Polyacetylenes

Polyacetylenes26 27 exhibit a number of fine-structure bands. Diacetylenes show medium intensity bands (e 100-350) in the 210-250 mp region. Tri- and tetra- and higher acetylenes, however, exhibit very high intensity bands (e 100,000) at lower wavelengths (200-280 mp) in addition to the medium intensity bands (240-390 mp). Absorption spectra of diphenyl-poiyacetylenes have been reported27 (see references 37 and 38 for naturally occurring acetylenic compounds). 

Despite the wide spread use of the artificial sweetener, Splenda , which is a synthetic chlorinated carbohydrate, Nature has provided very few halogenated carbohydrates. The antitumor metabolite FR 901463 (1598) was isolated from a Pseudomonas sp. along with two nonchlorinated epoxides. FR 901463 is not an isolation artifact, being present in the culture medium prior to extraction and isolation (1542-1544). 

Polyacetylene attracts constant attention as an excellent simple model of the polyconjugated polymer on which the main optical and electrical properties can be verified. The possibility of achieving metallic conductivities by doping opens real perspectives of practical application of conducting polymers. The complication is the strong interaction with oxygen. The reproducibility of results strongly depends on the synthesis and measurement conditions. 

Polyacetylene consists of CH-group chains which form a quasi one-dimensional (ID) lattice [104-107], There are cis and trans forms of polyacetylene. 

On the terminology of the band model c- bonds form the completely filled low band, and rc-bonds make the partially filled band, which defines the electronic properties of the polyacetylene. 

Active research of the electronic properties began in the early 1970s, when it was shown that polyacetylene may be synthesized as a flexible film with arbitrary and specially oriented fibrils. 

Two lower states of the frans-(CH) are energetically degenerated as follows from symmetry conditions. Theory shows that electron excitation invariably includes the lattice distortion leading to polaron or soliton formations. If polarons have analogs in the three dimensional (3D) semiconductors, the solitons are nonlinear excited states inherent only to ID systems. This excitation may travel as a solitary wave without dissipation of the energy. So the 1-D lattice defines the electronic properties of the polyacetylene and polyconjugated polymers. 

In 2000, the Nobel Prize in Chemistry was awarded for the discovery and development of conducting polymers, and the conversion of polyacetylene to a conducting material was integral in this effort [151]. Oligomeric polyacetylene has been produced via ADMET polymerization of 2,4-hexadiene with both Schrock s tungsten and molybdenum catalysts [63a]. Similarly, oligomers were also formed by ADMET polymerization of 2,4,6-octatriene, although no polymer resulted from the reaction of either 1,3-butadiene or 1,3,5-hexadiene. 

Copolymerization of conjugated and nonconjugated dienes has also been achieved with ADMET [63a]. Poly(acetylene-co-octenamer)s were produced by ADMET polymerization of mixtures of 2,4-hexadiene and 2,10-dodecadiene. NMR and UV spectra revealed that these polymers were blocky, with an average of 4-5 acetylene units per conjugated block. 

The polymerization of 1,4-divinylbenzene has been investigated with [Mo]2 catalyst [63d], but it was determined that the reaction ceased as the state of matter changed to a soUd, in this case with the formation of a trimer. A series of relatively high molecular weight copolymers of 1,3- and 1,4-divinylbenzene with substituted derivatives have been synthesized with [Mo]2 [156], all of which had high trans content. 

Furthermore, PPV nanoparticles have been synthesized [159] by ADMET polymerization of l,4-dipropoxy-2,5-divinylbenzene in aqueous emulsion. 

The classical catalyst system Re207/Al203/Bu Sn has been used to achieve the ADMET polymerization of various divinylsilanes to produce a-jt-conjugated oligo(silylene-vinylene)s [162]. These compounds could not be polymerized with Schrock s, Grubbs, and the WClg/SnBu classical catalysts. 

There are a variety of routes toward the synthesis of polyacetylene. These syntheses can be classified into four categories catalytic polymerization of acetylene, noncatalytic pofymerization of acetylene, catalytic polymerization of monomers other than acetylene, and, finally, precursor methods. 

The aforementioned procedure yields pofyacetylene that is mainly cis. Baker et al. [169] prepared almost pure trans polymer 

Single-Component Catalysts. There is not always a need for a catalyst and co-catalyst system. Several metal complexes alone, such as tetrabenzyltitanium, are reported to be active for acetylene polymerization. Hsu et al. [181] used a Ti-Ti complex to obtain cis-rich or trans-rich polyacetylene, depending on the temperature. Alt et al. [182] demonstrated that Cp2Ti(PMe3)2 reacts readily with acetylene. Martinez et al. [183] also found that titanium compounds polymerize acetylene to yield trans-rich polyacetylene at lower temperatures. 

