Catalyst-transfer Polycondensation Research Articles

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.

Kinetically Controlled Formation of Semi-crystalline Conjugated Polymer Nanostructures

Conjugated polymer (CP) materials that are considered as an attractive choice for various electronic and optoelectronic applications possess highly heterogeneous complex structures that can span several orders of magnitude in length scale. This is due to the intricate coupling effects of weak secondary interactions and entropic forces that determine molecular organization in bulk polymer materials. Understanding the influence of molecular interactions on the emergence and evolution of nano- and microscale structures is of paramount importance to the guided design and development of condensed CP materials. Such understanding is even more critical for the rational design of hierarchical systems away from equilibrium, where weak molecular interactions can guide the system along competing kinetic pathways toward local energy minima and metastable architectures. In this work, we studied a promising concept for accessing various kinetically stabilized semi-crystalline CP nanostructures that are formed in the process of controlled chain-growth Kumada catalyst-transfer polymerization initiated by a small-molecule catalytic species which can exist in equilibrium between free and aggregated states. Specifically, we chose a small-molecule perylenedicarboximide (PDCI) polymerization catalytic initiator which has a strong tendency toward reversible supramolecular assembly–disassembly that can be controlled by temperature. Addition of a thiophene monomer to a solution of the PDCI initiator starts the chain-growth polymerization process, which produces distinct nanoscale semi-crystalline polythiophene structures formed under predominant kinetic control. We demonstrated that, depending on the reaction temperature (which affects the fine balance between the position of PDCI assembly–disassembly equilibrium, rate of polymerization, and solvent–solute interactions for the growing polythiophene chains), a range of kinetically trapped hierarchically organized semi-crystalline CP systems with substantially varying optoelectronic properties could be obtained. In addition to spectroscopic and electron microscopic studies, in order to better reveal the structural aspects of the generated polymer nanoscale systems, we carried out a series of X-ray diffraction and neutron scattering experiments which indicated complex hierarchical organization in these kinetically stabilized CP assembled materials. We expect that this in situ polymerization-based approach can lead to a general way to expand access to various semi-crystalline CP nanostructured materials with broadly tunable electronic and optical properties.

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.

Pairing Suzuki–Miyaura cross-coupling and catalyst transfer polymerization

Borylation strategies to make AB Suzuki–Miyaura monomers for use in catalyst-transfer polymerization with nickel or palladium catalysts.

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.

Examining the Spin State and Redox Chemistry of Ni(Diimine) Catalysts during the Synthesis of π‐Conjugated Polymers

AbstractKumada catalyst transfer polymerization (KCTP) is currently an unparalleled catalytic method for preparing π‐conjugated materials with well‐defined end‐groups, targeted molecular weights, and narrow dispersity. Polymerization control is both catalyst and monomer dependent. Polymerizations using bidentate Ni(phosphine) catalysts and 3‐alkylthiophene (or related) monomers lead to the highest degrees of polymerization control; however the scope of monomers that Ni(phosphines) can polymerize is quite limited. Here, two possible mechanistic limitations of Ni(diimine) catalysts are evaluated: 1) the role of spin states, and 2) the redox non‐innocence of diimine ligands. Density functional theory (DFT) calculations are utilized to evaluate singlet and triplet spin states of Ni(diimine) species throughout the KCTP catalytic cycle. It is found that in the energetically favored triplet state, Ni(diimine) catalyst dissociation from polymer chains is competitive with intramolecular oxidative addition, which reduces polymerization control. The redox activity of Ni(diimine) catalysts during polymerization is investigated and it is determined that a Ni(diimine)X2 precatalyst is converted into Ni(diimine)2 under reductive conditions analogous to KCTP, but surprisingly this species is also active during polymerization which leads to an interesting off‐cycle pathway that has not been previously considered. These insights differentiate the mechanisms of Ni(diimine) and Ni(phosphine) polymerizations, highlighting catalyst spin and redox activity as additional factors in KCTP.

