Chemistry Reference
In-Depth Information
If the amount of glycerin is small, then the product is only branched. In addition, there might be only
one branch per molecule:
A
BB
BA
AB
BA
AB
BA
AB
BA
AB
BA
B
A
A
B
B
Statistical methods were developed for prediction of gelation [
7
]. These actually predict gelation at
a lower level than does the Carothers equation shown above. As an example, we can use a reaction of
three monomers, A, B, and C. We further assume that the functionality of two monomers,
f
A
and
f
B
,is
equal to two, while that of
f
C
is greater than two. The critical reaction conversion can then be written as:
1
=
2
p
c
¼
1
=½r þ rpðf
C
2
Þ
7.1.3 Ring Formation in Step-Growth Polymerizations
Step growth polymerizations can be complicated by cyclization reactions that accompany formations
of linear polymers. Such ring formations can occur in reactions of monomers with either the same type
of functional groups or with different ones. Following are some illustrations of cyclization reactions:
H
R
N
R
+
H
2
N
COOH
H
2
O
O
O
R
H
x
H
2
N
COOH
x
H
N
COOH
R
O
R
2
HO
COOH
O
O
OR
x-1
x
HO
COOH
O
O
Similarly, dicarboxylic acids can cyclize into anhydrides.
Whether ring formation is likely to take place or not depends upon the size of the ring that can
form. If cyclization results in rings with strained bond angles or repulsions due to crowding, the
probability of their formation is low. So, small rings, with less than five members, do not form readily.
Five-membered rings, however, are essentially free from bond angle distortion and have a greater
