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Synthesis of conjugated polymers. Polymerizability studies of bis-sulfonium salts
Abstract
Although the bis-sulfonium salt precursor route is one of the most versatile procedures available for the synthesis of conjugated polymers, little effort has been made to explore the scope of this polymerization reaction. In this study, a series of bis-sulfonium salts were synthesized and in order to assess their polymerizability, isotopic exchange at the benzylic positions as well as their reactivity was studied by 13C-NMR spectroscopy. Meanwhile the generation of the reactive intermediate was analyzed by UV spectroscopy. In addition, the reaction heats of the equilibria leading to the intermediate polymerizable species from the corresponding precursors were calculated by computational methods. The relationship between the chemical structure of the bis-sulfonium salts, their ability to polymerize, and the theoretical results was then analyzed in an effort to establish a reactivity pattern that could help to predict the type of bis-sulfonium salt that can be polymerized by this synthetic route and to contribute to the rational design of new conjugated polymers.
We report here on the experimental conditions for radical chlorination that provide improved yields of 2,5-bis(chloromethyl) pyridine and pyrazine, as well as on theoretical calculations which were performed in order to gain some insight regarding the factors that affect reactivity in these systems. The difficulties encountered previously in the methyl functionalization of N-heteroarenes are not due to radical selectivity that produces low yield of the dichlorides or to lower reactivity of this type of compounds, but to the post-halogenation reactions that occur on the desired products. Therefore, a careful handling during and after the reaction allowed us to find conditions for product optimization of α, α′ -dichlorinated monomers.
Summary The polymerizability of nitrogen-containing bissulfonium salts was analyzed. The poor homopolymerization results were attributed to sluggish quinodimethane formation. However, copolymerizations with the non-nitrogenated analogue sulfonium salt were successful. We report here the synthesis and characterization of a series of poly(1,4-phenylene vinylene)-co-(2,5-pyridylene vinylene)s. Elemental analysis, FTIR, UV-vis and fluorescence studies demonstrate that the pyridilene units can be incorporated by the water soluble precursor route in PPV-type polymers with free-standing film formation properties.
IntroductionPoly(arylene vinylene)s PAVs via Quinodimethane PolymerizationsPAVs via PolycondensationsHeck Coupling and other Transition-metal-mediated MethodsPAVs by Metathesis PolymerizationStructure–Property Relationships in PAVs Substituent Effects on Emission Color of PPVsControlling Emission Color via the Degree of Conjugation in PPVsEffect of the Aryl Group on the Emission from PAVsSubstituent Effects on Charge InjectionPoly(arylene ethynylene)sPolyarylenes Oxidative Coupling of ArenesReductive Coupling of Arenes (Yamamoto Coupling)Transition-metal-mediated Crosscoupling Reactions Suzuki PolycondensationRegioregular Polythiophenes by CrosscouplingPrecursor Routes to Poly(para-phenylene)Polyarylenes by Heterocyclic PolysynthesisStructure–Property Relationships Bridged versus Nonbridged PolyphenylenesEffect of Alkyl Substituents in PolythiophenesHomo- versus CopolymersOptimizing Charge Injection into PolyarylenesEL Polymers with Isolated Chromophores Polymers with Isolated Chromophores in the Main ChainPolymers with Emissive SidechainsStability of EL PolymersConclusion PAVs via Quinodimethane PolymerizationsPAVs via PolycondensationsHeck Coupling and other Transition-metal-mediated MethodsPAVs by Metathesis PolymerizationStructure–Property Relationships in PAVs Substituent Effects on Emission Color of PPVsControlling Emission Color via the Degree of Conjugation in PPVsEffect of the Aryl Group on the Emission from PAVsSubstituent Effects on Charge Injection Substituent Effects on Emission Color of PPVsControlling Emission Color via the Degree of Conjugation in PPVsEffect of the Aryl Group on the Emission from PAVsSubstituent Effects on Charge Injection Oxidative Coupling of ArenesReductive Coupling of Arenes (Yamamoto Coupling)Transition-metal-mediated Crosscoupling Reactions Suzuki PolycondensationRegioregular Polythiophenes by CrosscouplingPrecursor Routes to Poly(para-phenylene)Polyarylenes by Heterocyclic PolysynthesisStructure–Property Relationships Bridged versus Nonbridged PolyphenylenesEffect of Alkyl Substituents in PolythiophenesHomo- versus CopolymersOptimizing Charge Injection into Polyarylenes Suzuki PolycondensationRegioregular Polythiophenes by Crosscoupling Bridged versus Nonbridged PolyphenylenesEffect of Alkyl Substituents in PolythiophenesHomo- versus CopolymersOptimizing Charge Injection into Polyarylenes Polymers with Isolated Chromophores in the Main ChainPolymers with Emissive Sidechains
A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.