Synthesis of Conjugated Rod–Coil Block Copolymers by RuPhos Pd-Catalyzed Suzuki–Miyaura Catalyst-Transfer Polycondensation: Initiation from Coil-Type Polymers

A novel coil-first/grafting-from approach was developed for the synthesis of conjugated rod–coil block copolymers using RuPhos Pd-catalyzed Suzuki–Miyaura catalyst-transfer polycondensation (SCTP). First, aryl iodide end-functionalized polystyrene (PS) was prepared as a macroinitiator for SCTP via atom transfer radical polymerization (ATRP) followed by sequential end-group modifications, azidation, and click reactions. Then, RuPhos Pd-catalyzed SCTP using the PS macroinitiator was carried out in the presence of N-methyliminodiacetic acid boronate-containing 3-hexylthiophene monomer (M1), and this afforded well-defined PS-block-poly(3-hexylthiophene) with excellent control and high yield. The scope of this method was successfully expanded to include poly(methyl acrylate)-, poly(methyl methacrylate)-, and poly(ethylene oxide)-block-poly(3-hexylthiophene) with controlled molecular weight and low dispersity. Further, the combination of the conventional rod-first and newly developed coil-first approaches facilitated the straightforward synthesis of a unique ABC-type rod–coil–rod triblock copolymer that was limitedly accessible by other methods. We believe that this efficient and readily accessible synthetic platform would be highly useful for the preparation of novel conjugated rod–coil block copolymers that can be applied in optoelectronics, battery engineering, and chemical sensing.

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.

Direct Synthesis of Chain-end-functionalized Poly(3-hexylthiophene) without Protecting Groups Using a Zincate Complex.

Chain-end-functionalized poly(3-hexylthiophene)s (P3HTs) with benzyl alcohol (─PhCH2 OH), phenol (─PhOH), and benzoic acid (─PhCOOH) groups are directly synthesized based on the Negishi catalyst-transfer polycondensation method utilizing the zincate complex of t Bu4 ZnLi2 . In this system, neither protection nor deprotection steps are required, and also providing a living polymerization system to control the molecular weight while maintaining a low molar mass dispersity (ÐM ) of the obtained P3HT derivatives. Indeed, the chain-end-functionalized P3HTs can be synthesized along with controlled number-average molecular weights (Mn = 5100-20 000), low ÐM (1.06-1.14), and high chain-end functionality (Fn = 46-86%). The Fn values for the alcohol and phenol groups are found to be high (86% for ─PhCH2 OH and 71% for ─PhOH based on 1 H NMR, respectively), as also confirmed by matrix-assisted laser desorption/ionization time of flight mass spectroscopy. The easily synthesizable chain-end-functionalized P3HTs will be applicable for the facile synthesis of block and branched polymers containing P3HT as well as its related semiconducting polymer segments.

RuPhos Pd Precatalyst and MIDA Boronate as an Effective Combination for the Precision Synthesis of Poly(3-hexylthiophene): Systematic Investigation of the Effects of Boronates, Halides, and Ligands

Herein, we report detailed mechanistic studies of Suzuki-Miyaura catalyst-transfer polycondensation (SCTP) of thiophene. The effects of boronates, halides, ligands, and chain transfer agents (CTAs)...

Cross-coupling polymerization at iodophenyl thin films prepared by spontaneous grafting of a diazonium salt

Cross-coupling at aryl halide thin films has been well-established as a technique for the surface-initiated Kumada catalyst transfer polymerization (SI-KCTP), used to produce covalently bound conjugated polymer thin films. In this work, we report that the spontaneous grafting of 4-iodobenzenediazonium tetrafluoroborate on gold substrates creates a durable iodoarene layer which is effective as a substrate for cross-coupling reactions including SI-KCTP. Using cyclic voltammetry of a surface-coupled ferrocene terminating agent, we have measured initiator surface coverage produced by oxidative addition of Pd(t-Bu3P)2 and estimated the rate constant of the termination reaction in the SI-KCTP system with 2-chloromagnesio-5-bromothiophene and Pd(t-Bu3P)2. We used this system to prepare uniform polythiophene thin films averaging 90 nm in thickness.

Elucidating the Role of Catalyst Steric and Electronic Effects in Controlling the Synthesis of π-Conjugated Polymers

Kumada catalyst transfer polymerization (KCTP) has been widely adopted to synthesize π-conjugated polymers for various electronic and biological applications. However, the scope of monomers that ca...

Synthesis of side-chain regioregular and main-chain alternating poly(bichalcogenophene)s and an ABC-type periodic poly(terchalcogenophene).

