Chemistry Reference
In-Depth Information
commercially important materials early in this century and still find many uses in industry. The earliest
studies reported condensations of ethylene, trimethylene, hexamethylene, and decamethylene glycols
with malonic, succinic, adipic, sebacic, and
phthalic acids [
6
]. Later studies showed that such
condensations yield high molecular weight compounds [
44
]. Nevertheless, these polyesters exhibit
poor hydrolytic stability and are generally low melting. Subsequently, however, it was found that
aromatic dicarboxylic acids yield polymers with high melting points and poly(ethylene terephthalate),
which melts at 265
C, is now an important commercial material.
Physical properties of linear polyesters follow the general observation of the relationships between
glycols with aromatic rings in the structures yield polyesters with high melting points, while the
aliphatic ones yield low melting solids or viscous liquids. In addition, hydrogen bonding, dipole
interactions, polarizations, stiff interchain bonds, molecular symmetry or regularity, and the ability of
polymeric chains to undergo close packing raise the melting points. Conversely, bulky side chains
and flexible interchain bonds lower the melting points.
ortho
7.2.1.1 Synthetic Methods
The synthetic methods that are in general use for the preparations of linear polyesters [
15
] can be
summarized as follows:
1. Dibasic acids are reacted with glycols. Stoichiometric balance is strictly maintained. When low
boiling glycols are used, however, they are often added in a slight excess and the excess gradually
removed by vacuum [
6
] or by sweeping an inert gas over or through the reaction mixture [
6
,
8
].
This procedure is useful with dicarboxylic acids that otherwise require high temperatures and
strong catalysts. Running the reaction at high temperatures can cause the glycols to condense into
ethers and the dicarboxylic acids to decompose.
2. Diesters or half esters of dicarboxylic acids or amine salts of the acids are reacted with glycols [
19
]:
O
O
R"
n
R
R
+
HO
HO
n
O
O
R'
O
O
R"
+
ROH
O
O
R'
n
The above transesterification reaction is practical for use with high melting and poorly soluble
dicarboxylic acids. In addition, less energy is needed to remove alcohol than water.
3. Hydroxy acids, like
p
-hydroxyethoxybezoic acid or
o
-hydroxydecanoic acid, are capable of self-
condensation to form polyesters [
10
]:
O
O
n
HO
2
O
OH
2
n
