Kumada Catalyst-transfer Polycondensation Research Articles

A SNAr‐Active External Initiator that Enables Heterobifunctional Clickable Polythiophenes

AbstractPoly(3‐hexylthiophene) (P3HT) is a well‐studied conjugated polymer; however, the end‐group functionalization of these polymers is limited by fundamental synthetic challenges. Generally, end‐capping agents are used to terminate the Kumada catalyst transfer polycondensation (KCTP) to install functional end‐groups at the propagating chain end. In this work, it is shown that P3HT can be prepared with a novel SNAr‐active external initiator, followed by end capping to afford heterobifunctional end‐groups that exhibit orthogonal chemical reactivity. This orthogonal chemistry can be leveraged in sequential “click”‐type reactions to ultimately achieve the synthesis of triblock copolymers. More specifically, it is shown that the synthesis of a new pentafluorophenyl initiator allows one to append this readily functionalized group at the initiating end during P3HT synthesis. After reaching the desired degree of polymerization, an ethynyl group is incorporated at the polymer terminus using an end‐capping Grignard reagent. Critically, this heterobifunctional P3HT allows for both orthogonal para‐fluoro thiol reactions, a subset of nucleophilic aromatic substitution (SNAr), and copper‐catalyzed alkyne‐azide cycloadditions (CuAAC). The efficacy of this heterobifunctional polymer design is demonstrated by assessing sequential “click” reactions with different nonconjugated polymer blocks to either end of the P3HT polymer block.

Tellura(benzo)bithiophenes: Synthesis, Oligomerization, and Phosphorescence.

A series of planar π-extended Te-containing heteroacenes, termed tellura(benzo)bithiophenes, were synthesized. This new structural class of heterocycle features a tellurophene ring fused to a benzobithiophene unit with aromatic side groups (either -C6H4iPr or -C6H4OCH3) positioned at the 2- and 5-positions of the tellurophene moiety. Although attempts to enhance molecular rigidity and extend ring-framework π-delocalization in a cumenyl (-C6H4iPr)-capped tellura(benzo)bithiophene led to oxidation (and Te-C bond scission) to form a diene-one, the formation of an oligomeric tellura(benzo)bithiophene was possible via Kumada catalyst-transfer polycondensation (KCTP). Furthermore, one tellura(benzo)bithiophene derivative exhibits orange-red phosphorescence at room temperature in air when incorporated into a poly(methyl methacrylate) host; accompanying TD-DFT computations provided insight into a potential mechanism for the observed phosphorescence.

Preparing polythiophene derivative with alternating alkyl and thioalkyl side chains via Kumada coupling for efficient organic solar cells

A polythiophene, namely PTST with alternating alkyl and thioalkyl side chains, is prepared by Kumada catalyst-transfer polycondensation. PTST can hierarchically pre-aggregate in solution, and then form a favorable morphology in organic solar cells.

Finely Designed P3HT-Based Fully Conjugated Graft Polymer: Optical Measurements, Morphology, and the Faraday Effect.

In this work, our group synthesized and characterized a fully conjugated graft polymer comprising of a donor-acceptor molecular backbone and regioregular poly(3-hexylthiophene) (RRP3HT) side chains. Here, our macromonomer (MM) was synthesized via Kumada catalyst transfer polycondensation reaction based on ditin-benzodithiophene (BDT) initiator. The tin content of MM was then investigated by inductively coupled plasma-mass spectrometry (ICP-MS), which allowed for accurate control of donor/acceptor monomer ratio of 1:1 for the following Stille coupling polymerization toward our graft polymer (BP). The structures of the polymers were then characterized by gel permeation chromatography (GPC), NMR, and elemental analysis. This was followed by the characterization of optical, electrochemical, and physical properties. The magneto-optical activity of graft polymer BP was then measured. It was found that, despite the presence of the acceptor backbone, the characteristic large Faraday rotation of RRP3HT was maintained in polymer BP, which exhibited a Verdet constant of 2.39 ± 0.57 (104) °/T·m.

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.

