Polyacetylene isomerism

Substituted polyacetylenes usually show a softening point in the range of ca. 200 to 400 °C (Table 27). It should be noted that these softening points are higher than those of usual vinyl polymers. Poly(l-chloro-2-phenylacetylene) does not melt but only decomposes (dehydrochlorinate) at high temperature. It is known that polyacetylene isomerizes at 145 °C from cis to trans and undergoes an exothermic reaction at 325 °C, but does not melt below 400 °C during its differential thermal analysis 9S). 

The cis/trans isomerization of cw-polyacetylene, previously only disclosed from spectroscopic data, has recently been detected by cyclic voltammetry The analysis of the redox data reveals that the trans-form is thermodynamically more favorable in the charged than in neutral state. 

A route to processible polyacetylene, devised initially using classical initiators (Scheme 1i) 576-578 has been developed using well-defined molybdenum initiators to prepare conjugated polymers.579-585 They have also been employed to prepare polyacetylene via the polymerization of cyclooctate-traene, COT,586 and by the isomerization of poly(benzvalene).587 588 Substituted, and hence soluble, polyacetylene derivatives may be synthesized by polymerizing monosubstituted COT substrates.589-591... 

Figure 6.39 Isomerization of cis (A) and trans (B) sequences in polyacetylene that meet to form a soliton. Reprinted, by permission, from J. M. G. Cowie, Polymers Chemistry Physics of Modem Materials, 2nd ed., p. 417. Copyright 1991 by J. M. G. Cowie.

The process results in a cis-transoid structure. The formation of trans-polyacetylene is suggested to take place through isomerization of the new segment formed by cis insertion before it can crystallize.412... 

This isomerization is accompanied by a drop in conductivity. The polymer is also unstable in air, oxidising more rapidly than does Shirakawa polyacetylene. Both of these problems result from the allylic hydrogen atoms in the 3,5-positions of the monomer. In an effort to remove these hydrogens Gibson looked at at the polymer derived from propiolic anhydride ... 

The aromatic residue may be any of a large number of such units but the favourite for academic study has been the perfluoromethylxylene derivative shown, which smoothly eliminates at around room temperature to give a polyacetylene containing 25 % of trans- and 75 % of m-units. After transformation and isomerization at 80 °C, the polyacetylene produced is a continuous dense film. The physical chemistry of the transformation and isomerization reactions has been studied in detail229,230) and the properties of the polyacetylene are reviewed 231). The great advantage of this route is that the precursor is a soluble polymer so that it can be characterized and the physical form of the polyacetylene can be controlled. 

The crystallinity of m-polyacetylene has been estimated to be 76-84% by x-ray diffraction while trans-polyacetylene is 71-79 %6). Observations on Durham polyacetylene 445) showed that the diffraction peak narrowed, and the interchain -spacing decreased, during isomerization and annealing. The x-ray coherence length, a measure of the crystallite size perpendicular to the chains, increased from 2.6 nm to 7.1 nm, compared with 30 nm for polyethylene. 

The isomerization of polyacetylene from the cis- to the trans-form can be readily... 

The polymer of 138 is more stable than that of 136 but its conversion to polyacetylene is much more exothermic and not so easily controlled406. The double bonds B formed in the retro-Diels-Alder reaction are initially cis, as may be some of the double bonds A formed in the metathesis reaction, but these quickly isomerize to trans above 100 °C. In a DSC... 

As in any other chemical compound, different geometrical arrangements of substituent groups are possible in a polymer where rigid molecular units are involved. This gives rise to trans- and cis-configurational isomerism in polymers containing double bonds in their repeat units, as in polyacetylene and natural and synthetic rubbers. The structures of the trans- and m-isomers of polyacetylene and polybutadiene are illustrated in Fig. 1.8. 

Figure 1. TYie trans isomeric form of polyacetylene shovn in its planar trans and cis conformations, with pertinent si )stituents numbered.

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. 

NMR and IR spectra of the polyacetylenes formed support that their main chain consists of alternating double bonds. No evidence for monomer isomerization prior... 

Fig. 57 a and b. Change of the diffraction pattern of cis-polyacetylene upon heating from room temperature with 11 °C min" The cis/trans isomerization is indicated by the disappearance of the 201 reflection at about 150 °C. Above about 290 °C a broad peak indicates decomposition... 

G3(MP2) enthalpies of hydrogenation, isomerization, and formation of extended linear polyacetylenes ... 

Similar defect structures may be envisioned as arising during isomerization of cw-polyacetylene to frans-polyacetylene. If two trans sequences with opposite bond alternation approach each other along a chain containing an odd number of conjugated carbon atoms, an unpaired electron (radical) wiU be left at the point where the two sequences meet. This defect, which chemists call a free radical, is similar to the solitary charged defect produced by separation of a bipolaron, except that it is neutral. 

---