Precursor Routes to PA

The concept of the precursor method is akin to the process of adding protecting groups in organic synthesis. To be effective, the attachment and removal of the protecting group must proceed in a clean and efficient manner. Ideally the monomers used for the synthesis of the precursor polymers should be inexpensive and readily available. Furthermore, the formation of the precursor polymer should be easy, and its structure, well defined. Finally, the elimination of the small molecule from the precursor polymer should be efricient and quantitative. 

Because the metathesis catalysts used in this process are often nondiscriminating, the precursor polymer is an atactic mixture of cis- and trans-linkages. According to theoretical calculations, the elimination of the substituted benzene from Feast s precursor polymer is symmetry allowed [66]. At room temperature this precursor polymer spontaneously eliminates hexafluoroxylene, pro- 

Polystyrene (PS) and polyisoprene (PI) form block copolymers with PA in the presence of Ti(OBu)4 catalysts [83-85]. The two copolymers are prepared similarly. For instance, styrene is first initiated by n-BuLi (typically 0.05 M) in an anionic polymerization. A lithiated polystyiyl anion can then displace one butoxy group from the titanium center to form a new Ti-C bond, which serves as the active site for the subsequent acetylene polymerization. However, before acetylene was added, this polymeric catalyst was often aged for 1 day (PS) or 2 days (PI). The acetylene polymerization was then carried out under dilute conditions so as to minimize side reactions. In this manner, acetylene can be polymerized through the Ti(UI) catalyst, forming an AB diblock copolymer. In the case of polystyrene, less than 20 wt% of PA in the copolymer renders the copolymer soluble. Gels that were not soluble could be pressed into thin films for characterization. 

VSO can be used to form block copolymers with PS followed by VSO elimination to form EA-PS block copolymers [73,89,90]. th monomers can be polymerized anionical the s ene was initiated first and end-capped with 1,1-diphenyl ethylene (DPE). Vfith the addition of a few drops of DPE to the living styr solution, the styryl carbanions were converted to diphenyl methyl car-banions [91], which were bull, less reactive, and less nucleophilic than the styryl carbanions, thus minimizing any side reactions. The VSO monomer was then added to the DPE-capped styryl chains to generate a PS/PVSO copolymer. The resulting precursor copolymer had PDIs that were as low as 1.09. To thermally eliminate the benzenesulfenic acid moieties from the PVSO block, the copolymer was heated from room temperature to 80° C at l°C/min and then held at 80°C for 1 h [90]. It was then heated to 150°C at I°C/min to get the maximum amoimt of elimination (87-92%). Whereas the PS/PVSO precursor copolymer was soluble in many solvents, the PS/PA copolymer was soluble only when the PA content was less than 50 mol%. th 78 mol% PA content the conductivity of the copolymer was 8 x 10 S/cm after iodine doping. 

Copolymer type Polyacetylene source Comonomer wt% PA O, I2 doping (S/cm) Ref. 

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An early example of the precursor route to PA was through polyvinylchloride (PVC). A base was used to remove an equivalent of HCl from every repeating unit of the PVC backbone, leaving a conjugated structure (Scheme V) [57-59]. In a ical experiment. 

Scheme X. Precursor route to PA with the elimination of naphthalene.

Although the soluble precursor route to conducting PA has offered a number of advantages in terms of processability and stability, the fact remains that the doped polymer is still difficult to process and is unstable in air. 

A better product can be obtained if the precursor polymer contains some conjugated bonds which restrict the final structure and ensure extended conjugated-sequences. The most prominant example, is the Durham precursor-route to (equation 1). The precursor polymer is soluble in acetone, ethyl acetate, etc, and can be purified and cast to make films. The thermal elimination reaction produces cis-PA as the predominant form at 60 °C. Use of higher temperatures leads to higher proportions of trans-FA due to cis-trans isomerization. If the precursor film is unoriented the PA produced is amorphous. Stretching the precursor film during elimination leads to a highly oriented non-fibrous form of PA. trans-VA produced in this way exhibits a paracrystalline structure. 

Figure 12 (a) Optical absorption of trans-PA and a related oligomer. Solid lines polarized absorptions of an oriented PA film (precursor route), calculated from the reflection spectrum (from Ref. 113). The polymer chains are parallel to c. Dashed line absorption of the hexamer all-trans dodecahexaene, in solution in hexane (from Ref. 114). Dotted line hexamer emission in hexane, (b) Optical absorption and reflectivity of unoriented cis- and trans- A films (Shirakawa type) (from Ref. 116). 

The similar structural and catalytic properties of the SiOj-supported and unsupported samples prepared from the same precursor suggest that the same active surface is formed on both types of samples. The higher conversions obtained with the supported samples could be attributed to higher dispersions of the VPO compounds. The slightly lower maleic anhydride selectivity observed for catalyst A than B or the bulk catalyst could be due to some phosphorus atoms interacting with the silica surface rather than with vanadium atoms, such that the P/V ratio is less than two in the VPO compounds. Addition of phosphorus to catalyst B replenished this lost phosphorus. Previous studies of supported vanadium-phosphorus oxides have shown that some phosphorus atoms can be associated with the silica [2,8]. The catalytic properties of the supported samples as well as the LRS are similar to the SiOj-supported PA =2 VPO samples prepared previously [2,3]. These earlier samples were prepared by adding H3PO4 to PA =1 samples synthesized by various synthesis routes. Thus, for the supported samples, the method of preparation is much less important than the composition. 

The use of soluble precursor-polymers, which can subsequently be converted to a conjugated form, offers a route to pure materials with controlled morphology. It has been known for many years that poly(vinyl chloride) (PVC) can be thermally, chemically and photothermally dehydro-chlorinated to produce polyene (PA) chain sequences. Initiation occurs at random, and continuous conjugated-sequences are not obtained. As a result the maximum conductivity for iodine-doped dehydrochlorinated PVC is ca. 10 Scm" ... 

Compared to nonoptical routes toward cold molecules discussed in this book, the main advantage of the PA route is the ability to produce a large number of stable ultracold molecules, at the same translational temperature (a few ttK, or even less) than the precursor atoms. In contrast with the halo molecules produced by sweeping magnetic Feshbach resonances [8], the stable molecules are formed in vibrational levels, which can be relatively deeply bound. The drawback is that stabilization through the spontaneous emission process spreads the population into a variety of excited vibrational levels, so that the product molecules are not in a pure vibrational state. Schemes to cool down the vibrational and rotational degrees of freedom have to be implemented. 

An interesting copolymer of poly-p-phenylene and PA has also been produced as a conducting polymer. Poly(paraphenylene vinylene) (PPV) was made by a relatively new technique in which a soluble precursor polymer was prepared and processed prior to its conversion to the conjugated polymer [46]. Such a route allows variations to be made in the polymer morphology and hence the electrical conductivity. Before the development of this method, it was initially prepared as low-molecular-weight oligomeric powders [47] because of limitations in its solubility. 

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