Unsaturated polymers Polyacetylene

Composite matrix resin Conjugated and other unsaturated polymers Polyacetylene Polyaniline... 

Whereas selected unsaturated polymers can be made conductive, making saturated polymers like polyethylene or polypropylene conductive is another matter. Because of their low cost, availability and ease of processing, they are used as electrical insulators, and making them electrically conductive would seem at first thought difficult to do. In one approach, saturated polymers can be made conductive via polyacetylene chemistry by forming composites wherein the conductive phase can be varied in weight percent from 10 to 50 percent (4 - ). 

A growing array of different terminal and internal alkynes have been polymerized [8]. Many polyalkynes are air-stable, soluble materials, and not highly conjugated. As new catalysts allow the polymerization of alkynes with an increasing variety of substituents, an exploration of what properties unsaturated polymers have to offer is warranted. In general, substituted polyacetylenes may or may not be colored, and tend to be more rigid than saturated polymers. Selected materials are described below and compiled in Table 10-1. 

An example of an enantiomerically pure polymer is also shown [11]. Aoki et al. showed that films of a polyacetylene substituted with a (-)-p-pinene derivative formed an effective membrane for chromatographic resolutions of racemic mixtures. ( )-2-Butanol was resolved to 29.8% eje. and unsaturated polymers for both liquid-phase and gas-phase separation applications (8, 9, 79]. It has been suggested that the rigidity and irregularity of the highly substituted polyacetylene chain, combined with the presence of aliphatic substituents which reduce interchain interactions, are important for the polymers transport properties [10]. 

The first conducting polymers (named ICPs for intrinsically conducting polymers, or ECPs for electroactive conducting polymers) were discovered in the seventies by McDiarmid, Heeger, and Shirakawa [1] who showed that unsaturated conjugated polymers (polyacetylene, polyphenylene) became conductive when doped, corresponding to the chemical oxidation of the ethylene or polyphenyl carbon chain. 

CLASS Conjugated and other unsaturated polymers STRUCTURE ds-Polyacetylene H H H H... 

Figure 1 Evolution as a function of the number of unit cells (N) of the longitudinal polarizability per unit cell, AoIl(N) = l(N) - 0[l(N-1) for prototypes of saturated (polyethylene) and unsaturated (polyacetylene) polymers.

What would it take to make the polymer itself conducting To start, we would need to make a polymer with a long series of interacting double bonds (called conjugated unsaturation). Three examples are polyacetylene, poly(p-phenylene), and polythiophene ... 

It should be noted that the metal-poly-yne polymers except Pt-D -D1 are soluble in a variety of organic solvents such as benzene, toluene, tetrahydrofuran, and methylene chloride in spite of the fact that they contain a heavy metal in the backbone, whereas polymers containing conjugated unsaturation such as polyacetylene and poly(p-dieth-ynylbenzene) are practically insoluble in orgnaic solvents. Even a high molecular weight sample (Mw = 105) of Pt-D1 polymer dissolves in trichloroethylene at least up to a concentration of 50wt-%. 

Polymers with unsaturated carbon chain backbone form another important class of macromolecules, many of the compounds from this class having properties of elastomers. The most common polymers from this class are obtained from 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene) and their derivatives. Natural rubber, which is poly(c/s-isoprene), as well as the natural polymers gutta-percha and balata also have an unsaturated carbon chain backbone. For many practical applications, the polymers from this class are subject to a process known as vulcanization, which consists of a reaction with sulfur or S2CI2, and leads to the formation of bridges between the molecular chains of the polymer. This process significantly improves certain physical properties of practical interest. A separate subclass of polymers with unsaturated carbon chain backbone is formed by polyacetylene. 

A different subclass of unsaturated hydrocarbon type polymers is formed by polyacetylenes. This type of polymer contains conjugated double bonds in a linear structure, and due to their special electrical properties they have been the subjects of numerous studies including some on thermal stability. 

A number of other unsaturated poiyhydrocarbons have practical applications. These include poiy(phenyl acetylene) and poly((E,E)-[6.2]paracyclophane-1,5-diene), which have been studied as photoconducting polymers. The thermal decomposition of polyacetylenes and of poly((E,E)-[6.2]paracyclophane-1,5-diene) generates fragments summarized in Table 7.1.8 [19]. 

The most recent application of olefin metathesis to the synthesis of polyenes has been described by Tao and Wagener [105,117], They use a molybdenum alkylidene catalyst to carry out acyclic diene metathesis (ADMET) (Fig. 10-20) on either 2,4-hexadiene or 2,4,6-octatriene. The Wagener group had earlier demonstrated that, for a number of nonconjugated dienes [118-120], these polymerizations can be driven to high polymer by removal of the volatile product (e. g., 2-butene). To date, insolubility limits the extent of polymerization of unsaturated monomers to polyenes containing 10 to 20 double bonds. However, this route has some potential for the synthesis of new substituted polyacetylenes. Since most of the monomer unit is preformed before polymerization, it is possible that substitution patterns which cannot be incorporated into an alkyne or a cyclic olefin can be built into an ADMET monomer. 

Polymers appear at first sight to be most unlikely materials to exhibit electronic conduction. A polymer such as polyethylene contains only fully saturated chemical bonds, each with two electrons closely bound to particular atoms, which thus constitute a filled valence band there are no free electrons. Some polymers, however, contain unsaturated bonds in the backbone. An important example is polyacetylene, -(CH=CH, which has alternating single and double bonds in the backbone. Examples of other polymers with such alternation are given in fig. 9.6. Where a number of double bonds alternate in this way the phenomenon of resonance usually occurs and all the bonds tend to become similar. Notice, however, that, for all the molecules shown in fig. 9.6, except for truM5-polyacetylene (t-PA),... 

ROMP reactions are unique in that all the unsaturation present in the monomers is conserved in the polymeric product. This feature makes ROMP techniques especially attractive for the preparation of highly unsaturated, fully conjugated materials. One example is the direct preparation of polyacetylene by the ROMP technique through one of the double bonds of cyclooctatetraene. For further discussion of polyacetylene, see "Chemical Connections Organic Polymers That Conduct Electricity" earlier in this chapter. 

Polymers have been employed in many fields where their applications range from common daily use to sophisticated aerospace applications. They are primarily used for insulators in the electrical and electronic industries. Recently, however, conjugated unsaturated systems have received special attraction due to their ability to conduct electricity. Doped polyacetylene, the first well characterized and studied conducting polymer, possesses conductivities as high as 10 S/cm.l Such values are comparable to metal conductors. 

---