Conjugated Block Copolymers Research Articles

Recent Developments in Chain-Growth Polymerizations of Conjugated Polymers

In this Review, we discuss recent developments and advancements in chain-growth polymerizations for conjugated polymers. Controlled synthesis methods will produce conjugated polymers with finely tuned properties and facilitate their development in new industrial applications. We begin with a discussion on the inherent differences between step-growth, chain-growth, and controlled chain-growth polymerization mechanisms. We emphasize that verification of polymerization mechanisms by kinetic analyses is necessary for the development of new controlled polymerizations. Next, we highlight recent advances in this rapidly growing field. Controlled syntheses of conjugated polymers usually employ a catalyst transfer mechanism, but techniques utilizing different controlled mechanisms are also beginning to emerge. We then highlight reported limitations that serve as a platform for future studies. Finally, to emphasize the potential of controlled chain-growth polymerizations, the synthesis of conjugated block copolymer...

Multiple polarizations and nanostructure of double-stranded conjugated block copolymer for enhancing dielectric performance

The double-stranded conjugated triblock copolymers are synthesized by tandem metathesis cyclopolymerization and ring-opening metathesis polymerization. They display high permittivity of 28–26, low dielectric loss of 0.06–0.02, and good stored/released energy density of 2.96–2.89/2.58–2.75Jcm−3 at 245MVm−1 accompanied with the excellent charge-discharge efficiency of 87.2–95.2% when the copolymers self-assemble into a core-shell nanostructure, which are better than those of both homopolymer and copolymers without nanostructure.

Patchy Nanofibers from the Thin Film Self-Assembly of a Conjugated Diblock Copolymer.

An unexpected morphology comprising patchy nanofibers can be accessed from the self-assembly of an all-conjugated, polyselenophene-block-polythiophene copolymer. This morphology consists of very small (<10 nm), polythiophene- and polyselenophene-rich domains and is unprecedented for both conjugated polymers and diblock copolymers in general. We propose that the patchy morphology occurs from the enhanced miscibility of the blocks arising from the longer alkyl chains in comparison to similar block copolymers with shorter alkyl chains, which fully phase separate, as well as the difference in rigidity between the polythiophene and polyselenophene blocks. This work demonstrates a facile way to tune the self-assembly behavior of conjugated block copolymers by modification of the side chain substituents.

Patchy Nanofibers from the Thin Film Self‐Assembly of a Conjugated Diblock Copolymer

AbstractAn unexpected morphology comprising patchy nanofibers can be accessed from the self‐assembly of an all‐conjugated, polyselenophene‐block‐polythiophene copolymer. This morphology consists of very small (&lt;10 nm), polythiophene‐ and polyselenophene‐rich domains and is unprecedented for both conjugated polymers and diblock copolymers in general. We propose that the patchy morphology occurs from the enhanced miscibility of the blocks arising from the longer alkyl chains in comparison to similar block copolymers with shorter alkyl chains, which fully phase separate, as well as the difference in rigidity between the polythiophene and polyselenophene blocks. This work demonstrates a facile way to tune the self‐assembly behavior of conjugated block copolymers by modification of the side chain substituents.

Architectural Effects on Solution Self-Assembly of Poly(3-hexylthiophene)-Based Graft Copolymers.

While solution assembly of conjugated block copolymers has been widely used to produce long 1-D nanowires (NWs), it remains a great challenge to provide a higher level of control over structure and function of the NWs. Herein, for the first time, we report the solution assembly of graft copolymers containing a conjugated polymer backbone in a selective solvent and demonstrate that their self-assembly behaviors can be manipulated by the molecular structures of the graft copolymers. A series of poly(3-hexylthiophene)-graft-poly(2-vinylpyridine) (P3HT-g-P2VP) copolymers was designed with two different architectural parameters: grafting fraction (fg) and molecular weight of P2VP chains (Mn,P2VP) on the P3HT backbone. Interestingly, crystallization of the P3HT-g-P2VP copolymers was systematically modulated by changes in fg and Mn,P2VP, thus allowing for control of the growth kinetics and curvatures of solution-assembled NWs. When Mn,P2VP (4.4 to 15.1 kg/mol) or fg (2.8 to 9.2%) of the P3HT-g-P2VP polymers was increased, the crystallinity of the copolymers was reduced significantly. Steric hindrance from the grafted P2VP chains apparently modified the growth of NWs, leading to shorter NWs with a greater degree of curvature for graft copolymers with more hindrance. Therefore, we envision that such conjugated chain-based graft copolymers can be versatile building blocks for producing NWs with controlled length and shape, which can be important for tailoring the optical and electrical properties of NW-based devices.

