Thieno[3,4‐c]pyrrole‐4,6‐dione‐Based Polymers for Optoelectronic Applications

Over the last decade, great progress has been made in the field of organic electronics. Advancements in organic syntheses as well as in device engineering enabled preparation of polymer solar cells with power conversion efficiency (PCE) exceeding 8%–9%. In search for new polymers suitable for photovoltaic applications, push–pull polymers containing thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) motif as an electron deficient (pull) unit emerged as very promising candidates. This Trend Article summarizes research on TPD‐based polymers with a special emphasis on the structure–property relationships.     

Preparation of Poly(p‐phenylenepyromelliteimide) Microspheres with Rugged Surfaces Using Crystallization During Isothermal Polymerization

Preparation of discrete PPPI microspheres was examined using the crystallization of oligomers during isothermal polymerization of PMDA and p‐phenylene diisocyanate, and that of pyromellitic dithioanhydride and PPDA in poor solvents. In contrast to PPPI microspheres with smooth surfaces that were previously prepared by phase‐separation during the polymerization of PPDA and PMDA, the microspheres obtained here consisted of flake‐like lamellae with rugged surfaces. The diameters of the microspheres ranged from 0.5 to 6.7 µm. They exhibited excellent thermal stability depending on the monomers. These results provide a new preparative procedure of the PPPI microsphere with a rugged surface.

     

Grafting Polymers from Poly(2,6‐dimethyl‐1,4‐phenylene oxide) as New Thermoplastics

Graft polymers with poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) as backbones are successfully prepared via two convenient steps. The utilization of semiflexible PPO as backbones offers unique properties for the graft polymers. Thermal, rheological, and phase behaviors of these new graft polymers are well controlled via the precise design of architectural parameters. The disordered microphase separation in melt state and the proper composition of side grafts provide the ease of thermal processing for these graft polymers. The graft density shows impact on the relaxation and mechanical properties of the thermoplastics. This work shows the possibility to use lots of semiflexible engineering polymers as backbones to construct new thermoplastics.

     

Characterization and Properties of Poly[N‐(2‐cyanoethyl)pyrrole]

PNCPy was prepared by anodic polymerization and its properties in both doped and undoped state were characterized. The doping level of the oxidized material has been found to be larger than that of other conducting polymers; the more relevant electrochemical properties of the doped material were retained after undoping. SEM and AFM data are consistent with a lumpy surface and a multidirectional growing of the polymer chains. Finally, PNCPy has been combined with PEDOT to prepare three‐layer systems with enhanced electroactivity and electrostability. Results suggest that PNCPy is a potential candidate for the fabrication of electric circuit components that are able to block the current flow below a given potential.

     

A New Class of Organosoluble Rigid‐Rod Fully Aromatic Poly(1,3,4‐oxadiazole)s and Their Solid‐State Properties, 1. Synthesis

Since it is known, that poly(p‐phenylene‐1,3,4‐oxadiazole) exhibits reversible reduction behaviour in cyclovoltammetric measurements, it may be used as an electron transport material in electronic devices such as light emitting diodes (LEDs). However, the polymers are difficult to process, for example as thin films. By introducing linear or branched alkoxy side groups into the backbone it is possible to obtain rigid rod, fully aromatic poly(1,3,4‐oxadiazole)s which are soluble in organic solvents. Various polycondensation routes were investigated to prepare symmetrical and unsymmetrical dialkoxy‐substituted aromatic poly(1,3,4‐oxadiazole)s. Initially the interfacial polycondensation and the low‐temperature solution polycondensation between aromatic dihydrazides and diacid chlorides in N‐methylpyrrolidone were used. These methods were not successful for the synthesis of the polyoxadiazole with branched side chains. Thus, a new synthetic method for the preparation of such polymers was developed. The polycondensations are carried out in hot 1,2‐dichlorobenzene in the presence of a basic acceptor like pyridine. These conditions are unusual for such types of reaction.     