Synthesis of Polythiophene Derivatives with Oligoisobutylene Side Chains

In recent years, organic electronics including organic light emitting diode, organic photovoltaic cell and organic thin film transistor have received much attention due to their severals advantages based on organic materials, such as mechanical flexibility and solution-based low-cost processability. π-Conjugated polymers are known as mechanically soft and flexible material in organic semiconductors. However, their durability for stretched stress is not enough for practical application. Therefore, the molecular design is required for enabling stretchable organic semiconductor. The low strethcability of π-conjugated polymer is due to high crystallinity which induced their stiff main chain and strong intermolecular interactions. Here, we developed novel polythiophene derivatives, P3IBTs, which contain oligoisobutylene units in their side chains. We expect that several branched points on side chains weaken π-π stacking interaction and decrease crystallinity of the polymers. In addition, we tried to develop cross-linked P3IBT to realize the formation of an inherently stretchable π-conjugated polymer. We prepared the monomer of polythiophene derivatives with diisobutylene and triisobutylene side chains according to Scheme 1. The oligoisobutylene unit was synthesized based on a carbocationic addition reaction. Finally, polythiophene derivatives with diisobutylene sidechain (P3IB2T) (M n = 12,000, M w/M n = 1.17) and triisobutylene side chain (P3IB3T) (M n = 13,000, M w/M n = 1.21) could be synthesized by Kumada catalyst-transfer polycondensation. P3IB2T is dark red solid and P3IB3T is dark red chewing-gum like solid at room temperature. Subsequently, we characterized their thermal, optical properties and crystalline nature compared with poly(3-hexylthiophene)(P3HT) (M n = 10,000, M w/M n = 1.17). The thermal properties of the polymers were investigated by differential scanning calorimetry (DSC). The melting temperature of P3IB2T (T m = 130 ˚C) is lower than P3HT (T m = 242 ˚C). On the other hand, there are neigher endothermic nor exothermic peaks in the DSC curve of P3IB3T. Therefore, this polymer would be more amorphous-like than P3IB2T. These result suggests that oligoisobutylene side chains decrease crystallinity of polymers. The absorption spectrum of P3IB2T film showed lower absorption intensity around 620 nm than the P3HT film, indicating weaker π-π stacking interaction. Moreover, P3IB3T films showed the blue-shifted absorption spectrum compared to P3HT and P3IB2T ones probably due to larger torsion of main chains affected by triisobutylene side chain. Next, the crystalline nature was evaluated by Grazing incidence wide-angle x-ray scattering (GIWAXS). P3IB2T and P3IB3T films showed edge-on orientation similar to the P3HT ones, judged from the azimuth plots of (200) diffraction patterns. Moreover, (010) diffraction patterns were not observed for P3IB2T and P3IB3T films, probably due to less π-π stacking interaction by oligoisobutylene side chains. Finally, we cross-linked P3IB3T to realize the formation of an inherently stretchable p-conjugated polymer. 4,4’-Methylenebis(2,6-bis(methoxymethyl)phenol) and isopropyl p-toluenesulfonate were used as cross-linker and thermal acid generator, respectively. The cross-linking reaction was proceeded by heating at 200 ˚C for 3 h. The mechanical properties of the cross-linked P3IB3T was investigated by the tensile test using a TMA instrument. As a result, P3IB3T showed much lower tensile strength (0.68 MPa) and higher elongation at break (141%) than P3HT (984 MPa, 59 %). In conclusion, we have synthesized polythiophene derivatives with oligoisobutylene side chains. These polymer showed lower crystallinity and weaker π-π stacking interaction than P3HT. These properties might be caused by the effect of several branch points on oligoisobutylene side chains. In addition, the cross-linked P3IB3T film could be formed, possessing good mechanical durability. Figure 1

Exploring the Graft-To Synthesis of All-Conjugated Comb Copolymers Using Azide–Alkyne Click Chemistry

Copolymers with graft architectures possess interesting material properties distinct from their linear polymer counterparts. The effects of multidimensional architectures on the optoelectronic and physical properties of all-conjugated graft copolymers are not well-known, thus providing a large incentive for their study. In order to readily access these materials (hypothesized to have “comb” architectures), it is extremely important to investigate the methods used in their synthesis. Here we study the graft-to synthesis of comb copolymers composed of polythiophene backbones and polyselenophene side chains and identify the opportunities and challenges associated with copolymer formation. Azide-functionalized polythiophene “backbones” and acetylene-terminated polyselenophene “side chains” were synthesized in a controlled fashion using Kumada catalyst-transfer polycondensation (KCTP) polymerization and grafted together using copper-catalyzed azide–alkyne click chemistry (CuAAC). 1H NMR, GPC, and FTIR results ...

Synthesis of Structurally Defined Cationic Polythiophenes for DNA Binding and Gene Delivery.

