Polyacetylene is an organic polymer with -(C2H2)n repeating monomer

Structure  carbon atoms with alternating single and double bonds between them each with one hydrogen atom. It can be substituted with other functional group gives better rigidity than the saturated polymers double bonds can have either cis or trans geometry.

Segments Trans-Polyacetylene Google Image

Segment of Cis-polyacetylene Google Image

History The first conducting polymers- polyacetylenes Cuprene a high crosslinked exremely amorphous product in present of copper catalyst is the first known acetylene polymer

Synthesis Natta Routes (1958) Also called Ziegler- Natta Scheme  uses titanium and aluminum catalysts control over the structure and properties of the final polymer by varying temperature and catalyst loading

Yet Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa have changed this view with their discovery that a polymer, polyacetylene, can be made conductive almost like a metal.

X-ray diffraction studies demonstrated that the resulting polyacetylene was trans-polyacetylene preparation was an insoluble, air sensitive, and infusible black powder.

Hideki Shirakawa polyacetylene (1990) The polyacetylene film forms at the gas-liquid interface when acetylene gas passes through a heptane solution of the Ziegler-Natta catalyst. Cis polymer forms at low temperature (-78 C). Isomerization to the more stable trans form takes place on rising the temperature of the film. Conductivity of doped cis films is two or three times greater than the trans analogues.

(via the metal-carbene intermediate) Possible Polymerization Mechanism of Acetylene (via the metal-carbene intermediate) metal-carbene metallocycle Insoluble Infusible Intractable

What is conductivity? Conductivity can be defined simply by Ohms Law. V= IR Where R is the resistance,I the current and V the voltage present in the material. The conductivity depends on the number of charge carriers (number of electrons) in the material and their mobility.In a metal it is assumed that all the outer electrons are free to carry charge and the  impedance to flow of charge is mainly due to the electrons "bumping" in to each other.  Insulators however have tightly bound electrons so that nearly no electron flow occurs so they offer high resistance to charge flow.  So for conductance free electrons are needed.

What makes the material conductive? Three simple carbon compounds are diamond, graphite and polyacetylene. They may be regarded as three- two- and one-dimensional forms of carbon materials . Diamond, which contains only σ bonds, is an insulator and its high symmetry gives it isotropic properties. Graphite and acetylene both have mobile π electrons and are, when doped, highly anisotropic metallic conductors. Images from Wikipedia

Conducting Polymers Polymers are typically utilized in electrical and electronic applications as insulators where advantage is taken of their very high resistivities. Typical properties of polymeric materials: Strength, flexibility, elasticity, stability, mouldability, ease of handling, etc.

According to the paper Shirakawa’s to the Noble prize group the polymerization not only gave poly acetylene product but also gave the benzene ring The ratio between the benzene and poly acetylene depends on the species of Natta Catalyst. Not the concentration but Al and Ti ratio

Cis/ Trans Polyacetylene Higher concentration of the Ti will prodeced Cis - polyacetylene as the major products. The trans- polyacetylene is sysnthesized by lowering the reaction temperature at 150 ˚C 100% trans product but at -78 ˚C is 1.9% trans polyacetylene Trans is by the thermodynamic products and the Cis is the catalytically produced in the active site. Nobel lecture December 8, 2000. Shirakawa at University of Tuskuba

General electrical properties Polyacetylene (PA) or (CH)x is chemically the simplest A semiconductor in which chain conformation (structure) impacts band gap (as synthesized) (after thermal conversion) Image from 1

Conducting polymers behave as semiconductors Polyacetylene (After doping!) Silver Conductivity (Siemens/meter) Poly(p-phenylene vinylene) (A “highly” crystalline polymer host) 6.6 Å a   Even when doped to a highly conductive state most p-conjugated polymers behave as classic semiconductors (VRH-variable range hoping is the standard proposed mechanism) Temperature (K) + http://www.organicsemiconductors.com

Conventional Semiconductors at the atomic level p-type doping VB CB E acceptor Si Al - hole in valence band ``Ionization'' (hole in valence band) An unbonded electron + + n-type doping Egap e- Si P+ 5 valence electrons Phosphorous has VB CB Edonor At room temperature electron is delocalized in conduction band (CB) At room temperature hole is delocalized in valence band (VB) CB VB Edonor ef Eacceptor Band structure is essentially rigid Mobility is everything

Conjugation of π orbitals

Two conditions to become conductive: 1-The first condition for this is that the polymer consists of alternating single and double bonds, called conjugated double bonds. In conjugation, the bonds between the carbon atoms are alternately single and double. Every bond contains a localised “sigma” (σ) bond which forms a strong chemical bond. In addition, every double bond also contains a less strongly localised “pi” (π) bond which is weaker.

p-conjugated polymers have unusual charge excitations 5

Minding the gap Electronic states are split off from the valence and conduction bands All charge excitations involve local self-consistent structural distortions of the lattice 5

A one-dimensional chain (trans-polyacetylene) 5 From a tight-binding perspective: EA is the energy of a single atomic orbital A(R) is an overlap integral

Solubility Conductivity Substituent Effects: Coplanarity is the key for gaining high conductivity

Doping process The halogen doping transforms polyacetylene to a good conductor. nano-bio.ehu.es/.../conducting%20poly. Oxidation with iodine causes the electrons to be jerked out of the polymer, leaving "holes" in the form of positive charges that can move along the chain.

The iodine molecule attracts an electron from the polyacetylene chain and becomes I3ֿ. The polyacetylene molecule, now positively charged, is termed a radical cation, or polaron. The lonely electron of the double bond, from which an electron was removed, can move easily. As a consequence, the double bond successively moves along the molecule. The positive charge, on the other hand, is fixed by electrostatic attraction to the iodide ion, which does not move so readily. 6

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Conductivities Nobel Lecture 2000 7

Applications Conducting polymers have many uses.  The most documented are as follows: anti-static substances for photographic film Corrosion Inhibitors Compact Capacitors Anti Static Coating Electromagnetic shielding for computers "Smart Windows" A second generation of conducting polymers have been developed these have industrial uses like: Transistors Light Emitting Diodes (LEDs) Lasers used in flat televisions Solar cells  Displays in mobile telephones and mini-format television screens

Conclusion For conductance free electrons are needed. Conjugated polymers are semiconductor materials while doped polymers are conductors. The conductivity of conductive polymers decreases with falling temperature in contrast to the conductivities of typical metals, e.g. silver, which increase with falling temperature. Today conductive plastics are being developed for many uses.

Bibliography 7. nano-bio.ehu.es/.../conducting%20poly. 1. Floyd, L.K.; Grubbs, R. H.; Polycyclooctatetraene (polyacetylene): synthesis and propertiesJ. AM.Chem. Soc.,1988, 110(23),pp 7807-7813. 2. Saxon, A.; Leipins, F.; Aldissi, M. Polyacetylene: Its Synthesis, Doping and Structure. Prog. Polym. Sci: 11 57 3. Nobel lecture December 8, 2000. Shirakawa at University of Tuskuba 4. H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang and A. J. Heeger, J. Chem. Soc., Chem. Commun.1977, 578 5. Evaristo Riande and Ricardo Díaz-Calleja, Electrical Properties of Polymers 6. http://www.organicsemiconductors.com 7. nano-bio.ehu.es/.../conducting%20poly.

Question Describe the Natta Routes for the synthesis of the Poly acetylene ? Poly acetylene can be obtained in the Cis and trans geometry in the Sirakawa Synthesis. Explain the probability of such formation? Semiconducting polymers become the conductor. Explain the paradox with poly acetylene as an examples?