Films of poly(1,4-naphthalene vinylene) (P-1,4-NV) were obtained by the thermal elimination of a cycloalkylene sulphonium salt precursor polymer. The ultra-violet/visible (u.v./vis.), fluorescence and Raman spectra and the small-angle X-ray scattering diagrams were examined, and the oxidation and reduction potentials of P-1,4-NV were determined by cyclic voltammetry. The u.v./vis. spectra of P-1,4-NV films made from a sulphonium salt polymer based on the cyclic sulphide, tetrahydrothiophene, which was converted to P-1,4-NV at a temperature about 30°C lower than the temperature needed to obtain P-1,4-NV from the dimethylsulphide sulphonium salt polymer, showed phonon sidebands at room temperature as well as at −200°C. The phonon sidebands were apparently associated with a narrow distribution of crystallite sizes, which had an average long-period spacing of 140Å, and was obtained when P-1,4-NV was made from the cyclic sulphonium chloride precursor polymer. The oxidation potential was found to be 0.75 V vs. SCE (saturated calomel electrode), the reduction potential was found to be −1.60V vs. SCE and the band-gap energy was found to be around 2.05V. The high value for the oxidation potential of P-1,4-NV, in comparison to the oxidation potential of triiodide (0.52V vs. SCE), from which an approximate value of 2 × 10−8 for the oxidation equilibrium constant was obtained, explains why very low conductivities are obtained when P-1,4-NV is doped with iodine. Films of the cycloalkylene sulphonium salt precursor polymer could be drawn to as much as 1.6 times their original length, and the drawn films showed a significant degree of orientation according to X-ray diffraction and polarized infra-red spectroscopy.
1,4-Phenylenebis(methylene) sulfonium salts were polymerized under different conditions to yield p-xylylene sulfonium salt polyelectrolytes in a broad range of molecular weights. The aqueous reaction mixture formed a reversible gel at initial monomer concentrations higher than 1.0M, but at lower concentrations it remained as an emulsion until reaction completion. The effect of reaction time on intrinsic viscosities and polymer yields for both the emulsion and the gel state is discussed. The higher yields obtained when the reaction was carried out in the presence of a water-immiscible organic solvent were apparently due to the effect of this solvent on increasing the concentration of the reactive intermediate that led to the polyelectrolyte. Both the addition of an organic solvent and the variation of the initial monomer concentrations allowed some control over the molecular weight of the polyelectrolyte formed.
A new conducting polymer poly(benzo[1,2-b:4,5ib']dithiophene-4,8-d (1), has been synthesized by the pyrolysis of the soluble precursor polymer 2, which was itself prepared by a multistep synthesis. The polymer is burgundy, and a film has lambda(max) = 503 nm (2.47 eV) and a band-gap (band edge) of 1.89 eV. Upon doping (FeCl3) the polymer film becomes light blue-green and has a conductivity of 60 S cm(-1). The original absorption peak at 503 nm decreases, while two new near-IR absorptions at 860 and 1860 nm appear during doping, which is reversible. Quantum mechanical calculations show a rather low rotational barrier in the monomer, and additional calculations on various oligomers and the polymer suggest that the stable form of the polymer is aromatic and planar.