Tuning the synthesis of fully conjugated block copolymers to minimize architectural heterogeneity

Control of conversion, end group composition, and feed ratio is crucial to minimize homopolymer impurities in the synthesis of conjugated block copolymers for photovoltaics.

Thiophene-fused 1,10-phenanthroline toward a far-red emitting conjugated polymer and its polymer dots: synthesis, properties and subcellular imaging

A new far-red emitter based on a conjugated block copolymer was developed, leading to the formation of compact Pdots for specific subcellular imaging.

(D)n–σ–(A)m type partially conjugated block copolymer and its performance in single-component polymer solar cells

We synthesized two types of novel poly(3-alkylthiophene)-free (D)n-b-(A)m conjugated block copolymers: PTQI-block-PNDISs.

Orthogonal solubility in fully conjugated donor-acceptor block copolymers: Compatibilizers for polymer/fullerene bulk-heterojunction solar cells

Donor-acceptor (D-A) type fully conjugated block copolymer systems have been rarely reported due to the challenges in synthetic approaches to prepare well-defined low-polydispersity products. In this work, fully conjugated block copolymers are synthesized in a one-pot reaction through Stille coupling polycondensation, by utilizing the end-functional polymer copolymerization method. End-functional P3HT are copolymerized with AA (2,7-dibromo-9-(heptadecan-9-yl)-9H-carbazole) and BB (4,7-bis(5-(trimethylstannyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole, TBT) type monomers, respectively. The orthogonal solubility between the very soluble P3HT donor and the insoluble PCDTBT acceptor block improves the purity of block copolymers as well as distinct nano-scale phase-separation compared with other reports on miscibility of donor and acceptor polymer block. Further purification via preparative GPC is carried out to remove the excess of unreacted P3HT and free PCDTBT as well as to achieve low polydispersity of block copolymers. The chemical structure of the P3HT-b-PCDTBT block copolymers are verified via 1H-NMR, and further confirmed by FTIR spectra. The block copolymer shows broad absorption and moderate optical band gap of 1.8 eV. Furthermore, the fully conjugated block copolymer films exhibit significant fine structures, much smoother film morphology compared to P3HT/PCDTBT polymer blends. By adding a small amount of block copolymer P3HT-b-PCDTBT as a compatibilizer into the bulk-heterojunction of P3HT:PC61BM blends, polymer solar cells with an 8% increase of short circuit current (J sc and 10% increase of power conversion efficiency (PCE) are achieved owing to the improvement of the active-layer film morphology. To the best of our knowledge, this is the first report on donor-acceptor type fully conjugated block copolymer as an effective ternary additive in polymer: fullerene bulk heterojunction solar cells.

Role of Solution Structure in Self-Assembly of Conjugated Block Copolymer Thin Films

Conjugated block copolymers provide a pathway to achieve thermally stable nanostructured thin films for organic solar cells. We characterized the structural evolution of poly(3-hexylthiophene)-block-poly(diketopyrrolopyrrole–terthiophene) (P3HT-b-DPPT-T) from solution to nanostructured thin films. Aggregation of the DPPT-T block of P3HT-b-DPPT-T was found in solution by small-angle X-ray scattering with the P3HT block remaining well-solvated. The nanostructure in thin films was determined using a combination of wide and small-angle X-ray scattering techniques as a function of processing conditions. The structure in solution controlled the initial nanostructure in spin-cast thin films, allowing subsequent thermal annealing processes to further improve the ordering. In contrast to the results for thin films, nanostructural ordering was not observed in the bulk samples by small-angle X-ray scattering. These results suggest the importance of controlling solvent induced aggregation in forming nanostructured th...

A Heparin-Mimicking Block Copolymer Both Stabilizes and Increases the Activity of Fibroblast Growth Factor 2 (FGF2).

