the need for stoichiometric balance
. The highest gains
in molecular weight occur at the very end of the reaction. If you
are reacting A and B, and there is more B than A, then in the end, your
polymer chains will be capped with B units. An example is shown below.
In the following thought experiment, assume there are 8 A molecules and 8 B
molecules, which represents perfect stoichiometric balance. As you will
see, the theoretical limit to DP, 16, is achieved.
A A A A A A A A B B B B B B B B
(first round of reaction)
A-B A-B A-B A-B A-B A-B A-B A-B
(second round of reaction)
A-B-A-B A-B-A-B A-B-A-B A-B-A-B
(third round of reaction)
A-B-A-B-A-B-A-B A-B-A-B-A-B-A-B
(fourth round of reaction)
A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B
But of course, reactions occur continuously on the reaction coordinate.
The above scenario divided the reaction coordinate up into four equally
spaced intervals.
Now assume there are 7 A molecules and 9 B molecules
A A A A A A A B B B B B B B B B
(first round of reaction)
A-B A-B A-B A-B A-B A-B A-B B B
(second round of reaction)
B-A-B A-B-A-B A-B-A-B A-B-A-B B
(third round of reaction)
B-A-B A-B-A-B-A-B-A-B B-A-B-A-B
(fourth round of reaction)
B-A-B-A-B-A-B-A-B-A-B B-A-B-A-B
Here we have a large molecule and a small molecule.
It should make sense that when we are working with millions of monomer
molecules, that in the end, the B ---- B polymers (or A ------ A) should
have similar molecular weights.
In the previous 7:9 mismatch, we reacted one excess B in the second round
of reactions, and the other in the third round of reactions. Now we
do both in the second round of reactions.
A A A A A A A B B B B B B B B B
(first round of reaction)
A-B A-B A-B A-B A-B A-B A-B B B
(second round of reaction)
B-A-B A-B-A-B A-B-A-B B-A-B A-B
(third round of reaction)
B-A-B-A-B-A-B A-B-A-B B-A-B-A-B
(fourth round of reaction)
B-A-B-A-B-A-B-A-B-A-B B-A-B-A-B
And now, we prevent the excess B from reacting until the end
A A A A A A A B B B B B B B B B
(first round of reaction)
A-B A-B A-B A-B A-B A-B A-B B B
(second round of reaction)
A-B-A-B A-B-A-B A-B-A-B A-B B B
(third round of reaction)
A-B-A-B-A-B-A-B A-B-A-B-A-B B B
(fourth round of reaction)
A-B-A-B-A-B-A-B-A-B-A-B-A-B B B
(fifth round of reaction)
B-A-B-A-B-A-B-A-B-A-B-A-B-A-B B
Now, it is fair to criticize this model for deliberately holding back the
excess molecules away from the reaction until the end, but, what if
you had a trick to do this? The claim is made that
Interfacial polymerization
can be used to make a high molecular weight condensation polymer in spite of
stoichiometric imbalance.