Water-soluble conjugated polymers (WCPs) have prospective applications in the field of bioimaging, disease diagnosis, and therapy. However, the use of WCPs with controllability and regioregularity for bioapplications have scarcely been reported. In this work, we synthesized polythiophenes containing ester side chains (P3ET) via Kumada catalyst-transfer polycondensation (KCTP) and confirmed a quasi-"living" chain-growth mechanism. In addition, we obtained cationic regioregular polythiophenes (cPTs) by aminolysis of P3ET with varied chain lengths, and studied DNA binding capability and gene delivery performance. Benefiting from photocontrolled generation of intracellular reactive oxygen species (ROS), the cationic polythiophenes successfully delivered DNA into tumor cells without additional polymer species.

Balancing steric and electronic effects of bidentate, mixed P,N ligands to control Kumada catalyst transfer polycondensation of a sterically hindered thiophene

Screening various P,N ligands to control KCTP of a sterically hindered thiophene reveals an oxazoline-based ligand most effective.

Tunable Fluorescence of a Semiconducting Polythiophene Positioned on DNA Origami.

A novel approach for the integration of π-conjugated polymers (CPs) into DNA-based nanostructures is presented. Using the controlled Kumada catalyst-transfer polycondensation, well-defined thiophene-based polymers with controllable molecular weight, specific end groups, and water-soluble oligoethylene glycol-based side chains were synthesized. The end groups were used for the easy but highly efficient click chemistry-based attachment of end-functionalized oligodeoxynucleotides (ODNs) with predesigned sequences. As demonstrated by surface plasmon resonance spectroscopy, the prepared block copolymers (BCPs), P3(EO)3T-b-ODN, comprising different ODN lengths and specific or repetitive sequences, undergo specific hybridization with complementary, thiol-functionalized ODNs immobilized on a gold surface. Furthermore, the site-specific attachment of the BCPs to DNA origami structures is studied. We demonstrate that a nanoscale object, that is, a single BCP with a single ODN handle, can be directed and bound to the DNA origami with reasonable yield, site-specificity, and high spatial density. On the basis of these results, we are able to demonstrate for the first time that optical properties of CP molecules densely immobilized on DNA origami can be locally fine-tuned by controlling the attractive π-π-stacking interactions between the CPs. In particular, we show that the fluorescence of the immobilized CP molecules can be significantly enhanced by surfactant-induced breakup of π-π-stacking interactions between the CP's backbones. Such molecular control over the emission intensity of the CPs can be valuable for the construction of sophisticated switchable nanophotonic devices and nanoscale biosensors.

Formation of Polyelectrolyte Complexes from Cationic Polyfluorenes and ssDNA

Trimethylphosphonium-substituted polyfluorenes (PTMPHFs) with controlled molecular weights were synthesized using the Kumada catalyst-transfer polycondensation procedure, and their recognition properties towards various biomolecules were examined. Upon the formation of polyelectrolyte complexes with single stranded DNA (particularly polyadenine and polycytosine), the UV-vis absorption of PTMPHF was red-shifted and its fluo-rescence was quenched.

A Study of the Degree of Fluorination in Regioregular Poly(3-hexylthiophene)

We systematically varied the degree of fluorination along the backbone of a series of highly regioregular 3-hexylthiophene-based polymers, P3HT-50F, P3HT-33F, and P3HT-25F, in which 50%, 33%, and 25% of the thiophene units within the polymer chain contain fluorine atoms in the available 4-position, respectively. These materials were homopolymerized using the Kumada catalyst transfer polycondensation method from a set of mono-fluorinated bi-, ter-, and quarterthiophenes to ensure high polymer regioregularity and evenly spaced fluorine atoms along the conjugated thiophene backbone. The monomers were obtained from a synthetic route consisting of iterative Migita–Stille couplings of fluorinated and non-fluorinated 3-hexylthiophenes. The effect of the fluorine atoms on both polymer structure and properties is presented, with supporting quantum mechanical calculations that rationalize the intrinsic conformation preferences of the three P3HT derivatives. P3HT-50F (Mn = 34 kg/mol, 98.5% rr), P3HT-33F (Mn = 46 k...

