Chemistry Reference
In-Depth Information
stoichiometry of the reactants must be carefully controlled, and a close balance
is needed between the concentrations of functional groups of opposite kinds.
6.
It must be possible to control the reaction to produce target average molecular
weights and molecular weight distributions.
It is necessary to understand these requirements in order to be able to carry
out and control step-growth polymerizations. The requirements listed above are
considered in more detail in the following sections of this chapter.
7.3.1
Speed of Step-Growth Polymerizations
It can be seen from the preceding examples that the same macromolecular struc-
ture can be produced by alternative step-growth polymerizations of different
monomers. The rate of a polymerization reaction will depend on the reactivity of
the particular monomers under the experimental conditions. Most of the least
expensive monomers used for the production of large-volume polymers by step-
growth syntheses react very slowly at room temperature even when the reaction
responds to catalysis. The examples given also illustrate the general rule that it is
usually more practical to increase the reaction rate by moving to high tempera-
tures than by switching to more reactive monomers which will be more expensive
per se and may require extra finishing operations for the polymer. Hot reaction
temperatures also facilitate removal of volatile condensation products and help
drive the polymerization reaction to completion.
This conclusion is widely applicable. Where possible, step-growth polymeriza-
tions are carried out with the least expensive monomers and are accelerated by resort
to high reaction temperatures and catalysts, when these are available. Further consid-
eration of reaction engineering in such polymerizations is given in Chapter 12.
7.3.2
Side Reactions in Step-Growth Polymerization
Side reactions present a particular problem in step-growth polymerizations
because these syntheses are often carried out at high temperatures compared to
the comparable reactions in conventional organic chemistry. Thus, the acid-
catalyzed esterification of ethanol with acetic acid is performed commercially at
about 70
C whereas the polyesterification of ethylene glycol and terephthalic
acid must be finished at about 275
C to obtain a high conversion of functional
groups and a product with satisfactory molecular weight. Side reactions that are
of negligible importance in conventional esterifications can become very signifi-
cant at these higher temperatures.
The difficulties with side reactions are particularly important when linear poly-
mers are being synthesized from bifunctional monomers. When branched thermo-
setting polymers are being produced from polyfunctional monomers, the
functionality of the polymer increases as it grows in size, and some wastage of
functional groups is not too serious as long as the polymer can still be cross-linked
to convert it to its final, thermoset stage. Because of their application as adhesives
