C
2
H
2

• 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 .

Google Image

Google Image

• 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 transpolyacetylene
• preparation was an insoluble, air sensitive, and infusible black powder.

•
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.

Possible
Polymerization
Mechanism of Acetylene
(via the metal-carbene intermediate)
HC CH acetylene
WCl
6
WCl
6
/ Bu
4
Sn
Bu
4
Sn Bu
3
SnCl polyacetylene
CH
3
CH
2
CH
2
CH
2
WCl
5
Bu
4
Sn
"Metal carbene"
H
CH
3
CH
2
CH
2
C WCl
4
CH
3
CH
2
CH
2
CH
3
CH
3
CH
2
CH
2
H
C
Bu
3
SnCl
H
WCl
4
H
3
CH
2
CH
2
CH
2
C
HC CH
HC CH
H
H
7
C
3
C WL n metal-carbene
H
H
C
C C
3
H
7
HC
C
H
HC WL n
HC
CH
H
H
C
C
H
HC
C
C
3
H
7
C
WL n
H
CH
WL n
H
7
C
3 metallocycle
C
3
H
7
HC CH
C
H
C
3
H
7
WL n
HC CH
H
C
H C
3
H
7
C
H
C
CH
WL n
C
H
HC CH etc.
H
WL n termination
Polyacetylene
Insoluble
Infusible
Intractable 12

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

• 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

• 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

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

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

Conventional Semiconductors at the atomic level p -type doping
Si n -type doping
Phosphorous has
5 valence electrons
Si Si e -
P +
CB
Si
E gap
+
+
Si
E donor
0
Al
Si
An unbonded electron
VB
At room temperature electron is delocalized in conduction band (CB)
0 +
CB
VB
Si Si
Si Si
Al
-
+
(hole in valence band)
Si Si
E acceptor hole in valence band
CB CB
At room temperature hole is delocalized in valence band (VB)
E donor e e f f
E acceptor
Band structure is essentially rigid
Mobility is everything
VB
VB

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
5
All charge excitations involve local self-consistent structural distortions of the lattice

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

Substituent Effects:
Solubility
Conductivity doped with I
2
> 10 S/cm
R R R R R
R = Me, Br, Cl ......... etc.
doped with I
2
; < 0.001 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
Coplanarity is the key for gaining high conductivity
26

• 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 I
3
. 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

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

•
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.

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.

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?