The inherent insolubility and infusibility of polyacetylene coupled with its sensitivity to air imposes a barrier to the processibility of the polymer. As a result, considerable efforts have been directed toward obtaining polyacetylene from workable, easily processible polymers by means of polymer analogous transformations. In particular, the synthesis of polyacetylene from the dehydrohalogena-tion of poly(vinyl chloride) has attracted a lot of attention but the polymers prepared by this route generally possess relatively short conjugated segments and contain structural defects and cross-links [198,199], 

Another approach involves the use of prepolymers that can be thermally converted to polyacetylene. This approach was first followed by the group of Feast [200-211] (Fig. 4). 

In order to support the mechanism of photo-oxidation of this terpolymer, photolysis of model compounds such as dimethylhexane (3.134) and ethylidene cyclohexane (3.135) has been investigated [1390]  

The photo-oxidative degradation of benzoin [283] and benzophenone [291, 292] grafted EPDM rubbers has been investigated. 

Polyacetylenes are deep yellow-red coloured polymers because they contain very strong chromophoric =C—C—C—C= groups. 

Another possible mechanism, especially for the polyacetylenes without allylic hydrogens, is  

In this mechanism, radicals are formed by excitation of a double bond in the main chain by UV irradiation. 

Schrock, Wrighton, and coworkers have reported that ring-opening metathesis polymerization (ROMP) of norbomene monomers fhnctionalized with ferrocenyl groups allowed for the isolation of polymers such as 55-58. Alternatively, polynorbornenes capped with ferrocenyl units could be produced by using a molybdenum initiator functionalized with ferrocene. When this polymerization reaction was terminated through the addition of octamethyl-ferrocenecarboxaldehyde, low molecular weight polymers such as 58 were isolated. 

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In considering the molecules in Table 5.2 it should be remembered that the method of detection filters out any molecules with zero dipole moment. There is known to be large quantities of FI2 and, no doubt, there are such molecules as C2, N2, O2, FI—C=C—FI and polyacetylenes to be found in the clouds, but these escape detection by radioffequency, millimetre wave or microwave spectroscopy. 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

A second type of soHd ionic conductors based around polyether compounds such as poly(ethylene oxide) [25322-68-3] (PEO) has been discovered (24) and characterized. These materials foUow equations 23—31 as opposed to the electronically conducting polyacetylene [26571-64-2] and polyaniline type materials. The polyethers can complex and stabilize lithium ions in organic media. They also dissolve salts such as LiClO to produce conducting soHd solutions. The use of these materials in rechargeable lithium batteries has been proposed (25). 

Polyacetylenes. The first report of the synthesis of a strong, flexible, free-standing film of the simplest conjugated polymer, polyacetylene [26571-64-2] (CH), was made in 1974 (16). The process, known as the Shirakawa technique, involves polymerization of acetylene on a thin-film coating of a heterogeneous Ziegler-Natta initiator system in a glass reactor, as shown in equation 1. 

The resulting porous, fibrillar polyacetylene film is highly crystalline, so is therefore insoluble, infusible, and otherwise nonprocessible. It is also unstable in air in both the conducting and insulating form. 

Much effort has been expended toward the improvement of the properties of polyacetylenes made by the direct polymerization of acetylene. Variation of the type of initiator systems (17—19), annealing or aging of the catalyst (20,21), and stretch orientation of the films (22,23) has resulted in increases in conductivity and improvement in the oxidative stabiHty of the material. The improvement in properties is likely the result of a polymer with fewer defects. 

Even with improvement in properties of polyacetylenes prepared from acetylene, the materials remained intractable. To avoid this problem, soluble precursor polymer methods for the production of polyacetylene have been developed. The most highly studied system utilizing this method, the Durham technique, is shown in equation 2. 

A drawback to the Durham method for the synthesis of polyacetylene is the necessity of elimination of a relatively large molecule during conversion. This can be overcome by the inclusion of strained rings into the precursor polymer stmcture. This technique was developed in the investigation of the ring-opening metathesis polymerization (ROMP) of benzvalene as shown in equation 3 (31). 

