Polyacetylene preparation

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. 

Polyacetylene prepared by the Shirakawa route pyrolyses on heating, before showing any detectable crystal melting point. At the same time, it is insoluble in all known solvents. For these reasons it is essentially unprocessable. Until recently it has seemed to be a general rule that all conducting polymers were insoluble, which follows naturally from the conjugation of the double bonds along the chain which results in chain stiffness. 

TABLE 3. Cw-contents of polyacetylenes prepared at different temperatures ... 

The intractability of the early preparations of polyacetylene has severely hampered the establishment of clear-cut relationships between structure, morphology and (electrical) properties. An early example of an integrated approach to structure-property relations is a paper by Haberkom et al. [24], From a combination of x-ray data with NMR and IR investigations, these authors have found a relationship between the content of sp defects and crystallinity in polyacetylene prepared by the Shirakawa, Luttinger and other methods. Such defects are apparently expelled to the amorphous phase. The authors find a correlation with conductivity in both undoped and iodine-doped samples. 

Around the same time, Murthy et al. [94] also carried out a study of oriented, Li-doped polyacetylene, prepared by Lugli et al. [42]. These diflfiaction data can be indexed on a trigonal lattice, space group R3c, <7= 12.4 A, c = 7.5 A (Figure 1.10). Importantly, the... 

A spectacular example of a clear relationship between chemical and stnictural homogeneity and improved electrical properties can be found in polyacetylene. C CP MAS NMR studies of initially synthesized polyacetylene [22,23] indicated the presence of a small fraction of sp hybridized carbons in addition to sp carbons expected for a perfect polyene chain. Drastic reduction in the number of these sp defects observed in polyacetylene prepared by the method developed by Naarman and Theophilou, resulted in an important improvement of the conductivity of this polymer in the doped state [24],... 

ENDOR frequency determination with the aid of ENDOR-induced ESR in cis-rich samples gives the half-width of the spin distribution of 18 carbon sites and the ratio of the peak value of the negative density to that of the positive density of p(l)/p(0)%0.44 [104,105], Similar results to those in cis-rich samples, that is similar spectra with resolved structures for the stretch direction and similar spectral frequencies, have been reported for stretch-oriented trans-polyacetylene prepared by the Durham route using pulsed ENDOR techniques [99,106,107], The unpaired electrons observed in Durham samples were conjectured to be trapped solitons from nearly temperature-independent ENDOR spectra. In the studies of this system the first successful application of TRIPLE resonance in polyacetylene has been reported [99,107], Readers can find reviews on the results of Durham samples [2,99] as... 

The configuration of the double bonds strongly depends on the temperature of polymerization. The trans content of polyacetylene prepared by the Ziegler-Natta catalysts decreases with decreasing polymerization temperature, as listed in Table 29.3, determined by infrared spectroscopy. 

TABLE 29.3 The Trans Contents of Polyacetylene Prepared at Different Temperatures ... 

Mechanical stretching can be performed after polymerization, e.g. in non-crosslinked polymers [4]. In the case of polyacetylenes prepared with aged Ziegler-Natta catalysts [5], stretching increases conductivity to values as high as 100,000 S/cm. 

W. Deits, P. Cukor, M. Rubner, H. Jopson, Analogs of polyacetylene-preparation and properties, Industrial and Engineering Chemistry Product Research and Development 1981, 20, 696. 

The first illustration of the theme is provided by reference to the paradigm for the semiconducting polymer field, polyacetylene. This curious material was first produced by the obvious route namely, direct addition polymerisation of acetylene over a Ziegler-Natta catalyst, indeed the reaction was first conducted in Natta s laboratories. The product, an intractable, air sensitive powder, was shown to have a predominantly trans vinylene microstructure. Two American industrial research chemists. Berets and Smith, demonstrated that compressed pellets of polyacetylene prepared by Natta s route were semiconductors whose conductivity could be varied by exposure to volatile donors like anunonia (conductivity decreases) or acceptors like iodine (conductivity increases). Subsequently, apparently as a result of a serendipitous event in Shirakawa s laboratory, it was found that interfacial polymerisation of acetylene at the surface of a high... 

Cataldo, F., A spectroscopic study of polyacetylene prepared by using Rh( 1) catalysts. Polymer, 35, 5235-5240 (1994). 

Polyacetylenes are the most typical and basic r-conjugated polymers, and can ideally take four geometrical structures (trans-transoid, trans-cisoid, cis-transoid, cis-cisoid). At present, not only early transition metals, but also many late transition metals are used as catalysts for the polymerization of substituted acetylenes. However, the effective catalysts are restricted to some extent, and Ta, Nd, Mo, and W of transition metal groups 5 and 6, and Fe and Rh of transition metal groups 8 and 9 are mainly used. The polymerization mechanism of Ta, Nd, W, and Mo based catalysts is a metathesis mechanism, and that of Ti, Fe, and Rh based catalysts is an insertion mechanism. Most of the substituted polyacetylenes prepared with W and Mo catalysts provide trans-rich and cis-rich geometries respectively. Polymers formed with Fe and Rh catalysts selectively possess stereoregular cis main chains. 

Tabata, M. Sone, T. Sadahiro, Y Yokota, K. Nozaki, Y. Pressure-induced cis to trans isomerization of aromatic polyacetylenes prepared using a Rh complex catalyst A control of jt-conjugation length. J. Polym. ScL, Part A Polym. Chem. 1998, 36, 217-223. 

L. Ding, C. Chen, J. Deng, W. Yang, Optically active thermosensitive amphiphilic polymer bmshes based on helical polyacetylene preparation through click onto grafting method and self-assembly. Polym. Bull. 69, 1023-1040 (2012)... 

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