Chemistry Reference
In-Depth Information
Reaction (b) was once the preferred method for making poly(ethylene terephthal-
ate) because dimethyl terephthalate can be readily purified to the quality necessary
for production of this polymer. It should be remembered in this connection that any
impurities or structural imperfections produced during the polymerization process
will be left in the final polymer. An important side reaction in manufacture of this
polymer results from the formation of diethylene glycol (HOCH
2
CH
2
OCH
2
CH
2
OH),
the ether produced by dehydration of two molecules of ethylene glycol. This impu-
rity combines randomly in the polyester. It lowers the polymer melting temperature
compared to that of the pure homopolymer and affects its rate of crystallization. The
net results are adverse influences on fiber spinning and heat setting to stabilize fiber
dimensions. Also, inadvertent production of color-forming centers detracts from the
appearance of textile fibers made from this polymer.
The only practical means for ensuring the desired polymer quality is to use
scrupulously pure monomers. Purification of a polymer after it is synthesized
would be prohibitively expensive, because these materials are sparingly soluble
and are often difficult or impossible to crystallize or free of solvent. The overall
least expensive route to good quality poly(ethylene terephthalate) was therefore
through the dimethyl ester of terephthalic acid as shown in reaction (b). The
byproduct methanol was recovered to generate more diester from the acid. In
more recent years, methods have been developed to produce the diacid with satis-
factory purity, and reaction (a) is now the preferred route to this polymer because
the esterification step with methanol can be eliminated. Reactions (d), (e), and
others, which the reader may be able to write, will be more expensive in the final
analysis for the various reasons mentioned above.
The foregoing discussion illustrates one of the reasons why the great majority
of step-growth polymerizations which can be written on paper are not used in
fact: the expenses involved in the purchase of monomers, carrying out the reac-
tion, or preparing the polymer for further use. The overall process that produces a
final product of required quality at the lowest cost will be chosen.
Another reason why many theoretical step-growth syntheses are not employed
is because many of these polymerization reactions are not efficient or do not give
a good yield of polymer with the desired molecular weight.
A satisfactory step-growth polymerization reaction must satisfy the following
requirements:
1.
The reaction must proceed at a reasonably fast rate.
2.
The polymerization must be free of side reactions that produce cyclic or
otherwise undesirable products.
3.
The monomers that are employed must be free of deleterious impurities.
4.
It must be possible to drive the process almost to complete reaction of the
functional groups.
5.
Many important step-growth polymerizations are run under conditions in which
the reverse reaction between the polymer and condensation products is
significant [as in reactions (a) and (b) in
Fig. 7.2
]. In these cases, the
