Chemistry Reference
In-Depth Information
Two dimers can combine to form a tetramer:
O
R
O
O
R
O
O
R
O
OH
R'
R'
OH
O
OH
OH
O
O
O
R'
These step condensations continue slowly with the molecular weights of the polymers increasing
with each step. In such reactions, the monomeric species disappear early from the reaction mixtures,
long before any large molecular weight species develop. In most step-growth polymerizations, on a
weight basis, less than 1% of monomeric species remain by the time the average chain length attains
the size of ten combined monomeric units [
1
,
3
,
4
,
6
].
One important characteristic of step-growth polymerizations is that any functional group on any
one molecule is capable of reacting with any opposite functional group on any other molecule. Thus,
for instance, if it is a reaction of polyesterification, any carboxylic acid group on any one molecule,
regardless of size, can react with any hydroxy on another one. This is true of all other step-growth
polymerizations. It means that the rates of step-growth polymerizations are the sums of the rates of all
reactions between molecules of various sizes. A useful assumption that can be applied here is that the
reactivities of both functional groups remain the same throughout the reaction, regardless of the size
of the molecules to which they are attached. This allows treating step-growth polymerizations like
reactions of small molecules. General observations would suggest slower reactivity of functional
groups attached to large molecules. This, however, is usually due to lower diffusion rates of large
molecules. The actual reactivity of the functional groups depends upon collision frequencies (number
of collisions per unit of time) of the groups and not upon the rate of diffusion. Functional groups on
the terminal ends of large molecules have greater mobility than the remaining portions of the
molecules as a whole. In addition, the reactivity of one given functional group in a bifunctional
molecule is not altered by the reaction of the other group (if there is no neighboring group effect).
This implies that the reactivities of functional groups are not altered during the polymerization.
The kinetics of step-growth polymerization can be derived from a polyesterification reaction that
follows the same course as all acid-catalyzed esterifications [
2
].
1. Protonation step:
O
OH
k
1
+
+
HA
A
k
2
OH
OH
2. Reaction of the protonated carboxylic acid group with the alcohol:
OH
OH
k
3
k
5
+
OH
O
k
4
OH
OH
H
O
+
H
2
O
+
H
O
The above polyesterifications, like many other reactions, are equilibrium reactions. They must be
conducted in a way that allows the equilibrium to shift to the right to attain high molecular weights.
