All-Conjugated, All-Crystalline Donor–Acceptor Block Copolymers P3HT‑<i>b</i>‑PNDIT2 via Direct Arylation Polycondensation

The synthesis and characterization of all-conjugated, all-crystalline donor–acceptor block copolymers (BCPs) containing poly­(3-hexyl­thiophene) (P3HT) and poly­{[<i>N</i>,<i>N</i>′-bis­(2-octyl­dodecyl)­naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)} (PNDIT2) is presented. Direct arylation polycondensation (DAP) of dibromo­naphthalene­diimide and bithiophene is carried out in the presence of P3HT end-cappers to allow the in situ formation of BCPs P3HT-<i>b</i>-PNDIT2. As-prepared, well-defined H-P3HT-Br with hydrogen and bromine chain termini shows nonoptimal reactivity under the DAP conditions used. Therefore, H-P3HT-Br is converted into either H-P3HT-Th (thiophene) or H-P3HT-Mes (mesitylene), giving α,ω-hetero-C–H functionalized P3HT with modulated C–H reactivity. The influence of the different C–H chain termini of P3HT on the ability to act as end-capper and the resulting block structures is investigated in detail using wavelength-dependent size exclusion chromatography (SEC) and NMR spectroscopy. Different C–H reactivities of α,ω-hetero-C–H functionalized P3HT cause different contents of multiblocks, which in turn lead to varied degrees of crystallinity. These results show that careful tuning of C–H reactivity is a promising way to obtain well-defined, all-conjugated block copolymers via DAP

Rational Use of Aromatic Solvents for Direct Arylation Polycondensation: C–H Reactivity versus Solvent Quality

The solvent for direct arylation polycondensation (DAP) is of crucial importance. For conjugated polymers exhibiting reduced solubility, the choice of solvent decides on the maximum molecular weight that can be achieved, hence, good aromatic solvents are generally desirable. However, unintentional activation of C–H bonds present in aromatic solvents under DAP conditions leads to in situ solvent termination which competes with step growth. Here we evaluate relative C–H reactivity and solvent quality of seven aromatic solvents for the DAP of defect-free naphthalene diimide (NDI)-based copolymers of different solubility. C–H reactivity is strongly reduced with increasing degree of substitution for both chlorine and methyl substituents. Mesitylene is largely C–H unreactive and, thus, albeit being a moderate solvent, enables very high molecular weights at elevated temperature for NDI copolymers with limited solubility

Alkoxide-Initiated Regioselective Coupling of Carbon Disulfide and Terminal Epoxides for the Synthesis of Strongly Alternating Copolymers

The synthesis of highly regioregular and alternating polythiocarbonates from carbon disulfide and terminal epoxides has been achieved. The copolymerizations were performed under ambient and solvent-free conditions in the presence of LiO^<i>t</i>Bu (0.125–0.5 mol %) as initiator. At higher loadings the reaction pathway switched in favor to the catalytic formation of cyclic dithiocarbonates. Under optimized reaction conditions polymers with molecular weights up to 109 kg mol^–1 were isolated. The NMR spectroscopic analysis of the polythiocarbonates revealed that 94% of backbone structure is formed by strongly alternating head-to-head arrangement of epoxypropane and 1,2-epoxybutane monomers, respectively, at a thiocarbonate group −CHR–OC­(S)­O–CHR– and tail-to-tail arrangement at a trithiocarbonate group −CH₂–SC­(S)­S–CH₂–. Atactic polymers were obtained using racemic mixtures of the epoxides, but an isotactic polymer was obtained when chiral (<i>R</i>)-epoxy­propane was converted. A mechanism is proposed which rationalizes the regio- and stereochemistry observed for the alkoxide-initiated copolymerization of CS₂ and terminal epoxides

Defect-free Naphthalene Diimide Bithiophene Copolymers with Controlled Molar Mass and High Performance via Direct Arylation Polycondensation