Poly(propylene terephthalate) Modified with 2,2‐Bis[4‐(ethylenoxy)‐1,4‐phenylene]propane terephthalate) Units: Thermal Behaviour,Crystallisation Kinetics and Morphology

Summary: The thermal behaviour of poly(propylene terephthalate) modified with 2,2‐bis[4‐(ethylenoxy)‐1,4‐phenylene]propane terephthalate) units (PPT/BHEEBT copolymers) was investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and hot‐stage optical microscopy (MO). Good thermal stability was found for each sample. The thermal analysis carried out using the DSC technique showed that the T_m of the copolymers decreased with the increment in BHEEBT unit content. This was different from the T_g which, on the contrary, increased. Wide‐angle X‐ray diffraction measurements allowed the identification of the PPT crystalline structure in each semicrystalline sample. Multiple endotherms were shown in the PPT/BHEEBT samples, due to melting and recrystallisation processes, similarly to PPT. The of the copolymers was derived from the application of the Hoffman‐Weeks' method. The isothermal crystallisation kinetics were analysed according to Avrami's treatment. The introduction of BHEEBT units was found to decrease the crystallisation rate compared to pure PPT. Values of the Avrami's exponent n close to 3 were obtained for PPT/BHEEBT6 and PPT/BHEEBT12, regardless of T_c, in agreement with a crystallisation process originating from pre‐determined nuclei and characterised by three‐dimensional spherulitic growth. As a matter of fact, for these two copolymers, space‐filling spherulites were observed through optical microscopy at all T_cs. The heat of fusion (ΔH_m) was correlated to the specific heat increment (Δc_p) for samples with different degrees of crystallinity, and the results were interpreted on the basis of the existence of an interphase, whose amount was found to depend mainly on composition, despite the thermal treatment applied to the polymer also playing an important role.

Calorimetric curves of PPT, PBHEEBT homopolymers and their random copolymers after melt quenching.     

Synthesis and Characterisation of Side‐Chain Liquid Crystalline Poly[1‐({[(4‐cyano‐4′‐biphenyl)oxy]alkyl}oxy)‐2,3‐epoxypropane]

Summary: A new series of side‐chain liquid‐crystal polymers, the poly[1‐({[(4‐cyano‐4′‐biphenyl)oxy]alkyl}oxy)‐2,3‐epoxypropane], has been synthesized in which the spacer length is varied from 2 to 8 methylene units. The synthesis used for the chemical modification of polyepichlorohydrin (PECH) involved the phase transfer catalyzed etherification of the chloromethyl groups with sodium 4‐cyano‐4′‐biphenoxide and lithium n‐(4‐cyano‐4′‐oxybiphenyl)‐alkanoates. All the resulting polymers (except polymers 7 and 8), including that without spacer, characterized by ¹H NMR, and IR, exhibit thermotropic liquid crystalline mesomophism. The thermal behavior of the polymers has been characterized using differential scanning calorimetry (DSC) and polarized light microscopy (POM) equipped with hot‐stage. A more pronounced odd‐even effect in the clearing temperatures is observed on increasing the spacer length in which the odd members display slightly higher values, which were also dependent on the substitution degree of PECH. The flexible PECH chain assists nematic LC formation compared with other more rigid backbone polymers where a smectic phase is often encountered with the same mesogen. A comparison of the thermal properties of the cyanobiphenyl based series reported here. Polyacrylate (PA) and poly(methacrylate)‐based (PMA) SCLCPs containing 4‐cyanobiphenyl as the mesogenic unit are consistent with the general rule that increasing backbone flexibility for a given mesogenic unit and spacer length enhance the clearing temperature. This was not found from the PECH‐based series, which show the lower clearing temperature than the PMA and PA series, even though they have more flexible backbone than PA and PMA series. So the clearing temperature is not solely determined by the flexibility of the backbone.

