Xanthate precursor route

Abstract In situ spectroscopy is an important tool to characterize polymers synthesized via a precursor route. Highly conjugated polymers such as po y(p-phenylene vinylene) (PPV) and PPV derivatives are commonly prepared from a precursor polymer because the final polymers are very insoluble and intractable. Preparation in the precursor form enables the polymer materials to be cast as films. The PPV polymers are obtained from the precursor forms using a thermal elimination reaction. The exact conditions of the reaction are important as they influence the properties of the resultant polymer. The details of this thermal elimination reaction have been analyzed using thermal gravimetric analysis (TGA) coupled with infrared analysis of the evolved gas products. In situ infrared spectroscopy of the precursor films during thermal conversion to the polymers has provided further details about the elimination reaction. We have characterized PPV synthesized from a tetrahydrothiophenium monomer (sulfonium precursor route) and via the xanthate precursor route. PPV derivatives under study include poly(2,5-dimethoxy-p-phenylene vinylene) and poly(phenoxy phenylene vinylene). 

PPV and PPV derivatives have been synthesized using precursor routes because the final highly conjugated product is insoluble and intractable. The advantage of the precursor route is that the precursor polymer is soluble and the material can be readily cast as a film. Subsequently, the precursor film is thermally converted to the final conjugated PPV product. The earliest precursor route to PPV is known as the Wessling precursor route and involves a sulfonium precursor (also referred to as the sulfonium precursor route (SPR)). Other routes can be used to prepare PPV and PPV derivatives. These include the xanthate precursor route (XPR) and the chlorine precursor route (CPR). ... 

The solid state thermal elimination reaction is a very important step in the formation of the final PPV or PPV derivative. In situ infrared spectroscopy therefore plays a critical role in the ability to monitor the reaction that converts the precursor polymer to the final product. We have characterized the mechanism of this conversion reaction in the formation of PPV synthesized by both the sulfonium precursor route (SPR) and the xanthate precursor route (XPR). The polymerization reaction of PPV from the tetrahydrothiophenium monomer is shown in Figure 1. After polymerization of the precursor polymer, the material is thermally converted to the final PPV product. This SPR method involves the thermal elimination of the tetrahydrothiophenium (THT) group and HCl as shown. 

PPV can also be prepared via the xanthate precursor route as shown in Figure 2. During heating, the xanthic acid is lost and the conjugated PPV results. This route has been shown to produce a more amorphous PPV that has applications in light-emitting diodes. The differences between these two routes were investigated in the current analysis. Both of these routes (SPR and XPR) involve a thermal elimination step that is the focus of the in situ spectroscopic analysis. 

Figure 2. Synthesis of poly(p-phenylene vinylene)(PPV) via the xanthate precursor route...

We have previously reported the synthesis of PPV and the analysis of the thermal elimination reaction in the polymer prepared from the tetrahydrothiophenium (THT) monomer via the sulfonium precursor route. The reaction is shown in Figure 1. PPV prepared via the xanthate precursor route has been recently described.The reaction is shown in Figure 2. 

The xanthate precursor route is another method to prepare PPV. The PPV synthesized by the XPR is more amorphous that PPV synthesized via the SPR. The more amorphous PPV has been reported to exhibit better photoluminescence efficiency. Evolved gas infrared spectral analysis demonstrates loss of the xanthate group in the form of carbonyl sulphide (COS) and thiol (CH3CH2SH) byproducts. The loss of COS occurs first as the precursor is heated to -ISO ° C. The three-dimensional temperature-... 

The preparation of poly(2,5 dimethoxy phenylene vinylene)(DM-PPV) utilized the xanthate precursor route. Many similarities exist between the PPV xanthate thermal elimination and this PPV derivative. Previous studies had attempted to synthesis DM-PPV using the SPR method. The SPR resulted in gel formation during dialysis so that film casting was not possible. The XPR was successful in preparing precursor films that can be thermally converted to DM-PPV. The derivatives of PPV are useful to prepare a polymer with a shift in the absorbance and photoluminescence maximum. DM-PPV exhibits an 100 nm red shift as compared with PPV synthesized via the XPR. 

