Polyacetylene color

Acetylene (ethyne), C2H2, can be polymerized, (a) Draw the Lewis structure for acetylene and draw a Lewis structure for the polymer that results when acetylene is polymerized. The polymer has formula (CH), where n is large, (b) Consider the polymers polyacetylene and polyethylene. The latter has the formula (CH2)W and is an insulating material (plastic wrap is made of polyethylene), whereas polyacetylene is a darkly colored material that can conduct electricity when properly treated. On the basis of your answer to part (a), suggest an explanation for the difference in the two polymers. 

Optical properties are related to both the degree of crystallinity and the actual polymer structure. Most polymers do not possess color site units, so are colorless and transparent. But, some phenolic resins and polyacetylenes are colored, translucent, or opaque. Polymers that are transparent to visible light may be colored by the addition of colorants, and some become opaque as a result of the presence of additives such as fillers, stabilizers, moisture, and gases. 

As described in Chapter 6, Electric Properties of Polymers, there is a general relationship between the delocalization of electrons throughout a polymer chain or network and color so that the incidence of and darkness of color increases as electron delocalization increases. Thus polyethylene is colorless while polyacetylene is black. 

When heated, polyvinyl chloride (PVC) and polyvinyl alcohol (PVA) lose HC1 and H20, respectively, to produce dark-colored conductive polyacetylene. Superior polymers of acetylene can be made by the polymerization of acetylene with Ziegler-Natta catalysts. The conductivity of polyacetylene is increased by the addition of dopants, such as arsenic pentafluoride or sodium naphthenide. 

Instead of bemoaning the error, Shirakawa decided to learn more about this fascinating new version of an old familiar polymer, polyacetylene. He soon discovered that the silvery film consisted of the trans form of polyacetylene and that a comparable copper-colored film could he produced with large quantities of catalyst at different temperatures. The copper-colored film was found to consist of the cis form of the polymer. 

Fig. 2. A molecular data storage scheme based on an aromatic molecule (naphthalene) bonded to four gold electrodes by sulfur atoms and polyacetylene wires [37). For the surface an insulator has to be chosen to prevent cross-talk between the electrodes. The variables X, Y and Z could either be chemical substituents or, alternatively, connections to further electrodes. Some parts of the molecule, electrodes and variables are drawn in bright colors, which is meant to indicate an active state during a particular read-out. The darker parts are considered to be inactive.

Commercial use of "doped" polyacetylene electrochemical cells failed because of their air sensitivity and a long-term chemical degradation, which limited their shelf life to about six months this progressive degradation and color change of some conducting polymers was instead used as a tag to monitor the age of products with limited shelf life ... 

The 2000 Nobel Prize in Chemistry was awarded for work on poly acetylenes. Acetylene can be polymerized using a Ziegler-Natta catalyst. The cis or trans stereochemistry of the products can be controlled by careful selection and preparation of the catalyst. The resulting polyacetylene is an electrical semiconductor with a metallic appearance. cw-Polyacetylene has a copper color, and frawi-polyacetylene is silver. 

Unlike vinyl polymers, polyacetylenes which have alternating double bonds along the main chain often show the following unique properties i) electrical conductivity (semiconductivity), ii) paramagnetism, iii) chain stiffness, iv) geometrical isomerism, and v) color. Thus it seems interesting to elucidate the properties of polyacetylenes and develop their functions. 

Many properties of polyacetylenes with bulky substituents are substantially different from those of polyacetylene. For example, the substituted polyacetylenes are soluble because of the interaction between the substituents and solvent. Furthermore, such polymers are usually only lightly colored and are stable in air at room temperature. These properties arise from twisted conformations assumed by the main chain because of the presence of substituents. The electrically insulating and nonparamagnetic properties of substituted polyacetylenes are attributable also to the same cause. 

