Chemistry Reference
In-Depth Information
extrusions. These materials may contain up to 2-4 mole percent of di(oxyethylene)oxy units [
54
].
Presence of such units influences the degree of whiteness of the polyesters and the melting temperature.
A process was developed [
54
] to reduce the presence of di(oxyethylene)oxy units in poly(ethylene
terephthalate). The condensation reactions are still carried out in two steps. In the first one, or during
the period of precondensation, the material is prepared in the melt as described previously. In second
step, however, the reaction is carried out below the melting temperature. This still yields high
molecular weight polymers. The products, however, are low in di(oxyethylene)oxy linkages.
There has been continued interest in developing a process for direct esterification of terephthalic
acid with ethylene glycol. It does not appear, however, that this is currently practiced on commercial
scale in the U.S. In Japan, a process was commercialized where terephthalic acid is reacted with two
moles of ethylene oxide to form the dihydroxy ester in situ, as the starting material. One mole of
ethylene glycol is then removed under vacuum in the subsequent condensation process. Also it was
reported [
25
] that the polymer can be prepared by direct esterification at room temperature in the
presence of picryl chloride. The reaction can also be performed at about 120
C in the presence of
diphenyl chloro-phosphate or toluenesulfonyl chloride [
25
]. This is done in solution, where pyridine is
either the solvent or the cosolvent. Pyridine acts as a scavenger for HCl, which is a by-product of the
reaction and, perhaps, also as an activator (by converting the acid into a reactive ester intermediate).
Another commercially important high molecular weight polyester is poly(butylene terephthalate),
also called poly(tetramethylene terephthalate). The polymer is prepared by a catalyzed ester inter-
change of dimethyl terephthalate and 1,4-butane diol:
O
O
n
+
O
HO
n
OH
O
O
O
O
+
n
CH
3
OH
O
O
n
This synthesis is also carried out in two stages. In stage one, an excess of the diol is reacted with
dimethyl terephthalate (about 1.3:1) to insure complete removal of methanol. Zinc acetate is favored
as a catalyst for this reaction. A prepolymer mixture of bis(hydroxybutyl)terephthalate and higher
oligomers forms. Stage two is conducted in vacuum at 1 mmHg and high enough temperature (usually
at least 60
C above the melting temperature of the polyester) to remove excess diol and reach high
molecular weight. Zinc oxide is favored as the catalyst for this stage.
Prolonged heating of the reaction mixture at excessive temperatures results in formation of large
proportions of tetrahydrofuran. This is objectionable because it affects the properties of the product. It
also results in lower molecular weight polyesters.
A polyester from terephthalic acid and 1,2-dimethylol cyclohexane is produced mainly for use in
fibers. This polymer is also formed from dimethyl terephthalate and the diol by a transesterification
reaction. The material has the following structure:
O
O
O
O
n
The polymer is stiffer than poly(ethylene terephthalate) and higher melting.