Influence of the halogen and organometallic function in a KCTP (Co)polymerization

ABSTRACTThe effect of changing the halogen and the organometallic function in a Kumada Catalyst Transfer Polycondensation (KCTP) of poly(3‐alkylthiophene)s (P3AT) is investigated. On the one hand, the bromine substituent is replaced with chlorine in the commonly used 2‐bromo‐5‐chloromagnesio‐3‐hexylthiophene. The effect on the homopolymerization is clear, since the stickiness decreases remarkably, but copolymerizations are hardly affected when a chlorinated monomer is used. Second, the option of changing the organometallic function is considered. Because also organozinc compounds provide a controlled P3AT polymerization with Ni(dppp)Cl2, but are less reactive than organomagnesium compounds, the effect of using zinc in one monomer during a copolymerization is investigated. However, it is found that the organometallic functions exchange during mixing of the monomers. Consequently, no effect is observed during copolymerizations. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3701–3706

Synthesis of P3HT-b-PS donor-acceptor diblock copolymer carrying pendant fullerenes at precise positions along the PS block

We present a strategy for the synthesis of a well-defined rod-coil block copolymers of regioregular poly(3-hexylthiophene) and polystyrene (P3HT-b-PS), carrying pendant fullerenes (C60) at precise positions along the PS block. The synthesis is achieved by combining ‘living’ anionic polymerization, Kumada catalyst-transfer polycondensation, and click chemistry. Azide terminated polystyrene was synthesized via anionic polymerization, while C60 moieties were grafted along the coil block with a [4+2] Diels-Alder cycloaddition reaction. The regioregular poly(3-hexylthiophene) rod donor block was end-capped with alkyne and was coupled with the azide terminated polystyrene via a copper catalyzed alkyne-azide cycloaddition. Size exclusion chromatography (SEC), NMR, and FT-IR spectroscopies were employed to confirm the synthesis of the diblock copolymers. Furthermore, the end-capping of the ‘living’ polystyrene (PS) with the diphenylethylene cyclobutene (DPE-CB) end-group was confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). Thermogravimetric analysis (TGA) was employed for the estimation of the C60 content in the block copolymer. The C60 content at the coil block was 22wt% and a strong quenching of photoluminescence (PL) was observed as the result of a large interfacial area between the P3HT and C60 in the diblock copolymer. The bonding of C60 across the PS block offers new routes to engineering stable morphologies, where the degree of organization/aggregation and/or crystallization of the acceptor material, of paramount importance for stable and efficient electron transport, may be controlled.

All-conjugated donor–acceptor graft/block copolymers as single active components and surfactants in all-polymer solar cells

In this work, we report the synthesis of all-conjugated donor---acceptor (D---A) graft/block copolymers via an externally-initiated Kumada catalyst-transfer polycondensation (KCTP) method. In the first step, end-functional electron acceptor blocks, poly(naphthalene diimide)s (PNDIs), were prepared via the Stille coupling polycondensation. Then, P3HT blocks were polymerized via KCTP initiated by the Ni(COD)2-activated PNDI complexes. Therefore, a series of D---A graft copolymers and ABA-type (P3HTs were initiated from both ends of PNDI) triblock copolymers were successfully synthesized. The polymers were used to fabricate polymer thin film transistors and the results indicated that all the polymers exhibited moderate charge mobilities ranging from 10ź4 to 10ź3 cm2 Vź1 sź1, and they could be further improved by thermal annealing. Finally, all-polymer solar cells were fabricated by using the block copolymers as the single active components or as surfactants. A power conversion efficiency (PCE) of 0.22 % with open circuit voltage (Voc) = 0.55 V, short circuit current density (Jsc) = 0.64 mA/cm2, and fill factor (FF) = 0.62 was recorded by using the donor---acceptor all-conjugated block copolymer as acceptor, and a PCE of 2.28 % with Voc = 0.61 V, Jsc = 7.00 mA/cm2, and FF = 0.54 was obtained when PNDI was used as a surfactant.

Grafting Poly(3‐hexylthiophene) from Silicon Nanocrystal Surfaces: Synthesis and Properties of a Functional Hybrid Material with Direct Interfacial Contact

AbstractHybrid functional materials (HFMs) comprised of semiconductor nanoparticles and conjugated polymers offer the potential of synergetic photophysical properties. We have developed HFMs based upon silicon nanocrystals (SiNCs) and the conductive polymer poly(3‐hexylthiophene) (SiNC@P3HT) by applying surface‐initiated Kumada catalyst transfer polycondensation (SI‐KCTP). One unique characteristic of the developed SiNC@P3HT is the formation of a direct covalent bonding between SiNCs and P3HT. The presented method for obtaining direct interfacial attachment, which is not accessible using other methods, may allow for the development of materials with efficient electronic communication at the donor–acceptor interfaces. Systematic characterization provides evidence of a core–shell structure, enhanced interfacial electron and/or energy transfer between the P3HT and SiNC components, as well as formation of a type‐II heterostructure.

