Polyacetylene
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Polyacetylene. Polyacetylene is an organic polymer with -( C 2 H 2 )n repeating monomer. Structure.  carbon atoms with alternating single and double bonds  between them

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Polyacetylene

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Polyacetylene

Polyacetylene

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


Structure

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.


Segments trans polyacetylene

Segments Trans-Polyacetylene

Google Image


Segment of cis polyacetylene

Segment of Cis-polyacetylene

Google Image


History

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

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


Polyacetylene

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


Polyacetylene

  • 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

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.


Polyacetylene

Possible

Polymerization

Mechanism

of Acetylene

(via the metal-carbene

intermediate)

metallocycle

metal-carbene

Insoluble

Infusible

Intractable


What is conductivity

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 .

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


Conducting polymers

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.


Polyacetylene

  • 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

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


General electrical properties

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


Conducting polymers behave as semiconductors

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


Conventional semiconductors at the atomic level

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


Conjugation of orbitals

Conjugation of π orbitals


Two conditions to become conductive

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

p-conjugated polymers have unusual charge excitations

5


Minding the gap

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

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


Polyacetylene

Substituent Effects:

Solubility

Conductivity

Coplanarity

is the key for

gaining high

conductivity


Doping process

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.


Polyacetylene

  • 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


Polyacetylene

7


Conductivities

Conductivities

Nobel Lecture 2000

7


Applications

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

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

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.


Question

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?


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