Chemistry Reference
In-Depth Information
metal surfaces of antimony oxide [
18
]. It was suggested that a reaction of antimony trioxide with
ethylene glycol results in formation of antimony glycolates with a ligand number of 3 [
19
,
20
]:
O
HO
O
OH
Sb
O
O
Sb
O
O
O
O
O
Sb
Sb
H
OH
OO
O
3
A study of antimony glycolates as effective catalysts for preparation of poly(ethylene terephthal-
ate) with varying number of hydroxyethoxy ligands rated them in the following decreasing order of
effectiveness [
20
]:
5
ΒΌ
3
>
2
>
1
0
This led to a suggestion by Maerov [
20
] that a key step in condensation may involve a chemical
reaction that ties up a hydroxyl chain end with the catalyst molecule. Introduction of a second
hydroxyethoxy chain end is followed by the right electronic bond shift:
O
O
O
O
R
H
O
Sb
R
O
The role of antimony is to establish a favorable spatial configuration for the transition state [
20
].
An earlier study, however, resulted in a conclusion that antimony's activity is inversely proportional
to the hydroxyl group concentration [
21
].
Based on model reactions for the preparation of poly(ethylene terephthalate) by ester interchange,
the optimum molar ratio of ethylene glycol to dimethyl terephthalate is 2.4 to 1. This ratio allows
complete removal of methanol [
22
]. The overall polyesterification reaction is third order [
22
,
23
]. In
addition, high molecular weight polymerizations of poly(ethylene terephthalate) invariably produce
some cyclic oligomers as by-products [
24
,
25
]. Eight different cyclic species were identified in one
commercial polymer:
O
O
O
O
n
O
O
The most important side reaction, however, is formation of diethylene glycol. It becomes included in
the polymer as a di(oxyethylene)oxy link. Commercial polymerizations are carried out until molecular
weights of about 20,000 are reached, for use in fibers, and higher ones, for use in injection moldings or