A highly efficient, simple, and environmentally friendly protocol for the synthesis of an alternating naphthalene diimide bithiophene copolymer (PNDIT2) via direct arylation polycondensation (DAP) is presented. High molecular weight (MW) PNDIT2 can be obtained in quantitative yield using aromatic solvents. Most critical is the suppression of two major termination reactions of NDIBr end groups: nucleophilic substitution and solvent end-capping by aromatic solvents via C–H activation. In situ solvent end-capping can be used to control MW by varying monomer concentration, whereby end-capping is efficient and MW is low for low concentration and vice versa. Reducing C–H reactivity of the solvent at optimized conditions further increases MW. Chain perfection of PNDIT2 is demonstrated in detail by NMR spectroscopy, which reveals PNDIT2 chains to be fully linear and alternating. This is further confirmed by investigating the optical and thermal properties as a function of MW, which saturate at <i>M</i>_n ≈ 20 kDa, in agreement with controls made by Stille coupling. Field-effect transistor (FET) electron mobilities μ_sat up to 3 cm²/(V·s) are measured using off-center spin-coating, with FET devices made from DAP PNDIT2 exhibiting better reproducibility compared to Stille controls

Enhancing Phase Separation and Photovoltaic Performance of All-Conjugated Donor–Acceptor Block Copolymers with Semifluorinated Alkyl Side Chains

Phase separation of all-conjugated donor–acceptor block copolymers is more difficult to achieve compared to classical coil–coil systems owing the intrinsic similarity of the two blocks having both rigid conjugated backbones and alkyl side chains and their generally low degrees of polymerization. Here we demonstrate that side chain fluorination of a poly­(carbazole-<i>alt</i>-dithienylbenzothiadiazole) segment (SF-PCDTBT), to be used as electron acceptor block in combination with poly­(3-hexylthiophene) P3HT as donor block in all-conjugated donor–acceptor block copolymers of type SF-PCDTBT-<i>b</i>-P3HT, strongly increases dissimilarity between P3HT and SF-PCDTBT leading to phase separation for already moderate molar masses. Key to the successful synthesis of a new TBT-monomer with semifluorinated side chains is a direct arylation step that elegantly bypasses classical cross-coupling reactions in which the semifluorinated side chain causes low yields. Suzuki polycondensation of the semifluorinated TBT monomer with a suitable carbazole comonomer and in situ termination by P3HT-Br is optimized extensively with respect to the yield of the end-capping efficiency and molar mass control of the PCDTBT segment. When the fluorinated side chains are replaced by hydrogen (H-PCDTBT) or by <i>n</i>-hexyl chains (hex-PCDTBT), the tendency for phase separation with covalently connected P3HT is much reduced as shown by differential scanning calorimetry and grazing incidence small-angle scattering measurements on thin films. Favorably, of all the block copolymers made only SF-PCDTBT-<i>b</i>-P3HT is microphase separated, exhibits face-on orientation of P3HT domains, and additionally displays surface segregation of the SF-PCDTBT segment at the polymer/air interface. All of these properties are beneficial for single layer single component solar cells. SF-PCDTBT-<i>b</i>-P3HT exhibits the best solar cells performance with a high open-circuit voltage of 1.1 V and a power conversion efficiency of ∼1% which largely outperforms devices based on the analogous H-PCDTBT-<i>b</i>-P3HT and hex-PCDTBT-<i>b</i>-P3HT

Synthesis of Allyl-Terminated Polar Macromonomers by Metallocene-Catalyzed Polymerizations of 10-Undecene-1-ol

Poly­(10-undecene-1-ol) macromonomers were synthesized by metallocene-catalyzed polymerization using vinyl chloride as a chain transfer agent. Using this technique, predominantly allyl terminated polymers could be obtained, which was verified by NMR spectroscopy and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). By means of the mass spectra, a detailed interpretation of the nature of the head and end groups of these polymers was possible

