R7.1 Polymerization

Termination by
combination
375
Chap.
Figure R7.1-5 compares the molecular weight distribution for poly(hexamethylene
adipamide) calculated from Flory’s most probable distribution6
[Equation
(R7.1-35)] for a conversion of 99% with the experimental values obtained
by fractionation. One observes that the comparison is reasonably favorable.
For termination by combination, the mole fraction of polymers with j
repeating units is
while the corresponding weight fraction is
where p is given by Equation (R7.1-22) [i.e., p = B].
7.1.5 Anionic Polymerization
(R7.1-36)
(R7.1-37)
To illustrate the development of the growth of live polymer chains with time,
we will use anionic polymerization. In anionic polymerization, initiation takes
place by the addition of an anion, which is formed by dissociation of strong
bases such as hydroxides, alkyllithium, or alkoxides that react with the mono-
mer to form an active center, . The dissociation of the initiator is very rapid
and essentially at equilibrium. The propagation proceeds by the addition of
monomer units to the end of the chain with the negative charge. Because the
live ends of the polymer are negatively charged, termination can occur only by
charge transfer to either the monomer or the solvent or by the addition of a
6
W j × 10 4
P. J. Flory,
1953).
35
30
25
20
15
10
5
0
0 100
MOL.WT.
10,000 20,000 30,000
p = 0.990
j
40,000 50,000 60,000
200 300 400 500
Figure 7-1 Molecular distribution. [Adapted from G. Tayler, Journal of the
Fogler/Prenhall/F7.7 rev 12/11/97
American Chemical Society, 69, p. 638, 1947. Reprinted by permission.]
y j ( j � 1)
( 1 � p)
2 �
p
w j
1
2
Principles of Polymer Chemistry,
(Ithaca, N.Y.: Cornell University Press,
j 2 �
-- j ( 1 � p)
3 � ( j � 1)p
R 1 �
j 2 