The Synthesis of Poly(thiophene-co-fluorene) Gradient Copolymers

Statistical Copolymers of Thiophene-3-Carboxylates and Selenophene-3-Carboxylates; 77Se NMR as a Tool to Examine Copolymer Sequence in Selenophene-Based Conjugated Polymers

Herein, we demonstrate that homopolymerization and statistical copolymerization of 2-ethylhexyl thiophene-3-carboxylate and 2-ethylhexyl selenophene-3-carboxylate monomers is possible via Suzuki-Miyaura cross-coupling. A commercially available palladium catalyst ([1,3-bis(2,6-di-3-pentylphenyl)imidazol-2-ylidene](3-chloropyridyl)dichloropalladium(II) or PEPPSI-IPent) was employed...

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.

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).

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.

Exploring the Utility of Buchwald Ligands for C-H Oxidative Direct Arylation Polymerizations

Oxidative C-H/C-H cross-coupling polymerizations provide an opportunity to synthesize conjugated polymers with an increased ease of monomer preparation, reduced environmental impact, and increased sustainability. Considering these attributes, it is necessary to expand the diversity of monomers that readily and efficiently participate in this coupling strategy to enable the development of conjugated polymers with a wide range of properties. Herein, the oxidative direct arylation polymerization toolbox is expanded to include 3,4-propylenedioxythiophene being synthesized via C-H/C-H cross-coupling methodologies. In conjunction with these efforts, the utilization of Buchwald ligands in C-H/C-H cross coupling polymerizations also is reported, and variations in the ligand structure provide insight into the role ligand choice has on C-H cross-coupling polymerizations. Specifically, it is determined that the phosphine functionality affects the rate-determining, concerted metalation-deprotonation step of the catalytic cycle, while bulky isopropyl substituents on the ligand's lower aryl ring promote reductive elimination. By balancing these steric effects on the ancillary ligands, polymers are synthesized to exhibit molecular weights above the effective conjugation length, with recovered yields >90%. In addition to expanding the scope of conjugated polymers accessible via oxidative direct arylation polymerization, these results provide the foundational understanding for utilizing Buchwald-type ligands in C-H-activated polymerizations.

Gradient copolymer micelles: an introduction to structures as well as structural transitions

Exhibiting variation of the composition along a chain, gradient copolymers bring new blood to the old story of polymeric micelles. The gradient chain structure results in some special features in micellar structures and leads to unique structural transitions, potentially leading to new properties and applications. Henceforth, gradient copolymer micellar structures and their transitions from the viewpoint of soft matter physics will be reviewed. Concepts such as a diffuse interface, shrinkage-stretching of micelles, and intrinsic temperature responsiveness are summarized from current research, which highlight new characteristic structures, relaxation modes and novel properties of micelles, respectively.

Electronic and structural properties of fluorene–thiophene copolymers as function of the composition ratio between the moieties: a theoretical study

Through theoretical analysis, we study relevant properties of some molecular structures formed by oligothiophenes (T) and dioctylfluorenes (F) units, like the exciton binding energy (E_b) and dipole moment, important for the efficiency of different kinds of optical and electronic devices.

Synthesis of conjugated copolymers by combining different coupling reactions

Different coupling reactions are combined in the same copolymerization to tune the structure of the resulting conjugated copolymer.

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.

Limitations of Using Small Molecules to Identify Catalyst-Transfer Polycondensation Reactions

Catalyst-transfer polycondensation (CTP) is a relatively new method for synthesizing conjugated polymers in a chain-growth fashion using transition metal catalysis. Recent research has focused on screening catalysts to broaden the monomer scope. In this effort, small molecule reactions have played an important role. Specifically, when selective difunctionalization occurs, even with limiting quantities of reaction partner, it suggests an associative intermediate similar to CTP. Several new chain-growth polymerizations have been discovered using this approach. We report herein an attempt to use this method to develop chain-growth conditions for synthesizing poly(2,5-bis(hexyloxy)phenylene ethynylene) via Sonogashira cross-coupling. Hundreds of small molecule experiments were performed and selective difunctionalization was observed with a Buchwald-type precatalyst. Unexpectedly, these same reaction conditions led to a step-growth polymerization. Further investigation revealed that the product ratios in the small molecule reactions were dictated by reactivity differences rather than an associative intermediate. The lessons learned from these studies have broad implications on other small molecule reactions being used to identify new catalysts for CTP.