A major recent development in the field of molecular electronics has been the discovery of electroluminescent conjugated polymers-that is, fluorescent polymers that emit light when excited by the flow of an electric current. These materials may now challenge the domination by inorganic materials of the commercial market in light-emitting diodes. Conjugated polymers are particularly versatile because their physical properties (color, emission efficiency) can be fine-tuned by manipulation of their chemical structures. The development of polymer-based light-emitting devices has been accompanied by an increased understanding of the processes involved in device function and breakdown, and the systematic modification of the properties of the emissive polymers by synthetic design has become a vital component in the optimization of light-emitting devices. In this account we present an overview of the synthesis and properties of electroluminescent polymers and demonstrate through examples the development of the field. There are many aspects of fundamental science to be studied in order to realize commercial applications of this serendipitous discovery. Opportunities for synthesis abound.
Poly(phenyl-1,4-phenylene vinylene) (PhPPV) and a series of arylene vinylene copolymers containing phenyl-monosubstituted arylene units and either unsubstituted or 2,5-dialkoxy 1,4-phenylene vinylene units was prepared from water-soluble, sulfonium salt precursor polymers. The conversions of the respective precursor polymers to the conjugated polymers were studied by thermogravimetric analysis, and the arylene vinylene polymers so formed were characterized by elemental analysis and by FT-IR and UV-Vis spectroscopy. The substituted polymers could be drawn above their softening points even though some degree of crosslinking occurred during the thermal elimination step. Doping of the polymer films with either iodine or antimony pentafluoride vapor led to conductivities in the 10 S.cm range. The low level of conductivity observed may be a result of morphological perturbations that inhibit the interchain hopping of the charge carriers.
The first anthracene-containing conjugated polymer, poly(1,4-anthrylenevinylene) (PAV), was synthesized. Its processability was accomplished by using the precursor polymer route. Consideration of the mechanism responsible for the polymerization reaction led to selection of the adequate monomer to achieve polymerizability. UV spectra of PAV films showed a band-gap energy 0.3 eV smaller compared to that of poly(phenylenevinylene) (PPV) as a result of the minimization of the energy difference between aromatic and quinoid resonance structures by the anthracene unit. Model compound 13 was synthesized to exclude structural defects of PAV as well as to clarify the role of the steric and electronic effects on the conformation of the main chain. The optical properties of PAV thin films drastically changed upon reaction with a dienophile in the solid state.
Recent investigations indicate that the base-promoted polymerization reaction of a 1,4-phenylenebis(methylene)disulfonium salt in a aqueous solution involves the formation of a highly reactive p-xylylene intermediate, which polymerizes through formation of a sulfur ylide active center by an anionic mechanism. If so, it would be expected that the concentration of this intermediate should be very important in controlling both polymer yield and molecular weight, so it should be possible to increase the concentration of that intermediate by removing the organic sulfide released in the initial elimination reaction by extraction into an organic solvent. This expected effect was verified by the very large increases in both reaction conversion and polymer intrinsic viscosity obtained when the polymerization reaction was carried out in the presence of pentane. The effect was greater with the cyclic sulfide monomer than with the dimethyl sulfide monomer, presumably because of the higher water solubility of the latter.
In this paper we report on the synthesis and characterization of ABC-triblock copolymers poly(N-vinylcarbazole)-block-poly(4-(1-pyrenyl)butyl vinyl ether)-block-poly(2-[4-(2-phenyl-1,3,4-oxdiazolyl)phenyloxy]ethyl vinyl ether) 15. The ABC-triblock copolymers were synthesized by sequential living cationic polymerization of N-vinylcarbazole 11, 4-(1-pyrenyl)butyl vinyl ether 10 and 2-chloroethyl vinyl ether 3. In a second step, the reactive pendant chloro groups of the poly(2-chloroethyl vinyl ether) segment of the block copolymers were substituted with 2-(4-hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole 6 to form fully functionalized ABC'-triblock copolymers. The molecular weight of the block copolymers varied from Mn = 4800 to 14000. The thermal behavior is discussed with respect to the block copolymer composition and compared with corresponding polymer blends. LED characteristics of a single layer device for one of the block copolymers are presented as an example.
