Chemistry Reference
In-Depth Information
In 1929, Carothers [
19
] suggested a separation of all polymers into two classes,
condensation
and
addition
polymers. By condensation polymers he defined those polymers that lack certain atoms from
the monomer units from which they were formed or to which they may be degraded by chemical
means. An example would be a polyester:
O
OH
O
O
OH
+
HO
HO
O
O
O
n
He also defined addition polymers as polymers with identical structures of the repeat units to the
monomers from which they are derived. According to the above definition, an example of an addition
polymer can be polystyrene that is formed by addition of styrene monomers:
n
n
monomer
polymer
Note: The definition ignores loss of double bonds. The Carothers definition fails to describe all
the polymers that can fit into the category of condensation polymers, yet form without an evolution
of a byproduct. An example is polyurethane that can form from a reaction of a glycol with a
diisocyanate:
O
O
+
n
n
ONR
NCO
HO
R'
OH
C
N
R
N
C
O
R'
O
n
H
H
Flory proposed a superior definition [
20
]. It is based on the reaction mechanism involved in the
formation of the two classes of polymers. Into the first category (it includes all the condensation
polymers) falls the macromolecules that form through reactions that occur in
discreet steps
. They are,
therefore, called
step-growth
polymers. Such polymerizations require long periods of time for each
macromolecule to form, usually measured in hours. Into the second category belong all polymers that
form by
chain propagating reactions
. They are, therefore, called
chain-growth polymers
, as one
might expect. Such reactions depend upon the presence of active centers on the ends of the growing
chains. The chains grow by propagating these reactive sites through inclusion of monomers at such
sites. These inclusions are very rapid and chain-growth can take place in a fraction of a second, as the
chains successively add monomers.
The important features of step-growth polymerizations are:
1. The monomer is consumed early in the beginning of the reaction while the increase in molecular
weight occurs only slowly.
2. The growth of polymeric chains takes place by reactions between monomers, oligomers, and
polymers.