Padmanaban, G., and S. Ramakrishnan. 2001. An improved method for the control of conjugation length in MEH-PPV via a xanthate precursor route. Synth Met 119 533. 

Xanthate Precursor Route. Son et al. [994] published a modification of the Wessling precursor route. The sulfonium groups were replaced by xanthate groups in order to avoid typical side reactions of the Wessling approach (Fig. 81). The precursor is stable at room temperature and soluble in most common organic solvents. Elimination with formation of the polyconjugated system takes place between 160 and 250° C. 

The first decision in choosing a synthetic method for a PPV material is the way in which the material will be processed (Scheme 7.8). The precursor routes will enable the preparation of solvent-resistant and more durable thin films of PPV. This is particularly desirable if a multilayer device structure is required for the application. When choosing different precursor methods, it is important to assess the criteria of the application. Most precursor methods involve a thermal elimination step to convert the precursor polymer to the PPV material. Sul-fonium precursors require higher-temperature elimination compared to sulfinyl precursors. This makes the sulfinyl route compatible with deposition on plastic substrates. Another factor to consider in precursor methods is the nature of the elimination byproducts. Sulfonium precursors convert to PPV with elimination of acids, such as HCl or HBr, which has been shown to be detrimental to device performance. Xanthate and dithiocarbamate routes involve the elimination of amine and CO2 and CS2, respectively. 

The xanthate transfer process provides a simple and uniquely powerful route to a-tetralones, another family of important aromatic derivatives [69-71]. a-Tetralones are starting materials for the synthesis of a host of medicinally important compoimds. They are precursors to naphthalenes, naphthols, naphthylamines, and ring expansion through the Beckmann rearrangement provides access to benzazepine derivatives. The two examples in Scheme 34 illustrate, on one hand, the possibility of preparing a tetralone with a carbohydrate-derived appendage [69] and, on the other, the synthesis of a substituted naphthol 59 by aromatisation of tetralone 58 through acid... 

The xanthates are stable in the solid state, but in acidic aqueous solutions they are readily decomposed into the starting materials. The thermal decomposition of xanthates can be a useful route to metal sulfides,197 e.g., using the CVD technique.198 Nickel xanthates serve as precursors for photochemical (laser) and thermal vapor deposition of metal sulfide films.199... 

Conversion of the precursor 22 by initial cleavage of the xanthate unit, and subsequent acid-induced cyclization provided a route to the dihydrothiophene 23. Similar cyclization of substrates where the pivaloyl group was replaced with TBDPS or acetyl groups gave considerably lower yields of the corresponding dihydrothiophenes <05TL8053>. 

Alkoxycarbonyl radical cyclization leads to direct formation of lactones. For example, the lactone 154 was obtained from the selenocarbonate 153 in Corey s synthesis of atractyligenin (155) [103] (Scheme 53). 5-Alkoxycarbonyl xanthates are also viable precursors for alkoxycarbonyl radicals the lactone 157 was prepared from the xanthate 156 via group transfer radical cyclization en route to methyl-enolactocin (149) [104] (Scheme 54). 

Development of xanthate and dithiocarhamate derivatives overcomes several drawbacks of the sulfonium monomer (Scheme 7.2b and c). Xanthates and dithiocarbamates are easily prepared by the reaction of bis(halomethyl)benzene with alkylxanthate and dialkyldithiocarbamate salts respectively. Both precursors are stable at room temperature and soluble in organic solvents. This means the polymerization of these monomers can be performed in organic solvents e.g. THE) with the addition of alkoxide base e.g. potassium tert-butoxide). For the dithiocarhamate precursor, lithium bis(trimethylsilyl)amide can be used as the base and the polymerization proceeds at 35 The elimination temperature of these precursor polymers is typically lower than that of the sulfonium polymers with xanthate elimination at 160-250 °C and dithio-carbamate at 180 °C. It has been found that elimination of dithiocarbamate gave materials with reduced structural defects. Both xanthate and dithiocarbamate routes avoid the corrosive acid byproducts (HCl) present in the sulfonium elimination. This is particularly advantageous in device fabrication as adds have a negative impact on indium tin oxide electrodes and interfaces. ... 

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