Chemistry Reference
In-Depth Information
small quantities, to control molecular weight and to cap the reactive end groups. The condensation
reactions are carried out at 200-250
C in an inert atmosphere. To obtain a molecular weight of about
1,000, these reactions are run for only several hours. For higher molecular weights, however, the glycols
are added in excess and the initial products heated under vacuum (about 1 mmHg) for several hours.
Among the high molecular weight aliphatic-aromatic polyesters, the highest commercial volume
material is poly(ethylene terephthalate). Most of it is prepared from dimethylene terephthalate and
ethylene glycol by a transesterification reaction:
O
O
+
n
HO
n
O
O
OH
O
O
n
O
O
Often the reaction is carried out in two steps. An excess of two moles of ethylene glycol is used and
the first stage of the reaction is carried out at 150-210
C to form bis(2-hydroxyethyl)terephthalate, a
small amount of an oligomer, and methanol, which is removed:
O
O
+
HO
OH
O
O
O
O
3
HO
OH
O
O
HO
O
HO
OH
O
O
4
O
O
In the second stage, the temperature is maintained at 270-285
C and the reaction is carried out
under vacuum at about 1 mmHg. The bis(2-hydroxyethyl)terephthalate undergoes a transesteri-
fication reaction and the excess glycol is removed:
O
O
OH
O
O
2
HO
+
OH
O
O
HO
O
O
O
O
H
O
4
HO
O
In the process, ethanol forms and is removed. Various metal oxide or acetate catalysts are
employed in the first stage. These are antimony, barium, calcium, cobalt, lead, manganese, titanium,
or zinc oxides or acetates. Carbonates, alcoholates, and alkanoates can also be used. Based on
disclosures in the patent literature, it appears that antimony compounds, particularly the trioxide,
dominate the field of catalysts for the second stage of this reaction [
16
]. The exact mechanism by
which the antimony compounds act as catalysts for the syntheses of polyesters is still being
investigated. It was shown that bis(2-hydroxyethyl)-terephthalate competes successfully with oligo-
mer end groups for Sb
2
O
3
and that a complex of this compound with the metal oxide is unreactive in
these polymerizations [
17
]. In addition, during polymerizations under vacuum, there is an increase in