Schlieren textures of PECHOC2‐B taken at 110 °C (cooling from isotropic phase after annealing 1 h).     

Molecular Packing and Orientation Transition of Crystalline Poly(2,5‐dihexyloxy‐p‐phenylene)

The effect of thermal treatment on the transition of molecular packing and orientation of the semiconducting poly(2,5‐dihexyloxy‐p‐phenylene) (PPP) film is probed by the combination of in situ 2D grazing incidence X‐ray diffraction (2D GIXD) and selected area electron diffraction (SAED). The structure analysis indicates that PPP crystallizes in an orthorhombic unit cell with a = 21.20 Å, b = 3.78 Å, and c = 4.24 Å. A variation of the annealing temperature from 80 °C to 100 °C demonstrates that the worm‐like morphology in the pristine film melts and develops into nanowires. Furthermore, the molecular orientation transforms from face‐on to edge‐on via thermal annealing. Remarkably, a previously unknown metastable state of “slope” edge‐on with a tilt angle of 51.1° is observed. This orientation change arises from the heterogeneous nucleation and growth of edge‐on PPP crystal during crystallization from the melt.

     

Poly[2,7‐(9,9‐dihexylfluorene)]‐block‐Poly(2‐vinylpyridine) Rod–Coil Star‐block Copolymers: Synthesis,Micellar Structures,and Photophysical Properties

The first successful synthesis of conjugated rod–coil star block copolymer, (PF‐b‐P2VP)_n, containing conjugated poly[2,7‐(9,9‐dihexylfluorene)] (PF), and coil‐like poly(2‐vinylpyridine) (P2VP) by combining a Suzuki coupling reaction and living anionic polymerization is reported. With increasing methanol content in THF/methanol mixtures (PF‐b‐P2VP)_n symmetric star‐block copolymers maintain spherical micelles, but PF‐b‐P2VP asymmetric diblock copolymers vary from spherical micelles to vesicles. Both the absorption and emission spectra of PF‐b‐P2VP blue shift with increasing methanol content, suggesting an “H‐type” aggregation. However, (PF‐b‐P2VP)_n star‐block exhibits no shift in absorption but a red shift in the emission spectra, indicating a different type of aggregation. These results suggest the significance of polymer architectures on microphase‐separated morphologies and photophysical properties.

     

Synthesis,Characterization, and Properties of Poly(aryl)phosphinoboranes Formed via Iron‐Catalyzed Dehydropolymerization

The dehydropolymerization of the primary phosphine–boranes, RPH₂•BH₃ ( 1a – f ) (R = 3,4‐(OCH₂O)C₆H₃ ( a ), Ph ( b ), p‐(CF₃O)C₆H₄ ( c ), 3,5‐(CF₃)₂C₆H₃ ( d ), 2,4,6‐(CH₃)₃C₆H₂ ( e ), 2,4,6‐t Bu₃C₆H₂ ( f )) is explored using the precatalyst [CpFe(CO)₂OTf] ( I ) (OTf = OS(O)₂CF₃), based on the earth abundant element Fe. Formation of polyphosphinoboranes [RPH‐BH₂]_n ( 2a – e ) is confirmed by multinuclear NMR spectroscopy, but no conversion of 1f to 2f is detected. Analysis by electrospray ionization mass spectrometry confirms the presence of the anticipated polymer repeat units for 2a – e . Gel permeation chromatography (GPC) confirms the polymeric nature of 2a – e and indicates number‐average molecular weights (M _n) of 12 000–209 000 Da and polydispersity indices between 1.14 and 2.17. By contrast, thermal dehydropolymerization of 1a – e in the absence of added precatalyst leads to formation of oligomeric material. Interestingly, polyphosphinoboranes 2c and 2d display GPC behavior typical of polyelectrolytes, with a hydrodynamic radius dependent on concentration. The thermal transition behavior, thermal stability, and surface properties of thin films are also studied.     

