Page 389 - Elements of Chemical Reaction Engineering Ebook

360 Nonelernentary Reaction Kinetics Chap. 7

Ma-M
Recalling p = -
MO
Pj = M,(1 -p)*pj-1 (E7-3.13)

The mole fraction of polymer with a chain lengthj is just

Recalling M = Mo(l - p), we obtain
- 1 (7-27)

This is the Flory-Schulz distribution. We discuss thls distribution further after
we discuss chain reactions.

7.3.2 Chain Polymerizations Reactions

Chains (i.e., addition) polymerization requires an initiator (I) and pro-
ceeds by adding one repeating unit at a time.
Z+M -----+ R,
M+R, -----+ R2
M+R2 -----+ R,
M+R, ---+ R4
R,,
M+ R4 * etc.
Here the molecular weight in a chain usually builds up rapidly once a chain is
initiated. The formation of polystyrene,

n c6 H5CH=CH2 ---+ [-CHCH,-1,
I
c6 H5
is an example of chain polymerization. A batch process to produce polystyrene
for use in a number of molded objects is shown in Figure 7-4.
We can easily extend the concepts described in the preceding section to
polymerization reactions. In this section we show how the rate laws are formu-
lated so that one can use the techniques developed in Chapter 6 for multiple
reactions to determine the molecular weight distribution and other properties.
In the material that follows we focus on free-radical polymerization.

7.3.2.1 Steps in Free-Radical Polymerization

The basic steps in free-radical polymerization are initiation, propagation,
chain transfer, and termination.