Three unsymmetrical diiodobichalcogenophenes SSeI2, STeI2, and SeTeI2 and a diiodoterchalcogenophene SSeTeI2 were prepared. Grignard metathesis of SSeI2, STeI2, SeTeI2, and SSeTeI2 occurred regioselectively at the lighter chalcogenophene site because of its relatively lower electron density and less steric bulk. Nickel-catalyzed Kumada catalyst-transfer polycondensation of these Mg species provided a new class of side-chain regioregular and main-chain AB-type alternating poly(bichalcogenophene)s—PSSe, PSTe, and PSeTe—through a chain-growth mechanism. The ring-walking of the Ni catalyst from the lighter to the heavier chalcogenophene facilitated subsequent oxidative addition, thereby suppressing the possibility of chain-transfer or chain-termination. More significantly, the Ni catalyst could walk over the distance of three rings (ca. 1 nm)—from a thiophene unit via a selenophene unit to a tellurophene unit—to form PSSeTe, the first ABC-type regioregular and periodic poly(terchalcogenophene) comprising three different types of 3-hexylchalcogenophenes.

Suzuki–Miyaura catalyst-transfer polycondensation of triolborate-type fluorene monomer: toward rapid access to polyfluorene-containing block and graft copolymers from various macroinitiators

We demonstrated that the Suzuki–Miyaura catalyst transfer polycondensation of a triolborate-type fluorene monomer can be an efficient and versatile approach to the synthesis of polyfluorenes (PFs) and PF-containing block and graft copolymers.

Core@Corona Functional Nanoparticle-Driven Rod-Coil Diblock Copolymer Self-Assembly.

Herein, a novel strategy to overcome the influence of π-π stacking on the rod-coil copolymer organization is reported. A diblock copolymer poly(3-hexylthiophene)-block-poly(ethylene glycol methyl ether methacrylate) (P3HT-b-PEGMA) was synthesized by the Huisgen cycloaddition, so-called "click chemistry", combining the PEGMA and P3HT blocks synthesized by atom transfer radical polymerization and Kumada catalyst transfer polymerization, respectively. Using a dip-coating process, we controlled the original film organization of the diblock copolymer by the crystallization of the P3HT block via π-π stacking. The morphology of the P3HT-b-PEGMA films was influenced by the incorporation of gold nanoparticles (GNPs) coated by poly(ethylene glycol) ligands. Indeed, the crystalline structuration of the P3HT sequence was counterbalanced by the addition in the film of gold nanoparticles finely localized within the copolymer PEGMA matrix. Transmission electron microscopy and time-of-flight secondary ion mass spectrometry analysis validated the GNP homogeneous localization into the compatible PEGMA phase. Differential scanning calorimetry showed the rod block crystallization disruption. A morphological transition of the self-assembly is observed by atomic force microscopy from P3HT fibrils into out-of-plane cylinders driven by the nanophase segregation.

Homogenous Synthesis of Monodisperse High Oligomers of 3-Hexylthiophene by Temperature Cycling.

Whereas monodisperse polymers are ubiquitous in Nature, they remain elusive to synthetic chemists. Absolute control over polymer length and structure is essential to imparting chemical functionality, reproducible properties, and specific solid-state behavior. Precise polymer length has proven to be extremely difficult to control. The most successful examples are generally similar to solid-phase oligo nucleotide or peptide synthesis, wherein the polymer is built up one unit at a time with each sequential monomer addition requiring purification and deprotection (or other functional group activation) step. We have discovered a stepwise homogeneous catalyst-transfer polymerization to prepare monodisperse oligo(3-hexylthiophene) using temperature to limit additions to one unit per chain per cycle. This is the first reported example of a one-pot synthesis of monodisperse oligomers that requires no additional purification or intermediate steps. It is our hope that the strategy of temperature cycling to "freeze" intermediates will be generalizable to other living polymerization techniques, such as other catalyst-transfer polymerization systems, and those where a resting state involves an association between the catalyst and growing chain.

Electroactive Composite of FeCl3 -Doped P3HT/PLGA with Adjustable Electrical Conductivity for Potential Application in Neural Tissue Engineering.

Conducting polymers (CPs) is one of intelligent biomaterials with the specific properties of reversible redox states, which have a significant effects on the cell behaviors and nerve tissue regeneration. However, the effects of CPs with different electrical conductivity on the behaviors of nerve cells are rarely reported. Therefore, a kind of Poly(3-hexylthiophene) (P3HT) with certain molecular weight is synthesized by Kumada catalyst transfer polymerization (KCTP) method and employed to prepare bioabsorbable and electroactive intelligent composites of Poly(3-hexylthiophene)/Poly(glycolide-lactide) (P3HT/PLGA). FeCl3 doping electroactive membranes with different electrical conductivities are prepared to investigate the cell behaviors. On the substrate with higher electrical conductivity, the proliferation of rat adrenal pheochromocytoma cells (PC12 cells) is significantly promoted and neurite length is increased obviously. In particular, the most significant improvements are the neuron gene expression of Synapsin 1 and microtubule-associated protein 2 (MAP2) by the composites with high conductivity. These results suggest that P3HT/PLGA with suitable electrical conductivity have a positive role in promoting neural growth and differentiation, which is promising for advancing potential application of nerve repair and regeneration.

