Chemistry Reference
In-Depth Information
Chapter 7
Step-Growth Polymerization and Step-Growth Polymers
7.1 Mechanism and Kinetics of Step-Growth Polymerization
Two types of monomers can undergo step-growth polymerizations [
1
,
4
,
5
]. Both are polyfunctional,
but one type possesses only one kind of functionality. An example is adipic acid that has two
functional groups, but both are carboxylic acid groups. Another one is hexamethylene diamine
with two amine functional groups. To the second type belong monomers that have both functional
groups needed for condensation on the same molecule. An example of such a monomer is
-amino
benzoic acid, where both amino and carboxylic acid groups are present in the same molecule. Chain
growths proceed from reactions between two different functional groups with both types of
molecules. An exception is formation of polyanhydrides, where the polymeric chains are formed
from two carboxylic acid groups reacting with each other and splitting out water. There are some
other exceptions as well.
p
7.1.1 Reactions of Functional Groups
Kinetic considerations are of paramount importance in understanding the mechanism of step-growth
polymerization [
1
]. As stated in Chap.
1
,
chain-growth polymerizations take place in discreet steps.
Each step is a reaction between two functional groups, like, for instance, in a polyesterification
reaction, it is a reaction between -COOH and -OH. The increase in molecular weight is slow. The
first step is a condensation between two monomers to form a dimer:
O
R
O
OR O
R'
+
+
HO
OH
H
2
O
OH
OH
O
OH
HO
R'
A dimer can react next with another monomer to form a trimer:
O
R
O
O
R
O
O
R
O
+
+
H
2
O
OH
O
OH
O
O
R
OH
OH
HO
HO
R'
R'
O
O
