Page 655 - Carrahers_Polymer_Chemistry,_Eighth_Edition

618 Carraher’s Polymer Chemistry

Grubbs and others have used the ring-opening metathesis polymerization to produce thick fi lms
of polyacetylene and polyacetylene derivatives (Equation 19.7).

R
(19.7)
R
R R

Polyacetylene has good inert atmospheric thermal stability but oxidizes easily in the presence

of air. The doped samples are even more susceptible to air. Polyacetylene films have a lustrous,
silvery appearance, and some flexibility. Other polymers have been found to be conductive. These

include poly(p-phenylene) prepared by the Freidel–Crafts polymerization of benzene, polythio-

phene and derivatives, PPV, polypyrrole, and polyaniline. The first polymers commercialized as
conductive polymers were polypyrrole and polythiophene because of their greater stability to air
and the ability to directly produce these polymers in a doped form. While their conductivities
4

(often on the order of 10 S/m) are lower than that of polyacetylene, this is sufficient for many
applications.
Doped polyaniline is employed as a conductor and as an electromagnetic shielding for elec-
tronic circuits. Poly(ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonic acid is
used as an antistatic coating material to prevent electrical discharge exposure on photographic
emulsions and is also used as a hole-injecting electrode material in polymer light-emitting
devices. Organic soluble substituted polythiophenes with good conductivities have been pre-
pared. Poly(3-hexylthiophene) has a room temperature conductivity of about 100 S/cm; poly(3-
metnylthiophene) has a conductivity of 500 S/cm; and a poly(3-alkylether)thiphene with a
conductivity of about 1,000 S/cm reported. The unsubstituted polythiophene has a conductivity
in the range of 50–100 S/cm. The fact that all of these substituted polythiophenes have similar
conductivities indicates that there is little twisting of the backbone conjugation as alkyl substitu-
ents are added.
Polythiophene derivatives are being used in field-effect transistors. Polypyrrole is being used as

microwave-absorbing “stealth” screen coatings and in sensing devices. PPV derivatives are being
used in the production of electroluminescent displays.
Following are the structures of some of the more common conjugated polymers, along with
poly(acetylene), that can be made conductive through doping. As noted before, doping causes and
electrical imbalance that allows electrons to fl ow when an electrical potential is applied. The band
gap is the energy needed to promote an election from the valence band to the empty energy or con-
ductive band. Metals have zero band gaps while insulators such as polyethylene have large band gaps
meaning that a lot of energy is needed to promote an electron to an empty band. Semiconductors
have small band gaps where valence electrons can be moved into the conductance band through
application of relatively small potential energies.

R NH R
R N R R S R
H
Polyalanine (19.8) Polypyrrole (19.9) Polythiophene (19.10)

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