryl-bromides via Cu-catalyzed direct arylation polymerization (Cu-DArP)

The first report on direct arylation polymerization with copper catalysts and aryl-bromide monomers expands the sustainability and practicality of DArP.

Across the Board: Antonio Facchetti

In this series of articles, the board members of ChemSusChem discuss recent research articles that they consider of exceptional quality and importance for sustainability. This entry features Prof. Dr. Antonio Facchetti, who discusses the use of organic photovoltaic (OPV) devices as a source of renewable energy, and challenges that must be met for OPVs to serve as a viable fully sustainable technology for future energy production, taking into account the components used in such devices and their stability and durability.

Sustainable Synthesis of a Fluorinated Arylene Conjugated Polymer via Cu-Catalyzed Direct Arylation Polymerization (DArP)

Direct arylation polymerization (DArP) has enabled the more sustainable synthesis of conjugated polymers through the reduction of waste, elimination of toxic and hazardous byproducts, and through the reduction of overall synthetic steps. However, DArP methodologies have almost exclusively relied on the use of noble metal catalysts, such as those with Pd, counter to the efforts toward sustainability. Herein, we report the optimized synthesis of a flourinated arylene conjugated copolymer via DArP using a low-cost, sustainable copper catalyst. Through optimization of the polymerization conditions, we are able to lower the loading of the catalyst from 50 to 5 mol %. As an example, we are able to obtain molecular weights of 16.4 kDa, accompanied by a yield of 54%, with a loading of only 5 mol % for the Cu catalyst. The synthesized polymers were characterized using ¹H and ¹⁹F NMR spectroscopy, showing agreement with those previously prepared with a minimization or exclusion of defects. This work also demonstrates that Cu-catalyzed DArP methodologies can be compatible with substrates that do not possess directing groups, which can help to facilitate C-H functionalization, allowing for a broadening of substrate scope.