Synthesis of Magnetic Polystyrene Nanoparticles Using Amphiphilic Ionic Liquid Stabilized RAFT Mediated Miniemulsion Polymerization

Imidazole based amphiphilic ionic liquids (ILs) were used as surfactants in miniemulsion polymerization (MEP) of styrene using a free radical process as well as reversible addition–fragmentation chain transfer (RAFT). Monodisperse polystyrene (PS) nanoparticles were obtained, demonstrating the efficiency of the amphiphilic IL as surfactant in MEP. IL stabilized miniemulsion was furthermore used to prepare polystyrene based magnetic nanoparticles (MNP). A large increase of the possible MNP content associated with very good colloidal stability was achieved using IL stabilized RAFT mediated MEP where a carboxyl functionalized chain transfer agent (CTA) was applied, allowing interaction with the MNP surface. The molecular weight and dispersity index of polystyrene, the content of MNP, and the morphologies of the hybrid nanoparticles were controlled by proper optimization of the concentration of initiator and CTA. The materials have been analyzed by NMR, GPC, DLS, SEM, TEM, and TGA. Finally, the magnetic properties of the materials were determined by vibrating sample magnetometer (VSM) analysis

Formation of Oligomeric and Macrocyclic Ureas Based on 2,6-Diaminopyridine

The conversion of 1,3-bis-(6-amino-pyridin-2-yl)-urea (<b>1</b>) with <i>N</i>,<i>N</i>′-carbonyldiimidazole at high temperatures in DMSO yielded a mixture of defined cyclic trimers and tetramers. On the basis of model reactions, exchange reactions were evidenced, which convert the cyclic tetramer into a stable cyclic trimer. Linear even numbered oligomers were obtained in acetone under reflux where side reactions were suppressed. The pronounced tendency of cyclization is attributed to a preferred folded conformation of the urea bond between two pyridyl units

Network Formation, Properties, and Actuation Performance of Functionalized Liquid Isoprene Rubber

Due to some useful mechanical, dynamic, and dielectric properties along with the ease of processing and forming, liquid rubbers are ideal materials for fabricating dielectric elastomer actuators in various configurations and for many potential applications ranging from automation to automobile and medical industry. In this study, we present a cross-linkable liquid rubber composition where amine-catalyzed esterification reactions lead to the formation of a network structure based on anhydride functional isoprene rubber, carboxyl-terminated nitrile-butadiene rubber, and epoxy end-capped prepolymers. The success of this intricate network formation procedure was verified by HR-MAS NMR spectroscopy. The new isoprene-based elastomeric material exhibits actuation-relevant attributes including a low elastic modulus of 0.45 MPa, soft response to an applied load up to a large deformation of 300%, and a dielectric constant value (2.6) higher than the conventional Elastosil silicone (2.2). A dot actuator comprising of an isoprene dielectric elastomer film in unstretched state and carbon paste electrodes was fabricated that demonstrated an electrode deformation of 0.63%, which is nearly twice as high as for the commercial Elastosil 2030 film (∼0.30%) at 5 kV. Compared to the Elastosil silicone film, the enhanced performance is attributed to the low modulus and high dielectric constant value of the new isoprene elastomer

Fullerene-Functionalized Donor–Acceptor Block Copolymers through Etherification as Stabilizers for Bulk Heterojunction Solar Cells

A new synthetic method for the covalent linking of fullerenes to polymers is introduced. The Bingel-reaction was used to prepare bromine-functionalized fullerene building blocks that could be covalently linked to hydroxyl groups of model copolymers by the cesium carbonate promoted Williamson ether synthesis. Subsequently, block copolymers with a second block based on styrene and hydroxystyrene or hydroxyethyl methacrylate could be synthesized with a poly­(3-hexylthiophene)–TEMPO macroinitiator through NMRP. Fullerene derivatives were linked to these polymers in a controlled manner and donor–acceptor block copolymers with high fullerene contents of near 50 wt % were achieved