Molecular weights were studied by gel permeation chromatography of derivatized poly(α-tetrahydrothiophenio para-xylylene) chloride produced by aqueous or methanolic base-induced polymerization of 1,4-bis(tetrahydrothiopheniomethyl) benzene dichloride, both with and without a variety of added polymerization inhibiting agents. Efficient radical scavenging agents such as 2,2,6,6-tetramethylpiperidinoxyl and hydrogen atom donor 2,4,6-tri-tert-butylaniline reduced the degree of polymerization of the reactive α-(tetrahydrothiophenium chloride)-para-xylylene intermediate produced in this chemistry, and in some cases completely suppressed formation of high polymer. These two traps did not affect the equilibrium production of the para-xylylene by UV-Vis spectral analysis; hence they must affect the subsequent polymerization chain propagation steps in the mechanism. Electron spin resonance studies of polymerization in the presence of 0.00025 equiv of TEMPO showed disappearance of the spin label, a result consistent with a radical scavenging process. The results suggest that production of high molecular weight poly(α-tetrahydrothiophenio para-xylylene) chloride proceeds through a radical chain propagation sequence. © 1992 John Wiley & Sons, Inc.
The reaction of bis(4,7-tetrahydrothiopheniomethyl) benzofuran dibromide with aqueous tetramethylammonium hydroxide leads to a water-soluble polyelectrolyte which can be film cast and thermolytically eliminated to give poly(4,7-benzofuran vinylene) (PBFV). Subjection of bis(4,7-tetrahydrothiopheniomethyl) benzothiophene dibromide to the same reaction sequence gives poly(4,7-benzothiophene vinylene) (PBTV). UV-VIS studies show that PBFV has a band gap of 2.76 eV, while PBTV has a band gap of 2.92 eV. These polymers are members of a new class of conjugated poly (arylene vinylene)s, in which heterocyclic pseudoaromatic rings are fused onto a poly(1,4-phenylene vinylene) backbone. © 1994 John Wiley & Sons, Inc.
A new method for obtaining optimized parameters for semiempirical methods has been developed and applied to the modified neglect of diatomic overlap (MNDO) method. The method uses derivatives of calculated values for properties with respect to adjustable parameters to obtain the optimized values of parameters. The large increase in speed is a result of using a simple series expression for calculated values of properties rather than employing full semiempirical calculations. With this optimization procedure, the rate-determining step for parameterizing elements changes from the mechanics of parameterization to the assembling of experimental reference data.
The molecular mechanics (MMP2) program and procedures for the treatment of conjugated hydrocarbons, and some of the results which they can achieve are described. The program is an updated version of the similar MMP1 program, but contains some differences. It is based on an SCF π system calculation, rather than on the VESCF method used earlier. All parameters are compatible with those in the MM2 program. Hence it is possible to calculate heats of formation, resonance energies, and structures for conjugated hydrocarbons in a way that is consistent with the calculations on non-conjugated molecules. The overall results as far as structure and energy are somewhat better than they were with the MMP1 program.
High molecular weight poly(p-phenylene vinylene) (PPV) has been synthesized starting from the monomer p-xylylene-bis(dimethylsulphonium chloride). The latter was polymerized to yield a water-soluble sulphonium salt polyelectrolyte, which was converted to PPV by the thermal elimination of (CH3)2S and HCl from films cast from aqueous solution. The elimination reaction was studied by elemental analysis and by thermogravimetric analysis and mass spectrometry. The PPV films had good mechanical properties and could be n- and p-doped to yield material with electrical conductivities approaching those of highly doped polyacetylene. The degree of conversion of the intermediate polyelectrolyte to PPV could be controlled and the conductivities of these doped films could be related to the average conjugation length.
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