The 2000 Nobel Prize in Chemistry was awarded for work on polyacetylenes. 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. cis-Polyacetylene has a copper color, and trans-polyacetylene is silver.

(a) Draw the structures of cis- and trans-polyacetylene.

(b) Use your structures to show why these polymers conduct electricity.

(c) It is possible to prepare polyacetylene films whose electrical conductivity is anisotropic. That is, the conductivity is higher in some directions than in others. Explain how this unusual behavior is possible.

Ziegler-Natta catalysts are a mixture of alkyl aluminum and titanium halides, for example [(C₂H₅)₃AI/TiCl₄].

Cis- and trans-isomerism can be used only for the compounds in which two doubly bonded carbon atoms are having similar atoms or groups. When the similar atoms or groups lie on the same side, it is called as cis-isomer and when similar atoms or groups lie on the opposite sides, it is called as trans-isomer. These two geometric isomers cannot interconvert without breaking and forming the bonds again.

It is the property of a material which is direction dependent.

(b) As can be seen, both the structures (cis- and trans-polyacetylene) has fully conjugated system and hence, electrons could be transferred through the pi system.

(c) Electrons must flow along the pi system of the chain such that if the chains were aligned conductivity would be higher in the direction parallel to the polymer chain.