Chemistry Reference
In-Depth Information
polymerizations where the monomers add to the end of a macromolecule without
the intervention of an active center.)
It is not practical to conduct free-radical polymerizations under conditions
where there is an equilibrium between polymerization and depolymerization pro-
cesses. The polymer synthesis is effectively irreversible in normal radical poly-
merizations. The course of the reaction is then determined kinetically, and the
molecular weight distribution cannot be predicted statistically as was done for
equilibrium step-growth polymerizations described in Chapter 7.
This section develops the overall kinetics for an ideal free-radical polymeriza-
tion. Various details of the individual reactions in the kinetic sequence are treated
in subsequent sections of this chapter.
8.3.1
Initiation
Free radicals must be introduced into the system to start the reaction. There are
many ways to accomplish this, but the most common method involves the use of
a thermolabile compound, called an
initiator
, which decomposes to yield two free
radicals at the temperature of the reaction mixture. That is,
k
d
2R
(8-6)
Here the initiator I decomposes to yield two radicals R
. (These are called
pri-
mary radicals
.) The specific rate constant for this reaction at the particular tem-
perature is
k
d
. The initiation reaction
per se
follows if a radical R
adds to a
monomer as in
I
!
H
H
R
+ CH
2
C
(8-7)
C
R
CH
2
Cl
Cl
Reaction (8-7) may be abbreviated and generalized to
k
i
M
1
(8-8)
where M stands for monomer and M
1
denotes a monomer-ended radical (like that
in the preceding reaction). There is only one monomer in this radical. The rate
constant for reaction (8-8) is
k
i
.
Reaction (8-6) is a unimolecular decomposition, as written, so
k
d
will be a
first-order rate constant. The magnitude of this decomposition rate constant is
usually of the order of 10
2
4
R
1
M
!
to 10
2
6
sec
2
1
at the temperatures at which such
initiators are used.
The rate of radical production from reaction (8-6) is
R
=
d
½
dt
2
k
d
½
I
(8-9)
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