Fibroblast growth factor 2 (FGF2) is a protein involved in cellular functions in applications such as wound healing and tissue regeneration. Stabilization of this protein is important for its use as a therapeutic since the native protein is unstable during storage and delivery. Additionally, the ability to increase the activity of FGF2 is important for its application, particularly in chronic wound healing and the treatment of various ischemic conditions. Here we report a heparin mimicking block copolymer, poly(styrenesulfonate-co-poly(ethylene glycol) methyl ether methacrylate)-b-vinyl sulfonate) (p(SS-co-PEGMA)-b-VS, that contains a segment that enhances the stability of FGF2 and one that binds to the FGF2 receptor. The FGF2 conjugate retained activity after exposure to refrigeration (4 °C) and room temperature (23 °C) for 7 days, while unmodified FGF2 was inactive after these standard storage conditions. A cell study performed with a cell line lacking native heparan sulfate proteoglycans indicated that the conjugated block copolymer facilitated binding of FGF2 to its receptor similar to the addition of heparin to FGF2. A receptor-based enzyme-linked immunosorbant assay (ELISA) confirmed the results. The conjugate also increased the migration of endothelial cells by 80% compared to FGF2 alone. Additionally, the FGF2-p(SS-co-PEGMA)-b-VS stimulated endothelial cell sprouting 250% better than FGF2 at low concentration. These data verify that this rationally designed protein-block copolymer conjugate enhances receptor binding, cellular processes such as migration and tube-like formation, and stability, and suggest that it may be useful for applications in biomaterials, tissue regeneration, and wound healing.

Enhanced cellular uptake and phototoxicity of Verteporfin-conjugated gold nanoparticles as theranostic nanocarriers for targeted photodynamic therapy and imaging of cancers

Activatable theranostics with the capacity to respond to a given stimulus have recently been intensively explored to develop more specific, individualized therapies for various diseases, and to combine diagnostic and therapeutic capabilities into a single agent. In this work, we designed tumor-targeting ligand-conjugated block copolymer-gold nanoparticle (AuNP) conjugates as multifunctional nanocarriers of the hydrophobic photosensitizer (PS), verteporfin (Verte), for simultaneous photodynamic therapy and imaging of cancers. Folic acid (FA)-conjugated block copolymers composed of polyethylene glycol (PEG) and poly-β-benzyl-l-aspartate (PBLA) were attached to citrate-stabilized AuNPs through a bidentate dihydrolipoic acid (DHLA) linker. The resulting AuNP conjugates (FA-PEG-P(Asp-Hyd)-DHLA-AuNPs) were significantly more stable than unmodified AuNPs, and their optical properties were not affected by pH. The hydrophobic PS, Verte, was covalently incorporated onto the surfaces of the AuNP conjugates through a pH-sensitive linkage, which increased the water solubility of Verte from <1μg/ml to >2000μg/ml. The size of FA-PEG-P(Asp-Hyd)-DHLA-AuNPs-Verte as determined by light-scattering measurements was about 110.3nm, and FE-SEM and FE-TEM images showed that these nanoparticles were spherical and showed adequate dispersivity after modification. In particular, an in vitro cell study revealed high intracellular uptake of FA-PEG-P(Asp-Hyd)-DHLA-AuNPs-Verte (about 98.62%) and marked phototoxicity after laser irradiation compared with free Verte. These results suggest that FA-PEG-P(Asp-Hyd)-DHLA-AuNPs-Verte has great potential as an effective nanocarrier for dual imaging and photodynamic therapy.

Polymer-Chlorambucil Drug Conjugates: A Dynamic Platform of Anticancer Drug Delivery.

Recently, polymer drug conjugates (PDCs) have attracted considerable attention in the treatment of cancer. In this work, a simple strategy has been developed to make PDCs of an antitumor alkylating agent, chlorambucil, using a biocompatible disulphide linker. Chlorambucil-based chain transfer agent was used to prepare various homopolymers and block copolymers in a controlled fashion via reversible addition-fragmentation chain transfer polymerization. Chlorambucil conjugated block copolymer, poly(polyethylene glycol monomethyl ether methacrylate)-b-poly(methyl methacrylate), formed nanoaggregates in aqueous solutions, which are characterized by dynamic light scattering and field emission-scanning electron microscopy. Finally, the simplicity of the design is exemplified by performing a release study of chlorambucil under reducing condition by using D,L-dithiothreitol.

Molecular Rectification in Conjugated Block Copolymer Photovoltaics

We investigate the influence that covalent linkage of electron donating and accepting blocks in high performance fully conjugated block copolymer photovoltaics has on charge generation and recombination using ultrafast mid-infrared transient absorption spectroscopy. We show that block copolymer architectures containing a conjugated bridge between the donor and acceptor groups can be used to form ordered mesoscale morphologies that lead to improved photovoltaic performance without enhancing charge recombination. Judicious placement of an electron-rich moiety in the electron accepting block of the block copolymer creates a donor–bridge–acceptor architecture that slows intramolecular charge transfer across the covalent linkage. Charge recombination in such donor–bridge–acceptor block copolymer films proceeds at the same rate as it does in their corresponding homopolymer blends for which the donor and acceptor blocks are not covalently linked, indicating that recombination is dominated by intermolecular charg...

