Polyacetylene

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

2. 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 cisor trans geometry.

3. Segments Trans-Polyacetylene Google Image

4. Segment of Cis-polyacetylene Google Image

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

6. 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

7. Yet Alan J. Heeger, Alan G. MacDiarmid and HidekiShirakawahavechangedthisviewwiththeirdiscoverythat a polymer, polyacetylene, can bemadeconductivealmostlike a metal.

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

9. 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.

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

13. 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.

14. Three simple carbon compounds are diamond, graphite and polyacetylene. They may be regarded as three- two- and one-dimensional forms of carbon materials . What makes the material conductive? 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

15. 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.

16. 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

17. Cis/ Trans Polyacetylene • Higher concentration of the Ti will prodecedCis - 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 • Cisis the catalytically produced in the active site. Nobel lecture December 8, 2000. Shirakawa at University of Tuskuba

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

19. Conducting polymers behave as semiconductors Conductivity (Siemens/meter) Silver a 6.6 Å Polyacetylene (After doping!) + Poly(p-phenylene vinylene) (A “highly” crystalline polymer host) 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

20. n-type doping Phosphorous has 5 valence electrons Si Si p-type doping Si Si An unbonded electron Al e- P+ CB Si Si Si Si ``Ionization'' Egap Edonor + + 0 Si Si VB + - CB Al (hole in valence band) Si Si E acceptor 0 + hole in valence band VB CB CB ef Edonor Eacceptor ef VB VB Conventional Semiconductors at the atomic level At room temperature electron is delocalized in conduction band (CB) At room temperature hole is delocalized in valence band (VB) Band structure is essentially rigid Mobility is everything

21. Conjugation of π orbitals

22. 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.

23. p-conjugated polymers have unusual charge excitations 5

24. 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

25. 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

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

27. 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.

28. The iodine molecule attracts an electron from the polyacetylene chain and becomes I3ֿ. The polyacetylene molecule, now positively charged, is termed a radical cation, orpolaron. • 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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31. Conductivities Nobel Lecture 2000 7

32. 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

33. 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.

34. Bibliography 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.

35. 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?