Polyacetylene

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

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Segment of Cis-polyacetylene

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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.

PossiblePolymerization Mechanism of Acetylene(via the metal-carbene intermediate)

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HC CHWCl6 / Bu4Sn

polyacetyleneacetylene

WCl6 CH3CH2CH2CH2 WCl5

Bu4Sn Bu3SnCl

Bu4Sn

Bu3SnCl

CH3CH2CH2 C

WCl4

H

H

H3CH2CH2CH2C

CH3CH2CH2CH3

"Metal carbene"

C WCl4CH3CH2CH2

H

HC CH

C WLn

H7C3

H

HC CH HC

CH

CH

WLn

H7C3

HC CH

C WLn

C3H7

H

HC CH

C

C

H C3H7

H CH

WLnC

C

H

HHC C

CH

C3H7

WLn

C

H

CH C3H7

H

CC

C

C3H7

H

H

HHC

HC WLn

HC CH

etc.WLn

H

termination

Polyacetylene

InsolubleInfusibleIntractable

metallocyclemetal-carbene

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 metallicconductors. 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

(as synthesized)

(after thermal conversion)

A semiconductor in which chain conformation (structure) impacts band gap

Polyacetylene (PA) or (CH)x is chemically the simplest

Image from 1

Conducting polymers behave as semiconductors

Polyacetylene (After doping!)

Silver

Conductivity (Siemens/meter)

6.6 Å

Poly(p-phenylene vinylene) (A “highly” crystalline polymer host)

+

Temperature (K)Even when doped to a highly conductive state most conjugated polymers behave as classic semiconductors (VRH-variable range hoping is the standard proposed mechanism)

http://www.organicsemiconductors.com

Band structure is essentially rigid

Conventional Semiconductors at the atomic level

+

n-type doping

Egap

e-

Si Si

Si Si

P+

5 valence electronsPhosphorous has

VB

CB

0Edonor+

At room temperature electron is delocalized in conduction band (CB)

p-type doping

VB

CB

0 E acceptor

Si Si

Al

Si Si

Si Si

Al

Si Si

-

hole in valence band

``Ionization''(hole in valence band)

An unbonded electron

+

+

CB

VB

Edonor

f

CB

VB

Eacceptorf

At room temperature hole is delocalized in valence band (VB)

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.

-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 lattice5

A one-dimensional chain (trans-polyacetylene)

EA is the energy of a single atomic orbital

A(R) is an overlap integral

From a tight-binding perspective:5

Coplanarity is the key for gaining high conductivity

Substituent Effects: Solubility Conductivity

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full overlaping

Coplanarity gives best overlapingbetween orbitals

Distortion from coplanarityreduces the electron mobility

partial overlaping

no overlaping

90o distortion lead to conjugation defects

doped with I2

R R R R R

R = Me, Br, Cl ......... etc.doped with I2; < 0.001 S/cm> 10 S/cm

Steric hindrance effect of substituent is very important

Because,R group destroy the coplanarity of the conjugation system

Reduce electron mobility of intrachain and interchain

Doping process• The halogen doping transforms polyacetylene to a good conductor.

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.

nano-bio.ehu.es/.../conducting%20poly.

• The iodine molecule attracts an electron from the polyacetylene chain and becomes I3 ֿ. 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

7

Conductivities

Nobel Lecture 2000

7

ApplicationsConducting 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 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, 5785. Evaristo Riande and Ricardo Díaz-Calleja, Electrical Properties of Polymers

6. http://www.organicsemiconductors.com

7. nano-bio.ehu.es/.../conducting%20poly.

Question

1. Describe the Natta Routes for the synthesis of the Poly acetylene ?

2. Poly acetylene can be obtained in the Cis and trans geometry in the Sirakawa Synthesis. Explain the probability of such formation?

3. Semiconducting polymers become the conductor. Explain the paradox with poly acetylene as an examples?