Transcript Polyacetylene - University of Nebraska Omaha

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
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 transpolyacetylene
• 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.
Possible
Polymerization
Mechanism
of Acetylene
WCl6 / Bu4Sn
HC
CH
acetylene
polyacetylene
Bu4Sn
Bu3SnCl
CH3CH2CH2CH2 WCl5
WCl6
(via the metal-carbene
intermediate)
Bu4Sn
"Metal carbene"
Bu3SnCl
H
C WCl4
CH3 CH2CH2 C
H
CH3CH2CH2
HC
HC
H
CH3CH2CH2CH3
WCl 4
H3CH2CH2CH2C
CH
CH
HC
CH
CH
WLn
HC
CH
H
C
WLn
H7C3
H7C3
metal-carbene
WLn
C
H
C3H7
metallocycle
HC
H
H
C C3H7
H C
C
H
HC
HC WLn
CH
C
H
H
C3H7
C
HC
C
H C
H
H
WLn
C3H7
C
C
H
WLn
C
C3H7
CH
C
H
HC CH
termination
WLn
etc.
H
Polyacetylene
Insoluble
Infusible
Intractable
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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
(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
doping!)
(After
(K)
Even when doped toTemperature
a highly conductive
state most
p-conjugated polymers behave as classic
semiconductors (VRH-variable range hoping is the
standard proposed mechanism)
http://www.organicsemiconductors.com
+
Conventional Semiconductors at the atomic level
n-type doping
p-type Si
doping
Phosphorous has
5 valence electrons
Si
Si
Si
An unbonded electron
Al
Si
Si
eP+
CB
Egap
+
Si
Si
Si
Edonor
+
0
Al -
CB
VB
Si
CB
CB
hole in valence band
VB
Band structure is essentially rigid
ef
VB
Si
At room temperature hole is
delocalized in valence band (VB)
ef
Edonor
+
(hole in valence band)
E acceptor
0 +
At room temperature electron is
delocalized in conduction band (CB)
Si
Eacceptor
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.
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
EA is the energy of a
single atomic orbital
A(R) is an overlap
integral
Substituent Effects:
Solubility
Conductivity
R
doped with I 2
> 10 S/cm
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
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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
7
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
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
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?