Copolymerizations of benzvalene with norhornene have been used to prepare block copolymers that are more stable and more soluble than the polybenzvalene (32). Upon conversion to (CH), some phase separation of nonconverted polynorhornene occurs. Other copolymerizations of acetylene with a variety of monomers and carrier polymers have been employed in the preparation of soluble polyacetylenes. Direct copolymeriza tion of acetylene with other monomers (33—39), and various techniques for grafting polyacetylene side chains onto solubilized carrier polymers (40—43), have been studied. In most cases, the resulting copolymers exhibit poorer electrical properties as solubiUty increases. 

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

Eig. 3. Lattice distortions associated with the neutral and soHton states in polyacetylene. 

Although polyacetylene has served as an excellent prototype for understanding the chemistry and physics of electrical conductivity in organic polymers, its instabiUty in both the neutral and doped forms precludes any useful appHcation. In contrast to poly acetylene, both polyaniline and polypyrrole are significantly more stable as electrical conductors. When addressing polymer stabiUty it is necessary to know the environmental conditions to which it will be exposed these conditions can vary quite widely. For example, many of the electrode appHcations require long-term chemical and electrochemical stabihty at room temperature while the polymer is immersed in electrolyte. Aerospace appHcations, on the other hand, can have quite severe stabiHty restrictions with testing carried out at elevated temperatures and humidities. 

GLASER - CHODKIEWCZ Acetylene Coupling Polyacetylenes from monoacetylenes in the presence of copper salts. 

STEPHENS CASTRO Acetylene cycloptiane synthesis Polyacetylene cyclophane synthesis from an iodophenyl copper acetylide... 

The polymers which have stimulated the greatest interest are the polyacetylenes, poly-p-phenylene, poly(p-phenylene sulphide), polypyrrole and poly-1,6-heptadiyne. The mechanisms by which they function are not fully understood, and the materials available to date are still inferior, in terms of conductivity, to most metal conductors. If, however, the differences in density are taken into account, the polymers become comparable with some of the moderately conductive metals. Unfortunately, most of these polymers also have other disadvantages such as improcessability, poor mechanical strength, instability of the doped materials, sensitivity to oxygen, poor storage stability leading to a loss in conductivity, and poor stability in the presence of electrolytes. Whilst many industrial companies have been active in their development (including Allied, BSASF, IBM and Rohm and Haas,) they have to date remained as developmental products. For a further discussion see Chapter 31. 

Whilst the conductivity of these polymers is generally somewhat inferior to that of metals (for example, the electrical conductivity of polyacetylenes has reached more than 400 000 S/cm compared to values for copper of about 600 000 S/cm), when comparisons are made on the basis of equal mass the situation may be reversed. Unfortunately, most of the polymers also display other disadvantages such as improcessability, poor mechanical strength, poor stability under exposure to common environmental conditions, particularly at elevated temperatures, poor storage stability leading to a loss in conductivity and poor stability in the presence of electrolytes. In spite of the involvement of a number of important companies (e.g. Allied, BASF, IBM and Rohm and Haas) commercial development has been slow however, some uses have begun to emerge. It is therefore instructive to review briefly the potential for these materials. 

Growth mechanism of a (9n,0) tubule, over 24n coordination sites of the catalyst. The growth of a general (9 ,0) tubule on the catalyst surface is illustrated by that of the (9,0) tubule in Fig. 16 which shows the unsaturated end of a (9,0) tubule in a planar representation. At that end, the carbons bearing a vacant bond are coordinatively bonded to the catalyst (grey circles) or to a growing cis-polyacetylene chain (oblique bold lines in Fig. 16). Tlie vacant bonds of the six c/s-polyacetylene chains involved are taken to be coordinatively bonded to the catalyst [Fig. 16(b)]. These polyacetylene chains are continuously extruded from the catalyst particle where they are formed by polymerization of C2 units assisted by the catalyst coordination sites. Note that in order to reduce the number of representations of important steps, Fig. 16(b) includes nine new Cj units with respect to Fig. 16(a). 

The 12 catalyst coordination sites — drawn further away from the surface of the particle (closer to the tubule) — are acting in pairs, each pair being always coordinatively bonded to one carbon of an inserted (F) or of a to-be-inserted (2 ) Cj unit and to two other carbons which are members of two neighbouring cis-polyacetylene chains (3°). It should be emphasized that, as against the (5n,5n) tubule growth, the C2 units extruded from the catalyst particle are positioned in this case parallel to the tubule axis before their insertion. 

Once the pair of sites is disconnected from the growing tubule [Fig. 17(d)], an orthogonal unit is inserted below the five membered ring [4° in Fig. 17(e)]. The latter inserted unit and the remaining two ci.s-polyacetylene C2 segments are finally displaced by the arrival two orthogonal C-, units [.r in Fig. 18(f)]. 

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