Suspensions of polyacetylene were prepared as burrs or fibers (46) by using a vanadium catalyst. When the solvent was removed, films of polyacetylene were formed with densities greater than that prepared by the Shirakawa method. These suspensions were mixed with various fillers to yield composite materials. Coatings were prepared by similar techniques. Blends of polypyrrole, polyacetylene, and phthalocyanines with thermoplastics were prepared (47) by using the compounding techniques typically used to disperse colorants and stabilizers in conventional thermoplastics. Materials with useful antistatic properties were obtained with conductivities from 10" to 10" S/cm. The blends were transparent and had colors characteristic of the conducting polymer. For example, plaques containing frans-polyacetylene had the characteristic violet color exhibited by thin films of solid trans-polyacetylene. 

Zeolites. The weak Raman signals arising from the aluminosilicate zeolite framework allow for the detection of vibrational bands of adsorbates, especially below 1200 cm which are not readily accessible to infrared absorption techniques. Raman spectroscopy is an extremely effective characterization method when two or more colored species coexist on the surface, since the spectrum of one of the species may be enhanced selectively by a careful choice of the exciting line. A wide range of adsorbate/zeolite systems have been examined by Raman spectroscopy and include SO2, NO2, acety-lene/polyacetylene, dimethylacetylene, benzene, pyridine, pyrazine, cyclopropane, and halogens. Extensive discussions of these absorbate/zeolite studies are found in a review article by Bartlett and Cooney. ... 

When PVAPAC films, optimized with respect to conjugation length distribution and PAC concentration, are stretched in a controlled manner they turn into highly dichroic transparent neutral grey POLPAC filters (Fig. 1.10). The absence of coloration is indicative of an exceptionally highly ordered PAC state in these novel all polymer broadband polarizers based on polyacetylene (POLPAC ). 

The polymerization was done at room temperature. The sample eventually changed from the characteristic white color of microporous polypropylene to a deep purple due to the presence of polyacetylene. 

After acetylene polymerization and washing with toluene to remove catalyst, the sample weighed 1.1703 grams. Thus, the structure contained 18.0% polyacetylene. No change in flexibility was noted, and the structure was very dark in color with a detectable purple hue. 

The appearance of the sample was quite different from that in Procedure 7. After only one exposure, the sample was dark in color, indicating a more uniform deposition of polyacetylene throughout. Multiple exposures gave an even darker membrane. 

Polyacetylene is a colorful polymer. The cis isomer transmits red light, the trans isomer blue, and because these polymers often come in mixtures of cis and trans, various shades of purple result. If well formed films of polyacetylene are made, the surfaces reflect silver and, sometimes, gold. Polyacetylene in powder form appears black. The vacuum line experiments produced membranes which appeared purple when wet with solvent (transmitted light - the membrane is transparent when wet) and silver when dry (reflected light-the membrane is opaque when dry). 

Samples containing more than 2% of sulfur did not pick up any iodine even after a 72-hour period. The completely saturated EPDM portions of the blend seem to prevent any iodine molecules from permeating into the polyacetylene moieties. In order to circumvent this problem, we have doped the blend with iodine prior to the crosslinking procedure. Subsequently, the doped material having a conductivity of 60 ft-1 cm-1 was reacted with sulfur monochloride in a toluene solution for 10 minutes. The color of the solution turned from pale yellow to dark red while the polymer film remained insoluble in the toluene solution. 

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. 

Polyacetylene is black in the powdery state and shows a metallic luster in the filmy state. On the other hand, substituted polyacetylenes are variously colored depending on the number and kind of substituents (Table 10). Or/Zzo-substituted poly(phenylacetylenes) have deep colors, i.e. brown to purple. The color of poly(phenylacetylene) depends on the catalysts, and it is auburn with W catalysts. Polymers from aromatic disubstituted... 

Thermogravimetric curves for the polyacetylene, there are two exothermic peaks at 145 and 325°C [16]. The first of these corresponds to an irreversible cis-trans isomerization. Migration of hydrogen occurs at 325°C, open chain and cross-linking without the formation of polyacetylene volatile products. The color of the polymer becomes brown. A large number of defects appears. In the infrared spectrum there are absorption bands characteristic of the CH, CH3, -C=C- and -C2H5- groups [16]. 

The first two reactions lead to highly colored residues, indicating that the double bonds formed in the polymer backbone are primarily conjugated. Such elimination reactions are not satisfactory for synthesizing polyacetylene, however, since side reactions also occur. But nonchain scission has been... 

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