Chemistry Reference
In-Depth Information
(Wolfe
et al
., 1981). On the other hand, trans-PBO was ruled out
because the polycondensation did not proceed below the temperature of
monomer decomposition. Wolfe and coworkers also attempted the synthe-
sis of cis-PBT through polycondensation of 4,6-diamino-1,3-benzenedithiol
dihydrochloride and terephthalic acid. This attempt was not successful,
for only low molecular weight polymer was obtained. Nevertheless, high
molecular weight PBT and PBO were prepared and spun to high-strength
high-modulus fibers (Wolfe and Loo, 1980; Wolfe and Sybert, 1985; 1987).
PBT was prepared (Wolfe
et al
., 1981) in polyphosphoric acid by
the reaction of terephthalic acid (TA) and 2,5-diamino-1,4-benzenedithiol
dihydrochloride (DABDT). The monomer DABDT was better prepared
under argon by the addition of the dipotassium salt of 2,5-diamino-1,4-
benzenedithiol into a large volume of 6 N HCl. The resulting solid was
collected by filtration and transferred immediately into a large volume of
dilute HCl. Concentrated HCl was then added slowly to give large hexag-
onal prisms of DABDT. The monomer prepared in this manner gave PBT
prepared on a large scale with intrinsic viscosities between 18 dL/g and
30 dL/g. With other methods, the average crystal size of the monomer was
too small, giving a greater chance of contamination and making the con-
trol of dehydrochlorination for large scale production more di
cult. For
polymerization, an 85% polyphosphoric acid (PPA) was freshly prepared
by heating 1.52 times excess weight of P
2
O
5
with 86% H
3
PO
4
at 150
◦
C
for 5 hours to 8 hours. The monomer DABDT was first activated by dehy-
drochlorination with PPA at room temperature for 24 hours under a stream
of argon, followed by heating to 70
◦
C. The complete dehydrochlorination
is essential to obtain the monomer in a reactive form. The reaction tem-
perature should be controlled to
100
◦
C, otherwise a side reaction with
the evolution of H
2
S occurs. A stoichiometric amount of terephthalic acid
of suitable particle size was then added to the reactor. Additional PPA was
also added to adjust the polymer concentration to the desired value (5% to
10%). The mixture was heated to 150
◦
C to dissolve terephthalic acid, and
then to 175
◦
C-185
◦
C for 24 hr-48 hr. The resulting polymerization mixture
was precipitated in water, and washed successively with water, ammonium
hydroxide, and water. The polymer was generally of very high molecular
weight with intrinsic viscosity as high as 30 dL/g with a weight-average
of molecular weight of
≤
40
,
000. The conditions summarized by Wolfe and
coworkers for high molecular weight PBT included the use of high purity
DABDT, the use of terephthalic acid of small particle size, the complete
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