Chemistry Reference
In-Depth Information
CHAPTER
7
Step-Growth Polymerizations
Lest men suspect your tale untrue, keep probability in view.
—John Gay, The Painter Who Pleased Nobody and Everybody
7.1
Condensation and Addition Polymers
There are many possible ways to classify polymers. Each may be useful depending
on the interests of the classifier. Examples include typing according to the source
of the product (e.g., naturally occurring polymers, entirely synthetic macromole-
cules, or those derived by chemical modification of naturally occurring polymers),
chemical structure (e.g., polyolefin, polyamide, etc.), polymer texture during use
(rubbery, glassy, partially crystalline), area of application (adhesive, fiber, etc.),
and so on. An important classification divides macromolecules into addition and
condensation polymers. This distinction was made by W. H. Carothers
[1]
, who
invented nylon-6,6 and made many fundamental contributions to our knowledge
and control of polymerizations.
Carother's classification into condensation and addition polymers is no longer
valid for its intended purpose because of the many advances in technology since
the idea was proposed. It is nevertheless still deeply entrenched in current think-
ing, and it will be necessary for the reader to understand its current meaning in
order to read the polymer literature with ease. In this section we review the origi-
nal classification, show why it is no longer generally applicable, summarize the
current accepted meaning of the terms addition and condensation polymers, and
then turn our attention to a useful, alternative classification which focuses on
polymerization processes rather than the products of such processes. This line of
reasoning takes us into a more detailed consideration of polymerizations in this
and succeeding chapters.
A condensation polymer is one in which the repeating unit lacks certain atoms
which were present in the monomer(s) from which the polymer was formed or to
which it can be degraded by chemical means. Condensation polymers are formed
from bi- or polyfunctional monomers by reactions that involve elimination of
some smaller molecule. Polyesters (e.g., 1-5) and polyamides like 1-6 are exam-
ples of such thermoplastic polymers. Phenol-formaldehyde resins (
Fig. 7.1
) are