A Highly Efficient One‐Pot Method for the Synthesis of Carbon Black/Poly(4,4′‐Diphenylether‐1,3,4‐Oxadiazoles) Composites

A series of polyoxadiazoles (PODs) based composites containing carbon black (CB) are prepared through a fast polycondensation reaction of hydrazine sulfate and dicarboxylic acid monomers using poly(phosphoric acid) (PPA) as reaction medium. In order to achieve a homogeneous dispersion of CB, a predispersion step is applied to the CB prior to the synthesis. The moderate acidic nature and high viscosity of the PPA medium promote the oxidation of the CB and suppress reaggregation after dispersion. The carboxylic groups generated on the surface of the CB effectively anchored the growing of POD chains, improving the interfacial interaction between the CB and the surrounding matrix. An increase of 22% in storage modulus at 100 °C is observed. The tensile strength increases up to 48% with the CB addition and the elongation at break increases up to 118% at a low concentration of CB (0.5 wt.‐%). The resultant CB/POD composites, which can be cast as dense films, are soluble in several polar aprotic solvents and their POD matrix have molecular weights in the range of 10⁵ g · mol⁻¹. The high‐performance, high‐temperature light‐weight materials obtained are thermally stable at temperatures as high as 450 °C.

     

Tunable Microcellular Morphologies from Poly(ferrocenylsilane) Ceramic Precursors Foamed in Supercritical CO2

Summary: Poly(ferrocenylmethylphenylsilane) (PFMPS) and poly[ferrocenylmethyl(phenylacetylido)silane] (PFMPAS) homopolymers are organometallic polymers that can serve as precursors to magnetically‐active ceramics. In this work, PFMPS and PFMPAS films ranging in thickness from 30 to 220 μm have been exposed to scCO₂ over a broad range of saturation pressures, temperatures and times to produce surface‐constrained microcellular polymeric foams. Electron microscopy identifies viable foaming windows and reveals that foams with submicron cell sizes can be generated by judicious selection of exposure conditions. Cell shape anisotropy can likewise be varied from being nearly spherical to vertically elongated. Curvature in the films and non‐negligible surface roughness create paths that facilitate CO₂ diffusion, resulting in the formation of “V‐directional” cells oriented at approximately 30° to the film surface. The development of bimodal cell size distributions is attributed to temperature variation during depressurization and/or stepwise homogeneous and heterogeneous nucleation. These results are prerequisite for the production of magnetically‐active porous ceramics from pyrolyzed PFS foams.

SEM image of a PFMPS foam generated in scCO₂ illustrating considerable cell anisotropy: the cells are tilted with respect to the film surface.     

Rigid‐Rod Polyamides from 3,3′‐bis­(trifluoromethyl)‐4,4′‐diamino‐1,1′‐biphenyl

A diamine monomer with trifluoromethyl groups, 3,3′‐bis(trifluoromethyl)‐4,4′‐diamino‐1,1′‐biphenyl, is synthesized from 5‐bromo‐2‐nitrobenzotrifluoride with two steps. The model reaction with monofunctional benzoyl chloride at room temperature gives the model compound with a quantitative yield. The results of the model reaction indicate that the amine groups have sufficient reactivity in spite of the presence of electron‐withdrawing and bulky trifluoromethyl group at the ortho position. The monomer is polymerized with terephthaloyl chloride and isophthaloyl chloride in N,N‐dimethylacetamide (DMAc) or DMAc containing LiCl using pyridine as an acid acceptor at room temperature. All synthesized polymers are somewhat crystalline and colorless. While the meta‐linked polyamide ( PA2 ) shows excellent solubility in polar aprotic solvents, the para‐linked one ( PA1 ) is dissolved in polar aprotic solvents containing LiCl and concentrated H₂SO₄. They show good thermal and thermooxidative stability as well as high melting temperatures.

