Page 31 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN001F-4 May 7, 2001 16:19

Acetylene 83

acidprecursor,whichisthenreactedwithm-aminophenyl- for which it is claimed to have great potential are rechar-
acetylene to end-cap the prepolymer. Azeotropic removal gable, lightweight automobile batteries and rechargable,
of water (refluxing benzene or toluene) converts the portable energy-storage devices, solar batteries, radio and
polyamic to polyimide acetylenic prepolymer. The latter EMIshielding,fuelcells,Schottkydevices,andwire-cable
◦
is then cured at 200 C. applications.Companiesthathavecarriedoutresearchand
Gulf Chemical Company purchased the technology for commercial development activities with polyacetylene are
these polymers and marketed the HR-600 product under Rohm and 2Haas, Allied, BASF, Xerox, IBM, GTE, and
the trade name Thermid 600. The applications recom- Showa Denko. Rohm and Haas has actually produced pilot
mended for the product included aircraft and missile struc- quantities of undoped polyacetylene film for commercial
tures, structural adhesives, low-friction bearings, nuclear- sampling.
radiation-resistant parts, and circuit boards. The Thermid To date, no significant markets have developed for
600 line included a polyimide molding powder (MC-600), polyacetylene. The following problems are associated
an N-methylpyrrolidone solution of the polyamic polymer with its production and the utilization of doped polyace-
(50% solids, LR-600), and a fast-drying alcohol solution tylene:
of the polyamic ester form (Thermid AL-600). Gulf has
sold this specialty acetylenic polymer business to National 1. Difficulty in melt processing and extruding
Starch. 2. Deterioration and loss of conductivity in air and in the
presence of moisture
3. Need for expensive processing equipment
D. Polyacetylene ( CH ) x
4. Lack of compatibility with other polymeric materials
McDiarmid and Shirakawa observed that the polymeriza-
Polyacetylene, on a weight basis, is an outstanding con-
tion of acetylene in the vapor state on glass surfaces with
ductive material, but its chemical environments and con-
Ziegler–Natta catalysts, followed by doping with a vari-
ditions of use must be carefully controlled. New dopants
ety of additives such as halogens, antimony pentafluoride,
−
such as hexafluorophosphide ion (PF ) render the poly-
perchloric acid, sodium, and lithium, resulted in greatly 6
mer more stable to oxidation and hydrolysis, and further
increased conductivity. A typical increase in conductivity
research along this line may be fruitful. Also, copolymer-
from that of the original undoped polyacetylene to that of
3
the doped polymer was of the order of 10 −9 to 10 (doped) ization with new acetylenic monomers or the formation
−1
−1 cm . of lower molecular weight, linear polyacetylenes may im-
prove the melt processability of the polymer.
Polymerization of acetylene at room temperature yields
Polyacetylene doped with lithium (as the anode) is
a mixed cis–trans product, approximately 70–80% cis iso-
a highly conductive system, which because of poly-
mer. Polymerization at low temperature gives an all-cis
acetylene’s highly unsaturated (polyene) structure has the
◦
polymer. Heating the cis isomer to 200 C for 1 hr con-
unique property of storing and releasing electrons. This
verts it completely to the trans form. The conductivities of
property, together with the low unit weight of the cell,
the undoped cis and trans isomers are essentially identical
−9
(cis, trans, 10 ), while both doped isomers give conduc- makes rechargable, portable batteries a commercial pos-
3
tivities up to 10 −1 cm −1 in resistivity. sibility. However, such a cell is highly vulnerable to air
oxidation and traces of moisture. Also, the other difficul-
Polyacetylene film is a fibrillar mat of randomly ori-
ties cited above for doped polyacetylene have not yet been
ented strands. The dopants used are classified as elec-
resolved. At some future time in new space–age applica-
tron acceptors (bromine, iodine, arsenic pentafluoride)
tions using the inertness of outer space, polyacetylene may
or electron donors (lithium, sodium, potassium). A poly-
prove its worth.
mer treated with donor or acceptor dopants yields n-type
The year 2000 Nobel Prize in Chemistry was awarded
or p-type semiconductors, respectively. Polyacetylene
to Alan J. Heeger, Alan G. MacDiarmid, and Hideki
dopedwithSbF 5 orAsF 5 isabetterconductorofelectricity
Shirakawa for the discovery of and development of con-
on a weight basis than mercury. The doped, random film
ductive polymers, which among the first were the poly-
mat can be stretched oriented by thermal and mechani-
cal treatment to give conductivities as high as 2000 −1 actylenes.
−1
cm . The conjugated polyene chain of polyacetylene and
its formation of charge-transfer complexes with dopants
are responsible for the greatly enhanced conductivities XIV. ACETYLENE RESEARCH IN RUSSIA
observed.
Polyacetylene has been given much publicity in pub- Russia has maintained a strong interest in acetylene chem-
lications, trade magazines, and the press. Applications istry since the discovery of the Favorsky reaction, in which