Grafting Poly(3-hexylthiophene) from Silicon Nanocrystal Surfaces: Synthesis and Properties of a Functional Hybrid Material with Direct Interfacial Contact.

Hybrid functional materials (HFMs) comprised of semiconductor nanoparticles and conjugated polymers offer the potential of synergetic photophysical properties. We have developed HFMs based upon silicon nanocrystals (SiNCs) and the conductive polymer poly(3-hexylthiophene) (SiNC@P3HT) by applying surface-initiated Kumada catalyst transfer polycondensation (SI-KCTP). One unique characteristic of the developed SiNC@P3HT is the formation of a direct covalent bonding between SiNCs and P3HT. The presented method for obtaining direct interfacial attachment, which is not accessible using other methods, may allow for the development of materials with efficient electronic communication at the donor-acceptor interfaces. Systematic characterization provides evidence of a core-shell structure, enhanced interfacial electron and/or energy transfer between the P3HT and SiNC components, as well as formation of a type-II heterostructure.

Controlled Growth of Well-Defined Conjugated Polymers from the Surfaces of Multiwalled Carbon Nanotubes: Photoresponse Enhancement via Charge Separation.

The installation of heterojunctions on the surfaces of carbon nanotubes (CNTs) is an effective method for promoting the charge separation processes needed for CNT-based electronics and optoelectronics applications. Conjugated polymers are proven state-of-the-art candidates for modifying the surfaces of CNTs. However, all previous attempts to incorporate conjugated polymers to CNTs resulted in unordered interfaces. Herein we show that well-defined chains of regioregular poly(3-hexylthiophene) (P3HT) were successfully grown from the surfaces of multiwalled CNTs (MWNTs) using surface-initiated Kumada catalyst-transfer polycondensation. The polymerization was found to proceed in a controlled manner as chains of tunable lengths were prepared through variation of the initial monomer-to-initiator ratio. Moreover, it was determined that large-diameter MWNTs afforded highly ordered P3HT aggregates, which exhibited a markedly bathochromically shifted optical absorption due to a high grafting density induced planarization of the polymer chains. Using ultrafast spectroscopy, the heterojunctions formed between the MWNTs and P3HT were shown to effectively overcome the binding energy of excitons, leading to photoinduced electron transfer from P3HT to MWNTs. Finally, when used as prototype devices, the individual MWNT-g-P3HT core-shell structures exhibited excellent photoresponses under a low illumination density.

Detrimental Ni(0) transfer in Kumada catalyst transfer polycondensation of benzo[2,1‐b:3,4‐b']dithiophene

ABSTRACTThis article deals with the Kumada Catalyst Transfer Polycondensation (KCTP) of 4,7‐dioctylbenzo[2,1‐b:3,4‐b']dithiophene (BDP‐Oct) using Ni(II) catalyst or In/cat combination. A combination of MALDI MS, GPC, and 31P NMR spectroscopy is used to reveal the failure of the KCTP of this particular monomer. Intermolecular transfer reactions to monomer appeared to prevent the formation of polymer. This result is remarkable, since isomeric benzo[1,2‐b:4,5‐b']dithiophene polymerizes in a controlled way. The presence of a “non‐aromatic double bond” in annulated monomers is discussed. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1706–1712

Influence of the Grafting Density on the Self-Assembly in Poly(phenyleneethynylene)-g-poly(3-hexylthiophene) Graft Copolymers

Conjugated graft copolymers consisting of a chiral poly(phenyleneethynylene) (PPE) backbone and poly(3-hexylthiophene) side chains (P3HT) with different grafting degrees were synthesized. While PPE was prepared by classical Sonogashira couplings, the end-functionalized P3HT was prepared by a controlled Kumada catalyst transfer polycondensation (KCTP) allowing the installation of an acetylene end group. After some postpolymerization reactions on the PPE to introduce azide groups, the P3HT was clicked to the PPE through the CuAAC coupling reaction. Subsequently, the (chiral) self-assembly of these materials was studied by means of UV–vis and CD spectroscopy, AFM, and DSC. Finally, fluorescence spectroscopy is used to study the quenching of the PPE fluorescence by P3HT.