     

Synthesis,Characterization and Field‐Effect Transistor Performance of Poly[2,6‐bis(3‐alkylthiophen‐2‐yl)benzo[1,2‐b;4,5‐b′]diselenophene]s

Benzo[1,2‐b:4,5‐b′]diselenophene (BDS) has been incorporated for the first time in a polymer. bis(Stannyl)‐functionalized BDS was copolymerized with 3,3′‐bis(alkyl)‐5,5′‐bithiophenes (dodecyl and tetradecyl side chains) through Stille copolymerization, to yield p‐type polymer semiconductors for organic field‐effect transistor application. The electronic and structural effect of the selenium atoms, compared to sulphur atoms in analogous copolymers, is described. The molecular weight has a decisive influence on the photophysical properties and supramolecular ordering, expressed in field‐effect transistor measurements. Saturation mobilities around 10^?2 cm² · V^?1s^?1 are obtained on standard silicon substrates.

     

Phase Separation Dynamics of Aqueous Poly [(2‐ethoxy) ethoxy ethyl vinyl ether] Solutions as Explored using the Laser T‐Jump Technique Combined With Photometry

Poly[(2‐ethoxy) ethoxy ethyl vinyl ether] (poly(EOEOVE)) is a representative thermoresponsive polymer in aqueous solution for which the time constants of the phase separation (PS) are determined with high accuracy. It is revealed that the PS dynamics of the polymer are entirely different from those of the poly(N‐isopropylacrylamide) (PNIPAM) system, which is an alternative representative thermoresponsive polymer. Poly(EOEOVE) exhibits complicated PS behavior that is described using double exponential functions. The PS of poly(EOEOVE) is much faster than the PS of PNIPAM in aqueous solutions and becomes faster with increasing concentration of the polymer. PS behavior that is particular to the present system is successfully understood within the framework of the aggregation mechanism.     

Preparation and Characterization of Novel Temperature Sensitive Poly(N‐isopropylacrylamide‐co‐acryloyl beta‐cyclodextrin) Hydrogels with Fast Shrinking Kinetics

Summary: Novel temperature sensitive poly(N‐isopropylacrylamide‐co‐acryloyl beta‐cyclodextrin) (P(NIPA‐co‐A‐CD)) hydrogels with fast shrinking rates were prepared by radical polymerization of NIPA, A‐CD and crosslinker in a mixture of water/1,4‐dioxane as solvent. Because the mixed solvent was a poor solvent for the copolymers, phase separation occurred during the polymerization, which resulted in a heterogeneous, porous structure of the hydrogels. In contrast to the normal PNIPA hydrogel and the homo P(NIPA‐co‐A‐CD) gel prepared in water, the P(NIPA‐co‐A‐CD) hydrogels synthesized in water/1,4‐dioxane as solvent exhibited higher swelling ratios at the temperature below the lower critical solution temperature (LCST) and shrunk rapidly to equilibrium within shorter time when the temperature was increased above LCST. Increasing the acryloyl beta‐cyclodextrin content in the gels led to a slight decrease of the swelling ratio at lower temperature and had no marked influence on the shrinking kinetics. The gels prepared in water/1,4‐dioxane, at different v/v ratios of 1.0/0.2, 0.8/0.4 and 0.6/0.6, showed similar properties.

SEM photos of the heterogeneous P(NIPA‐co‐A‐CD) hydrogel prepared in water/1,4‐dioxane.     

Synthesis and Characterization of Aromatic Poly(benzobisthiazole)s and Poly[oxy(bisbenzothiazole)]s Containing Flexible Linkages

Aromatic poly(benzothiazole)s (PBT)s were prepared by direct polycondensation of multi‐ring dicarboxylic acids containing aryl ether or 2,2′‐hexafluoroisopropylidene flexible groups and 2,5‐diamino‐1,4‐benzenedithiol (PBT I) or 3,3′‐oxybis(6‐aminobenzenethiol) (PBT II) using poly(phosphoric acid) or poly(phosphoric acid)/methanesulfonic acid as condensing agent and solvent. Inherent viscosities were in the range 0.62–2.68 dL/g, indicating moderate polymerization degrees. The molecular structure was checked by FTIR and ¹H NMR spectroscopy. Good solubility in polar aprotic organic solvents was observed for PBTs derived from monomers containing flexibilizing groups in both moieties. Glass transition temperatures ranged between 268 and 312°C for PBTs I, and between 219 and 233°C for PBTs II. Both PBTs I and II possessed good thermal and thermooxidative stability.     

