Aqueous Palladium‐Catalyzed Direct Arylation Polymerization of 2‐Bromothiophene Derivatives

Abstract

Conjugated polyelectrolytes have been utilized in applications including chemical/biosensing, [30][31][32] cell imaging, [33][34][35] and disease diagnosis/therapy. [36,37] Their synthesis comprises the preparation of organic-soluble conjugated polymers followed by post-functionalization. A typical post-functionalization reaction is the nucleophilic substitution of an alkyl-bromide with amines to afford ammonium salts, which render the resultant conjugated polyelectrolytes aqueous soluble. However, post-functionalization exhibits poor conversion with many unreacted reaction sites. Aqueous polymerization of water-soluble monomers is an alternate approach realized through Heck, [38,39] Sonogashira, [40,41] Suzuki, [42,43] and FeCl 3 -mediated oxidative [44,45] polymerizations. These reactions are successful when the active species tolerates the presence of water.Reports of aqueous palladium-catalyzed DArP are scarce. [9,46,47] DArP also requires a carbonate base and carboxylic acid additive. Compared to water, carbonate bases display higher Lewis basicities, while carboxylic acids are more acidic. Palladium species preserve the reactivity in the presence of carbonates and carboxylic acids, making them tolerant to water. This study examines the aqueous palladium-catalyzed DArP of 2-bromothiophene derivatives, T1 and T2. The initially selected ligand, triphenylphosphine-3,3′,3″-trisulfonic acid trisodium salt (m-TPPTs), was functional in the DArP; however, its separation from the resultant polymers by dialysis was challenging due to its strong aggregation in water and N,N-dimethylacetamide (DMAc). This was supported by dynamic light scattering (DLS), X-ray crystallography, and gel permeation chromatography (GPC). Thus, pyrimidine-Pd(OAc) 2 was employed in the DArP of T1 to afford PT1 with no ligand (pyrimidine) contamination. Density functional theory (DFT) binding energy calculations revealed that the coordinating ability to palladium was of the order carboxylate > pivalic acid (PivOH) > water, rationalizing the feasibility of executing DArP in water. Finally, the polyelectrolyte molecular-weight overestimation by GPC in water was attributed to the polyelectrolyte effect. Experimental results indicated that the conjugated polyelectrolytes aggregate in DMAc to form a folding structure with the polar groups pointing outwards and conjugated moieties directed inwards, corresponding to a reduced hydrodynamic volume. The polyelectrolyte molecular weight and dispersity (Đ) values determined by GPC in DMAc were, therefore, questionable.

Water-Processable Self-Doped Conducting Polymers via Direct (Hetero)arylation Polymerization

“…Cross-coupling polymerization methods such as Stille, Suzuki, and direct arylation are better choices to ensure the synthesis of well-defined alternating copolymers. However, few of these coupling reactions are adapted for the synthesis of water-soluble materials. , Recently, some studies showed that the addition of water or the use of polar solvents is compatible with direct (hetero)­arylation polymerization (DHAP) reactions. − For instance, the addition of water has shown to allow the synthesis of materials with good molecular weight even in the presence of air in the system . Biphasic conditions also permit to eliminate the use of anhydrous solvents.…”

Thiophene α-Chain-End-Functionalized Oligo(2-methyl-2-oxazoline) as Precursor Amphiphilic Macromonomer for Grafted Conjugated Oligomers/Polymers and as a Multifunctional Material with Relevant Properties for Biomedical Applications

“…Further, favorable intermolecular interactionsmay be slowed down by the “melt-like” state in which the reaction is developing. Consequently, performing polymerization in bulk at a higher temperature than 150 °C in solution (in the presence of Lewis [ 122 ] or Bronsted [ 123 ] acids) or alternatively, based on its aqueous self-dispersibility, applying direct arylation in an aqueous medium [ 126 ] is expected to have better performance creating a polythiophene-g-OMeOx, and these ideas will be explored in future studies.…”

Strategies for the synthesis of water-soluble conjugated polymers

“…However, the development of efficient water-only polymerization conditions is still a work in progress [31,57]. As an example, Lin and colleagues [57] recently reported DHAP conditions enabling the polymerization of both cationic and anionic polymers by using water as the only solvent.…”