Design of bicontinuous donor/acceptor morphologies for use as organic solar cell active layers

In recent works, we demonstrated the achievement of bicontinuous donor/acceptor morphologies by the addition of conjugated block copolymers to a blend of conjugated homopolymer donors and fullerene acceptors. However, the domain sizes resulting in experiments were much larger than those of interest for high-performance organic solar cells. Moreover, a significant concentration of fullerene acceptors was present in the donor domains. Here, we utilize simulations to study the bicontinuous donor/acceptor morphologies that result for different parametric conditions. Using such results, we provide guidelines for how to blend polymer materials to give rise to bicontinuous phases with the smaller and more compositionally pure domains that are desirable for organic photovoltaic applications. Our results can be generalized to treat a large range of donor and acceptor monomers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 884–895

Fully conjugated block copolymers for single-component solar cells: synthesis, purification, and characterization

All-polymer solar cells using the preparative GPC separated block copolymer P3HT-b-PBIT2 as a simple active layer show a power conversion efficiency of 1.0%.

Challenges and Opportunities in the Development of Conjugated Block Copolymers for Photovoltaics

Fully conjugated block copolymers that covalently link donor and acceptor units within the molecular structure have the potential to simultaneously control the mesoscale assembly and interfacial structure. As a consequence, utilizing block copolymers as the active layer provides a unique opportunity to tune charge and energy transfer processes while retaining a mesoscale scaffold for efficient charge extraction within photovoltaic devices. Recent progress in synthesis, structural characterization, modeling, and device fabrication suggest significant advances in block copolymer materials are in the near future—both as model materials for systematic studies of charge and energy transfer and as high performance materials for photovoltaic devices. In this Perspective, we describe recent efforts, current challenges, and opportunities in the development of fully conjugated block copolymers for organic photovoltaics.

Conjugated polymer/fullerene nanostructures through cooperative non-covalent interactions for organic solar cells

We have previously reported the formation of polymer/fullerene core–shell composite nanofibers (NFs) through self-assembly of functionalized conjugated block copolymers and fullerene derivatives, by cooperation of orthogonal non-covalent interactions including block copolymer phase separation, fullerene aggregation, poly(3-hexylthiophene) (P3HT) crystallization and complementary hydrogen bonding. These NFs displayed improved solar cell performances and the core–shell type structures are generally insensitive to polymer/fullerene weight ratios. In order to better understand the self-assembly mechanisms of such complex systems, we have prepared a new block copolymer (P3) having a different block length ratio and studied its solution and thin film morphologies in the presence of fullerene derivatives at different weight ratios. Worm-like micelles are observed for P3 in mixed-solvent solutions, distinctly different from the well-defined NFs observed previously. Upon fullerene additions, the solution morphologies gradually change into hierarchically complex nanostructures with varying fullerene contents. Thin film morphologies are studied by atomic force microscopy (AFM) and the blends are applied in organic solar cell (OSC) devices. Our present studies provide detailed information on the self-assembly behaviors of conjugated block copolymers and fullerenes under cooperative orthogonal non-covalent interactions.

Conjugated Polymers: Linking Group Influences Charge Separation and Recombination in All‐Conjugated Block Copolymer Photovoltaics (Adv. Funct. Mater. 35/2015)

Fully conjugated block copolymers with donor and acceptor polymer blocks can function as the active layer of an organic photo-voltaic device. On page 5578, R. Verduzco and co-workers demonstrate that the linking group at the donor-acceptor junction is critical for charge separation. Proper design of the linking group is essential for producing viable block copolymer photovoltaic devices.

Rational Design of Thermally Stable, Bicontinuous Donor/Acceptor Morphologies with Conjugated Block Copolymer Additives.

The bicontinuous microemulsion (BμE) phase is an equilibrium morphology characterized by cocontinuous domains, high interfacial areas, and nanoscale domain dimensions. These characteristics make the BμE potentially suitable for use in organic photovoltaic applications. Here, we use a combination of simulations and experiments to investigate the equilibrium morphologies formed by a ternary blend of conjugated polymer, all-conjugated diblock copolymer, and fullerene derivative PCBM. Using coarse-grained simulations, we identify the blend compositions that are most likely to result in donor/acceptor morphologies resembling the BμE. Experimentally, we probe these compositions through transmission electron microscopy and grazing-incidence X-ray scattering measurements. We demonstrate that all-conjugated block copolymer additives can be used to produce thermally stable, cocontinuous donor/acceptor morphologies at higher additive contents and longer annealing times than previously reported. These results demonstrate that conjugated BCP compatibilizers can be used as a means to achieve equilibrium, cocontinuous morphologies in donor/acceptor blends.