A robust nickel catalyst with an unsymmetrical propyl-bridged diphosphine ligand for catalyst-transfer polymerization

A new nickel diphosphine catalyst has been synthesized in which the bidentate ligand has two different phosphine donors, a typical PPh2 group and a stronger σ-donating PEt2 group. The catalyst was highly effective for the chain-growth polymerization of a 3-alkylthiophene monomer using a Suzuki–Miyaura cross-coupling. The catalyst is particularly effective for this polymerization in the presence of excess free ligand. The unsymmetrical diphosphine nickel complex reported here represents a new approach to tuning metal-ligand reactivity in the chain-growth polymerization of aromatic monomers. In addition, this new nickel catalyst exhibited increased hydrolytic resistance in the polymerization as compared to commercially available 1,3-bis(diphenylphosphino)propane nickel dichloride. A new nickel diphosphine catalyst was synthesized and evaluated for Suzuki–Miyaura cross-coupling polymerization. The diphosphine is comprised of two electronically and sterically distinct phosphine donors, a PPh2 group and a PEt2 group. The catalyst was employed to bring about controlled polymerization of a 3-hexylthiophene monomer to afford poly(3-hexylthiophene). The catalyst was particularly effective for bringing about this polymerization in the presence of excess free ligand. The catalyst resting state was also probed using NMR spectroscopy and an externally initiated catalyst.

Highly Water‐Soluble Rod–Coil Conjugated Block Copolymer for Efficient Humidity Sensor

AbstractIn this report, the preparation of highly water‐soluble rod–coil conjugated block copolymer poly(3‐hexylthiophene)‐b‐polystyrenesulfonic acid (P3HT‐b‐PSSA) is demonstrated using a facile method with its moisture sensing properties. The block copolymer synthesis method comprises Kumada catalyst transfer polymerization and atom transfer radical polymerization from a bifunctional initiator followed by sulfonation of polystyrene using moderate reaction conditions. The polymerization results in the synthesis of well‐defined block copolymers with controllable block length. The successful synthesis of the block copolymer is studied by NMR and FTIR spectroscopy while optical and structural properties of the block copolymer are investigated using UV–vis, photoluminescence spectroscopy, XRD, and FESEM. In water, the block copolymer shows aggregated structure with crystalline core formed by rod‐like P3HT chain with absorption maxima at 558 nm, whereas in solid state the absorption maxima is blue shifted to 548 nm. The proton conductivity of the block copolymer P3HT‐b‐PSSA with ≈91% of PSSA (by weight) is measured from impedance study, and the values for bulk and grain conductivities are 5.25 × 10−4 and 4.66 × 10−6 S cm−1, respectively, at room temperature. The as‐synthesized block copolymer shows a very high water uptake with maximum ≈80% in comparison with its initial weight. The I–V measurement of the device made from block copolymer shows nonlinear, rectifying characteristic and the current increases with increase of relative humidity (RH%). The block copolymer device shows well‐correlated systemic and reversible resistance change with RH both in doped and undoped state. It is believed that the interesting and highly reversible moisture‐sensitive electronic properties of this block copolymer will be useful for the fabrication of moisture‐sensitive polymer‐based flexible electronic devices.

Photostable Helical Polyfurans

This report describes the design and synthesis of a new class of polyfurans bearing ester side chains. The macromolecules can be synthesized using catalyst-transfer polycondensation, providing precise control over molecular weight and molecular weight distribution. Such obtained furan ester polymers are significantly more photostable than their alkyl analogues owing to the electron-withdrawing nature of the attached subunit. Most interestingly, they spontaneously fold into a compact π-stacked helix, yielding a complex multilayer cylindrical nanoparticle with a hollow, rigid, conjugated core composed of the polyfuran backbone and a soft, insulating outer layer formed by the ester side chains. The length of polymer side chains dictates the outer diameter of such nanoparticles, which for the hexyl ester groups used in the present study is equal to ∼2.3 nm. The inner cavity of the conjugated core is lined with oxygen atoms, which set its effective diameter to 0.4 nm. Furthermore, installation of bulkier, branched chiral ester side chains on the repeat unit yields structures that, upon change of solvent, can reversibly transition between an ordered chiral helical folded and disordered unfolded state.