Preparation of Block Copolymers with a Single Tg Based on Segments of Poly(oxy‐2,6‐dimethyl‐1,4‐phenylene) and Polycarbonate of Bisphenol A

Summary: The synthesis and the properties of block copolymers based on PPO and PC segments are reported. Copolymers that have a multi‐block structure are synthesized by a polycondensation reaction that employs oligomeric PPO and PC diol terminated with phosgene or bischloroformate of bisphenol A. In the reaction scheme two steps are involved: first, the reaction of one of the oligomeric diols (PPO or PC) with the bischloroformate or phosgene; second, the oligomeric bischloroformate is reacted with the other diol. The molecular characteristics of the prepared samples are studied by SEC, ¹H and ¹³C NMR, and FT‐IR spectroscopy. The thermal and rheological properties and the thermal stability have also been investigated by means of DSC, rotational rheometry, and TGA, respectively. Polymers that have a single glass transition temperature are obtained if low‐molecular‐weight segments are used. From a rheological point of view, these materials show a remarkably lower melt viscosity compared with a PPO homopolymer that has a comparable average molecular weight.

Rheometric curves of PPO‐b‐PC (3) compared with a commercial PPO.     

Synthesis of π‐Conjugated Polymer Possessing Mercapto‐Substituted 1,3‐Butadiene‐1,4‐diyl Units by Reaction of Regioregular Organometallic Polymer Having Titanacyclopentadiene Moieties in the Main Chain

A π‐conjugated polymer having 1,4‐bismercapto‐substituted 1,3‐diene units in the main chain was prepared by the novel polymer reaction of a regioregular organometallic polymer having titanacyclopentadiene‐2,5‐diyls units with benzenesulfenyl chloride. The number‐average molecular weight and the molecular weight distribution of the polymer derived from 1,4‐diethynyl‐2,5‐di(2‐ethylhexyloxy)benzene were estimated to be 5 700 and 1.6, respectively, from the GPC analysis. The π‐conjugated character, photoluminescence behavior both in solid‐state and solutions, and the electron‐donating properties of the π‐conjugated polymer are discussed on the basis of its UV‐vis spectrum, photoluminescence spectrum, and CV analysis.

     

Effect of Varying Thiophene Units on Charge‐Transport and Photovoltaic Properties of Poly(phenylene ethynylene)‐alt‐poly(phenylene vinylene) Polymers

Poly(phenylene ethynylene)‐alt‐poly(phenylene vinylene)s (PPE‐PPVs) with various thiophene units (thiophene, bithiophene, and 3,4‐ethylenedioxythiophene) at the X position, with the general backbone design (Ph? C?C? X ? C?C? Ph? CH?CH? Ph? CH?CH? ), bearing identical solubilizing side chains at the phenylene rings of the polymers, are synthesized to study the effect of this structural alteration on the properties such as the photophysics, the electrochemical properties, the charge‐carrier mobility, and the morphology of the materials and its impact on their photovoltaic performance. The polymers are obtained in good yields with reasonable molecular weights and show solubility in ordinary organic solvents required for solution‐processing applications. The polymer with a basic thiophene ring at the X positions shows the highest open‐circuit voltage (V_OC of 930 mV) and the polymer with a bithiophene unit at the X position shows the highest short‐circuit current density and charge‐carrier mobility, whereas the polymer with 3,4‐ethylenedioxythiophene shows the lowest photovoltaic performance.